[Jvm]程序员的精进之路～JDK代理源码初探

矫情吗；* 2024-02-17 12:48 9阅读 0赞

> 从模仿到反编译 jdk 代理源码

Jdk代理的最简单模拟， 由前文可知 JDK动态代理需要实现接口，所以基于此，进行最简单的模拟。

package com.example.proxy;
    
    public class Jdk {
        
    
        interface Foo {
        
            void foo();
        }
    
    
        static class Target implements Foo {
        
    
            @Override
            public void foo() {
        
                System.out.println("foo");
            }
        }
    
        // 代理类
        static class $Proxy0 implements Foo {
        
    
            @Override
            public void foo() {
        
                // 1. 功能增强
                System.out.println("before");
                // 2. 调用目标
                new Target().foo();
            }
        }
    
        public static void main(String[] args) {
        
            Foo f = new $Proxy0();
            f.foo();
        }
    }

虽然简单实现了代理，但是目前增强是固定的，但是在实际应用中，使用到代理类，方法是不可能固定的，所以接下来进行优化一下。使用抽象类+模版方法设置代理的执行逻辑。

package com.example.proxy;
    
    public class Jdk {
        
    
        interface Foo {
        
            void foo();
        }
    
    
    
        static abstract class InvokeHandler {
        
            abstract Object invoke();
        }
    
        // 代理类
        static class $Proxy0 implements Foo {
        
    
            private final InvokeHandler invokeHandler;
    
            $Proxy0(InvokeHandler invokeHandler) {
        
                this.invokeHandler = invokeHandler;
            }
    
            @Override
            public void foo() {
        
                // 1. 功能增强
                System.out.println("before");
                // 2. 调用目标
                invokeHandler.invoke();
            }
        }
        
    
        public static void main(String[] args) {
        
            Foo f = new $Proxy0(new InvokeHandler() {
        
                @Override
                Object invoke() {
        
                    System.out.println(">>>>>>>> foo");
                    return null;
                }
            });
            f.foo();
        }
    }

至此，方法就是可以不再固定。但是很显然，代理的对象不可能永远只有一个方法，所以想办法动态设置。

package com.example.proxy;
    
    import java.lang.reflect.InvocationTargetException;
    import java.lang.reflect.Method;
    
    public class Jdk {
        
    
        interface Foo {
        
            void foo() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException;
    
            void bar() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException;
        }
    
    
    
        static abstract class InvokeHandler {
        
            abstract Object invoke(Method method, Object[] params) throws InvocationTargetException, IllegalAccessException;
        }
    
        // 代理类
        static class $Proxy0 implements Foo {
        
    
            private final InvokeHandler invokeHandler;
    
            $Proxy0(InvokeHandler invokeHandler) {
        
                this.invokeHandler = invokeHandler;
            }
    
            @Override
            public void foo() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        
                // 1. 功能增强
                System.out.println("before");
                // 2. 调用目标
                invokeHandler.invoke(Foo.class.getMethod("foo"), new Object[0]);
            }
    
            @Override
            public void bar() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        
                // 1. 功能增强
                System.out.println("before");
                // 2. 调用目标
                invokeHandler.invoke(Foo.class.getMethod("bar"), new Object[0]);
            }
        }
    
        static class Target implements Foo {
        
            @Override
            public void foo() {
        
                System.out.println("target foo");
            }
    
            @Override
            public void bar() {
        
                System.out.println("target bar");
            }
        }
    
    
        public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        
            Foo f = new $Proxy0(new InvokeHandler() {
        
                @Override
                Object invoke(Method method, Object[] params) throws InvocationTargetException, IllegalAccessException {
        
                    // 传入代理对象
                    method.invoke(new Target(), params);
                    return null;
                }
    
            });
            f.foo();
            f.bar();
        }
    }
    
    /**
    运行结果
    before
    target foo
    before
    target bar
    **/

到这里，可以发现，多方法的代理对象也可以正常执行。但是如果执行方法有值返回呢，这个也简单，小修改一波。

package com.example.proxy;
    
    import java.lang.reflect.InvocationTargetException;
    import java.lang.reflect.Method;
    
    public class Jdk {
        
    
        interface Foo {
        
            Object foo() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException;
    
            Object bar() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException;
        }
    
    
    
        static abstract class InvokeHandler {
        
            abstract Object invoke(Method method, Object[] params) throws InvocationTargetException, IllegalAccessException;
        }
    
        // 代理类
        static class $Proxy0 implements Foo {
        
    
            private final InvokeHandler invokeHandler;
    
            $Proxy0(InvokeHandler invokeHandler) {
        
                this.invokeHandler = invokeHandler;
            }
    
            @Override
            public Object foo() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        
                // 1. 功能增强
                System.out.println("before");
                // 2. 调用目标
                return invokeHandler.invoke(Foo.class.getMethod("foo"), new Object[0]);
            }
    
            @Override
            public Object bar() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        
                // 1. 功能增强
                System.out.println("before");
                // 2. 调用目标
                return invokeHandler.invoke(Foo.class.getMethod("bar"), new Object[0]);
            }
        }
    
        static class Target implements Foo {
        
            @Override
            public Integer foo() {
        
                System.out.println("target foo");
                return 1;
            }
    
            @Override
            public String  bar() {
        
                System.out.println("target bar");
                return "hello";
            }
        }
    
    
        public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        
            Foo f = new $Proxy0(new InvokeHandler() {
        
                @Override
                Object invoke(Method method, Object[] params) throws InvocationTargetException, IllegalAccessException {
        
                    // 传入代理对象
                    return method.invoke(new Target(), params);
                }
    
            });
            System.out.println(f.foo());
            System.out.println(f.bar());
        }
    }
    
    /**
    运行结果
    before
    target foo
    1
    before
    target bar
    hello
    
    **/

在源码实现中，方法还可以被缓存复用，不需要每次都重新创建。

package com.example.proxy;
    
    import java.lang.reflect.InvocationTargetException;
    import java.lang.reflect.Method;
    import java.util.HashMap;
    import java.util.Map;
    
    public class Jdk {
        
    
        interface Foo {
        
            Object foo() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException;
    
            Object bar() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException;
        }
    
    
    
        static abstract class InvokeHandler {
        
            abstract Object invoke(Method method, Object[] params) throws InvocationTargetException, IllegalAccessException;
        }
    
        // 代理类
        static class $Proxy0 implements Foo {
        
    
            private final InvokeHandler invokeHandler;
    
            private final Map<String, Method> cache = new HashMap<>();
    
            $Proxy0(InvokeHandler invokeHandler) {
        
                this.invokeHandler = invokeHandler;
            }
    
            @Override
            public Object foo() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        
                // 1. 功能增强
                System.out.println("before");
                // 2. 调用目标
                Method foo = cache.getOrDefault("foo", null);
                if(foo == null) {
        
                    foo = Foo.class.getMethod("foo");
                    System.out.println(">>>>>> 新创建方法");
                    cache.put("foo", foo);
                }
    
                return invokeHandler.invoke(foo, new Object[0]);
            }
    
            @Override
            public Object bar() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        
                // 1. 功能增强
                System.out.println("before");
                // 2. 调用目标
                Method bar = cache.getOrDefault("bar", null);
                if(bar == null) {
        
                    bar = Foo.class.getMethod("foo");
                    System.out.println(">>>>>> 新创建方法");
                    cache.put("bar", bar);
                }
                return invokeHandler.invoke(bar, new Object[0]);
            }
        }
    
        static class Target implements Foo {
        
            @Override
            public Integer foo() {
        
                System.out.println("target foo");
                return 1;
            }
    
            @Override
            public String  bar() {
        
                System.out.println("target bar");
                return "hello";
            }
        }
    
    
        public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
        
            Foo f = new $Proxy0(new InvokeHandler() {
        
                @Override
                Object invoke(Method method, Object[] params) throws InvocationTargetException, IllegalAccessException {
        
                    // 传入代理对象
                    return method.invoke(new Target(), params);
                }
    
            });
            System.out.println(f.foo());
            System.out.println(f.bar());
            System.out.println(f.foo());
            System.out.println(f.bar());
        }
    }
    
    /**
    before
    >>>>>> 新创建方法
    target foo
    1
    before
    >>>>>> 新创建方法
    target foo
    1
    before
    target foo
    1
    before
    target foo
    1
    **/

到此，代理方法只会被寻找一次。  
JDK 动态代理生成的代理类是以字节码的形式存在的，并不存在所谓的 .java 文件，但也不是说就没办法看到生成的代理类信息了。不过可以使用 arthas反编译，看到字节码。  
比如：

package com.example.proxy;
    
    import java.io.IOException;
    import java.lang.reflect.Proxy;
    
    public class Jdk1 {
        
        interface Foo {
        
            void foo();
        }
    
        static final class Target implements Foo {
        
            @Override
            public void foo() {
        
                System.out.println("target foo");
            }
        }
    
        public static void main(String[] args) throws IOException {
        
            // 原始对象
            Target target = new Target();
    
            // 用来加载在运行期间动态生成的字节码
            ClassLoader classLoader = Jdk1.class.getClassLoader();
            Foo proxy = (Foo) Proxy.newProxyInstance(classLoader, new Class[]{
        Foo.class}, (p, method, params) -> {
        
                System.out.println("before...");
                // 目标.方法(参数) --> 方法.invoke(目标, 参数)
                Object result = method.invoke(target, params);
                System.out.println("after...");
                // 也返回目标方法执行的结果
                return result;
            });
    
            // 打印代理类的全限定类名
            System.out.println(proxy.getClass());
    
            proxy.foo();
    
            // 只要不在控制台上输入并回车，程序就不会终端
            System.in.read();
        }
    
    
    }

打印的结果是：

class com.example.proxy.$Proxy0
    before...
    target foo
    after...

arthas 反编译的结果是：

[arthas@60054]$ jad com.example.proxy.$Proxy0
    
    ClassLoader:
    +-jdk.internal.loader.ClassLoaders$AppClassLoader@251a69d7
      +-jdk.internal.loader.ClassLoaders$PlatformClassLoader@17747fbe
    
    Location:
    
    /*
     * Decompiled with CFR.
     *
     * Could not load the following classes:
     *  com.example.proxy.Jdk1$Foo
     */
    package com.example.proxy;
    
    import com.example.proxy.Jdk1;
    import java.lang.invoke.MethodHandles;
    import java.lang.reflect.InvocationHandler;
    import java.lang.reflect.Method;
    import java.lang.reflect.Proxy;
    import java.lang.reflect.UndeclaredThrowableException;
    
    final class $Proxy0
    extends Proxy
    implements Jdk1.Foo {
        
        private static final Method m0;
        private static final Method m1;
        private static final Method m2;
        private static final Method m3;
    
        private static MethodHandles.Lookup proxyClassLookup(MethodHandles.Lookup lookup) throws IllegalAccessException {
        
            if (lookup.lookupClass() == Proxy.class && lookup.hasFullPrivilegeAccess()) {
        
                return MethodHandles.lookup();
            }
            throw new IllegalAccessException(lookup.toString());
        }
    
        public $Proxy0(InvocationHandler invocationHandler) {
        
            super(invocationHandler);
        }
    
        static {
        
            try {
        
                m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
                m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
                m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
                m3 = Class.forName("com.example.proxy.Jdk1$Foo").getMethod("foo", new Class[0]);
                return;
            }
            catch (NoSuchMethodException noSuchMethodException) {
        
                throw new NoSuchMethodError(noSuchMethodException.getMessage());
            }
            catch (ClassNotFoundException classNotFoundException) {
        
                throw new NoClassDefFoundError(classNotFoundException.getMessage());
            }
        }
    
        public final void foo() {
        
            try {
        
                this.h.invoke(this, m3, null);
                return;
            }
            catch (Error | RuntimeException throwable) {
        
                throw throwable;
            }
            catch (Throwable throwable) {
        
                throw new UndeclaredThrowableException(throwable);
            }
        }
    
        
    
        public final boolean equals(Object object) {
        
            try {
        
                return (Boolean)this.h.invoke(this, m1, new Object[]{
        object});
            }
            catch (Error | RuntimeException throwable) {
        
                throw throwable;
            }
            catch (Throwable throwable) {
        
                throw new UndeclaredThrowableException(throwable);
            }
        }
    
        public final String toString() {
        
            try {
        
                return (String)this.h.invoke(this, m2, null);
            }
            catch (Error | RuntimeException throwable) {
        
                throw throwable;
            }
            catch (Throwable throwable) {
        
                throw new UndeclaredThrowableException(throwable);
            }
        }
    
        public final int hashCode() {
        
            try {
        
                return (Integer)this.h.invoke(this, m0, null);
            }
            catch (Error | RuntimeException throwable) {
        
                throw throwable;
            }
            catch (Throwable throwable) {
        
                throw new UndeclaredThrowableException(throwable);
            }
        }
    
    }