本文是转载，原博主地址http://rejoy.iteye.com/blog/1627405
java 动态代理实例
MyInvocationHandler.java-实现自己的InvocationHandler
public class MyInvocationHandler implements InvocationHandler {
//目标对象（代理的真实对象）
private Object target;
/**
* 构造方法
*
* @param target 目标对象
*/
public MyInvocationHandler(Object target) {
this.target = target;
}
/**
* 执行目标对象的方法
*
* @param proxy
* @param method
* @param args
* @return
* @throws Throwable
*/
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
// 在目标对象的方法执行之前简单的打印一下
System.out.println("------------------before------------------");
// 执行目标对象的方法
Object result = method.invoke(target, args);
// 在目标对象的方法执行之后简单的打印一下
System.out.println("-------------------after------------------");
return result;
}
/**
* 获取目标对象的代理对象
*
* @return 代理对象
*/
public Object getProxy() {
return Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(),
target.getClass().getInterfaces(), this);
}
}
UserService.java-目标对象实现的接口，用JDK来生成代理对象一定要实现一个接口
public interface UserService {
/**
* 目标方法
*/
void add();
}
UserServiceImpl.java-目标对象
public class UserServiceImpl implements UserService {
public void add() {
System.out.println("add");
}
}
ProxyTest.java-测试类
public class ProxyTest {
public static void main(String[] args) {
// 实例化目标对象
UserService userService = new UserServiceImpl();
// 实例化InvocationHandler
MyInvocationHandler invocationHandler = new MyInvocationHandler(userService);
// 根据目标对象生成代理对象
UserService proxy = (UserService) invocationHandler.getProxy();
System.out.println(proxy.getClass().getName());
// 调用代理对象的方法
proxy.add();
}
}输出
com.sun.proxy.$Proxy0
------------------before------------------
add
-------------------after------------------既然生成代理对象是用的Proxy类的静态方newProxyInstance，那么我们就去它的源码里看一下它到底都做了些什么？
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
//判断InvocationHandler是否为空
Objects.requireNonNull(h);
//拷贝接口数组
final Class<?>[] intfs = interfaces.clone();
final SecurityManager sm = System.getSecurityManager();
//校验代理类的访问问权限 具体的请去查java安全管理器。
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
*/
//查找或生成指定的代理类。
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
//获取代理类构造函数
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
//构造函数不是public，则设置当前构造函数为访问权限
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
// 生成代理类的实例并把MyInvocationHandler的实例传给它的构造方法
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);
}
}我们再进去getProxyClass0方法看一下
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
return proxyClassCache.get(loader, interfaces);
}我们再进去proxyClassCache.get(loader, interfaces)方法看一下
public V get(K key,P parameter){
//parameter不能weinull
Objects.requireNonNull(parameter);
//删除老节点
expungeStaleEntries();
Object cacheKey=CacheKey.valueOf(key,refQueue);
// lazily install the 2nd level valuesMap for the particular cacheKey
ConcurrentMap<Object, Supplier<V>>valuesMap=map.get(cacheKey);
if(valuesMap==null){
//添加cacheKey - ConcurrentMap<Object, Supplier<V>对象到map中
ConcurrentMap<Object, Supplier<V>>oldValuesMap=map.putIfAbsent(cacheKey,valuesMap=new ConcurrentHashMap<>());
if(oldValuesMap!=null){
valuesMap=oldValuesMap;
}
}
//利用subKeyFactory生成subKey
Object subKey=Objects.requireNonNull(subKeyFactory.apply(key,parameter));
//获取valuesMap中subKey所对应的值，此时为cachevalue <V>实例（步骤1：cachevalue <V>实例的来源）
Supplier<V> supplier=valuesMap.get(subKey);
Factory factory=null;
while(true){
if(supplier!=null){
//步骤1：supplier可能是一个工厂对象或一个cachevalue <V>实例
V value=supplier.get();
if(value!=null){
return value;
}
}
// else no supplier in cache
// or a supplier that returned null (could be a cleared CacheValue
// or a Factory that wasn't successful in installing the CacheValue)
// lazily construct a Factory
if(factory==null){
//构造一个工厂对象(步骤1：工厂对象的来源）
factory=new Factory(key,parameter,subKey,valuesMap);
}
//
if(supplier==null){
//factory添加在valuesMap中
supplier=valuesMap.putIfAbsent(subKey,factory);
if(supplier==null){
// 添加成功，则factory赋值给supplier，此时supplier为一个工厂对象
supplier=factory;
}
// else retry with winning supplier
//失败则重试
}else{
//将valuesMap中subKey所对应的值替换为factory
if(valuesMap.replace(subKey,supplier,factory)){
// successfully replaced
// cleared CacheEntry / unsuccessful Factory
// with our Factory
supplier=factory;
}else{
// retry with current supplier
supplier=valuesMap.get(subKey);
}
}
}此时supplier返回一个工厂对象factory,并调用factory.get()方法
@Override
public synchronized V get() { // serialize access
//校验是否为同一个
Supplier<V> supplier = valuesMap.get(subKey);
if (supplier != this) {
return null;
}
//创建一个新值
V value = null;
try {
//调用valueFactory的applay方法生成一个新值
value = Objects.requireNonNull(valueFactory.apply(key, parameter));
} finally {
if (value == null) { //如果生成失败，从valuesMap中删除值
valuesMap.remove(subKey, this);
}
}
// the only path to reach here is with non-null value
assert value != null;
// wrap value with CacheValue (WeakReference)
CacheValue<V> cacheValue = new CacheValue<>(value);
//尝试更换我们cachevalue（这应该总是成功的)此时
if (valuesMap.replace(subKey, this, cacheValue)) {
// 放在reversemap
reverseMap.put(cacheValue, Boolean.TRUE);
} else {
throw new AssertionError("Should not reach here");
}
//成功取代当前工厂对象为一个新的CacheValue（包裹着我们的需要对象：代理类）
return value;
}进入valueFactory.apply(key, parameter)方法
@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
for (Class<?> intf : interfaces) {
//验证intf接口类Class对象是否为给定的classloder解析的
Class<?> interfaceClass=null;
try{
//指定类加载器获取接口类class对象，
interfaceClass=Class.forName(intf.getName(),false,loader);
}catch(ClassNotFoundException e){
}
//验证是否为同一个接口类对象，
if(interfaceClass!=intf){
throw new IllegalArgumentException(
intf+" is not visible from class loader");
}
//验证类对象是否为接口
if(!interfaceClass.isInterface()){
throw new IllegalArgumentException(
interfaceClass.getName()+" is not an interface");
}
/*
* Verify that this interface is not a duplicate.
* 验证此接口不是副本。
*/
if(interfaceSet.put(interfaceClass,Boolean.TRUE)!=null){
throw new IllegalArgumentException(
"repeated interface: "+interfaceClass.getName());
}
}
//定义代理类的包
String proxyPkg = null;
//定义代理类的修饰符：public和final类型
int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
/*
* 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.
* 记录一个非公共代理接口的包，以便在同一个包中定义代理类。验证所有非公共代理接口都在同一个包中。
*/
for (Class<?> intf : interfaces) {
//获取修饰符
int flags = intf.getModifiers();
if (!Modifier.isPublic(flags)) { //不是public修饰符
//则定义代理类的修饰符符：final
accessFlags = Modifier.FINAL;
//获取接口类名称，如：com.text.MyInterface
String name = intf.getName();
int n = name.lastIndexOf('.');
//获取包名，如：com.text
String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
if (proxyPkg == null) {
//获取包名称
proxyPkg = pkg;
} else if (!pkg.equals(proxyPkg)) { //interfaces含有来自不同包的非公共接口,抛错
throw new IllegalArgumentException(
"non-public interfaces from different packages");
}
}
}
if (proxyPkg == null) {
//如果没有非公开的代理接口，使用com.sun.proxy作为包名称
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
//选择要生成的代理类的名称。
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
//真正生成指定的代理类字节码的地方
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
//生成Calss对象
return defineClass0(loader, proxyName, proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
/*
* A ClassFormatError here means that (barring bugs in the
* proxy class generation code) there was some other
* invalid aspect of the arguments supplied to the proxy
* class creation (such as virtual machine limitations
* exceeded).
*/
throw new IllegalArgumentException(e.toString());
}
}
接下来去看真正生成指定的代理类字节码的地方ProxyGenerator.generateProxyClass()方法。
public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
//动态生成代理的字节码，
final byte[] var4 = var3.generateClassFile();
//如果saveGeneratedFiles的值为true，则会把所生成的代理类的字节码保存到硬盘上
if (saveGeneratedFiles) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
try {
int var1 = var0.lastIndexOf(46);
Path var2;
if (var1 > 0) {
Path var3 = Paths.get(var0.substring(0, var1).replace('.', File.separatorChar));
Files.createDirectories(var3);
var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
} else {
var2 = Paths.get(var0 + ".class");
}
Files.write(var2, var4, new OpenOption[0]);
return null;
} catch (IOException var4x) {
throw new InternalError("I/O exception saving generated file: " + var4x);
}
}
});
}
return var4;
}