给目标对象提供一个代理对象,并由代理对象控制对目标对象的引用。
假设我们想请明星来唱歌,我们是先联系他的经纪人,在经纪人告诉明星,然后明星唱歌。其中经纪人就是代理。
静态代理与动态代理的区别:
静态代理中的代理类是需要用户自己写的,这样代理类在程序运行前就已经存在了。
动态代理中的代理类是在程序运行中自动生成的。
//静态代理
//接口
public interface Fruit {
void eat();
}
//实现类
public class Apple implements Fruit {
public void eat() {
System.out.println("吃苹果);
}
}
//代理类
public class AppleProxy implements Fruit {
private Apple apple;
public AppleProxy (Apple apple) {
this.apple = apple;
}
public void eat() {
System.out.println("吃前先洗手");
apple.eat();
System.out.println("吃后也洗手");
}
public static void main(String[] args) {
AppleProxy appleProxy = new AppleProxy(new Apple());
appleProxy.eat();
}
}
输出
吃前先洗手
吃苹果
吃后也洗手
JDK动态代理(包含一个类和一个接口)
InvocationHandler接口
public interface InvocationHandler {
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable;
}
参数说明:
可以将实现InvocationHandler接口的子类想象成一个最终操作类,替换掉AppleProxy。
Proxy类
Proxy类是专门完成代理的操作类,可以通过此类为一个或多个接口动态的生成实现类
public static Object newProxyInstance(ClassLoader loader,
Class[] interfaces,
InvocationHandler h);
参数说明:
//实现类
public class Apple implements Fruit {
@Override
public void eat() {
System.out.println("吃苹果");
}
}
//代理类
public class AppleProxy implements InvocationHandler {
public Object object;
public Object bind(Object object) {
this.object = object;
return Proxy.newInstance(object.getClass().getClassLoader(),
object.getClass().getInterfaces(),
this);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
Object result = null;
System.out.println("执行方法之前");
result = method.invoke(object, args);
System.out.println("执行方法之后");
return result;
}
public static void main(String[] args) {
AppleProxy appleProxy = new AppleProxy();
Fruit fruitProxy = (Fruit) appleProxy.bind(new Apple());
fruitProxy.eat();
}
}
输出
执行方法之前
吃苹果
执行方法之后
JDK的动态代理依靠接口实现,如果有些类并没有实现接口,则不能使用JDK代理,这就要使用CGLIB动态代理了。
CGLIB动态代理
cglib是针对类来实现代理的,他的原理是对指定的目标类生成一个子类,并覆盖其中方法实现实现增强,但是因为采用是继承,所以不能对final类进行代理。
接口
public interface Fruit {
void eat();
}
实现类
public class Apple implements Fruit {
public void eat() {
System.out.println("吃苹果");
}
}
代理类
public class AppleProxy implements MethodInterceptor {
private Object object;
public Object getInstance(Object object) {
this.object = object;
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(this.object.getClass());
//回调方法
enhancer.setCallback(this);
//创建代理对象
return enhancer.create();
}
public Object intercept(Object object, Method method,
Object[] args, MethodProxy methodProxy) throws Throwable {
System.out.println("执行方法之前");
methodProxy.invokeSuper(object, args);
System.out.println("执行方法之后");
return null;
}
public static void main(String[] args) {
AppleProxy appleProxy = new AppleProxy();
Apple apple = (Apple) appleProxy.getInstance(new Apple());
apple.eat();
}
}
输出
执行方法之前
吃苹果
执行方法之后
JDK动态代理原理
生成代理对象的newProxyInstance()方法分析
public static Object newProxyInstance(ClassLoader loader,
Class[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
//如果h为null则抛出NUllPointerException异常
//之后所有的判断时候为null都用此方法
Objects.requireNonNull(h);
// 拷贝类实现的所有接口
final Class[] intfs = interfaces.clone();
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
//检查创建Proxy类所需的权限
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
* 查找或产生代理类
*/
Class cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
* 调用它的构造函数
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
// 获取代理类的构造器对象
final Constructor cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doPrivileged(new PrivilegedAction() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
//根据代理类的构造器对象创建代理类对象并返回
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);
}
}
可以看到主要做了3件事情:
生成代理类
/**
* 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) {//接口数不能超过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
// 如果缓存命中代理类则直接返回,否则用代理类工厂ProxyClassFactory创建代理类
return proxyClassCache.get(loader, interfaces);
}
/**
* @param key 类加载器(可能为null)
* @param parameter 接口数组(不能为null)
*/
public V get(K key, P parameter) {
// 在缓存中没有找到会创建新的代理类
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
Supplier supplier = valuesMap.get(subKey);
V value = supplier.get();
return value;
}
apply方法的实现源码如下:
public Class apply(ClassLoader loader, Class[] interfaces) {
Map, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
// 验证
for (Class intf : interfaces) {
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");
}
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
String proxyPkg = null; // 代理类所在包
int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
// 验证所有非公共的接口在同一个包内,公共的就无需处理
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 + ".";
}
// 生成代理类名,以$Proxy开头
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
// 生成代理类字节码
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
// 加载字节码,生成代理类对象
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
throw new IllegalArgumentException(e.toString());
}
}
可以看到通过generateProxyClass方法生成代理类字节码,跟进该方法,发现又调用了generateClassFile()方法生成字节码,源码如下:
private byte[] generateClassFile() {
// 添加hashCode、equals、toString方法
this.addProxyMethod(hashCodeMethod, Object.class);
this.addProxyMethod(equalsMethod, Object.class);
this.addProxyMethod(toStringMethod, Object.class);
Class[] var1 = this.interfaces;
int var2 = var1.length;
int var3;
Class var4;
// 添加接口中的方法
for(var3 = 0; var3 < var2; ++var3) {
var4 = var1[var3];
Method[] var5 = var4.getMethods();
int var6 = var5.length;
for(int var7 = 0; var7 < var6; ++var7) {
Method var8 = var5[var7];
this.addProxyMethod(var8, var4);
}
}
Iterator var11 = this.proxyMethods.values().iterator();
List var12;
while(var11.hasNext()) {
var12 = (List)var11.next();
checkReturnTypes(var12);
}
Iterator var15;
try {
// 生成代理类的构造函数
this.methods.add(this.generateConstructor());
var11 = this.proxyMethods.values().iterator();
while(var11.hasNext()) {
var12 = (List)var11.next();
var15 = var12.iterator();
while(var15.hasNext()) {
ProxyGenerator.ProxyMethod var16 = (ProxyGenerator.ProxyMethod)var15.next();
this.fields.add(new ProxyGenerator.FieldInfo(var16.methodFieldName, "Ljava/lang/reflect/Method;", 10));
this.methods.add(var16.generateMethod());
}
}
this.methods.add(this.generateStaticInitializer());
} catch (IOException var10) {
throw new InternalError("unexpected I/O Exception", var10);
}
if (this.methods.size() > 65535) {
throw new IllegalArgumentException("method limit exceeded");
} else if (this.fields.size() > 65535) {
throw new IllegalArgumentException("field limit exceeded");
} else {
// 编写最终类文件
this.cp.getClass(dotToSlash(this.className));
this.cp.getClass("java/lang/reflect/Proxy");
var1 = this.interfaces;
var2 = var1.length;
for(var3 = 0; var3 < var2; ++var3) {
var4 = var1[var3];
this.cp.getClass(dotToSlash(var4.getName()));
}
this.cp.setReadOnly();
ByteArrayOutputStream var13 = new ByteArrayOutputStream();
DataOutputStream var14 = new DataOutputStream(var13);
try {
var14.writeInt(-889275714);
var14.writeShort(0);
var14.writeShort(49);
this.cp.write(var14);
var14.writeShort(this.accessFlags);
var14.writeShort(this.cp.getClass(dotToSlash(this.className)));
var14.writeShort(this.cp.getClass("java/lang/reflect/Proxy"));
var14.writeShort(this.interfaces.length);
Class[] var17 = this.interfaces;
int var18 = var17.length;
for(int var19 = 0; var19 < var18; ++var19) {
Class var22 = var17[var19];
var14.writeShort(this.cp.getClass(dotToSlash(var22.getName())));
}
var14.writeShort(this.fields.size());
var15 = this.fields.iterator();
while(var15.hasNext()) {
ProxyGenerator.FieldInfo var20 = (ProxyGenerator.FieldInfo)var15.next();
var20.write(var14);
}
var14.writeShort(this.methods.size());
var15 = this.methods.iterator();
while(var15.hasNext()) {
ProxyGenerator.MethodInfo var21 = (ProxyGenerator.MethodInfo)var15.next();
var21.write(var14);
}
var14.writeShort(0);
return var13.toByteArray();
} catch (IOException var9) {
throw new InternalError("unexpected I/O Exception", var9);
}
}
}
可以看到主要做了3件事:
生成的代理类class文件为$Proxy0.class.通过对其反编译得到其源码如下:
package com.sun.proxy;
import com.test.Fruit;
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 Fruit {
// 保存Method对象,其中m3对应我们的eat方法,其余三个为hashCode、equals、toString方法
private static Method m1;
private static Method m3;
private static Method m2;
private static Method m0;
// 代理类的构造函数
// Proxy.newInstance方法就是通过通过这个构造函数来创建代理实例的
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
// hashCode、equals、toString方法
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);
}
}
// 代理方法
public final void eat() throws {
try {
super.h.invoke(this, m3, (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 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);
}
}
// 静态代码块进行变量初始化
static {
// 每个方法对象与实际的类或接口的方法绑定
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m3 = Class.forName("com.test.Fruit").getMethod("eat");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
可以看到以下特点:
至此JDK动态代理结束