Thursday, December 31, 2020

Java Semaphore With Examples

Semaphore is one of the synchronization aid provided by Java concurrency util in Java 5 along with other synchronization aids like CountDownLatch, CyclicBarrier, Phaser and Exchanger.

The Semaphore class present in java.util.concurrent package is a counting semaphore in which a semaphore, conceptually, maintains a set of permits. Semaphore class in Java has two methods that make use of permits-

  • acquire()- Acquires a permit from this semaphore, blocking until one is available, or the thread is interrupted. It has another overloaded version acquire(int permits).
  • release()- Releases a permit, returning it to the semaphore. It has another overloaded method release(int permits).

Wednesday, December 30, 2020

Java Exchanger With Examples

Exchanger in Java is one of the Synchronization class added along with other synchronization classes like CyclicBarrier, CountDownLatch, Semaphore and Phaser in java.util.concurrent package.

How does Exchanger in Java work

Exchanger makes it easy for two threads to exchange data between themselves. Exchanger provides a synchronization point at which two threads can pair and swap elements. Exchanger waits until two separate threads call its exchange() method. When two threads have called the exchange() method, Exchanger will swap the objects presented by the threads.

Tuesday, December 29, 2020

Creating Custom Exception Class in Java

In this post we'll see how to create a custom exception in Java.

custom exception in Java

Though Java's exception handling covers the whole gamut of errors and most of the time it is recommended that you should go with standard exceptions rather than creating your own custom exception classes in Java, but you might need to create custom exception types to handle situations which are specific to your application.

Monday, December 28, 2020

Lazy Initialization in Spring Using lazy-init And @Lazy Annotation

In Spring framework, by default all the singleton beans are eagerly created and configured by ApplicationContext as part of the initialization process. Though this behavior of pre-instantiation is desirable most of the time as you can detect errors in the configuration immediately. But sometimes you may have a large object graph and loading all those beans in the beginning itself may be expensive. In that kind of scenario you can prevent pre-instantiation of a singleton bean by configuring the Spring bean to be initialized lazily.

If you mark a bean as lazy-initialized in Spring that means IoC container will create a bean instance when it is first requested, rather than at startup.

Here note that when a lazy-initialized bean is a dependency of a singleton bean that is not lazy initialized, the ApplicationContext creates the lazy-initialized bean at startup, because it must satisfy the singleton’s dependencies.

Saturday, December 26, 2020

Injecting Inner Beans in Spring

This post shows how to inject inner beans in Spring and what are the scenarios when you would need to inject a bean as an inner bean.

Let’s say you have a Java class which refers to another class object and the access type for that object is private. In that case what do you think is the best way to provide bean definition in Spring. Let me provide class (Employee.java) here so you can understand what I am saying.

Friday, December 25, 2020

ServiceLocatorFactoryBean in Spring Framework

ServiceLocatorFactoryBean in Spring framework as the name suggests is an implementation of service locator design pattern and helps with locating the service at run time.

ServiceLocatorFactoryBean helps if you have more than one implementation of the same type and want to use the appropriate implementation at the run time i.e. you have an interface and more than one class implementing that interface and you want to have a factory that will return an appropriate object at run time.

registerShutdownHook() Method in Spring Framework

In the post Spring Bean Life Cycle we have already seen that you can provide destroy methods for your beans to do clean up. One problem is that these destroy callback methods are not executed automatically you do have to call AbstractApplicationContext.destroy or AbstractApplicationContext.close before shutting down the IOC container. There is another option to ensure graceful shutdown, you can register a shutdown hook with the JVM using registerShutdownHook() method in Spring.

Note that this problem is only with non-web applications. Spring’s web-based ApplicationContext implementations already have code in place to shut down the Spring IoC container gracefully when the relevant web application is shut down.

Thursday, December 24, 2020

Batch Processing in Java JDBC - Insert, Update Queries as a Batch

If you have a large number of similar queries it is better to process them in a batch rather than as individual queries. Processing them as a batch provides better performance as you send a group of queries in a single network communication rather than sending individual queries one by one.

Batch support in JDBC

JDBC provides batch support in the form of addBatch() and executeBatch() methods.

If you are using Statement interface then use addBatch(String Sql) method to add queries to the batch.

For PreparedStatement and CallableStatement use addBatch() method as parameters for the query are provided later.

Once you have added queries to a batch you can call executeBatch() method to execute the whole batch of queries.

DB you are using may not support batch updates. You should use the supportsBatchUpdates() method of the DatabaseMetaData interface to check whether the target database supports batch updates or not.

The method returns true if this database supports batch updates; false otherwise.

Wednesday, December 23, 2020

Transaction Management in Java-JDBC

This post provides detail about JDBC transaction management with examples for starting a transaction in JDBC, committing and rolling back a transaction, setting savepoint for transaction rollback.


DataSource in Java-JDBC

In the examples given in the previous post Java JDBC Steps to Connect to DB we have seen how to get a connection using DriverManager class. That’s ok for sample code where you just need to test using a connection and close it. But in a real life application creating connection object every time DB interaction is needed will be very time consuming. What you need is a connection pool where a given number of connection objects are created in the beginning itself and are reused.

In this post we’ll see another way of connecting to DB from your Java application using a DataSource object which provides the connection pooling. There are other advantages of using DataSource in JDBC too.

Tuesday, December 22, 2020

Java Lambda Expression as Method Parameter

As we have already seen in the post about functional interface that lambda expression provides implementation for the abstract method present in the functional interface and the target type for the lambda expression is the functional interface. In the simple terms we can say that lambda expression is an object that can be used where ever the reference of functional interface is required. One of these uses is passing lambda expression as an argument. To pass a lambda expression as a method parameter in Java, the type of the method argument, which receives the lambda expression as a parameter, must be of functional interface type.

Java Lambda Expressions Interview Questions And Answers

In this post some of the Java Lambda Expressions interview questions and answers are listed. This compilation will help the Java developers in preparing for their interviews especially when asked interview questions about Java 8.

  1. What is lambda expression?

    Lambda expression in itself is an anonymous method i.e. a method with no name which is used to provide implementation of the method defined by a functional interface.

    A new syntax and operator has been introduced in Java for Lambda expressions.

    General form of lambda expression

    (optional) (Arguments) -> body
    

Monday, December 21, 2020

Connection Pooling Using Apache DBCP in Java

In this post we’ll see how to configure connection pooling in your Java application using Apache DBCP datasource. The DB we are connecting to is MySQL.

Jars needed

If you are using Maven then you can add the following dependency.

<dependency>
  <groupId>org.apache.commons</groupId>
  <artifactId>commons-dbcp2</artifactId>
  <version>2.8.0</version>
</dependency>

Connection Pooling Using C3P0 in Java

In this post we’ll see how to configure connection pooling using C3P0 datasource in your Java application. The DB we are connecting to is MySQL.

Jars needed for C3P0

You need the following jars in your project’s classpath, check the versions as per your Java and DB versions.

lib/c3p0-0.9.5.5.jar
lib/mchange-commons-java-0.2.19.jar

If you are using Maven then you can add the following dependency.

<dependency>
  <groupId>com.mchange</groupId>
  <artifactId>c3p0</artifactId>
  <version>0.9.5.5</version>
</dependency>

Sunday, December 20, 2020

Spring WebFlux Example Using Functional Programming - Spring Web Reactive

In this post we’ll see a Spring web reactive example using Spring WebFlux functional programming model. The application built here is a RESTful web service with Spring Webflux and also includes a WebClient consumer of that service. Application uses Spring Boot and run on the default Netty server.

Saturday, December 19, 2020

Spring Web Reactive Framework - Spring WebFlux Tutorial

This Spring WebFlux tutorial gives an overview of the new reactive web framework- Spring WebFlux added in Spring version 5.0. It is fully non-blocking, helps you to write event driven, asynchronous logic and supports Reactive Streams back pressure.


Friday, December 18, 2020

Spring WebFlux Example Using Annotation-Based Programming - Spring Web Reactive

The post Spring Web Reactive Framework - Spring WebFlux Tutorial gives an overview of Spring web reactive. Building on that knowledge in this post we’ll see a Spring web reactive example using Spring WebFlux annotation-based programming model where @Controller and @RestController components use annotations to express request mappings, request input, exception handling, and more. The application built here is a RESTful web service with Spring Webflux and also includes a WebClient consumer of that service. Application uses Spring Boot and runs on the default Netty server.

Class And Object in Python

Python being an object oriented programming language works on the concept of creating classes and using objects of the class to access the properties and methods defined by the class.

What is a class

In object oriented programming class defines a new type. Class encapsulates data (variables) and functionality (methods) together. Once a class is defined it can be instantiated to create objects of that class. Each class instance (object) gets its own copy of attributes for maintaining its state and methods for modifying its state.

You can create multiple class instances, they all will be of the same type (as defined by the class) but they will have their own state i.e. different values for attributes.

Thursday, December 17, 2020

Constructor in Python - __init__() function

Constructor is a special method provided to initialize objects (assign values to its data members) when they are created. In Python __init__() special method is the constructor and this method is always called when an object is created.

Syntax of init() in Python

First argument in __init__() method is always self. Constructor may or may not have other input parameters i.e. other input parameters are optional.

def __init__(self, input_parameters):
 #initialization code
 ....
 ....

Wednesday, December 16, 2020

Connection Pooling Using C3P0 Spring Example

This post shows how to provide JDBC connection pooling using C3P0 data source in Spring framework. DB used in this example is MySQL.

Maven dependency for C3P0

<dependency>
  <groupId>com.mchange</groupId>
  <artifactId>c3p0</artifactId>
  <version>0.9.5.2</version>
</dependency>
Alternatively you can download the following jars and put them in the application’s classpath.
c3p0-0.9.5.2.jar
mchange-commons-java-0.2.11.jar

Tuesday, December 15, 2020

Spring JdbcTemplate With ResultSetExtractor Example

In the post Select Query Using JDBCTemplate in Spring Framework we have already seen an example of extracting data from ResultSet using RowMapper. A RowMapper is usually a simpler choice for ResultSet processing, mapping one result object per row but there is another option in Spring framework known as ResultSetExtractor which gives one result object for the entire ResultSet. In this post we’ll see an example of Spring JdbcTemplate with ResultSetExtractor.

Since ResultSetExtractor is a callback interface used by JdbcTemplate's query methods so you can use an instance of ResultSetExtractor with JdbcTemplate’s query method.

Monday, December 14, 2020

Spring JdbcTemplate Insert, Update And Delete Example

In the post Data access in Spring framework we have already seen how Spring provides templates for various persistence methods and how templates divide the data access code into fixed part and variable part. Where Spring framework manages the fixed part and custom code which is provided by the user is handled through callbacks. In this post we’ll see how to use Spring JdbcTemplate to insert, update and delete data from the database.

Note that JdbcTemplate needs a DataSource in order to perform its management of fixed part like getting a DB connection, cleaning up resources.
In this post Apache DBCP is used for providing pooled datasource and MYSQL is used as the back end.

Async Pipe in Angular With Examples

Of all the built-in pipes in Angular, Async pipe is a little different as it subscribes to an Observable or Promise and returns the latest value it has emitted. When the component gets destroyed, the async pipe unsubscribes automatically to avoid any memory leaks.


Saturday, December 12, 2020

Types of JDBC Drivers

JDBC API standardizes the way any Java application connects to DB. JDBC API is a collection of interfaces and JDBC drivers implement these interfaces in the JDBC API enabling a Java application to interact with a database.

The JDBC driver gives out the connection to the database and implements the protocol for transferring the query and result between client and database.

JDBC Driver Types

JDBC drivers can be categorized into four types.

Friday, December 11, 2020

Connection Pooling With Apache DBCP Spring Example

This post shows how to provide JDBC connection pooling using Apache DBCP data source in Spring framework. DB used in this example is MySQL.

Maven dependency for DBCP

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-dbcp2</artifactId>
    <version>2.1</version>
</dependency>

Alternatively you can download the following jars and put them in the classpath.

commons-dbcp2-2.1.1.jar
commons-pool2-2.5.0.jar
commons-logging-1.2.jar

Thursday, December 10, 2020

Pure and Impure Pipes in Angular

When you create a custom pipe in Angular there is one more attribute of @Pipe decorator which you can assign a value as true or false, that attribute is pure. In this tutorial we’ll see what are pure and impure pipes in Angular and what are the differences between pure and impure pipes.

@Pipe({
  name: 'myCustomPipe', 
  pure: false/true
})
export class MyCustomPipe {

}

By default, pipes are defined as pure so you don't explicitly need to assign value of pure as true.

Wednesday, December 9, 2020

Custom Pipe in Angular With Example

Though there are many Angular built-in pipes for data transformation but Angular framework also gives you an option to create a custom pipe to cater to custom data transformation. In this tutorial we’ll see how to create a custom pipe in Angular.

Creating custom pipe in Angular

To mark a class as a pipe apply the @Pipe decorator to the class. In the name attribute of the of @Pipe provide the name of the pipe. Use name in template expressions as you would for a built-in pipe.

Your custom pipe class should also implement the PipeTransform interface.

Sunday, December 6, 2020

CopyOnWriteArraySet in Java With Examples

In Java 5 many concurrent collection classes are added as a thread safe alternative for their normal collection counterparts which are not thread safe. Like ConcurrentHashMap as a thread safe alternative for HashMap, CopyOnWriteArrayList as a thread safe alternative for ArrayList. Same way CopyOnWriteArraySet in Java is added as a thread safe alternative for HashSet in Java.

CopyOnWriteArraySet class in Java

CopyOnWriteArraySet implements the Set interface (Actually it extends the AbstractSet class which in turn implements the set interface). Since CopyOnWriteArraySet implements the Set interface so basic functionality of the Set that only unique elements can be added applies to CopyOnWriteArraySet too.

Tuesday, December 1, 2020

Using Angular Pipes in Component or Service Classes

In this tutorial we’ll see how to use Angular pipe in Component classes and Service classes in Angular.

Steps to use angular pipes in component classes (or Service)

If you want to use the pipe in more than one component class then configure it in app.module.ts class-

  1. Import the required Angular pipe and add it to the providers array.
  2. In the Component class inject the pipe and use it.

You can do these steps in Component class itself if specific Angular pipe is used in that class only.

Using Angular pipes in component class example

We’ll create a simple reactive form with fields to enter date and amount. Date has to be transformed using the DatePipe and amount using the CurrencyPipe in Component. For DatePipe configuration is done in App module where as for CurrencyPipe it is done in the Component class itself.

Tuesday, November 24, 2020

Custom Async Validator in Angular Template-Driven Form

In this tutorial we’ll see how to create a custom asynchronous validator to be used with Angular Template-Driven form.

For custom async validator in Reactive form refer this post - Custom Async Validator in Angular Reactive Form

Custom Asynchronous validator for Template-Driven form

If you want to write a custom async validator for a template-driven form that has to be written as an Angular directive which should implement the AsyncValidator interface.

interface AsyncValidator extends Validator {
  validate(control: AbstractControl): Promise<ValidationErrors | null> | Observable<ValidationErrors | null>

  // inherited from forms/Validator
  validate(control: AbstractControl): ValidationErrors | null
  registerOnValidatorChange(fn: () => void)?: void
}

Your Async validator class has to implement the validate() function which must return a Promise or an Observable.

Monday, November 23, 2020

Custom Async Validator in Angular Reactive Form

In this tutorial we’ll see how to create a custom asynchronous validator to be used with Angular Reactive form.

For custom async validator in Template-Driven form refer this post- Custom Async Validator in Angular Template-Driven Form


Types of Validator functions

Validator functions can be either synchronous or asynchronous.

  • Sync validators: Synchronous validator functions are passed a FormControl instance as argument and immediately return either a set of validation errors or null. You can pass these in as the second argument when you instantiate a FormControl.
  • Async validators: Asynchronous validator functions are passed a FormControl instance as argument and return a Promise or Observable that later emits a set of validation errors or null. You can pass these in as the third argument when you instantiate a FormControl.

Sunday, November 22, 2020

CallableStatement Interface in Java-JDBC

In the post Statement Interface in Java-JDBC and PreparedStatement Interface in Java-JDBC we have already seen how you can use Statement to execute static SQL statements and PreparedStatement to execute precompiled parameterized SQL statements. On the same lines the JDBC API provides CallableStatement interface that extends PreparedStatement and used to execute SQL stored procedures.

Stored Procedure

Stored procedure is a subroutine which resides with in the database and may have DB specific way of writing it. If you have a huge SQL statement or a group of SQL statements involving more than one table, checking for conditions, looping it is better to write it as a stored procedure. That way you will need to make just one call to the DB server and your pre-compiled procedure would be executed in the same space as your DB server.

That brings you the advantages like efficiency as it is already compiled, reduced network traffic as its full execution happens in the DB server.

Obtaining JDBC CallableStatement object

CallableStatement object can be created using the prepareCall() method of the Connection interface.

CallableStatement callableStatement = connection.prepareCall(“{call PROCEDURE_NAME(?, ?, ?)}”); 
Here ‘?’ is a place holder used to register IN, OUT and INOUT parameters.

You can also also call functions using Callable statement, in that case general form would be like -

CallableStatement callableStatement = connection.prepareCall(“? = {call PROCEDURE_NAME(?, ?, ?)}”);

How to use CallableStatement in JDBC

In order to pass values to the IN and INOUT parameters of the stored procedure you need to use the appropriate setter method. CallableStatement inherits setter methods from PreparedStatement and there are different setter methods for different data types i.e. setInt(), setString(), setDate() etc.

You also need to register OUT parameters of the stored procedure. For that you need to use registerOutParameter method which takes column index or column name and type as parameters. It has other overloaded methods too.

There are also various getter methods (like getString(), getLong(), getTime()) for getting the values from the OUT parameters.

In order to execute the CallableStatement you can use execute() methods -

  • execute()- Any SQL statement. Returns a boolean which is true if the first result is a ResultSet object; false if it is an update count or there are no results.
  • executeUpdate()- For DML statements like Insert, Update or DDL statements like Create.
  • ExecuteQuery()- For SQL statement that returns ResultSet.

Java CallableStatement examples

Let’s see some examples using CallableStatement in JDBC. Database used is MySql, schema is netjs and table employee with columns id, age and name, where id is auto-generated.

1. CallableStatement example-Executing stored procedure having IN params

In this example let’s execute a stored procedure that has only IN params using CallableStatement. The stored procedure inserts a new row into the table.

insert_employee_proc.sql

CREATE PROCEDURE `insert_employee_proc`(IN param_name VARCHAR(35), IN param_age int)
BEGIN
  INSERT into EMPLOYEE (name, age) values 
  (param_name, param_age);
END

You can see in the stored procedure that there are two IN parameters in the stored procedure.

Java Code

import java.sql.CallableStatement;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class JDBCCallableStmt {

  public static void main(String[] args) {
    Connection connection = null;
    try {
      // Loading driver
      Class.forName("com.mysql.jdbc.Driver");
      
      // Creating connection
      connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/netjs", 
                          "root", "admin");
      
      // Getting CallableStatement object
      CallableStatement cStatement = connection.prepareCall(
         "{call insert_employee_proc(?, ?)}");
      // Setting params
      cStatement.setString(1, "Jackie");
      cStatement.setInt(2, 45);
      
      int count = cStatement.executeUpdate();
      System.out.println("Count of rows inserted " + count);
   
    } catch (ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (SQLException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }finally{
      if(connection != null){
        //closing connection 
        try {
          connection.close();
        } catch (SQLException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
      } // if condition
    }// finally
  }
}

2. CallableStatement example-Executing stored procedure having IN and OUT params

In this Java CallableStatement example let’s execute a stored procedure that has both IN and OUT params using CallableStatement. The stored procedure has a select query to which id is passed as an IN parameter and age and name for that id are send in OUT parameters.

select_employee_proc.sql

CREATE PROCEDURE `select_employee_proc`(IN param_id int, 
    OUT param_name varchar(35), OUT param_age int)
BEGIN
 SELECT name, age INTO param_name, param_age
 from EMPLOYEE where id = param_id;
END

Java Code

import java.sql.CallableStatement;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.JDBCType;
import java.sql.SQLException;

public class JDBCCallableStmt {

  public static void main(String[] args) {
    Connection connection = null;
    try {
      // Loading driver
      Class.forName("com.mysql.jdbc.Driver");
      
      // Creating connection
      connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/netjs", 
                          "root", "admin");
      
      // Getting CallableStatement object
      CallableStatement cStatement = connection.prepareCall(
        "{call select_employee_proc(?, ?, ?)}");
      // Setting params
      cStatement.setInt(1, 26);
      // Registering OUT parameters Using 
      // JDBCType enum which is added in Java 8
      cStatement.registerOutParameter(2, JDBCType.VARCHAR);

      cStatement.registerOutParameter(3, JDBCType.INTEGER);

      cStatement.executeQuery();

      // Reading the OUT paramter here 
      System.out.println("Fetched Result " + "Name: " + cStatement.getString(2) + 
        " Age: " + cStatement.getInt(3));
   
    } catch (ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (SQLException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }finally{
      if(connection != null){
        //closing connection 
        try {
          connection.close();
        } catch (SQLException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
      } // if condition
    }// finally
  }
}

3. CallableStatement example-Executing stored procedure returning multiple rows

Let’s see an example where stored procedure returns multiple rows as result. In that case you can use execute or executeQuery to execute the procedure using CallableStatement and that will return the resultset. In this example execute method is used in order to show how it uses other methods like getResultSet and getMoreResults.

all_employee_proc.sql

CREATE PROCEDURE `all_employee_proc`(IN param_age int)
BEGIN
  SELECT * from employee where age > param_age;
END

The stored procedure returns all employees whose age is greater than the passed age integer parameter.

Java code

import java.sql.CallableStatement;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;

public class JDBCCallableStmt {

  public static void main(String[] args) {
    Connection connection = null;
    try {
      // Loading driver
      Class.forName("com.mysql.jdbc.Driver");
      
      // Creating connection
      connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/netjs", 
                          "root", "admin");
      
      // Getting CallableStatement object
      CallableStatement cStatement = connection.prepareCall("{call all_employee_proc(?)}");
      // Setting params
      cStatement.setInt(1, 30);
    
      boolean hasResults = cStatement.execute();
      while(hasResults){
        ResultSet rs = cStatement.getResultSet();
        while(rs.next()){
          System.out.println("id : " + rs.getInt("id") + " Name : " 
            + rs.getString("name") + " Age : " + rs.getInt("age")); 
        }
        hasResults = cStatement.getMoreResults();
      }   
    } catch (ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (SQLException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }finally{
      if(connection != null){
        //closing connection 
        try {
          connection.close();
        } catch (SQLException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
      } // if condition
    }// finally
  }
}

Output

id : 6 Name : Tim Age : 40
id : 8 Name : Johnny Age : 35
id : 17 Name : Johnny Age : 35
id : 18 Name : Bob Age : 45
id : 25 Name : Jacky Age : 50
id : 26 Name : Jackie Age : 45

That's all for this topic CallableStatement Interface in Java-JDBC. If you have any doubt or any suggestions to make please drop a comment. Thanks!

>>>Return to Java Advanced Tutorial Page


Related Topics

  1. JDBC Tutorial - Java JDBC Overview
  2. Java JDBC Steps to Connect to DB
  3. ResultSet Interface in Java-JDBC
  4. DataSource in Java-JDBC
  5. Transaction Management in Java-JDBC

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Saturday, November 21, 2020

PreparedStatement Interface in Java-JDBC

In the post Statement interface in Java we have already seen how you can create a Statement using connection object and execute SQL statements. One problem though is that Statement interface is used to execute static SQL statements with no option to send parameters. Though you can always use a StringBuilder to append parameters to the SQL and use it with Statement but there is a better option provided by JDBC itself in the form of PreparedStatement which is a sub-interface of Statement. In this post we'll see how to use PreparedStatement in Java with examples.

Obtaining JDBC PreparedStatement object

You can get the PreparedStatement object by calling the prepareStatement method of the Connection class.

PreparedStatement preparedStatement = connection.prepareStatement(sql);

Advantages of using PreparedStatement in JDBC

As stated above one advantage of PreparedStatement is that you can use PreparedStatement object for SQL statement with or without parameters. The advantage of using SQL statement with parameters is that you can use the same statement and supply it with different parameter values each time you execute it.

That brings us to the second advantage of PreparedStatement, it is more efficient. Unlike Statement object, PreparedStatement is given the SQL statement when it is created. So the SQL is sent to the DB right away where it is already compiled. When you come to execute() method to actually execute the SQL that SQL is pre-compiled making it more efficient for repeated executions.

Java PreparedStatement example

Let’s see an example using PreparedStatement in JDBC. DB used here is MySql, schema is netjs and table is employee with columns id, age and name, where id is auto-generated.

In the code there are methods for insert, update, delete and select from the table.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class JDBCPrepStmt {
  public static void main(String[] args) {
    Connection connection = null;
    try {
      // Loading driver
      Class.forName("com.mysql.jdbc.Driver");
    
      // Creating connection
      connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/netjs", 
                         "root", "admin");
      JDBCPrepStmt prep = new JDBCPrepStmt();
      prep.insertEmployee(connection, "Kate", 24);
      prep.updateEmployee(connection, 22, 30);
      prep.displayEmployee(connection, 22);
    
      //prep.deleteEmployee(connection, 24);
    } catch (ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (SQLException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }finally{
      if(connection != null){
        //closing connection 
        try {
          connection.close();
        } catch (SQLException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
      } // if condition
    }// finally
  }
 
  // Method to insert
  private void insertEmployee(Connection connection, String name, int age) 
        throws SQLException{
    String insertSQL = "Insert into employee (name, age) values (?, ?)";
    PreparedStatement prepStmt = null;
    try {
      prepStmt = connection.prepareStatement(insertSQL);
      prepStmt.setString(1, name);
      prepStmt.setInt(2, age);
      int count = prepStmt.executeUpdate();
      System.out.println("Count of rows inserted " + count);
    }finally{
      if(prepStmt != null){
        prepStmt.close();
      }
    }
  }
 
 // Method to update
 private void updateEmployee(Connection connection, int id, int age) throws SQLException{
  String updateSQL = "Update employee set age = ? where id = ?";
  PreparedStatement prepStmt = null;
  try {
   prepStmt = connection.prepareStatement(updateSQL);
   prepStmt.setInt(1, age);
   prepStmt.setInt(2, id);
   int count = prepStmt.executeUpdate();
   System.out.println("Count of rows updated " + count);
  }finally{
    if(prepStmt != null){
     prepStmt.close();
    }
  }
 }
 
 // Method to delete
 private void deleteEmployee(Connection connection, int id) throws SQLException {
  String deleteSQL = "Delete from employee where id = ?";
  PreparedStatement prepStmt = null;
  try {
   prepStmt = connection.prepareStatement(deleteSQL);
   prepStmt.setInt(1, id);
   int count = prepStmt.executeUpdate();
   System.out.println("Count of rows deleted " + count);
  }finally{
    if(prepStmt != null){
     prepStmt.close();
    }
  }
 }
 

 // Method to retrieve
 private void displayEmployee(Connection connection, int id) throws SQLException{
  String selectSQL = "Select * from employee where id = ?";
  PreparedStatement prepStmt = null;
  try {
   prepStmt = connection.prepareStatement(selectSQL);
   prepStmt.setInt(1, id);
   ResultSet rs = prepStmt.executeQuery();
   while(rs.next()){
     System.out.println("id : " + rs.getInt("id") + " Name : " 
                   + rs.getString("name") + " Age : " + rs.getInt("age")); 
   }
  }finally{
    if(prepStmt != null){
     prepStmt.close();
    }
  }
 }
 
}

Points to note here

Taking this example as reference let’s go through some of the points you will have to keep in mind when using PreparedStatement in JDBC.

  • Parameterized statement– In the example you can see that all the SQL statements are parameterized and ‘?’ is used as a placeholder in parameterized statements.

    As example-

    String insertSQL = "Insert into employee (name, age) values (?, ?)";
    
  • Setter methods– Values for these placeholders are provided through setter methods. PreparedStatement has various setter methods for different data types i.e. setInt(), setString(), setDate() etc.

    General form of the setter method is setXXX(int parameterIndex, value)

    Here parameterIndex is the index of the parameter in the statement, index starts from 1.

    As example-

    String insertSQL = "Insert into employee (name, age) values (?, ?)";
    

    For this sql where the first parameter is String (name) and second parameter is of type int (age), you need to set the parameters on the PreparedStatement object as follows -

    prepStmt.setString(1, name);
    prepStmt.setInt(2, age);
    
  • Executing PreparedStatement objects– You can use execute methods for executing the queries.
    1. boolean execute()- Executes the SQL statement in this PreparedStatement object, (it can be any kind of SQL query), which may return multiple results.
      Returns a boolean which is true if the first result is a ResultSet object; false if it is an update count or there are no results.
    2. ResultSet executeQuery(String sql)- Executes the SQL statement in this PreparedStatement object, which returns a single ResultSet object. If you want to execute a Select SQL query which returns results you should use this method.
    3. int executeUpdate()- Executes the SQL statement in this PreparedStatement object, which may be an INSERT, UPDATE, or DELETE statement or an SQL statement that returns nothing, such as an SQL DDL statement.
      Returns an int denoting either the row count for the rows that are inserted, deleted, updated or returns 0 if nothing is returned.

That's all for this topic PreparedStatement Interface in Java-JDBC. If you have any doubt or any suggestions to make please drop a comment. Thanks!

>>>Return to Java Advanced Tutorial Page


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Name Mangling in Python

In Python there are no explicit access modifiers so you can’t mark a class member as public/private. Then the question is how to restrict access to a variable or method outside the class, if required. Class member can be made private (Close to private actually) using a process called name mangling in Python.


Name mangling (Making member private)

In Python name mangling process any identifier with at least two leading underscores, at most one trailing underscore is textually replaced with _classname__identifier where classname is the current class name. For example if there is a variable __var it is rewritten by the Python interpreter in the form _classname__var.

Since the name of any such class member (with at least two leading underscores, at most one trailing underscore) changes internally thus it can’t be accessed using the given name. That is the closest Python goes for making a class member private.

Python Name mangling example

Let’s try to clarify name mangling process with examples.

class Person:
  def __init__(self, name, age=0):
    self.name = name
    self.__age = age

  def display(self):
    print(self.name)
    print(self.__age)

person = Person('John', 40)
#accessing using class method
print('Displaying values using class method')
person.display()
#accessing directly from outside
print('Trying to access variables from outside the class ')
print(person.name)
print(person.__age)

Output

Displaying values using class method
John
40
Traceback (most recent call last):
File "F:/NETJS/NetJS_2017/Python/Test/Person.py", line 21, in <module>
Trying to access variables from outside the class 
John
    print(person.__age)
AttributeError: 'Person' object has no attribute '__age'

As you can see variable __age (having two leading underscores) is not accessible from outside the class. Using a method with in the class it can still be accessed.

Same way for a method with two leading underscores.

class Person:
  def __init__(self, name, age=0):
    self.name = name
    self.__age = age

  def __displayAge(self):
    print(self.name)
    print(self.__age)

person = Person('John', 40)
person.__displayAge()

Output

Traceback (most recent call last):
  File "F:/NETJS/NetJS_2017/Python/Test/Person.py", line 15, in <module>
    person.__displayAge()
AttributeError: 'Person' object has no attribute '__displayAge'

As you can see method is not accessible from outside the class.

How does name change in Name mangling

If you want to verify the rewriting of name in Python name mangling process you can do so using the dir() function.

When a class object is passed as an argument to dir() function, it returns a list of valid attributes for that object.

class Person:
  def __init__(self, name, age=0):
    self.name = name
    self.__age = age

person = Person('John', 40)
print(dir(person))

Output

['_Person__age', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', 
'__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', 
'__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', 
'__str__', '__subclasshook__', '__weakref__', 'name']

From the output of dir() function for Person object you can see that the __age is rewritten as _Person__age.

Name mangling and method overriding

As per Python docs stated objective of name mangling is to avoid name clashes of names with names defined by subclasses. Name mangling is helpful for letting subclasses override methods without breaking intraclass method calls.

For example consider the following scenario where Parent class is subclassed and there is an overridden method test is the Child class too. From the constructor of Parent class there is a call to test method- self.test()

class Parent:
  def __init__(self):
    print('in init')
    self.test()
  def test(self):
    print('In Parent test method')

class Child(Parent):
  def test(self):
    print('In Child test method')

obj = Child()
obj.test()

Output

in init
In Child test method
In Child test method

As you can see Child test method is getting called both of the times. To avoid that name clash you can create a private copy of the original method.

class Parent:
  def __init__(self):
    print('in init')
    self.__test()
  def test(self):
    print('In Parent test method')

  # private copy
  __test = test

class Child(Parent):
  def test(self):
    print('In Child test method')

obj = Child()
obj.test()

Output

in init
In Parent test method
In Child test method

Accessing name mangled class members

As already stated Python name mangling process rewrites the member name by adding _classname to the member. Thus it is still possible to access the class member from outside the class by using the rewritten name. That is why it is said that Name mangling is the closest to private not exactly private.

class Person:
  def __init__(self, name, age=0):
    self.name = name
    self.__age = age

  def display(self):
    print(self.name)
    print(self.__age)

person = Person('John', 40)
print('Trying to access variables from outside the class ')
print(person.name)
print(person._Person__age)

Output

Trying to access variables from outside the class 
John
40

As you can see private class member is accessed from outside the class by using the name mangled form _ClassName__var.

That's all for this topic Name Mangling in Python. If you have any doubt or any suggestions to make please drop a comment. Thanks!

>>>Return to Python Tutorial Page


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Friday, November 20, 2020

ResultSet Interface in Java-JDBC

java.sql.ResultSet interface in JDBC API represents the storage for the data you get by executing a SQL statement that queries the database.

A ResultSet object maintains a cursor pointing at the result data. Initially the cursor is positioned before the first row. The next method moves the cursor to the next row, it returns false when there are no more rows in the ResultSet object.

Creating and iterating a ResultSet Example

ResultSet rs = stmt.executeQuery("Select * from Employee");
   
// Processing Resultset
while(rs.next()){
 System.out.println("id : " + rs.getInt("id") + " Name : " +  rs.getString("name") + " Age : " + rs.getInt("age")); 
}

By default, ResultSet object is not updatable and has a forward moving cursor only. Thus, you can iterate through it only once and only from the first row to the last row. But ResultSet interface provides parameters that can produce ResultSet objects that are scrollable and/or updatable.

Fields for scrollable ResultSet

ResultSet interface in Java has fields that determine whether ResultSet object will be scrollable or not and will it be sensitive to the changes to the data that is represented by ResultSet or not.

  • TYPE_FORWARD_ONLY- The constant indicating the type for a ResultSet object whose cursor may move only forward.
  • TYPE_SCROLL_INSENSITIVE- The constant indicating the type for a ResultSet object that is scrollable but generally not sensitive to changes to the data that underlies the ResultSet. Which means you can move the cursor to an absolute position or relative to the current cursor position. If the data in the DB is changed by another thread/process that change won’t be reflected in the data stored in the ResultSet.
  • TYPE_SCROLL_SENSITIVE- The constant indicating the type for a ResultSet object that is scrollable and generally sensitive to changes to the data that underlies the ResultSet. Which means you can move the cursor to an absolute position or relative to the current cursor position. If the data in the DB is changed by another thread/process that change is reflected in the data stored in the ResultSet.

Fields for updatable ResultSet

  • CONCUR_READ_ONLY- The constant indicating the concurrency mode for a ResultSet object that may NOT be updated.
  • CONCUR_UPDATABLE- The constant indicating the concurrency mode for a ResultSet object that may be updated.

Java ResultSet interface example

Let’s see an example with scrollable resultset, DB used here is MySql, schema is netjs and table is Employee.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class JDBCResultSet {

 public static void main(String[] args) {
  try(Connection connection = DriverManager.getConnection(
      "jdbc:mysql://localhost:3306/netjs", "root", "admin")){
   // creating Statement
   Statement stmt = connection.createStatement(
                           ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE);  
   
   // Executing Query
   ResultSet rs = stmt.executeQuery("Select * from Employee");
   System.out.println("Displaying all rows");
   // Processing Resultset
   while(rs.next()){
       System.out.println("id : " + rs.getInt("id") + " Name : " 
        + rs.getString("name") + " Age : " + rs.getInt("age")); 
   }
   // moving to 3rd row
   rs.absolute(3);
   System.out.println("Displaying 3rd row");
   System.out.println("id : " + rs.getInt("id") + " Name : " 
                          + rs.getString("name") + " Age : " + rs.getInt("age")); 
  }catch (SQLException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
 }
}

Output

Displaying all rows
id : 5 Name : Tom Age : 35
id : 6 Name : Tim Age : 20
id : 7 Name : John Age : 25
id : 8 Name : Johnny Age : 35
id : 17 Name : Johnny Age : 65
Displaying 3rd row
id : 7 Name : John Age : 25

Getter Methods in ResultSet

You would have noticed in the examples how appropriate data type getter method is used (i.e. getInt, getString) for retrieving column values from the current row. You can retrieve value using either the index number of the column or the name of the column.

In general, using the column index will be more efficient. Columns are numbered from 1. Drawback is, any alteration in the table structure will mean change in the indexes in the Java code.

Updater methods in ResultSet

There are also updater methods corresponding to the data types which are used when your ResultSet is updatable. Using updater methods you can update the column values then update the row in the DB. Updater methods are used in conjunction with updateRow and insertRow methods.

Let’s see an example to update a row and insert a row using ResultSet methods.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class JDBCResultSetUpdate {

 public static void main(String[] args) {
  try(Connection connection = DriverManager.getConnection(
                   "jdbc:mysql://localhost:3306/netjs", "root", "admin")){
   // creating Statement
   Statement stmt = connection.createStatement(
                          ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE);  
   
   // Executing Query
   ResultSet rs = stmt.executeQuery("Select * from Employee");
   System.out.println("Displaying all rows");
   // Processing Resultset
   while(rs.next()){
       System.out.println("id : " + rs.getInt("id") + " Name : " 
                               + rs.getString("name") + " Age : " + rs.getInt("age")); 
   }
   // moving to 3rd row
   rs.absolute(3);
   // updating age column for 3rd row
   rs.updateInt("age", 28);
   rs.updateRow();
   System.out.println("Displaying 3rd row");
   System.out.println("id : " + rs.getInt("id") + " Name : " 
                            + rs.getString("name") + " Age : " + rs.getInt("age"));
   
   /*** Inserting row  ***/
   // moves cursor to the insert row
   rs.moveToInsertRow(); 
     
   //rs.updateInt("id",18); //updates the first column using column name
   rs.updateString(2, "Bob"); //updates the second column using column index
   rs.updateInt("age",45);
   rs.insertRow();
   rs.moveToCurrentRow();
   
  }catch (SQLException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
 }
}

Other fields of ResultSet interface

Some of the fields are already mentioned with examples, ResultSet in Java has some other fields which are as follows-

  • CLOSE_CURSORS_AT_COMMIT- This constant indicates that open ResultSet will be closed when the current transaction is commited.
  • HOLD_CURSORS_OVER_COMMIT- This constant indicates that open ResultSet will remain open when the current transaction is commited.
  • FETCH_FORWARD- The constant indicating that the rows in a result set will be processed in a forward direction; first-to-last.
  • FETCH_REVERSE- The constant indicating that the rows in a result set will be processed in a reverse direction; last-to-first.
  • FETCH_UNKNOWN- The constant indicating that the order in which rows in a result set will be processed is unknown.

Methods of the ResultSet

Most of the often used methods of the ResultSet are already covered with the examples. Some of the other methods which are used for moving the cursor are as follows-

  • afterLast()- Moves the cursor to the end of this ResultSet object, just after the last row.
  • beforeFirst()- Moves the cursor to the front of this ResultSet object, just before the first row.
  • first()- Moves the cursor to the first row in this ResultSet object.
  • last()- Moves the cursor to the last row in this ResultSet object.
  • moveToCurrentRow()- Moves the cursor to the remembered cursor position, usually the current row.
  • moveToInsertRow()- Moves the cursor to the insert row.
  • next()- Moves the cursor froward one row from its current position.
  • previous()- Moves the cursor to the previous row in this ResultSet object.
  • relative(int rows)- Moves the cursor a relative number of rows, either positive or negative.

Reference: https://docs.oracle.com/en/java/javase/12/docs/api/java.sql/java/sql/ResultSet.html

That's all for this topic ResultSet Interface in Java-JDBC. If you have any doubt or any suggestions to make please drop a comment. Thanks!

>>>Return to Java Advanced Tutorial Page


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Thursday, November 19, 2020

Statement Interface in Java-JDBC

In the post Java JDBC Steps to Connect to DB we have already seen a complete example using the interfaces Driver, Connection, Statement and ResultSet provided by the JDBC API. In this post we’ll see Java Statement interface in detail.

Statement interface in JDBC

java.sql.Statement interface in JDBC API is used to execute a static SQL statement and returning the result of the executed query. Statement interface has two sub-interfaces CallableStatement and PreparedStatement.

PreparedStatement– PreparedStatement object stores the SQL statement in its pre-compiled state. That way it can efficiently execute the same SQL statement multiple times with different parameters.

CallableStatement- This interface is used to execute SQL stored procedures.

You can get a Statement object by calling the Connection.createStatement() method on the Connection object.

Frequently used methods of the Statement interface

Mostly you will use the execute methods of the Java Statement interface to execute queries.

  1. boolean execute(String sql)- Executes the given SQL statement (it can be any kind of SQL query), which may return multiple results.
    Returns a boolean which is true if the first result is a ResultSet object; false if it is an update count or there are no results.
  2. ResultSet executeQuery(String sql)- Executes the given SQL statement, which returns a single ResultSet object. If you want to execute a Select SQL query which returns results you should use this method.
  3. int executeUpdate(String sql)- Executes the given SQL statement, which may be an INSERT, UPDATE, or DELETE statement or an SQL statement that returns nothing, such as an SQL DDL statement.
    Returns an int denoting either the row count for the rows that are inserted, deleted, updated or returns 0 if nothing is returned.
    Note:This method cannot be called on a PreparedStatement or CallableStatement.
  4. int[] executeBatch()- Submits a batch of commands to the database for execution and if all commands execute successfully, returns an array of update counts.

Java Statement example

Let’s see an example where SQL statements are executed using execute(), executeUpdate and executeQuery methods. In the example-

Using execute() method a SQL statement is executed and then the boolean value is checked.

Using executeUpdate() method insert, update and delete statements are executed and row count of the affected rows is displayed.

Using executeQuery() method select statement is executed and the returned ResultSet is processed.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class JDBCStmt {
  public static void main(String[] args) {
    Connection connection = null;
    try {
      // Loading driver
      Class.forName("com.mysql.jdbc.Driver");
   
      // Creating connection
      connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/netjs", 
                   "root", "admin");
  
      // creating Statement
      Statement stmt = connection.createStatement();  
            
      /** execute method **/
      boolean flag = stmt.execute("Update Employee set age = 40 where id in (5, 6)");
      if(flag == false){
        System.out.println("Updated rows " + stmt.getUpdateCount() );
      }
            
      /** executeUpdate method **/
      // Insert
      int count = stmt.executeUpdate("Insert into employee(name, age) values('Kim', 23)");
      System.out.println("Rows Inserted " + count);
            
      // update
      count = stmt.executeUpdate("Update Employee set age = 35 where id = 17");
      System.out.println("Rows Updated " + count);
            
      //delete
      count = stmt.executeUpdate("Delete from Employee where id = 5");
      System.out.println("Rows Deleted " + count);
            
      /** executeQuery method **/
      // Executing Query
      ResultSet rs = stmt.executeQuery("Select * from Employee");

      // Processing Resultset
      while(rs.next()){
        System.out.println("id : " + rs.getInt("id") + " Name : " 
          + rs.getString("name") + " Age : " + rs.getInt("age")); 
      }    
    } catch (ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (SQLException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }finally{
      if(connection != null){
        //closing connection 
        try {
          connection.close();
        } catch (SQLException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
      } // if condition
    }// finally
  }
}

Reference: https://docs.oracle.com/en/java/javase/12/docs/api/java.sql/java/sql/Statement.html

That's all for this topic Statement Interface in Java-JDBC. If you have any doubt or any suggestions to make please drop a comment. Thanks!

>>>Return to Java Advanced Tutorial Page


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Custom Validator in Angular Template-Driven Form

Angular framework provides many built-in validators that can be used with forms but sometimes you may need a validation in your application that can’t be catered by a built-in validators. For such scenario Angular also gives the option to create a custom validator. In this tutorial we’ll see how to create a custom validator to be used with Angular template-driven form.

Custom validator for Template-Driven form

If you want to write a custom validator for a template-driven form that has to be written as an Angular directive which should implement the Validator interface. This Validator interface is implemented by classes that perform synchronous validation.

interface Validator {
  validate(control: AbstractControl): ValidationErrors | null
  registerOnValidatorChange(fn: () => void)?: void
}

Wednesday, November 18, 2020

DatabaseMetaData Interface in Java-JDBC

DatabaseMetaData in Java, which resides in java.sql package, provides information about the database (DB meta data) you are connected to.

Using the methods provided by Java DatabaseMetaData interface you can get information about-

  • Database like DB name and version
  • JDBC driver like the driver’s name and version,
  • names of DB schemas,
  • name of tables in any DB schema,
  • names of views,
  • information about the procedures.

In this post we’ll see examples of some of the commonly used methods. You can get the list of full methods here- https://docs.oracle.com/javase/9/docs/api/java/sql/DatabaseMetaData.html


How to get DatabaseMetaData object in JDBC

You can get the DatabaseMetaData instance by calling the getMetaData() method of the Connection class.

DatabaseMetaData dbMetaData = connection.getMetaData();

DatabaseMetaData example-Getting DB product and version information

This example code shows how you can get DB name and version information using DatabaseMetaData in JDBC.

import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.DriverManager;
import java.sql.SQLException;

public class DBMetaData {

 public static void main(String[] args) {
  Connection connection = null;
   try {
    // Loading driver
    Class.forName("com.mysql.jdbc.Driver");
    
    // Creating connection
    connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/netjs", 
                        "root", "admin");
    
    DatabaseMetaData dbMetaData = connection.getMetaData();
    
    System.out.println("Database Name - " + dbMetaData.getDatabaseProductName());
    System.out.println("Database Version - " + dbMetaData.getDatabaseProductVersion());
    System.out.println("Database Major Version - " + dbMetaData.getDatabaseMajorVersion());
    System.out.println("Database Minor Version - " + dbMetaData.getDatabaseMinorVersion());
    System.out.println("Database User - " + dbMetaData.getUserName());
    
   } catch (ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
   } catch (SQLException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
   }finally{
       if(connection != null){
         //closing connection 
         try {
           connection.close();
         } catch (SQLException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         }
       } // if condition
   }// finally
 }
}

DatabaseMetaData example - Getting driver information

import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.DriverManager;
import java.sql.SQLException;

public class DBMetaData {
  public static void main(String[] args) {
    Connection connection = null;
    try {
      // Loading driver
      Class.forName("com.mysql.jdbc.Driver");
      
      // Creating connection
      connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/netjs", 
                       "root", "admin");
      
      DatabaseMetaData dbMetaData = connection.getMetaData();
      
      System.out.println("Driver Name - " + dbMetaData.getDriverName());
      System.out.println("Driver Version - " + dbMetaData.getDriverVersion());
      System.out.println("Driver Major Version - " + dbMetaData.getDriverMajorVersion());
      System.out.println("Driver Minor Version - " + dbMetaData.getDriverMinorVersion());
      System.out.println("JDBC Major Version - " + dbMetaData.getJDBCMajorVersion());
      System.out.println("JDBC Minor Version - " + dbMetaData.getJDBCMinorVersion());
    } catch (ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (SQLException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
   }finally{
      if(connection != null){
        //closing connection 
        try {
          connection.close();
        } catch (SQLException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
      } // if condition
    }// finally
  }
}

Example to get tables using DatabaseMetaData in JDBC

For getting tables you can use getTables(String catalog, String schemaPattern, String tableNamePattern, String[] types) method. You can provide null as value for all the parameters, that way you don’t narrow the search and all the tables are returned. If you want to narrow your search to get specific tables then you can provide values for these parameters.

import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;

public class DBMetaData {
  public static void main(String[] args) {
    Connection connection = null;
    try {
      // Loading driver
      Class.forName("com.mysql.jdbc.Driver");
    
      // Creating connection
      connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/world", 
                        "root", "admin");
    
      DatabaseMetaData dbMetaData = connection.getMetaData();
      
      ResultSet rs = dbMetaData.getTables(null, null, null, null);
      while (rs.next()){
        System.out.println("Table name " + rs.getString(3));
      }
    } catch (ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (SQLException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }finally{
      if(connection != null){
        //closing connection 
        try {
          connection.close();
        } catch (SQLException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
      } // if condition
    }// finally
  }
}

Output

Table name city
Table name country
Table name countrylanguage

Here I am connecting to “world” schema in MySQL and getting all the tables.

Each table description in the returned ResultSet has the following columns:

Table Description Columns
Column Name Type Description
TABLE_CAT String table catalog (may be null)
TABLE_SCHEM String table schema (may be null)
TABLE_NAME String table name
TABLE_TYPE String table type. Typical types are "TABLE", "VIEW" etc.
REMARKS String explanatory comment on the table (may be null)
TYPE_CAT String the types catalog (may be null)
TYPE_SCHEM String the types schema (may be null)
TYPE_NAME String type name (may be null)
SELF_REFERENCING_COL_NAME String name of the designated "identifier" column of a typed table (may be null)
REF_GENERATION String specifies how values in SELF_REFERENCING_COL_NAME are created.

That’s why column index is 3 while getting result from ResultSet as TABLE_NAME is at number 3.

Example to get Procedures using DatabaseMetaData in JDBC

For getting procedures you can use getProcedures(String catalog, String schemaPattern, String procedureNamePattern) method. Again you can pass null as value for all the parameters if you don’t want to narrow the search.

import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;

public class DBMetaData {

 public static void main(String[] args) {
  Connection connection = null;
   try {
    // Loading driver
    Class.forName("com.mysql.jdbc.Driver");
    
    // Creating connection
    connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/netjs", 
                        "root", "admin");
    
    DatabaseMetaData dbMetaData = connection.getMetaData();
    
    ResultSet rs = dbMetaData.getProcedures(null, null, null);
    
    while (rs.next()){
     System.out.println("Procedure name " + rs.getString(3));
    }
    
   } catch (ClassNotFoundException e) {
       // TODO Auto-generated catch block
       e.printStackTrace();
   } catch (SQLException e) {
       // TODO Auto-generated catch block
       e.printStackTrace();
   }finally{
       if(connection != null){
          //closing connection 
          try {
            connection.close();
          } catch (SQLException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
          }
       } // if condition
   }// finally
 }
}

Each procedure description in the returned ResultSet has the following columns:

Procedure Description Columns
Column Name Type Description
PROCEDURE_CAT String procedure catalog (may be null)
PROCEDURE_SCHEM String procedure schema (may be null)
PROCEDURE_NAME String procedure name
reserved for future use
reserved for future use
reserved for future use
REMARKS String explanatory comment on the procedure
PROCEDURE_TYPE short type name (may be null)
SPECIFIC_NAME String The name which uniquely identifies this procedure within its schema.

That’s why column index is 3 while getting result from ResultSet as PROCEDURE_NAME is at number 3.

That's all for this topic DatabaseMetaData Interface in Java-JDBC. If you have any doubt or any suggestions to make please drop a comment. Thanks!

>>>Return to Java Advanced Tutorial Page


Related Topics

  1. DataSource in Java-JDBC
  2. Java JDBC Steps to Connect to DB
  3. Statement Interface in Java-JDBC
  4. ResultSet Interface in Java-JDBC
  5. Data access in Spring framework

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