谈到动态代理，大家估计都非常熟悉，本文旨在深入探讨java动态代理的实现技术。
先看看一种jdk动态代理的简单实现：
定义一个接口如下：
1.BookOrder
package com.xxx.dynamicproxy.jdk;
public interface BookOrder {
public void order();
}
接口的实现类，亦即目标类
2.BookOrderImpl
package com.xxx.dynamicproxy.jdk;
public class BookOrderImpl implements BookOrder {
public void order() {
//todo
System.out.println("order successfully !!! ");
}
}
3.实现InvocationHandler接口。
package com.xxx.dynamicproxy.jdk;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
public class BookOrderProxy implements InvocationHandler {
private Object target;
public BookOrderProxy(Object target)
{
this.target = target;
}
public Object invoke( Object proxy, Method method, Object[] args ) throws Throwable
{
//在调用具体目标对象的方法之前，执行一些处理，
System.out.println("before order ......");
//调用具体目标对象的方法
Object result = method.invoke(target, args);
//在调用具体目标对象之后，执行一些处理
System.out.println("after order ......");
return result;
}
}
4.测试类，我们顺便生成代理后字节码文件
public class DynamicProxy {
public static void main( String args[] )
{
BookOrderImpl bookOrderImpl = new BookOrderImpl();
BookOrder proxyBookOrder = (BookOrder)Proxy.newProxyInstance(BookOrder.class.getClassLoader(),
new Class[]{BookOrder.class},
new BookOrderProxy(bookOrderImpl));
proxyBookOrder.order();
try {
//生成代理后的字节码文件
doGenerateProxyClassFile();
} catch (IOException e) {
e.printStackTrace();
}
}
public static void doGenerateProxyClassFile() throws IOException {
String name = "ProxyBookOrder";
FileOutputStream out = null;
byte[] data = ProxyGenerator.generateProxyClass( name, new Class[] { BookOrder.class } );
try
{
out = new FileOutputStream( name + ".class" );
out.write( data );
out.close();
}
catch( Exception e )
{
e.printStackTrace();
}finally{
if(out !=null){
out.close();
}
}
}
}
执行后：
before order …
order successfully !!!
after order …
这里我们先睹为快，看看生成的ProxyBookOrder.class到底是怎么实现的，反编译后代码如下：
import com.xxx.dynamicproxy.jdk.BookOrder;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class ProxyBookOrder
extends Proxy
implements BookOrder
{
private static Method m1;
private static Method m0;
private static Method m3;
private static Method m2;
public ProxyBookOrder(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 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);
}
}
public final void order()
throws
{
try
{
this.h.invoke(this, m3, null);
return;
}
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);
}
}
static
{
try
{
m1 = Class.forName(“java.lang.Object”).getMethod(“equals”, new Class[] { Class.forName(“java.lang.Object”) });
m0 = Class.forName(“java.lang.Object”).getMethod(“hashCode”, new Class[0]);
m3 = Class.forName(“com.xxx.dynamicproxy.jdk.BookOrder”).getMethod(“order”, new Class[0]);
m2 = Class.forName(“java.lang.Object”).getMethod(“toString”, new Class[0]);
return;
}
catch (NoSuchMethodException localNoSuchMethodException)
{
throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
}
catch (ClassNotFoundException localClassNotFoundException)
{
throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
}
}
}
ok,我们主要看jdk动态代理实现的过程，我们首先跟踪Proxy.newProxyInstance方法,笔者跟踪的是jdk1.7的源码 ，如下图所示：
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.
/
Class<?> cl = getProxyClass0(loader, intfs);
/
* Invoke its constructor with the designated invocation handler.
*/
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() {
public Object run() {
return newInstance(cons, ih);
}
});
} else {
return newInstance(cons, ih);
}
} catch (NoSuchMethodException e) {
throw new InternalError(e.toString());
}
}
仅仅有上述代码，我们发现其实Proxy类的newProxyInstance方法主要做了两件事：一个就是通过getProxyClass0方法生成代理的字节码对象，第二个就是通过生成代理字节码对象获取它的Constructor对象，而后通过反射实例化生成的代理对象。
我们看看生成字节码对象的方法getProxyClass0：
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);
}
这里通过注释我们也理解：如果已经生成过该地代理字节码对象直接从缓存中返回一份，否则通过ProxyClassFactory生成代理字节码对象。
我们继续跟进去：
private final BiFunction<K, P, ?> subKeyFactory;
public V get(K key, P parameter) {
Objects.requireNonNull(parameter);
//删除过时的实体
expungeStaleEntries();
Object cacheKey = CacheKey.valueOf(key, refQueue);
// lazily install the 2nd level valuesMap for the particular cacheKey
ConcurrentMap<Object, Supplier> valuesMap = map.get(cacheKey);
if (valuesMap == null) {
ConcurrentMap<Object, Supplier> oldValuesMap
= map.putIfAbsent(cacheKey,
valuesMap = new ConcurrentHashMap<>());
if (oldValuesMap != null) {
valuesMap = oldValuesMap;
}
}
// create subKey and retrieve the possible Supplier stored by that
// subKey from valuesMap
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
Supplier supplier = valuesMap.get(subKey);
Factory factory = null;
while (true) {
if (supplier != null) {
// supplier might be a Factory or a CacheValue instance
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) {
factory = new Factory(key, parameter, subKey, valuesMap);
}
if (supplier == null) {
supplier = valuesMap.putIfAbsent(subKey, factory);
if (supplier == null) {
// successfully installed Factory
supplier = factory;
}
// else retry with winning supplier
} else {
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);
}
}
}
}
这里面没有贴出来，其实ProxyClassFactory是BiFunction的实现类，我们直接观察其apply方法：
// 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
/*
* 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.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.
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces);
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());
}
}
}
这里，通过一些相关的校验，动态代理类的名称，包名的确定，通过ProxyGenerator.generateProxyClass生成字节码，然后通过本地方法defineClass0将方法加载到JVM中去，我们更关注
generateProxyClass方法如下：
public static byte[] generateProxyClass(final String var0, Class[] var1) {
ProxyGenerator var2 = new ProxyGenerator(var0, var1);
final byte[] var3 = var2.generateClassFile();
if(saveGeneratedFiles) {
AccessController.doPrivileged(new PrivilegedAction() {
public Void run() {
try {
FileOutputStream var1 = new FileOutputStream(ProxyGenerator.dotToSlash(var0) + “.class”);
var1.write(var3);
var1.close();
return null;
} catch (IOException var2) {
throw new InternalError("I/O exception saving generated file: " + var2);
}
}
});
}
return var3;
}
private byte[] generateClassFile() {
this.addProxyMethod(hashCodeMethod, Object.class);
this.addProxyMethod(equalsMethod, Object.class);
this.addProxyMethod(toStringMethod, Object.class);
int var1;
int var3;
for(var1 = 0; var1 < this.interfaces.length; ++var1) {
Method[] var2 = this.interfaces[var1].getMethods();
for(var3 = 0; var3 < var2.length; ++var3) {
this.addProxyMethod(var2[var3], this.interfaces[var1]);
}
}
Iterator var7 = this.proxyMethods.values().iterator();
List var8;
while(var7.hasNext()) {
var8 = (List)var7.next();
checkReturnTypes(var8);
}
Iterator var11;
try {
this.methods.add(this.generateConstructor());
var7 = this.proxyMethods.values().iterator();
while(var7.hasNext()) {
var8 = (List)var7.next();
var11 = var8.iterator();
while(var11.hasNext()) {
ProxyGenerator.ProxyMethod var4 = (ProxyGenerator.ProxyMethod)var11.next();
this.fields.add(new ProxyGenerator.FieldInfo(var4.methodFieldName, “Ljava/lang/reflect/Method;”, 10));
this.methods.add(var4.generateMethod());
}
}
this.methods.add(this.generateStaticInitializer());
} catch (IOException var6) {
throw new InternalError(“unexpected I/O Exception”);
}
if(this.methods.size() > ‘\uffff’) {
throw new IllegalArgumentException(“method limit exceeded”);
} else if(this.fields.size() > ‘\uffff’) {
throw new IllegalArgumentException(“field limit exceeded”);
} else {
this.cp.getClass(dotToSlash(this.className));
this.cp.getClass(“java/lang/reflect/Proxy”);
for(var1 = 0; var1 < this.interfaces.length; ++var1) {
this.cp.getClass(dotToSlash(this.interfaces[var1].getName()));
}
this.cp.setReadOnly();
ByteArrayOutputStream var9 = new ByteArrayOutputStream();
DataOutputStream var10 = new DataOutputStream(var9);
try {
var10.writeInt(-889275714); //其实这个有符号的负数转换成十六进制就是Oxcafebabe，java class文件的魔数
var10.writeShort(0);//写入两个字节的次版本号
var10.writeShort(49);//写入两个字节的主版本号，49，说明是1.5版本的class
this.cp.write(var10);//这个cp 其实就是constant pool 常量池对象，此处是将常量池写入
var10.writeShort(49);//
var10.writeShort(this.cp.getClass(dotToSlash(this.className)));
var10.writeShort(this.cp.getClass(“java/lang/reflect/Proxy”));
var10.writeShort(this.interfaces.length);
for(var3 = 0; var3 < this.interfaces.length; ++var3) {
var10.writeShort(this.cp.getClass(dotToSlash(this.interfaces[var3].getName())));
}
var10.writeShort(this.fields.size());
var11 = this.fields.iterator();
while(var11.hasNext()) {
ProxyGenerator.FieldInfo var12 = (ProxyGenerator.FieldInfo)var11.next();
var12.write(var10);
}
var10.writeShort(this.methods.size());
var11 = this.methods.iterator();
while(var11.hasNext()) {
ProxyGenerator.MethodInfo var13 = (ProxyGenerator.MethodInfo)var11.next();
var13.write(var10);
}
var10.writeShort(0);
return var9.toByteArray();
} catch (IOException var5) {
throw new InternalError(“unexpected I/O Exception”);
}
}
}
可见最终，还是通过直接通过字节码流，实现动态代理类。