[Java]关于动态加载及卸载jar后的一些思考

master336

发布于 2026-06-15 19:29:35

550

实现

编写要加载/卸载的测试jar

package cn.com.test;

public class JarTest {
    static {
        System.out.println("I am JarTest's static code");
    }
    public static void run(){
        System.out.println("I am JarTest's static method");
    }
    public void run1(){
        System.out.println("I am JarTest's  method>>>"+this);
    }
}

将以上class 打包成jartest.jar 假定打包文件位置放在/Users/tmp/jartest.jar

使用jar -cvf jartest.jar **path 打包或者使用IDE打包都行

编写加载/卸载测试工具

package jar;

import sun.misc.ClassLoaderUtil;

import java.io.File;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;

public class LoadJar {
    static Object jarTestInstance = null;
    static ClassLoader myClassLoader1;
    static Class<?> jarTest;

    public static void main(String[] args) throws MalformedURLException, InterruptedException {
        System.out.println("befor laod jar");
        loadClassAndRun();
        Thread.sleep(1000);
        System.out.println("laod jar");
        loadJar();
        Thread.sleep(1000);
        System.out.println("after load jar");
        loadClassAndRun();
        Thread.sleep(1000);
        System.out.println("start unload jar");
        unLoad();
        Thread.sleep(1000);
        System.out.println("after unload jar");
        loadClassAndRun();
        System.out.println("laod jar");
        loadJar();
        Thread.sleep(1000);
        System.out.println("after load jar");
        loadClassAndRun();
    }

    private static void loadClassAndRun() {
        if(myClassLoader1 == null) {
            myClassLoader1 = LoadJar.class.getClassLoader();
        }
        try {
            jarTest = myClassLoader1.loadClass("cn.com.test.JarTest");
            jarTestInstance = jarTest.newInstance();
            for (Method method : jarTest.getMethods()) {
                try {
                    method.invoke(jarTestInstance);
                } catch (InvocationTargetException e) {
                    e.printStackTrace();
                }
            }
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (Exception e){
            e.printStackTrace();
        }
    }

    private static void loadJar() throws MalformedURLException {
        URL url = new File("/Users/tmp/jartest.jar").toURI().toURL();
        myClassLoader1 = new URLClassLoader(new URL[] { url});
    }

    // 卸载jar包的代码如下：
    public static void unLoad() {

        if (null != myClassLoader1 && myClassLoader1 instanceof URLClassLoader) {
            System.out.println("unload URLClassLoader ");
            URLClassLoader loader = (URLClassLoader) myClassLoader1;
            try {
            	// 关注下这里，这里一会儿要改
                ClassLoaderUtil.releaseLoader(loader);
                loader.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

    }
}

执行结果

思考

URLClassLoader.close()释放掉资源后，刚加载过的class仍然可以继续使用？官方解释：

*** 从官方的解释中来看，确实如此 *** 那么在什么时候会被回收呢？反思类的加载机制，类在没有引用时可被GC，改动unLoad() 方法如下：

    // 卸载jar包的代码如下：
    public static void unLoad() {

        if (null != myClassLoader1 && myClassLoader1 instanceof URLClassLoader) {
            System.out.println("unload URLClassLoader ");
            URLClassLoader loader = (URLClassLoader) myClassLoader1;
            try {
                jarTest = null;
                jarTestInstance = null;
                myClassLoader1 = null;
                System.gc();
                Thread.sleep(2000);
                ClassLoaderUtil.releaseLoader(loader);
                loader.close();
            } catch (IOException e) {
                e.printStackTrace();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

    }

再次测试：