Error unable to load driver class

After you've installed the appropriate driver, it is time to establish a database connection using JDBC.

After you’ve installed the appropriate driver, it is time to establish a database connection using JDBC.

The programming involved to establish a JDBC connection is fairly simple. Here are these simple four steps −

  • Import JDBC Packages − Add import statements to your Java program to import required classes in your Java code.

  • Register JDBC Driver − This step causes the JVM to load the desired driver implementation into memory so it can fulfill your JDBC requests.

  • Database URL Formulation − This is to create a properly formatted address that points to the database to which you wish to connect.

  • Create Connection Object − Finally, code a call to the DriverManager object’s getConnection( ) method to establish actual database connection.

Import JDBC Packages

The Import statements tell the Java compiler where to find the classes you reference in your code and are placed at the very beginning of your source code.

To use the standard JDBC package, which allows you to select, insert, update, and delete data in SQL tables, add the following imports to your source code −

import java.sql.* ;  // for standard JDBC programs
import java.math.* ; // for BigDecimal and BigInteger support

Register JDBC Driver

You must register the driver in your program before you use it. Registering the driver is the process by which the Oracle driver’s class file is loaded into the memory,
so it can be utilized as an implementation of the JDBC interfaces.

You need to do this registration only once in your program. You can register a driver in one of two ways.

Approach I — Class.forName()

The most common approach to register a driver is to use Java’s Class.forName() method, to dynamically load the driver’s class file into memory, which automatically registers it. This method is preferable because it allows you to make the driver registration configurable and portable.

The following example uses Class.forName( ) to register the Oracle driver −

try {
   Class.forName("oracle.jdbc.driver.OracleDriver");
}
catch(ClassNotFoundException ex) {
   System.out.println("Error: unable to load driver class!");
   System.exit(1);
}

You can use getInstance() method to work around noncompliant JVMs, but then you’ll have to code for two extra Exceptions as follows −

try {
   Class.forName("oracle.jdbc.driver.OracleDriver").newInstance();
}
catch(ClassNotFoundException ex) {
   System.out.println("Error: unable to load driver class!");
   System.exit(1);
catch(IllegalAccessException ex) {
   System.out.println("Error: access problem while loading!");
   System.exit(2);
catch(InstantiationException ex) {
   System.out.println("Error: unable to instantiate driver!");
   System.exit(3);
}

Approach II — DriverManager.registerDriver()

The second approach you can use to register a driver, is to use the static DriverManager.registerDriver() method.

You should use the registerDriver() method if you are using a non-JDK compliant JVM, such as the one provided by Microsoft.

The following example uses registerDriver() to register the Oracle driver −

try {
   Driver myDriver = new oracle.jdbc.driver.OracleDriver();
   DriverManager.registerDriver( myDriver );
}
catch(ClassNotFoundException ex) {
   System.out.println("Error: unable to load driver class!");
   System.exit(1);
}

Database URL Formulation

After you’ve loaded the driver, you can establish a connection using the DriverManager.getConnection() method. For easy reference, let me list the three
overloaded DriverManager.getConnection() methods −

  • getConnection(String url)

  • getConnection(String url, Properties prop)

  • getConnection(String url, String user, String password)

Here each form requires a database URL. A database URL is an address that points to your database.

Formulating a database URL is where most of the problems associated with establishing a connection occurs.

Following table lists down the popular JDBC driver names and database URL.

RDBMS JDBC driver name URL format
MySQL com.mysql.jdbc.Driver jdbc:mysql://hostname/ databaseName
ORACLE oracle.jdbc.driver.OracleDriver jdbc:oracle:thin:@hostname:port Number:databaseName
DB2 COM.ibm.db2.jdbc.net.DB2Driver jdbc:db2:hostname:port Number/databaseName
Sybase com.sybase.jdbc.SybDriver jdbc:sybase:Tds:hostname: port Number/databaseName

All the highlighted part in URL format is static and you need to change only the remaining part as per your database setup.

Create Connection Object

We have listed down three forms of DriverManager.getConnection() method to create a connection object.

Using a Database URL with a username and password

The most commonly used form of getConnection() requires you to pass a database URL, a username, and a password −

Assuming you are using Oracle’s thin driver, you’ll specify a host:port:databaseName value for the database portion of the URL.

If you have a host at TCP/IP address 192.0.0.1 with a host name of amrood, and your Oracle listener is configured to listen on port 1521, and your database name is EMP, then complete database URL would be −

jdbc:oracle:thin:@amrood:1521:EMP

Now you have to call getConnection() method with appropriate username and password to get a Connection object as follows −

String URL = "jdbc:oracle:thin:@amrood:1521:EMP";
String USER = "username";
String PASS = "password"
Connection conn = DriverManager.getConnection(URL, USER, PASS);

Using Only a Database URL

A second form of the DriverManager.getConnection( ) method requires only a database URL −

DriverManager.getConnection(String url);

However, in this case, the database URL includes the username and password and has the following general form −

jdbc:oracle:driver:username/password@database

So, the above connection can be created as follows −

String URL = "jdbc:oracle:thin:username/password@amrood:1521:EMP";
Connection conn = DriverManager.getConnection(URL);

Using a Database URL and a Properties Object

A third form of the DriverManager.getConnection( ) method requires a database URL and a Properties object −

DriverManager.getConnection(String url, Properties info);

A Properties object holds a set of keyword-value pairs. It is used to pass driver properties to the driver during a call to the getConnection() method.

To make the same connection made by the previous examples, use the following code −

import java.util.*;

String URL = "jdbc:oracle:thin:@amrood:1521:EMP";
Properties info = new Properties( );
info.put( "user", "username" );
info.put( "password", "password" );

Connection conn = DriverManager.getConnection(URL, info);

Closing JDBC Connections

At the end of your JDBC program, it is required explicitly to close all the connections to the database to end each database session. However, if
you forget, Java’s garbage collector will close the connection when it cleans up stale objects.

Relying on the garbage collection, especially in database programming, is a very poor programming practice. You should make a habit of always closing the connection with the close() method associated with connection object.

To ensure that a connection is closed, you could provide a ‘finally’ block in your code. A finally block always executes, regardless of an exception occurs or not.

To close the above opened connection, you should call close() method as follows −

conn.close();

Explicitly closing a connection conserves DBMS resources, which will make your database administrator happy.

For a better understanding, we suggest you to study our JDBC — Sample Code tutorial.

Источник

Я работаю над веб-проектом и недавно установил postgres 9.1.1.

Сервер postgresql запущен и работает. Я могу подключиться через psql, как обычно, и все загружается и правильно сохраняется из дампа базы данных, которую я сделал из 8.5.

Итак, я также загрузил драйвер JDBC4 для версии postgres 9.1 здесь:
http://jdbc.postgresql.org/download/postgresql-jdbc-9.1-901.src.tar.gz

Я добавил его в путь сборки java, используя свойства проекта через eclipse.

Это код, который я использую для обеспечения соединения db с другими классами (то есть это одноэлементный, я получаю новое соединение только в том случае, если существующее либо закрыто, либо равно null, только от одного объекта за раз)

public abstract class DBConnection {
private static Connection connection = null;

public static void connect() {
    try {
        if (connection == null) {
            String host = "127.0.0.1";
            String database = "xxxxx";
            String username = "xxxxx";
            String password = "xxxxx";
            String url = "jdbc:postgresql://" + host + "/" + database;
            String driverJDBC = "org.postgresql.Driver";
            Class.forName(driverJDBC);
            connection = DriverManager.getConnection(url, username,
                    password); //line firing the class not found exception

        } else if (connection.isClosed()) {
            connection = null;
            connect();
        }
    } catch (SQLException e) {
        e.printStackTrace(System.err);
    } catch (Exception e) {
        e.printStackTrace(System.err);
    }
}

public static void disconnect() {
    if (connection != null) {
        try {
            connection.close();
        } catch (SQLException e) {
            Logger.getLogger(DBConnection.class.getName()).log(
                    Level.SEVERE, null, e);
        }
        }
    }

    public static Connection getConnection() {

        try {
            if (connection != null && !connection.isClosed()) {
                return connection;
            } else {
                connect();
                return connection;
            }
        } catch (SQLException e) {
            Logger.getLogger(DBConnection.class.getName()).log(Level.SEVERE,
                    null, e);
            return null;
        }
    }

    @Override
    public void finalize() {
        if (connection != null) {
            try {
                connection.close();
            } catch (SQLException e) {
                Logger.getLogger(DBConnection.class.getName()).log(
                        Level.SEVERE, null, e);
            }
        }
    }

}

Как я писал в заголовке, когда я запускаю проект и класс запрашивает подключение к этому классу, я всегда получаю исключение Class Not Found, поскольку он, по-видимому, не может загрузить org.postgresql.Driver.class. Драйвер находится в вложенная папка проекта ~ / lib / org.postgresql-9.1-901.jdbc4.jar и, как я уже сказал, добавлена ​​в путь сборки через свойства проекта eclipse.

Я также предоставляю образец запроса, чтобы увидеть обычное поведение моих классов при доступе к DBConnection:

public static final User validateUserCredentials(String id, String pswd) {
    Connection connection = DBConnection.getConnection();
    Logger.getLogger(Credentials.class.getName()).log(Level.SEVERE, (connection!=null)?"connection not null":"connection null");
    Statement stmt = null;
    Logger.getLogger(Home.class.getName()).log(Level.SEVERE, "validating credentials for user: username : " + id + " password : " + pswd);
    String sql = "Select * from fuser where id = '" + id + "'";
    ResultSet resultset = null;
    try {
        stmt = connection.createStatement();
        resultset = stmt.executeQuery(sql);
        Logger.getLogger(Credentials.class.getName())
                .log(Level.SEVERE, sql);
        resultset.next();
        String password = resultset.getString("pswd");
        if (pswd.equals(password))
            return new User(id, pswd);
    } catch (SQLException ex) {

        Logger.getLogger(Credentials.class.getName()).log(Level.SEVERE,
                null, ex);
    } finally {
        if (stmt != null)
            stmt = null;

        if (resultset != null)
            resultset = null;
        if (connection != null) {
            try {
                connection.close();
            } catch (SQLException e) {

            }
            connection = null;
        }
    }
    return null;
}
Источник

Ошибка:

org.hibernate.service.spi.ServiceException: Unable to create requested service [org.hibernate.engine.jdbc.env.spi.JdbcEnvironment]
	at org.hibernate.service.internal.AbstractServiceRegistryImpl.createService(AbstractServiceRegistryImpl.java:275)
	at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:237)
	at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:214)
	at org.hibernate.id.factory.internal.DefaultIdentifierGeneratorFactory.injectServices(DefaultIdentifierGeneratorFactory.java:152)
	at org.hibernate.service.internal.AbstractServiceRegistryImpl.injectDependencies(AbstractServiceRegistryImpl.java:286)
	at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:243)
	at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:214)
	at org.hibernate.boot.internal.InFlightMetadataCollectorImpl.<init>(InFlightMetadataCollectorImpl.java:175)
	at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.complete(MetadataBuildingProcess.java:118)
	at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.build(MetadataBuildingProcess.java:83)
	at org.hibernate.boot.internal.MetadataBuilderImpl.build(MetadataBuilderImpl.java:473)
	at org.hibernate.boot.internal.MetadataBuilderImpl.build(MetadataBuilderImpl.java:84)
	at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:689)
	at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:724)
	at cn.itcast.hibernate.HibernateDemo.testAdd(HibernateDemo.java:25)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
	at org.junit.vintage.engine.execution.RunnerExecutor.execute(RunnerExecutor.java:39)
	at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.Iterator.forEachRemaining(Iterator.java:116)
	at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
	at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
	at org.junit.vintage.engine.VintageTestEngine.executeAllChildren(VintageTestEngine.java:79)
	at org.junit.vintage.engine.VintageTestEngine.execute(VintageTestEngine.java:70)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:220)
	at org.junit.platform.launcher.core.DefaultLauncher.lambda$execute$6(DefaultLauncher.java:188)
	at org.junit.platform.launcher.core.DefaultLauncher.withInterceptedStreams(DefaultLauncher.java:202)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:181)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:128)
	at org.eclipse.jdt.internal.junit5.runner.JUnit5TestReference.run(JUnit5TestReference.java:89)
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:41)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:541)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:763)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:463)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:209)
Caused by: org.hibernate.boot.registry.classloading.spi.ClassLoadingException: Unable to load class [com.mysql.jdbc.Driver]
	at org.hibernate.boot.registry.classloading.internal.ClassLoaderServiceImpl.classForName(ClassLoaderServiceImpl.java:136)
	at org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl.loadDriverIfPossible(DriverManagerConnectionProviderImpl.java:149)
	at org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl.buildCreator(DriverManagerConnectionProviderImpl.java:105)
	at org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl.buildPool(DriverManagerConnectionProviderImpl.java:89)
	at org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl.configure(DriverManagerConnectionProviderImpl.java:73)
	at org.hibernate.boot.registry.internal.StandardServiceRegistryImpl.configureService(StandardServiceRegistryImpl.java:107)
	at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:246)
	at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:214)
	at org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator.buildJdbcConnectionAccess(JdbcEnvironmentInitiator.java:145)
	at org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator.initiateService(JdbcEnvironmentInitiator.java:66)
	at org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator.initiateService(JdbcEnvironmentInitiator.java:35)
	at org.hibernate.boot.registry.internal.StandardServiceRegistryImpl.initiateService(StandardServiceRegistryImpl.java:101)
	at org.hibernate.service.internal.AbstractServiceRegistryImpl.createService(AbstractServiceRegistryImpl.java:263)
	... 57 more
Caused by: java.lang.ClassNotFoundException: Could not load requested class : com.mysql.jdbc.Driver
	at org.hibernate.boot.registry.classloading.internal.AggregatedClassLoader.findClass(AggregatedClassLoader.java:210)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:348)
	at org.hibernate.boot.registry.classloading.internal.ClassLoaderServiceImpl.classForName(ClassLoaderServiceImpl.java:133)
	... 69 more

Решение:

Добавьте в проект пакет драйвера JDBC для mysql.

mysql-connector-java

：

Источник

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