java - 使用 Class 和/或 ClassLoader 实例化 Java 类

Question

我正在尝试根据 JFileChooser 返回给我的文件实例化一个类。到目前为止，我已经使 JFileChooser 正常工作，并且可以成功获取我正在查找的 .class 文件。但是，我在实际获取 .class 文件并用它创建对象时遇到了麻烦。

这是我正在编写的方法...

public static Agent loadAgentFromFile(File file){
        Agent agent = null;
        String fileName = file.getName();
        String filePath = file.getAbsolutePath();
        String parentDirectory = file.getParentFile().getName();

        String agentClassName = fileName.substring(0, fileName.lastIndexOf('.'));

        Object object = null;
        //System.out.println(filePath.contains(parentDirectory));
        //System.out.println(filePath.substring(0, filePath.indexOf(parentDirectory)));

        try {
            File f = new File(filePath.substring(0, filePath.indexOf(parentDirectory)));
            URL[] cp = {f.toURI().toURL()};
            URLClassLoader urlcl = new URLClassLoader(cp);

            Class agentDefinition = null;
            if (parentDirectory.equals("Agents"))
            {
                System.out.println(PACKAGE + agentClassName);
                agentDefinition = urlcl.loadClass(PACKAGE + agentClassName);
                //agentDefinition = Class.forName(PACKAGE + agentClassName);
            }
            else
            {
                System.out.println(PACKAGE + parentDirectory + "." + agentClassName);
                agentDefinition = urlcl.loadClass(parentDirectory + "." + agentClassName);
            }
            object = agentDefinition.newInstance();

            agent = (Agent)object;
        }
        catch (InstantiationException e) {

            JOptionPane.showMessageDialog(THIS,
                "The chosen class must not be an interface or be abstract, " +
                "and it must not have any arguments in its constructor.");
        }
        catch (IllegalAccessException e) {

            JOptionPane.showMessageDialog(THIS,
                "The chosen class's constructor must be public.");
        }
        catch (ClassNotFoundException e) {

            JOptionPane.showMessageDialog(THIS,
                "The chosen class cannot be found.  Make sure the .java file is compiled.");
        }
        catch (ClassCastException e) {

            JOptionPane.showMessageDialog(THIS,
                "The chosen class does not extend from the Agent class.");
        }
        catch (Exception e) {
            e.printStackTrace(); //prints the stack to see what exception occurred that we didn't catch

            JOptionPane.showMessageDialog(THIS,
                "The chosen class does not conform to the \"Agent\" class.");
        }

        return agent;
    }

但是，当调用此方法时，我实际上得到一个 NoClassDefFoundError，它被我的包罗万象的异常捕获:

Exception in thread "AWT-EventQueue-0" java.lang.NoClassDefFoundError: TestAgents/TestDFSAgent (wrong name: WumpusEnvironment/Model/Agent/TestAgents/TestDFSAgent)
    at java.lang.ClassLoader.defineClass1(Native Method)
    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$100(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at WumpusEnvironment.Model.Agent.AgentLoader.loadAgentFromFile(AgentLoader.java:41)
    at WumpusEnvironment.View.MainWindow.ApplicationWindow.actionPerformed(ApplicationWindow.java:270)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.AbstractButton.doClick(Unknown Source)
    at javax.swing.plaf.basic.BasicMenuItemUI.doClick(Unknown Source)
    at javax.swing.plaf.basic.BasicMenuItemUI$Handler.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.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$200(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(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)

我查看了以前的同类答案，并且我最初将 .class 文件放在整个包结构之外，因此我移动了它们。我的类文件现在与源文件位于同一文件夹中，但我仍然收到此错误。值得注意的是，我正在尝试在 Eclipse 中运行它，尽管将来整个项目将导出到 .jar 文件，该文件将实例化不在 Eclipse 中的类。包结构(因为此时所有内容都在 .jar 内)。

有人能给我指出正确的方向吗？我还希望您能指导我在导出到 .jar 文件时如何使您的建议继续发挥作用。我意识到 Eclipse 设置一些与 .java 文件相关的 .class 文件的方式有时会把这些事情弄乱。

谢谢!

Answer 1

Could someone point me in the right direction here?

我认为正确的方向是不要这样做。我建议您:

1. 将插件代理类放入 JAR 文件中(JAR 中包含正确的路径)。

2. 为每个 JAR 文件创建单独的类加载器或为所有 JAR 文件创建一个类加载器...具体取决于代理类是否需要彼此隔离。

您遇到的异常是因为类包名称与类加载器根目录中的“.class”文件的路径不匹配。在最坏的情况下，如果不移动“.class”文件，或者编写一个执行一些棘手操作的自定义类加载器，则无法使其匹配。即便如此，您仍然会遇到(潜在的)问题，即处理对其他“.class”文件的所需或不需要的依赖关系。

将插件放入 JAR 中可以避免这些问题。