JDK动态代理的深入理解
每一个Proxy实例对象都有一个与之相关联的InvocationHandler实例，当在Proxy对象上调用代理的方法时，该方法就会被分发（dispatched）到这个InvocationHandler对象的invoke()方法上。
package java.lang.reflect;
/**
* {@code InvocationHandler} is the interface implemented by
* the <i>invocation handler</i> of a proxy instance.
*
* <p>Each proxy instance has an associated invocation handler.
* When a method is invoked on a proxy instance, the method
* invocation is encoded and dispatched to the {@code invoke}
* method of its invocation handler.
*
* @author Peter Jones
* @see Proxy
* @since 1.3
*/
public interface InvocationHandler {
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable;
}
一、入门示例
1. 目标类和代理类需要实现的共同接口IPayment
package com.yxf.dynamicproxy;
public interface IPayment {
long pay(int amount);
long price();
}
2. 目标类CreditCard
package com.yxf.dynamicproxy;
public class CreditCard implements IPayment {
@Override
public long pay(int amount) {
return price() * amount;
}
@Override
public long price() {
return 99;
}
}
3. 调用者需要提供的InvocationHandler实例
package com.yxf.dynamicproxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
public class MyInvocationHandler implements InvocationHandler{
private final Object target;
public MyInvocationHandler(Object object) {
this.target = object;
}
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if(target == null) {
return null;
}
return method.invoke(target, args);
}
}
4. 使用方法
package com.yxf.dynamicproxy;
import java.lang.reflect.Proxy;
public class Client {
public static void main(String[] args) {
testDynamicProxy();
}
private static void testDynamicProxy() {
IPayment target = new CreditCard();
//System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
//sun.misc.Launcher$AppClassLoader
System.out.println("IPayment.class.getClassLoader: " + IPayment.class.getClassLoader().getClass().getName());
IPayment proxy = null;
try {
proxy = (IPayment) Proxy.newProxyInstance(IPayment.class.getClassLoader(), new Class[]{IPayment.class},
new MyInvocationHandler(target));
System.out.println("proxy: " + proxy.getClass().getName()); //com.sun.proxy.$Proxy0
long money = proxy.pay(2000);
System.out.println("proxy.price: " + money);
} catch (Exception e) {
e.printStackTrace();
}
}
}
五、生成的Proxy实例class
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
package com.sun.proxy;
import com.yxf.dynamicproxy.IPayment;
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 IPayment {
private static Method m1;
private static Method m4;
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 long price() throws {
try {
return (Long)super.h.invoke(this, m4, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
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 long pay(int var1) throws {
try {
return (Long)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"));
m4 = Class.forName("com.yxf.dynamicproxy.IPayment").getMethod("price");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m3 = Class.forName("com.yxf.dynamicproxy.IPayment").getMethod("pay", Integer.TYPE);
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
二、Proxy的newProxyInstance()方法
public class Proxy implements java.io.Serializable {
private static final long serialVersionUID = -2222568056686623797L;
/** parameter types of a proxy class constructor */
private static final Class<?>[] constructorParams =
{ InvocationHandler.class };
...
@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.
*/
//获取代理类的class对象。
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
//获取代理类的构造方法，其中constructorParams为构造方法的参数。
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;
}
});
}
//使用Constructor反射的方式创建代理类的实例，其中构造方法的参数为new Object[]{h}.
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);
}
}
/**
* A factory function that generates, defines and returns the proxy class given
* the ClassLoader and array of interfaces.
*/
//给定一个ClassLoader和一个Interface的Class[]数组，生成代理类。
private static final class ProxyFactory
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();
public Class<?> apply(ClassLoader classLoader, Class<?>[] classes) {
//...
String proxyPkg = null; // package to define proxy class in
/*
* 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, accessFlags);
try {
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
throw new IllegalArgumentException(e.toString());
}
}
}
//JVM native方法, 用于将字节流转化为Class对象。
private static native Class<?> defineClass0(ClassLoader loader, String name,
byte[] b, int off, int len);
}
三、ProxyGenerator类的generateClassFile()方法
关键代码：
[sun/misc/ProxyGenerator.java]
package sun.misc;
/**
* ProxyGenerator contains the code to generate a dynamic proxy class
* for the java.lang.reflect.Proxy API.
*
* The external interfaces to ProxyGenerator is the static
* "generateProxyClass" method.
*
* @author Peter Jones
* @since 1.3
*/
public class ProxyGenerator {
/* 预加载的java.lang.Object类的Method实例。*/
private static Method hashCodeMethod;
private static Method equalsMethod;
private static Method toStringMethod;
static {
try {
hashCodeMethod = Object.class.getMethod("hashCode");
equalsMethod = Object.class.getMethod("equals", new Class<?>[] { Object.class });
toStringMethod = Object.class.getMethod("toString");
} catch (NoSuchMethodException e) {
throw new NoSuchMethodError(e.getMessage());
}
}
/** 保存生成class文件的调试开关 */
private final static boolean saveGeneratedFiles =
java.security.AccessController.doPrivileged(
new GetBooleanAction(
"sun.misc.ProxyGenerator.saveGeneratedFiles")).booleanValue();
/**
* Generate a proxy class given a name and a list of proxy interfaces.
*
* @param name the class name of the proxy class
* @param interfaces proxy interfaces
* @param accessFlags access flags of the proxy class
*/
public static byte[] generateProxyClass(final String name,
Class<?>[] interfaces,
int accessFlags)
{
ProxyGenerator gen = new ProxyGenerator(name, interfaces, accessFlags);
final byte[] classFile = gen.generateClassFile();
if (saveGeneratedFiles) {
//调试使用
}
return classFile;
}
/**
* 生成Proxy类的.class文件
*
* @return
*/
private byte[] generateClassFile() {
//生成Java.lang.Object的hashCode、equals和toString方法.
addProxyMethod(hashCodeMethod, Object.class);
addProxyMethod(equalsMethod, Object.class);
addProxyMethod(toStringMethod, Object.class);
for (Class<?> intf : interfaces) {
for (Method m : intf.getMethods()) {
addProxyMethod(m, intf);
}
}
for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
checkReturnTypes(sigmethods);
}
try {
methods.add(generateConstructor());
for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
for (ProxyMethod pm : sigmethods) {
//添加static的Method字段变量
fields.add(new FieldInfo(pm.methodFieldName,
"Ljava/lang/reflect/Method;",
ACC_PRIVATE | ACC_STATIC));
methods.add(pm.generateMethod());
}
}
methods.add(generateStaticInitializer());
} catch (IOException e) {
throw new InternalError("unexpected I/O Exception", e);
}
if (methods.size() > 65535) {
throw new IllegalArgumentException("method limit exceeded");
}
if (fields.size() > 65535) {
throw new IllegalArgumentException("field limit exceeded");
}
cp.getClass(dotToSlash(className));
cp.getClass(superclassName);
for (Class<?> intf : interfaces) {
cp.getClass(dotToSlash(intf.getName()));
}
cp.setReadOnly();
ByteArrayOutputStream bout = new ByteArrayOutputStream();
DataOutputStream dout = new DataOutputStream(bout);
try {
dout.writeInt(0xCAFEBABE); // u4 magic;
dout.writeShort(CLASSFILE_MINOR_VERSION); // u2 minor_version;
dout.writeShort(CLASSFILE_MAJOR_VERSION); // u2 major_version;
cp.write(dout); // (write constant pool)
dout.writeShort(accessFlags); // u2 access_flags;
dout.writeShort(cp.getClass(dotToSlash(className))); // u2 this_class;
dout.writeShort(cp.getClass(superclassName)); // u2 super_class;
dout.writeShort(interfaces.length); // u2 interfaces_count;
// u2 interfaces[interfaces_count];
for (Class<?> intf : interfaces) {
dout.writeShort(cp.getClass(
dotToSlash(intf.getName())));
}
// u2 fields_count;
dout.writeShort(fields.size());
// field_info fields[fields_count];
for (FieldInfo f : fields) {
f.write(dout);
}
// u2 methods_count;
dout.writeShort(methods.size());
// method_info methods[methods_count];
for (MethodInfo m : methods) {
m.write(dout);
}
// u2 attributes_count;
dout.writeShort(0); // (no ClassFile attributes for proxy classes)
} catch (IOException e) {
throw new InternalError("unexpected I/O Exception", e);
}
return bout.toByteArray();
}
}
参见代码：
https://github.com/JetBrains/jdk8u_jdk/blob/master/src/share/classes/sun/misc/ProxyGenerator.java
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