动态代理玩不明白？别紧张，你只是缺少这个demo
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概念
JDK动态代理与静态代理是相对的，静态代理是在编译期就确定了代理类和被代理类的关系，且生成代理类。而且静态代理要求被代理的接口必须有其实现，因为代理类实现接口方法实际上是调用接口实现类的方法实现的。而动态代理是在运行期利用JVM的反射机制生成代理类,这里是直接生成类的字节码，然后通过类加载器载入JAVA虚拟机执行。
demo
不做赘述，直接上demo。
被代理类接口
public interface ISubject {
void doSomething();
}
被代理类实现
public class SubjectImpl implements ISubject {
@Override
public void doSomething() {
System.out.println("doSomething...");
}
}
InvocationHandler工具类
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
public class JDKDynamicProxy implements InvocationHandler{
private Object target;
public JDKDynamicProxy(){
}
public JDKDynamicProxy(Object target){
this.target=target;
}
public <T>T getProxy(){
System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
// return (T) Proxy.newProxyInstance(target.getClass().getClassLoader(),target.getClass().getInterfaces(),this);
return (T) Proxy.newProxyInstance(target.getClass().getClassLoader(),target.getClass().getInterfaces(),this);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("before...");
Object result=method.invoke(target,args);
System.out.println("after...");
return result;
}
}
测试类
public class TestProxy {
public static void main(String[] args) {
System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true");
ISubject subject=new JDKDynamicProxy(new SubjectImpl()).getProxy();
//ISubject subject= (ISubject) Proxy.newProxyInstance(subjectImpl.getClass().getClassLoader(),subjectImpl.getClass().getInterfaces(),new JDKDynamicProxy(subjectImpl));
//ISubject subject= (ISubject) Proxy.newProxyInstance(ISubject.class.getClassLoader(),new Class[]{ISubject.class},new JDKDynamicProxy(subjectImpl));
subject.doSomething();
}
}
运行结果
before...
doSomething...
after...
生成的代理类文件
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
package com.sun.proxy;
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 ISubject {
private static Method m1;
private static Method m3;
private static Method m2;
private static Method m0;
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
public final boolean equals(Object var1) throws {
try {
return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final void doSomething() throws {
try {
super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final String toString() throws {
try {
return (String)super.h.invoke(this, m2, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final int hashCode() throws {
try {
return (Integer)super.h.invoke(this, m0, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m3 = Class.forName("ISubject").getMethod("doSomething");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
原理
流程
获取代理类——调用Proxy.newProxyInstance获取代理类的Class实例getProxyClass0(loader, intfs)获取代理类实例的构造方法，并确保其访问权限final Constructor cons = cl.getConstructor(constructorParams)利用反射机制调用构造方法，返回代理类实例，参数是调用Proxy.newProxyInstance时传进来的InvocationHandler实例h 。cons.newInstance(new Object[]{h})
方法实现前面利用反射机制调用代理类的构造方法时传入了InvocationHandler实例h ，代理类$Proxy0的构造方法如下：
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
这是调用父类的构造函数，并传入参数h。所以在获取到代理类实例并执行方法时如测试类的subject.doSomething()，实际调用的是代理类内的doSomething()，如下所示。
public final void doSomething() throws {
try {
super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
所以JDK动态代理的接口方法实现逻辑是完全由InvocationHandler实例的invoke方法决定的 。
获取代理类的Class实例探究
getProxyClass0(loader, intfs)方法
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);
}
在Proxy类getProxyClass0(loader, intfs)方法中并无核心代码，主要是方法最后一行去缓存对象中获取代理类实例。
java.lang.reflect.WeakCache 类private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
proxyClassCache是Proxy类的静态变量，调用WeakCache类的构造函数初始化时，传入keyFactory和proxyClassFactory两个实例。注意WeakCache对象的数据结构包含两级键值。接下来就进入WeakCache类的get方法查看一番。
WeakCache类get方法代码
public V get(K key, P parameter) {
Objects.requireNonNull(parameter);
expungeStaleEntries();
Object cacheKey = CacheKey.valueOf(key, refQueue);
// lazily install the 2nd level valuesMap for the particular cacheKey
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
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;
while (true) {
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
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);
}
}
}
}
get方法核心代码解释获取一级和二级键。
//根据类加载器计算一级键 cacheKey=f(loader);
Object cacheKey = CacheKey.valueOf(key, refQueue);
//根据接口计算二级键 subKey=g(interfaces);
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
注意这里的subKeyFactory是Proxy初始化WeakCache类对象proxyClassCache时的入参new KeyFactory()；KeyFactory是Proxy的内部类。不妨进入apply简单看一下：@Override
public Object apply(ClassLoader classLoader, Class<?>[] interfaces) {
switch (interfaces.length) {
case 1: return new Key1(interfaces[0]); // the most frequent
case 2: return new Key2(interfaces[0], interfaces[1]);
case 0: return key0;
default: return new KeyX(interfaces);
}
}
循环 接下来是一个while循环，第一次循环前factory和supplier都为null ，所以循环体内只是做了一些初始化的工作，并将factory赋值给了supplier（Factory类是WeakCache的内部类，且继承了Supplier类）。第二次循环时，执行V value = supplier.get()，实际上就是调用factory.get()方法获取动态代理类的Class实例 。factory.get()方法方法完整代码如下：
@Override
public synchronized V get() { // serialize access
// re-check
Supplier<V> supplier = valuesMap.get(subKey);
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 {
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);
// try replacing us with CacheValue (this should always succeed)
if (valuesMap.replace(subKey, this, cacheValue)) {
// put also in reverseMap
reverseMap.put(cacheValue, Boolean.TRUE);
} else {
throw new AssertionError("Should not reach here");
}
// successfully replaced us with new CacheValue -> return the value
// wrapped by it
return value;
}
第4行代码Supplier supplier = valuesMap.get(subKey)通过subKey获取方才的supplier也就是factory。然后在第18行调用了valueFactory.apply方法创建动态代理类并将结果赋值给变量value。 valueFactory是初始化WeakCache对象调用构造方法时传入的参数 即Prxoy的内部类ProxyClassFactory对象。
ProxyClassFactory类进入apply方法
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.
*/
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());
}
}
该方法前边包括一些校验、计算出访问标志和包名、代理类定名 ，然后在73行创建代理类的字节码文件，并defineClass0将字节码文件加载到虚拟机和返回代理类的Class实例。
如上就是第一次创建代理类的大致流程。
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