加载并连接到 mysql jdbc 驱动程序运行时

Question

我目前正在处理需要加载 mysql 驱动程序运行时并使用 java 连接到数据库的需求。

我正在使用URLClassLoader来加载jar文件

File f = new File("D:/Pallavi/workspace/WarInstallation/mysql-connector-java-5.0.4-bin.jar"); //Jar path

URLClassLoader urlCl = new URLClassLoader(new URL[] { f.toURL()},System.class.getClassLoader()); 
Class sqldriver = urlCl.loadClass("com.mysql.jdbc.Driver"); // Runtime loading

Driver ds = (Driver) sqldriver.newInstance(); //Compilation failing as "sqldriver" class of type Driver is not found

//I am using now java.sql.Driver to remove the compilation error

sqldriver = Class.forName("com.mysql.jdbc.Driver", true, sqldriver.getClassLoader()).newInstance(); //Runtime fail.. "Driver" Class not Found Exception.

尽管类加载良好，但无论我尝试哪种驱动程序，我都无法建立数据库连接（找不到适合的驱动程序...）。

请建议一种加载 jdbc "com.mysql.jdbc.Driver" 类运行时的方法。 如果您需要任何进一步的信息，请告诉我，因为这很紧急。

提前致谢。

Answer 1

在回答您的问题之前，我有三个问题：

1. 声明 1：

   ya, I have set the classpath of mysql jar in the environment variables, do we need to set it through system properties?

   Q1：当系统类加载器可以从类路径中轻松获得一个类时，为什么要依赖自定义类加载器？
   您不需要明确的类路径到 mysql***.jar 来使用自定义类加载器。

2. 声明 2：

   Class sqldriver = urlCl.loadClass("com.mysql.jdbc.Driver"); // Runtime loading
//Compilation failing as "sqldriver" class of type Driver is not found
Driver ds = (Driver) sqldriver.newInstance();

   Q2：声称 Compilation failing ... 非常矛盾。您的编译器是否正在寻找这样的类来生成您的类！？
   我确定不是。可能是错误在运行时使用java.lang.ClassNotFoundException: com.mysql.jdbc.Driver。而且我还怀疑评论应该与您的 Statement 3 放在下面。
   如果是CNFE，则您指向mysql***.jar 的文件路径错误。先修好了。

3. 声明 3：

   //I am using now java.sql.Driver to remove the compilation error
//Runtime fail.. "Driver" Class not Found Exception. sqldriver = Class.forName("com.mysql.jdbc.Driver", true, sqldriver.getClassLoader()).newInstance();

   Q3：声称 ... "Driver" Class not Found Exception 是可疑的。因为，这个语句不会被编译。那么它怎么可能是一个运行时失败。 ..！？
   我还怀疑评论应该与您上面的 Statement 2 一起使用。
   在这里，您需要调用newInstance()，然后转换为java.sql.Driver，然后再分配给sqlDriver 变量。因为Class.forName( ... 只返回一个Class object associated with the class or interface with the given string name。
   如果上述声明 2 中的问题得到修复，您可以应用此修复来进一步测试。

希望您能澄清这些陈述。

---

我在下面有一个工作示例代码，并为您显示了经过测试的输出。

import java.io.File; // and others as required

public class MySQLDriveClassLoader {
  public static void main( String [] args ) throws Exception {
    //File f = new File( "/home/ravinder/soft-dump/mysql-connector-java-5.1.18-bin.jar" );
    File f = new File( "E:\\Soft_Dump\\mysql-connector-java-5.0.4\\mysql-connector-java-5.0.4-bin.jar" );
    URLClassLoader urlCl = new URLClassLoader( new URL[] { f.toURI().toURL() }, System.class.getClassLoader() );

    Class mySqlDriver = urlCl.loadClass( "com.mysql.jdbc.Driver" );

    //*** Start: DEBUG *************************
    //mySqlDriver.con // On pressing CTRL+SPACEBAR, after .con, IDE shows "No default proposals"
    // meaning it still is not an instance of Driver, and hence can't call a method from Driver class.

    //Incompatible conditional operand types Class and Driver
    //System.out.println( mySqlDriver instanceof java.sql.Driver ) );

    System.out.println( "mySqlDriver: " + mySqlDriver );
    System.out.println( "Is this interface? = " + mySqlDriver.isInterface() );

    Class interfaces[] = mySqlDriver.getInterfaces();
    int i = 1;
    for( Class _interface : interfaces ) {
      System.out.println( "Implemented Interface Name " + ( i++ ) + " = " + _interface.getName() );
    } // for(...)

    Constructor constructors[] = mySqlDriver.getConstructors();
    for( Constructor constructor : constructors ) {
      System.out.println( "Constructor Name = " + constructor.getName() );
      System.out.println( "Is Constructor Accessible? = " + constructor.isAccessible() );
    } // for(...)
    //*** End  : DEBUG *************************

    Driver sqlDriverInstance = ( Driver ) mySqlDriver.newInstance();
    System.out.println( "sqlDriverInstance: " + sqlDriverInstance );

    Connection con = null;
    try {
      /******************************************************************
      // You may fail to register the above driver
      // hence don't depend on DriverManager to get Connected
      //DriverManager.registerDriver( sqlDriverInstance );
      //Driver driver = DriverManager.getDriver( "com.mysql.jdbc.Driver" ); // ( "jdbc:mysql" );
      Enumeration<Driver> enumDrivers = DriverManager.getDrivers();
      while ( enumDrivers.hasMoreElements() ) {
        Driver driver = enumDrivers.nextElement();
        System.out.println( "driver: " + driver );
      } // while drivers
      //******************************************************************/

      String dbUrl = "jdbc:mysql://:3306/test";
      Properties userDbCredentials = new Properties();
      userDbCredentials.put( "user", "root" );
      userDbCredentials.put( "password", "password" );

      // No suitable driver found for ...
      //con = DriverManager.getConnection( dbUrl, "root", "password" );

      // safely use driver to connect
      con = sqlDriverInstance.connect( dbUrl, userDbCredentials );
      System.out.println( "con: " + con );

      Statement stmt = con.createStatement();
      String sql = "select now()";
      ResultSet rs = stmt.executeQuery( sql );
      if ( rs.next() ) {
        System.out.println( rs.getString( 1 ) );
      } // if rs
    } catch( Exception e ) {
      e.printStackTrace(); // only for quick debug
    } finally {
      try { if ( con != null ) con.close(); } catch ( Exception ignoreThis ) {}
    }
  } // psvm(...)
} // class MySQLDriveClassLoader

编译运行成功，输出如下：

mySqlDriver: class com.mysql.jdbc.Driver
Is this interface? = false
Implemented Interface Name 1 = java.sql.Driver
Constructor Name = com.mysql.jdbc.Driver
Is Constructor Accessible? = false
sqlDriverInstance: com.mysql.jdbc.Driver@1270b73
con: com.mysql.jdbc.Connection@32fb4f
2012-05-29 03:52:12.0

---

Answer 2

DriverManager 忽略在运行时加载的类，它只对系统类加载器加载的类起作用。

您可以创建一个 Dummy 驱动程序类来封装您的实际数据库驱动程序。源码可以在here找到。

题外话：

File.toURL 已弃用，而是使用 toURL 上的 URI 从 File 获取 URL

URLClassLoader urlCl = new URLClassLoader(new URL[] { f.toURI().toURL()},System.class.getClassLoader());