Java编程极限考验：ClassLoader类装载策略

public class WrapperServlet extends Servlet {
   public void init(ServletConfig config) {
     m_config = config; // stored as you don't want this to change
   }

   public void doGet(...) throws ServletException {
     getServlet().doGet(...);
   }

   private Servlet getServlet() throws ServletException {
     if ((m_servlet == null) || haveClassesChanged()) {
         创造一个独立的classloader 确保它的父是我们的自己的ClassLoader<br>
        path_to_separate_classes是将被装载的路

       URLClassLoader loader = new URLClassLoader(new URL[] {
         new URL("path_to_separate_classes");
       }, WrapperServlet.class.getClassLoader());

       从被装载的路径使用自己装载器来装载Servlet类       
       Class servletClass =
         loader.loadClass(m_config.getInitParameter("servletClass");
       m_servlet = (Servlet)servletClass.newInstance();
       m_servlet.init(m_config);
     }
     return m_servlet;
   }

   private boolean haveClassesChanged() {
     // determine whether classes have changed somehow
   }

   private ServletConfig m_config;
   private Servlet m_servlet = null;
}

// 摘自javax.xml.parsers.FactoryFinder
public final class ClassLoaderFinder {
  private ClassLoaderFinder() {
  }

  /**
   * Figure out which ClassLoader to use.  For JDK 1.2 and later use the
   * context ClassLoader if possible.  Note: we defer linking the class
   * that calls an API only in JDK 1.2 until runtime so that we can catch
   * LinkageError so that this code will run in older non-Sun JVMs such
   * as the Microsoft JVM in IE.
   */
  public static ClassLoader findClassLoader() throws ConfigurationError {
    ClassLoader classLoader;
    try {
      // Construct the name of the concrete class to instantiate
      Class clazz = Class.forName(ClassLoaderFinder.class.getName()
                                  + "$ClassLoaderFinderConcrete");
      ContextClassLoaderFinder clf = (ContextClassLoaderFinder) clazz.
          newInstance();
      classLoader = clf.getContextClassLoader();
    }
    catch (LinkageError le) {
      // Assume that we are running JDK 1.1, use the current ClassLoader
      classLoader = ClassLoaderFinder.class.getClassLoader();
    }
    catch (ClassNotFoundException x) {
      // This case should not normally happen.  MS IE can throw this
      // instead of a LinkageError the second time Class.forName() is
      // called so assume that we are running JDK 1.1 and use the
      // current ClassLoader
      classLoader = ClassLoaderFinder.class.getClassLoader();
    }
    catch (Exception x) {
      // Something abnormal happened so throw an error
      throw new ConfigurationError(x.toString(), x);
    }
    return classLoader;
  }

  static class ConfigurationError
      extends Error {
    private Exception exception;

    /**
     * Construct a new instance with the specified detail string and
     * exception.
     */
    ConfigurationError(String msg, Exception x) {
      super(msg);
      this.exception = x;
    }

    Exception getException() {
      return exception;
    }
  }

  /*
   * The following nested classes allow getContextClassLoader() to be
   * called only on JDK 1.2 and yet run in older JDK 1.1 JVMs
   */

  private static abstract class ContextClassLoaderFinder {
    abstract ClassLoader getContextClassLoader();
  }

  static class ClassLoaderFinderConcrete
      extends ContextClassLoaderFinder {
    ClassLoader getContextClassLoader() {
      return Thread.currentThread().getContextClassLoader();
    }
  }

  //Sample
  public static void main(String args[]) {
    try {

      final String CLASS_NAME = "oracle.jdbc.driver.OracleDriver";
      Class spiClass;
      ClassLoader classLoader = ClassLoaderFinder.findClassLoader();
      if (classLoader == null) {
        System.out.println("From Class.forName");
        spiClass = Class.forName(CLASS_NAME);
      }
      else {
        System.out.println("From ClassLoaderFinder");
        spiClass = classLoader.loadClass(CLASS_NAME);
        spiClass.newInstance();
      }

      java.sql.Connection conn = java.sql.DriverManager.getConnection(
          "jdbc:oracle:thin:@localhost:1521:ORCL",
          "scott",
          "tiger");
      java.sql.Statement stmt = conn.createStatement();
      java.sql.ResultSet rs = stmt.executeQuery("select * from cat");
      while (rs.next()) {
        System.out.println(rs.getString(2));
      }
    }
    catch (Exception e) {
      e.printStackTrace();
    }
  }

}

package cloader;

public final class ClassLoaderFinder {
  private ClassLoaderFinder() {
  }
  
  /**
   * The following nested classes extends the <tt>ClassLoader</tt> so that we
   * can create our ClassLoader freely.
   */
  private static class PersonalClassLoader extends ClassLoader {
    public PersonalClassLoader(ClassLoader parentLoader) {
      super(parentLoader);
    }
    
    public PersonalClassLoader() {
      //Use the external class's <tt>ClassLoader</tt>
      super(ClassLoaderFinder.class.getClassLoader());
    }
  }
  /**
   * Figure out which ClassLoader to use.  For JDK 1.2 and later use the
   * context ClassLoader if possible.  Note: we defer linking the class
   * that calls an API only in JDK 1.2 until runtime so that we can catch
   * LinkageError so that this code will run in older non-Sun JVMs such
   * as the Microsoft JVM in IE.
   */
  public static ClassLoader findClassLoader() throws ConfigurationError {
    ClassLoader classLoader;
    try {
      // Construct the name of the concrete class to instantiate
      PersonalClassLoader pLoader = new PersonalClassLoader();
      Class clazz = pLoader.loadClass(ClassLoaderFinder.class.getName()
                                         + "$ClassLoaderFinderConcrete");
      //Class clazz = Class.forName(ClassLoaderFinder.class.getName()
      //                            + "$ClassLoaderFinderConcrete");
      ContextClassLoaderFinder clf = (ContextClassLoaderFinder) clazz.
          newInstance();
      classLoader = clf.getContextClassLoader();
    }
    catch (LinkageError le) {
      // Assume that we are running JDK 1.1, use the current ClassLoader
      classLoader = ClassLoaderFinder.class.getClassLoader();
    }
    catch (ClassNotFoundException x) {
      // This case should not normally happen.  MS IE can throw this
      // instead of a LinkageError the second time Class.forName() is
      // called so assume that we are running JDK 1.1 and use the
      // current ClassLoader
      classLoader = ClassLoaderFinder.class.getClassLoader();
    }
    catch (Exception x) {
      // Something abnormal happened so throw an error
      throw new ConfigurationError(x.toString(), x);
    }
    return classLoader;
  }

  static class ConfigurationError
      extends Error {
    private Exception exception;

    /**
     * Construct a new instance with the specified detail string and
     * exception.
     */
    ConfigurationError(String msg, Exception x) {
      super(msg);
      this.exception = x;
    }

    Exception getException() {
      return exception;
    }
  }

  /*
   * The following nested classes allow getContextClassLoader() to be
   * called only on JDK 1.2 and yet run in older JDK 1.1 JVMs
   */

  private static abstract class ContextClassLoaderFinder {
    abstract ClassLoader getContextClassLoader();
  }

  static class ClassLoaderFinderConcrete
      extends ContextClassLoaderFinder {
    ClassLoader getContextClassLoader() {
      return Thread.currentThread().getContextClassLoader();
    }
  }

  //Sample
  
}