一、代理模式是什么？
代理模式就是给一个对象提供一个代理对象，并由代理对象管理着被代理对象的引用。就像生活中的代理律师，你只需要找好代理律师，剩下的都交给代理律师来打理。
Spring MVC 有两大特性，IoC 和 AOP。IoC为控制反转，这里不做介绍；AOP（Aspect Oriented Programming 面向切面编程）的实现就是基于代理技术。
二、静态代理
先来看下什么是静态代理。静态代理在程序运行前，就已经编写好代理类，实现静态代理需要以下：
1、定义业务接口
2、被代理对象实现业务接口
3、代理对象实现业务接口并持有被代理对象的引用
码上看
2.1 业务接口
package cn.zjm.show.proxy;
/**
* @author Zhao JinMing
* @date 2018年9月14日 09:49
*/
public interface IUserService {
void add(String name);
}2.2 被代理对象实现接口
package cn.zjm.show.proxy;
/**
* @author Zhao JinMing
* @date 2018年9月14日 09:49
*/
public class UserServiceImpl implements IUserService {
@Override
public void add(String name) {
System.out.println("添加用户“" + name + "”成功。");
}
}
2.3 代理对象实现接口并持有代理对象引用
package cn.zjm.show.proxy;
/**
* @author Zhao JinMing
* @date 2018年9月14日 09:50
*/
public class UserServiceProxy implements IUserService {
private IUserService proxy;
public UserServiceProxy(IUserService proxy) {
this.proxy = proxy;
}
@Override
public void add(String name) {
System.out.println("开始插入");
proxy.add(name);
System.out.println("结束插入");
}
}
2.4 代码测试
package cn.zjm.show.proxy;
public class SourceProxy {
public static void main(String[] args) {
IUserService userService = new UserServiceImpl();
IUserService proxy = new UserServiceProxy(userService);
proxy.add("王五");
}
}
2.5 输出结果
开始插入
添加用户“王五”成功。
结束插入2.6 总结
既然已经有动态代理了，那为什么还需要动态代理。毋庸置疑肯定是静态代理与动态代理相比有着很多的缺陷。从扩展性的角度考虑，如果业务接口添加或者删除方法，那么不仅仅是被代理类需要修改，代理类也需要修改。而且对于不同的业务接口，需要写不同的代理类，这么麻烦的操作，怕麻烦的大佬肯定受不了，于是乎动态代理来了。
三、动态代理
所谓的动态代理，就是在程序运行的时候，根据需要动态的创建代理类及其实例来完成具体的操作。动态代理又分为JDK动态代理和cglib动态代理两大类，下面介绍的是JDK动态代理。
3.1 使用JDK动态代理的步骤
①定义业务接口
②被代理对象实现业务接口
③通过Proxy的静态方法newProxyInstance( ClassLoaderloader, Class[] interfaces, InvocationHandler h)创建一个代理对象
④使用代理对象
3.1 定义业务接口
package cn.zjm.show.proxy;
/**
* @author Zhao JinMing
* @date 2018年9月14日 09:49
*/
public interface IUserService {
void add(String name);
}
3.2 被代理对象实现业务接口
package cn.zjm.show.proxy;
/**
* @author Zhao JinMing
* @date 2018年9月14日 09:49
*/
public class UserServiceImpl implements IUserService {
@Override
public void add(String name) {
System.out.println("添加用户“" + name + "”成功。");
}
}3.3 创建代理对象并使用
package cn.zjm.show.proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
public class SourceProxy {
public static void main(String[] args) {
IUserService userService = new UserServiceImpl();
IUserService proxy = (IUserService) Proxy.newProxyInstance(SourceProxy.class.getClassLoader(), userService.getClass().getInterfaces(), new InvocationHandler() {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("准备使用动态代理插入数据");
Object invoke = method.invoke(userService, args);
System.out.println("使用动态代理插入对象结束");
return invoke;
}
});
proxy.add("王五");
}
}
3.4 运行结果
准备使用动态代理插入数据
添加用户“王五”成功。
使用动态代理插入对象结束运行结果和静态代理一样，说明成功了。但是，我们注意到，我们并没有像静态代理那样去自己定义一个代理类，并实例化代理对象。实际上，动态代理的代理对象是在内存中的，是JDK根据我们传入的参数生成好的。那动态代理的代理类和代理对象是怎么产生的呢？
四、源码分析
首先来看 newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h)
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
//判断h是否为空
Objects.requireNonNull(h);
//对接口数组的拷贝
final Class<?>[] intfs = interfaces.clone();
//安全检查
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
* 查询（在缓存中已经有）或生成指定的代理类的class对象。
* --核心代码
*/
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
* 用invocationHandler生成构造函数
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
//得到代理对象的构造函数
//private static final Class<?>[] constructorParams ={ InvocationHandler.class };
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;
}
});
}
//生成代理对象
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);
}
}代码核心是通过getProxyClass0(loader, intfs)得到代理类的Class对象，然后通过Class对象获得构造方法，进而创建代理对象。再看getProxyClass0(loader, intfs)方法。
getProxyClass0(loader, intfs)
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
//接口数量不能大于65535
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
/*
如果由实现给定接口的给定加载器定义的代理类存在，则这将简单地返回缓存的副本；
否则，它将通过ProxyClassFactory创建代理类
*/
return proxyClassCache.get(loader, interfaces);
}
// private static final WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
// proxyClassCache是WeakCache类的实例，WeakCache又是什么？再看看 WeakCache 是什么
//K为key类型，P为参数类型，V为value类型
//根据传进来的参数，K为ClassLoader类型，P为Class<?>[]类型，V为Class<?>类型
final class WeakCache<K, P, V> {
private final ReferenceQueue<K> refQueue
= new ReferenceQueue<>();
// the key type is Object for supporting null key
private final ConcurrentMap<Object, ConcurrentMap<Object, Supplier<V>>> map
= new ConcurrentHashMap<>();
private final ConcurrentMap<Supplier<V>, Boolean> reverseMap
= new ConcurrentHashMap<>();
private final BiFunction<K, P, ?> subKeyFactory;
private final BiFunction<K, P, V> valueFactory;
public WeakCache(BiFunction<K, P, ?> subKeyFactory,
BiFunction<K, P, V> valueFactory) {
this.subKeyFactory = Objects.requireNonNull(subKeyFactory);
this.valueFactory = Objects.requireNonNull(valueFactory);
}map变量为实现缓存的核心变量，他是一个双重map结构（key，sub-key。
在进入proxyClassCache.get(loader, interfaces)
public V get(K key, P parameter) {
//检查参数是否为空
Objects.requireNonNull(parameter);
//清除无效的缓存
expungeStaleEntries();
//cacheKey 为上述map中的一级key
Object cacheKey = CacheKey.valueOf(key, refQueue);
// lazily install the 2nd level valuesMap for the particular cacheKey
//根据一级key得到ConcurrentMap<Object, Supplier<V>>对象，如果不存在，如果不存在就新建一个ConcurrentMap<Object, Supplier<V>>对象
ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
if (valuesMap == null) {
ConcurrentMap<Object, Supplier<V>> oldValuesMap
= map.putIfAbsent(cacheKey,
valuesMap = new ConcurrentHashMap<>());
if (oldValuesMap != null) {
valuesMap = oldValuesMap;
}
}
// create subKey and retrieve the possible Supplier<V> stored by that
// subKey from valuesMap
// 创建sub-key、获取存储的Supplier<V>
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
// 通过sub-key得到supplier
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;
while (true) {
//如果缓存中有supplier，就直接通过get方法得到代理对象返回
if (supplier != null) {
// supplier might be a Factory or a CacheValue<V> 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
//面的所有代码目的就是：如果缓存中没有supplier，则创建一个Factory对象，把factory对象在多线程的环境下安全的赋给supplier。
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);
}
}
}
}再来看Factory中的 get() 方法
public synchronized V get() { // serialize access
// re-check
Supplier<V> supplier = valuesMap.get(subKey);
//检查得到的supplier是不是当前对象
if (supplier != this) {
// something changed while we were waiting:
// might be that we were replaced by a CacheValue
// or were removed because of failure ->
// return null to signal WeakCache.get() to retry
// the loop
return null;
}
// else still us (supplier == this)
// create new value
V value = null;
try {
//代理类在这里得到代理对象
//重点方法valueFactory.apply(key, parameter)
value = Objects.requireNonNull(valueFactory.apply(key, parameter));
} finally {
if (value == null) { // remove us on failure
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);
// put into reverseMap
reverseMap.put(cacheValue, Boolean.TRUE);
// try replacing us with CacheValue (this should always succeed)
if (!valuesMap.replace(subKey, this, cacheValue)) {
throw new AssertionError("Should not reach here");
}
// successfully replaced us with new CacheValue -> return the value
// wrapped by it
return value;
}再看 ProxyClassFactory 的 apply() 方法
//这里的BiFunction<T, U, R>是个函数式接口，可以理解为用T，U两种类型做参数，得到R类型的返回值
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// prefix for all proxy class names
//所有代理类名字的前缀
private static final String proxyClassNamePrefix = "$Proxy";
// next number to use for generation of unique proxy class names
//用于生成代理类名字的计数器
private static final AtomicLong nextUniqueNumber = new AtomicLong();
@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
//验证代理接口，可不看
for (Class<?> intf : interfaces) {
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
Class<?> interfaceClass = null;
try {
interfaceClass = Class.forName(intf.getName(), false, loader);
} catch (ClassNotFoundException e) {
}
if (interfaceClass != intf) {
throw new IllegalArgumentException(
intf + " is not visible from class loader");
}
/*
* Verify that the Class object actually represents an
* interface.
*/
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* Verify that this interface is not a duplicate.
*/
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
//生成的代理类的包名
String proxyPkg = null; // package to define proxy class in
//代理类访问控制符: public ,final
int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
/*
* Record the package of a non-public proxy interface so that the
* proxy class will be defined in the same package. Verify that
* all non-public proxy interfaces are in the same package.
*/
//验证所有非公共的接口在同一个包内；公共的就无需处理
//生成包名和类名的逻辑，包名默认是com.sun.proxy，类名默认是$Proxy 加上一个自增的整数值
//如果被代理类是 non-public proxy interface ，则用和被代理类接口一样的包名
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();
//代理类的完全限定名，如com.sun.proxy.$Proxy0.calss
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
*/
//核心部分，生成代理类的字节码
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
//把代理类加载到JVM中，至此动态代理过程基本结束了
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
/*
* A ClassFormatError here means that (barring bugs in the
* proxy class generation code) there was some other
* invalid aspect of the arguments supplied to the proxy
* class creation (such as virtual machine limitations
* exceeded).
*/
throw new IllegalArgumentException(e.toString());
}
}
}获取字节码文件
package cn.zjm.show.proxy;
import sun.misc.ProxyGenerator;
import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
public class SourceProxy {
public static void main(String[] args) {
IUserService userService = new UserServiceImpl();
IUserService proxy = (IUserService) Proxy.newProxyInstance(SourceProxy.class.getClassLoader(), userService.getClass().getInterfaces(), new InvocationHandler() {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("动态代理开始");
Object invoke = method.invoke(userService, args);
System.out.println("动态代理结束");
return invoke;
}
});
proxy.add("王五");
String path = "D:/$Proxy0.class";
byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0", userService.getClass().getInterfaces());
FileOutputStream out = null;
try {
out = new FileOutputStream(path);
out.write(classFile);
out.flush();
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
out.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}反编译后
import cn.zjm.show.proxy.IUserService;
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 IUserService
{
private static Method m1;
private static Method m2;
private static Method m4;
private static Method m3;
private static Method m0;
public $Proxy0(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 del(String paramString)
throws
{
try
{
this.h.invoke(this, m4, new Object[] { paramString });
return;
}
catch (Error|RuntimeException localError)
{
throw localError;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
public final void add(String paramString)
throws
{
try
{
this.h.invoke(this, m3, new Object[] { paramString });
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]);
m4 = Class.forName("cn.zjm.show.proxy.IUserService").getMethod("del", new Class[] { Class.forName("java.lang.String") });
m3 = Class.forName("cn.zjm.show.proxy.IUserService").getMethod("add", new Class[] { Class.forName("java.lang.String") });
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());
}
}
}