卸载类加载器

Question

我的最终目标是能够在已经加载到JVM之后重新加载类.

阅读下面的答案后在java中卸载类？,我一直在尝试实现自己的Class-Loader,它本身为它加载的每个类创建一个不同的Class-Loader实例(它自己的类型).

所以,结果是每个Class-Loader一个类.

目的是能够GC类,意味着它的所有实例,然后卸载它的类加载器,并能够从其字节重新加载相同的类.

问题是 - 我可以看到我的类实例使用finalize()方法进行垃圾收集,但是我不能让我的类加载器卸载,或者被垃圾收集.
是否有任何代码示例,一个简单的测试,显示它是如何完成的？

谢谢,任何帮助将不胜感激

编辑:

更清楚的是,我对代码示例感兴趣,其中新对象的实例化是通过'new()'操作数,而类加载器没有显式重新加载主要的类,但是在下一个'new(之后) )' 叫做.

Answer 1

如果没有更多的引用,则应该对类加载器进行垃圾回收.我从@PeterLawrey那里获取了这个代码(谢谢)(它与你的相同),在自定义类加载器finalize()方法中添加一个日志,然后,在加载的类为gc之后,类加载器被垃圾收集:

 /* Copyright (c) 2011.  Peter Lawrey
 *
 * "THE BEER-WARE LICENSE" (Revision 128)
 * As long as you retain this notice you can do whatever you want with this stuff.
 * If we meet some day, and you think this stuff is worth it, you can buy me a beer in return
 * There is no warranty.
 */


import java.lang.reflect.Field;
import java.net.URL;
import java.net.URLClassLoader;

public class LoadAndUnloadMain {
    public static void main(String... args) throws ClassNotFoundException, NoSuchFieldException, IllegalAccessException, InterruptedException {
        URL url = LoadAndUnloadMain.class.getProtectionDomain().getCodeSource().getLocation();
        final String className = LoadAndUnloadMain.class.getPackage().getName() + ".UtilityClass";
        {
            ClassLoader cl;
            Class clazz;

            for (int i = 0; i < 2; i++) {
                cl = new CustomClassLoader(url);
                clazz = cl.loadClass(className);
                loadClass(clazz);

                cl = new CustomClassLoader(url);
                clazz = cl.loadClass(className);
                loadClass(clazz);
                triggerGC();
            }
        }
        triggerGC();
    }

    private static void triggerGC() throws InterruptedException {
        System.out.println("\n-- Starting GC");
        System.gc();
        Thread.sleep(100);
        System.out.println("-- End of GC\n");
    }

    private static void loadClass(Class clazz) throws NoSuchFieldException, IllegalAccessException {
        final Field id = clazz.getDeclaredField("ID");
        id.setAccessible(true);
        id.get(null);
    }

    private static class CustomClassLoader extends URLClassLoader {
        public CustomClassLoader(URL url) {
            super(new URL[]{url}, null);
        }

        @Override
        protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
            try {
                return super.loadClass(name, resolve);
            } catch (ClassNotFoundException e) {
                return Class.forName(name, resolve, LoadAndUnloadMain.class.getClassLoader());
            }
        }

        @Override
        protected void finalize() throws Throwable {
            super.finalize();
            System.out.println(this.toString() + " - CL Finalized.");
        }
    }
}

class UtilityClass {
    static final String ID = Integer.toHexString(System.identityHashCode(UtilityClass.class));
    private static final Object FINAL = new Object() {
        @Override
        protected void finalize() throws Throwable {
            super.finalize();
            System.out.println(ID + " Finalized.");
        }
    };

    static {
        System.out.println(ID + " Initialising");
    }
}