什么是动态代理？
代理模式(Proxy Pattern)是程序设计中的一种设计模式，定义为其他对象提供一种代理以控制对这个对象的访问。代理模式分为静态代理和动态代理，静态代理是手动创建或工具生成代理类的源码，动态代理则是在运行阶段动态生成代理类。
proxy-pattern.png
动态代理的实现
- 创建被代理的接口和实现类，Java中的动态代理基于接口
- 创建
InvocationHandler接口的实现类，在invoke方法中实现代理逻辑 - 通过
Proxy类的静态方法newProxyInstance(ClassLoader loader,Class<?>[] interfaces,InvocationHandler h)创建代理对象 - 使用代理对象
创建被代理的接口
//Subject.java
public interface Subject {
void hello(String str);
}
创建接口实现类
//RealSubject.java
public class RealSubject implements Subject {
@Override
public void hello(String str) {
System.out.println("hello: " + str);
}
}
创建InvocationHandler接口实现类
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
public class DynamicProxy implements InvocationHandler {
private Object object;
public DynamicProxy(Object object)
{
this.object = object;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable
{
Object result=method.invoke(subject,args);
return result;
}
}
测试
RealSubject realSubject = new RealSubject();
InvocationHandler handler = new DynamicProxy(realSubject);
Subject subject = (Subject) Proxy.newProxyInstance(Subject.class.getClassLoader(), new Class<?>[] { Subject.class }, handler);
subject.hello("jack");
输出结果为hello: jack
动态代理原理分析
动态代理是通过Proxy类的newProxyInstance方法动态创建代理对象，然后将接口方法"委托"给InvocationHandler实例完成的，newProxyInstance方法的定义如下：
public static Object newProxyInstance(ClassLoader loader,Class<?>[] interfaces,InvocationHandler h)
- ClassLoader：接口类对应的ClassLoader
- Class<?>[] ：代理类要实现的接口列表
- InvocationHandler: 方法调用处理类
对应的代码实现为：
/**
* Returns an instance of a proxy class for the specified interfaces
* that dispatches method invocations to the specified invocation
* handler.
*/
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
final Class<?>[] intfs = interfaces.clone();
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
*/
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
return cons.newInstance(new Object[]{h});
} catch (IllegalAccessException|InstantiationException e) {
throw new InternalError(e.toString(), e);
} catch (InvocationTargetException e) {
Throwable t = e.getCause();
if (t instanceof RuntimeException) {
throw (RuntimeException) t;
} else {
throw new InternalError(t.toString(), t);
}
} catch (NoSuchMethodException e) {
throw new InternalError(e.toString(), e);
}
}
核心代码如下：
- 找到代理类对于的Class
Class<?> cl = getProxyClass0(loader, intfs);
- 通过反射获取代理类的构造函数，参数为
InvocationHandler实例
private static final Class<?>[] constructorParams =
{ InvocationHandler.class };
final Constructor<?> cons = cl.getConstructor(constructorParams);
- 生成代理类的实例
return cons.newInstance(new Object[]{h});
生成的动态代理类如下：
//com.sun.proxy.$Proxy0.java
package com.sun.proxy;
import io.github.kongpf8848.pattern.proxy.Subject;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class $Proxy0 extends Proxy implements Subject {
private static Method m1;
private static Method m2;
private static Method m3;
private static Method m0;
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
public final boolean equals(Object var1) throws {
try {
return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final String toString() throws {
try {
return (String)super.h.invoke(this, m2, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final void hello(String var1) throws {
try {
super.h.invoke(this, m3, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final int hashCode() throws {
try {
return (Integer)super.h.invoke(this, m0, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m2 = Class.forName("java.lang.Object").getMethod("toString");
m3 = Class.forName("io.github.kongpf8848.pattern.proxy.Subject").getMethod("hello", Class.forName("java.lang.String"));
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
在该代理类中，通过反射找到代理接口的hello方法，当调用代理类的hello方法时，委托InvocationHandler实例去处理。
