java - 使用反射和 ClassLoaders 创建类的实例时出现 ClassCastException

Question

首先，这是Java 1.4(项目限制)。我正在尝试创建一个应用程序管理器。它使用自己的自定义类加载器实例加载每个应用程序的主类。之后，它使用反射创建主类的实例。每个应用程序都实现一个公共(public)接口(interface)，因此在创建实例后，它会运行应用程序的预定义方法。

但是，我在 CRASH POINT 1 遇到了一些麻烦(见代码)。该类未被识别为其接口(interface)的一种实现。如果我注释这个代码块，我会在崩溃点 2 处得到 ClassCastException。

我想这两个错误都与同一个问题有关(当然)。

谁能帮帮我？代码的相关部分如下(导入已删除)...

非常感谢。

马库斯

//AppManager.java

public class AppManager {
    public ThreadGroup threadGroup;
    private Class appClass;
    private AppInstance appInst;
    public AppContextImpl context;

    private AppManager(CustomClassLoader cl, String mainClass) throws ClassNotFoundException {
        final String className = mainClass;
        final CustomClassLoader finalLoader = cl;

        appClass = cl.loadClass(mainClass);

        // DEBUG CODE:
        Class[] k1 = AppInstance.class.getInterfaces();
        System.out.println(k1.length + " interfaces for AppInstance.class:");
        for (int ii = 0; ii < k1.length; ii++) {
            System.out.println("   " + ii + " - " + k1[ii].getName() + " (" + k1[ii].getClassLoader() + ")");
        }

        Class[] k2 = appClass.getInterfaces();
        System.out.println(k2.length + " interfaces for appClass instance:");
        for (int ii = 0; ii < k2.length; ii++) {
            System.out.println("   " + ii + " - " + k2[ii].getName() + " (" + k2[ii].getClassLoader() + ")");
        }

        // CRASH POINT 1
        if (!(AppInstance.class.isAssignableFrom(appClass))) {
            throw new IllegalArgumentException("Attempt to run a non-AppInstance class: " + appClass);
        }

        context = new AppContextImpl(mainClass, this);
        cl.setAppManager(this);
        Constructor m;
        try {
            m = appClass.getConstructor(new Class[0]);
           // CRASH POINT 2
            appInst = (AppInstance) m.newInstance(new Object[0]);
            appInst.init(context);
        } catch (Exception e) {
            System.out.println("Got ClassCastException here!\n");
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        App app1;

        String path1 = "/home/user/workspace/MultiTaskTest/bin/";
        String app1Name = "App1";

        Vector v1 = new Vector();
        try {
            v1.add(new URL(path1));
        } catch (MalformedURLException e1) {
            final File file1 = new File(path1);
            try {
                URL path1aux = (URL) AccessController.doPrivileged(
                    new PrivilegedExceptionAction() {
                        public Object run() throws IOException {
                            if (!file1.exists()) {
                                System.out.println("Warning: \"" + file1.getPath() + "\" not found");
                                return null;
                            }
                        return file1.toURI().toURL();
                        }
                    });

                if (path1aux != null) {
                    v1.add(path1aux);
                }
            } catch (PrivilegedActionException e) {
                e.getException().printStackTrace();
            }
    }

        final URL[] array1 = (URL[]) v1.toArray(new URL[v1.size()]);
        CustomClassLoader cl1 = (CustomClassLoader) AccessController.doPrivileged(
            new PrivilegedAction() { public Object run() {
                return new CustomClassLoader(array1);
            }});
        System.out.println("ClassLoader 1 created: " + cl1);
        try {
            app1 = new App(cl1, app1Name);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
            System.out.println("Cannot find class App1.");
        }
    }
}

//应用实例.java

public interface AppInstance {
    public void init(ContextImpl context);
}

//App1.java

public class App1 implements AppInstance {
    private AppContextImpl contextObj;

    public void init(AppContextImpl context) {
        this.contextObj = context;
        System.out.println("Running App1...");
    }
}

//AppContextImpl.java

public class AppContextImpl {
    public String mainClass;
    public AppManager app;

    public AppContextImpl(String mainClass, AppManager app) {
        this.mainClass = mainClass;
        this.app = app;
    }
}

//自定义类加载器.java

public class CustomClassLoader extends URLClassLoader {
    AppManager appInst;

    public CustomClassLoader(URL[] paths) { super(paths, null); }
    public void setAppManager(AppManager app) { this.appInst = app; }
}

AppManager.java 文件中调试代码的输出是:

0 interfaces for AppInstance.class:
1 interfaces for appClass instance:
   0 - AppInstance (CustomClassLoader@480457)

Answer 1

您的 AppInstance 类可能由每个自定义类加载器单独加载。由于类对象依赖于实际的类和类加载器，它们实际上是不同的类。所以来自类加载器 1 的 AppInstance 与来自类加载器 2 的 AppInstance 不同。

您需要做的是使用标准的类加载器层次结构:为您的应用程序使用根类加载器，并确保 AppInstance 可由类加载器加载。然后让你的自定义类加载器从根开始。每当他们需要访问 AppInstance 类时，他们将使用从根加载的内容。

所以，而不是这个:

public CustomClassLoader(URL[] paths) { super(paths, null); }

您需要为您的 CustomClassLoader 提供父级