Applet 和动态类加载问题

Question

首先，这是我第一次问 java 问题，所以我希望我能让自己理解:-) 提前感谢任何研究这个问题的人。

我有一个小程序，我需要根据用户与小程序的交互来加载类。我已经签署了该小程序，因此安全性不应该成为问题（如果我理解正确的话）。我的所有文件都位于 applet.jar 中，其中包含一个名为 p6applet 的包，所有 .class 文件都位于该包内。 我使用以下代码来测试小程序：

<APPLET 
     CODE="p6applet.Main.class" 
     ARCHIVE = applet.jar 
         WIDTH=0 
         HEIGHT=0>
</APPLET>

需要加载的类位于用户的硬盘上。它需要导入一个接口，该接口位于 p6applet 包下的 applet.jar 文件中。这个接口称为AlgorithmExecutable。

当我运行我的应用程序时，它会执行以下代码，然后发生错误： 当程序执行以下代码行时会发生错误：

    className = ...;
try {
    System.out.println("Test1");
    URL[] urlClass = {new URL("file:/C:/.../")};
    System.out.println("Test2");
    ClassLoader cl = new URLClassLoader(urlClass);
    System.out.println("Test3");
    algorithmClass = cl.loadClass(className); //This is where the error occurs
    System.out.println("Test4");
    algorithm = (AlgorithmExecutable) algorithmClass.newInstance();
    System.out.println("The " + className + " has been loaded.");
} catch (MalformedURLException ex) {
    System.out.println("MalformedURLException: " + ex);
} catch (ClassNotFoundException ex) {
    System.out.println("ClassNotFoundException: " + ex);
} catch (InstantiationException ex) {
    System.out.println("InstantiationException: " + ex);
} catch (IllegalAccessException ex) {
    System.out.println("IllegalAccessException: " + ex);
}

我收到以下错误：

Exception in thread "AWT-EventQueue-7" java.lang.NoClassDefFoundError: p6applet/AlgorithmExecutable
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(Unknown Source)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at java.security.SecureClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.access$000(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at p6applet.AlgorithmHandler.getAlgorithm(AlgorithmHandler.java:52)
    at p6applet.AlgorithmInterface.actionPerformed(AlgorithmInterface.java:671)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: p6applet.AlgorithmExecutable
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    ... 38 more

我的小程序在 netbeans 小程序查看器中运行良好，如果这可以帮助查明问题。我怀疑问题是需要加载的文件无法成功导入接口AlgorithmExecutable。任何帮助将不胜感激。

Answer 1

我认为主要问题是您没有使用 file:// 的安全权限，即使小程序已签名并给出了安全权限也是如此。据我所知，您需要访问本地服务器上的 URL，或者类路径上的 jar 内的 URL。这是一个复杂的问题。

http://www.securingjava.com/chapter-two/chapter-二-7.html

http://forums.sun .com/thread.jspa?threadID=5427882&tstart=0

http://mindprod. com/jgloss/applet.html