[Java]关于动态加载及卸载jar后的一些思考

柔光的暖阳◎ 2022-11-05 08:36 424阅读 0赞

# 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 #

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L21hc3RlcjMzNg_size_16_color_FFFFFF_t_70]

# 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：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L21hc3RlcjMzNg_size_16_color_FFFFFF_t_70 1]  
\*\*\* 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：  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L21hc3RlcjMzNg_size_16_color_FFFFFF_t_70 2]  
需要注意的是GC不是立即执行的，可能你的验证结果跟我不一样，注意区分！

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L21hc3RlcjMzNg_size_16_color_FFFFFF_t_70]: /images/20221023/ae25c5c5d84f46cfacbe65215024fa23.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L21hc3RlcjMzNg_size_16_color_FFFFFF_t_70 1]: /images/20221023/009bc459ceae473ba63030ee9364dee0.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L21hc3RlcjMzNg_size_16_color_FFFFFF_t_70 2]: /images/20221023/6873e4f562d244bd8b215e2d562cf649.png