Java自带的动态代理
大家都知道，代理模式是23中设计模式中的一种，代理模式又分为动态代理和静态代理，今天主要介绍下动态代理。目前流行的动态代理，一种是JDK自带的动态代理，另外一种就是cglib动态代理。所为的代理就是构造一个新对象，实现对原有对象所有行为的支持，同时又可以支持新的功能，比如常见的AOP。
JDK动态代理
先上代码
[java]
package com.learn.core.proxy;
public interface Subject {
public void doSth();
}
[/java]
真实的对象
[java]
package com.learn.core.proxy;
public class RealSubject implements Subject {
@Override
public void doSth() {
System.out.println("call doSth()");
}
}
[/java]
调用处理器
[java]
package com.learn.core.proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
public class ProxyHandler implements InvocationHandler {
private Object proxied;
public ProxyHandler(Object proxied) {
this.proxied = proxied;
}
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("do something before call");
Object obj = method.invoke(proxied, args);
System.out.println("do something after call");
return obj;
}
}
[/java]
测试入口
[java]
package com.learn.core.proxy;
import java.io.FileOutputStream;
import java.lang.reflect.Proxy;
import sun.misc.ProxyGenerator;
public class DynamicProxy {
public static void main(String args[]) {
RealSubject real = new RealSubject();
Subject proxySubject = (Subject) Proxy.newProxyInstance(Subject.class.getClassLoader(),
new Class[] { Subject.class }, new ProxyHandler(real));
proxySubject.doSth();
// write proxySubject class binary data to file
createProxyClassFile();
}
public static void createProxyClassFile() {
String name = "ProxySubject";
byte[] data = ProxyGenerator.generateProxyClass(name, new Class[] { Subject.class });
try {
FileOutputStream out = new FileOutputStream(name + ".class");
out.write(data);
out.close();
} catch (Exception e) {
e.printStackTrace();
}
}
}
[/java]
–输出
do something before call
call doSth()
do something after call
大家看了上边的实现和输出，应该很快就明白了动态代理的效果。一般JDK的动态代理就几个步骤
Interface Proxy = (Interface)constructor.newInstance(new Object[] (handler))
生成的ProxySubject继承Proxy类实现Subject接口，实现的Subject的方法实际调用处理器的invoke方法，而invoke方法利用反射调用的是被代理对象的的方法（Object result=method.invoke(proxied,args)）
那JDK自带的动态代理是如何做到的呢？
所有的核心都在newInstance方法里，如下
[java]
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);
}
//拿到代理类，调用构造器去生成一个实例
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);
}
}
[/java]
目前JDK8里用的ProxyClassFactory，它负责生成代理类的字节码
[java]
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// prefix for all proxy class names
private static final String proxyClassNamePrefix = "$Proxy";
// next number to use for generation of unique proxy class names
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
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.
*/
for (Class<?> intf : interfaces) {
int flags = intf.getModifiers();
if (!Modifier.isPublic(flags)) {
accessFlags = Modifier.FINAL;
String name = intf.getName();
int n = name.lastIndexOf(‘.’);
String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
if (proxyPkg == null) {
proxyPkg = pkg;
} else if (!pkg.equals(proxyPkg)) {
throw new IllegalArgumentException(
"non-public interfaces from different packages");
}
}
}
if (proxyPkg == null) {
// if no non-public proxy interfaces, use com.sun.proxy package
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
/*
* Choose a name for the proxy class to generate.
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
*/
//上边都是验证，ProxyGenerator生成代理类的字节码才是核心。
//sun.misc.ProxyGenerator 不是标准的java类库
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
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());
}
}
}
[/java]
我反编译了如何生成字节码
[java]
private byte[] generateClassFile()
{
addProxyMethod(hashCodeMethod, Object.class);
addProxyMethod(equalsMethod, Object.class);
addProxyMethod(toStringMethod, Object.class);
Object localObject4;
for (localObject4 : this.interfaces) {
for (Method localMethod : ((Class)localObject4).getMethods()) {
addProxyMethod(localMethod, (Class)localObject4);
}
}
for (??? = this.proxyMethods.values().iterator(); ((Iterator)???).hasNext();)
{
localList = (List)((Iterator)???).next();
checkReturnTypes(localList);
}
try
{
List localList;
this.methods.add(generateConstructor());
for (??? = this.proxyMethods.values().iterator(); ((Iterator)???).hasNext();)
{
localList = (List)((Iterator)???).next();
for (localIterator = localList.iterator(); localIterator.hasNext();)
{
localObject4 = (ProxyMethod)localIterator.next();
this.fields.add(new FieldInfo(((ProxyMethod)localObject4).methodFieldName, "Ljava/lang/reflect/Method;", 10));
this.methods.add(((ProxyMethod)localObject4).generateMethod());
}
}
Iterator localIterator;
this.methods.add(generateStaticInitializer());
}
catch (IOException localIOException1)
{
throw new InternalError("unexpected I/O Exception", localIOException1);
}
if (this.methods.size() > 65535) {
throw new IllegalArgumentException("method limit exceeded");
}
if (this.fields.size() > 65535) {
throw new IllegalArgumentException("field limit exceeded");
}
this.cp.getClass(dotToSlash(this.className));
this.cp.getClass("java/lang/reflect/Proxy");
for (localObject4 : this.interfaces) {
this.cp.getClass(dotToSlash(((Class)localObject4).getName()));
}
this.cp.setReadOnly();
??? = new ByteArrayOutputStream();
DataOutputStream localDataOutputStream = new DataOutputStream((OutputStream)???);
try
{
localDataOutputStream.writeInt(-889275714);
localDataOutputStream.writeShort(0);
localDataOutputStream.writeShort(49);
this.cp.write(localDataOutputStream);
localDataOutputStream.writeShort(this.accessFlags);
localDataOutputStream.writeShort(this.cp.getClass(dotToSlash(this.className)));
localDataOutputStream.writeShort(this.cp.getClass("java/lang/reflect/Proxy"));
localDataOutputStream.writeShort(this.interfaces.length);
for (Object localObject6 : this.interfaces) {
localDataOutputStream.writeShort(this.cp.getClass(
dotToSlash(localObject6.getName())));
}
localDataOutputStream.writeShort(this.fields.size());
for (??? = this.fields.iterator(); ((Iterator)???).hasNext();)
{
localObject5 = (FieldInfo)((Iterator)???).next();
((FieldInfo)localObject5).write(localDataOutputStream);
}
Object localObject5;
localDataOutputStream.writeShort(this.methods.size());
for (??? = this.methods.iterator(); ((Iterator)???).hasNext();)
{
localObject5 = (MethodInfo)((Iterator)???).next();
((MethodInfo)localObject5).write(localDataOutputStream);
}
localDataOutputStream.writeShort(0);
}
catch (IOException localIOException2)
{
throw new InternalError("unexpected I/O Exception", localIOException2);
}
return ((ByteArrayOutputStream)???).toByteArray();
}
[/java]
最后让大家看看一开始代理类的反编译代码
[java]
import com.learn.core.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 ProxySubject
extends Proxy
implements Subject
{
private static Method m1;
private static Method m2;
private static Method m3;
private static Method m0;
public ProxySubject(InvocationHandler paramInvocationHandler)
throws
{
super(paramInvocationHandler);
}
public final boolean equals(Object paramObject)
throws
{
try
{
return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
}
catch (Error|RuntimeException localError)
{
throw localError;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
public final String toString()
throws
{
try
{
return (String)this.h.invoke(this, m2, null);
}
catch (Error|RuntimeException localError)
{
throw localError;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
public final void doSth()
throws
{
try
{
this.h.invoke(this, m3, null);
return;
}
catch (Error|RuntimeException localError)
{
throw localError;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
public final int hashCode()
throws
{
try
{
return ((Integer)this.h.invoke(this, m0, null)).intValue();
}
catch (Error|RuntimeException localError)
{
throw localError;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
static
{
try
{
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
m3 = Class.forName("com.learn.core.proxy.Subject").getMethod("doSth", new Class[0]);
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
return;
}
catch (NoSuchMethodException localNoSuchMethodException)
{
throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
}
catch (ClassNotFoundException localClassNotFoundException)
{
throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
}
}
}
[/java]
总结：
Java自带的动态代理其实就是Java帮我们生成一个代理对象，它实现了所有我们需要的接口，所有的调用都会进入InvocationHandler，在InvocationHandler里通过反射区调用真实对象的方法。Proxy确实一个很漂亮的设计，尤其在我们不能改已有代码的前提下实现对已有对象行为的修改，但是美中不足的就是所有的依赖都是接口。