先抛出一个问题，JDK的动态代理为什么不支持对实现类的代理，只支持接口的代理？？？
首先来看一下如何使用JDK动态代理。JDK提供了Java.lang.reflect.Proxy类来实现动态代理的，可通过它的newProxyInstance来获得代理实现类。同时对于代理的接口的实际处理，是一个java.lang.reflect.InvocationHandler，它提供了一个invoke方法供实现者提供相应的代理逻辑的实现。
下面实现一个jdk动态代理的例子：
1.被代理的接口，编写一个接口HelloTest
package com.proxy.test2;
public interface HelloTest {
void say(String name);
}
2.HelloTestImpl 实现接口HelloTest
package com.proxy.test2;
public class HelloTestImpl implements HelloTest {
@Override
public void say(String name) {
System.out.println("Hello:"+name);
}
}
3.JDK的动态代码需要实现InvocationHandler
package com.proxy.test2;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
public class CustomInvocationHandler implements InvocationHandler {
private Object target;
public CustomInvocationHandler(Object target) {
this.target = target;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("Before invocation");
Object retVal = method.invoke(target, args);
System.out.println("After invocation");
return retVal;
}
}
4.编写一个测试类ProxyTest
package com.proxy.test2;
import java.lang.reflect.Proxy;
public class ProxyTest {
public static void main(String[] args) {
//设置为true,会在工程根目录生成$Proxy0.class代理类（com.sun.proxy.$Proxy0.class）
System.getProperties().put(
"sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
String saveGeneratedFiles = System.getProperty("sun.misc.ProxyGenerator.saveGeneratedFiles");
System.out.println(saveGeneratedFiles);
HelloTest helloWord = new HelloTestImpl();
CustomInvocationHandler customInvocationHandler = new CustomInvocationHandler(
helloWord);
//通过Proxy.newProxyInstance生成代理对象
HelloTest proxy = (HelloTest) Proxy.newProxyInstance(
HelloTest.class.getClassLoader(),
helloWord.getClass().getInterfaces(), customInvocationHandler);
//调用say方法
proxy.say("test");
}
}
5.运行测试类
true
Before invocation
Hello:test
After invocation
以上5步编写了一个JDK动态代理的例子，到底是如果代理的呢？？
首先看ProxyTest 类中
HelloTest proxy = (HelloTest) Proxy.newProxyInstance(
HelloTest.class.getClassLoader(),
helloWord.getClass().getInterfaces(), customInvocationHandler);
查看Proxy.newProxyInstance源码：（JDK版本为jdk1.7.0_80）
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
if (h == null) {
throw new NullPointerException();
}
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.
*/
//动态生成class的地方，重点是看这里面的方法
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
//获取代理类的实例
try {
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
if (sm != null && ProxyAccessHelper.needsNewInstanceCheck(cl)) {
// create proxy instance with doPrivilege as the proxy class may
// implement non-public interfaces that requires a special permission
return AccessController.doPrivileged(new PrivilegedAction<Object>() {
public Object run() {
return newInstance(cons, ih);
}
});
} else {
return newInstance(cons, ih);
}
} catch (NoSuchMethodException e) {
throw new InternalError(e.toString());
}
}
查看getProxyClass0方法
/**
* Generate a proxy class. Must call the checkProxyAccess method
* to perform permission checks before calling this.
*/
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
if (interfaces.length > 65535) {//
throw new IllegalArgumentException("interface limit exceeded");
}
// If the proxy class defined by the given loader implementing
// the given interfaces exists, this will simply return the cached copy;
// otherwise, it will create the proxy class via the ProxyClassFactory
// JDK对代理进行了缓存，如果已经存在相应的代理类，则直接返回，否则才会通过ProxyClassFactory来创建代理
return proxyClassCache.get(loader, interfaces);
}
点击proxyClassCache
/**
* a cache of proxy classes
*/
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
点击ProxyClassFactory
/**
* A factory function that generates, defines and returns the proxy class given
* the ClassLoader and array of interfaces.
*/
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
// 所有代理类名字的前缀
private static final String proxyClassNamePrefix = "$Proxy";
///用于生成唯一代理类名称的下一个数字
private static final AtomicLong nextUniqueNumber = new AtomicLong();
@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
for (Class<?> intf : interfaces) {
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
Class<?> interfaceClass = null;
try {
interfaceClass = Class.forName(intf.getName(), false, loader);
} catch (ClassNotFoundException e) {
}
if (interfaceClass != intf) {
throw new IllegalArgumentException(
intf + " is not visible from class loader");
}
/*
* Verify that the Class object actually represents an
* interface.
*/
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* Verify that this interface is not a duplicate.
*/
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
String proxyPkg = null; // package to define proxy class in
/*
* Record the package of a non-public proxy interface so that the
* proxy class will be defined in the same package. Verify that
* all non-public proxy interfaces are in the same package.
*/
for (Class<?> intf : interfaces) {
int flags = intf.getModifiers();
if (!Modifier.isPublic(flags)) {
String name = intf.getName();
int n = name.lastIndexOf('.');
String pkg = ((n == -1) ? "" : name