[Jvm]程序员的精进之路～JDK代理源码初探
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);
}
}
}