JDK 动态代理
基本用法
public class JDKProxy implements InvocationHandler {
private ISomeInterface someInterface;
private JDKProxy(ISomeInterface someInterface) {
this.someInterface = someInterface;
}
public ISomeInterface getProxy() {
return (ISomeInterface) Proxy.newProxyInstance(this.getClass().getClassLoader(), someInterface.getClass().getInterfaces(), this);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
before();
Object obj = someInterface.doSomething();
after();
return obj;
}
private void after() {}
private void before() {}
}
interface ISomeInterface {
Object doSomething();
}
Proxy 和 InvocationHandler
动态代理明面上就这两个关键类
InvocationHandler 里面就一个invoke方法，专门用于代理类的回调。
Proxy.newProxyInstance 用于生成的代理类，重点分析下内部的复杂实现。
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.
*/
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
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;
}
});
}
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);
}
}
三个参数：
ClassLoader loader：用哪个类加载器来加载；
interfaces：都需要代理哪些接口的方法；
InvocationHandler ：回调的类
第一步：生成代理类
第二步：实例化代理类对象
getProxyClass0
getProxyClass0用于生成代理类
/**
* 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) {
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
// 如果当前类加载器和接口已经被缓存，就直接返回，否则就用ProxyClassFactory创建代理类
return proxyClassCache.get(loader, interfaces);
}
第一个if判断接口数量，这里的65535是受java字节码文件结构中对接口数量的限制，u2的大小。具体另行查阅java字节码文件的构成。
private static final WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
proxyClassCache 是一个弱引用的缓存
proxyClassCache.get(loader, interfaces)
/**
* Look-up the value through the cache. This always evaluates the
* {@code subKeyFactory} function and optionally evaluates
* {@code valueFactory} function if there is no entry in the cache for given
* pair of (key, subKey) or the entry has already been cleared.
*
* @param key possibly null key 类加载器
* @param parameter parameter used together with key to create sub-key and
* value (should not be null) 接口对象列表
* @return the cached value (never null)
* @throws NullPointerException if {@code parameter} passed in or
* {@code sub-key} calculated by
* {@code subKeyFactory} or {@code value}
* calculated by {@code valueFactory} is null.
*/
public V get(K key, P parameter) {
Objects.requireNonNull(parameter);
expungeStaleEntries();
//1.
Object cacheKey = CacheKey.valueOf(key, refQueue);
//2.
// 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;
}
}
//3.
// 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;
//4.
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);
}
}
}
}
整个方法就是围绕着代理类缓存来做处理。
第一部分：先将类加载器构建成一个CacheKey的弱引用对象
private static final class CacheKey<K> extends WeakReference<K> {
// a replacement for null keys
private static final Object NULL_KEY = new Object();
static <K> Object valueOf(K key, ReferenceQueue<K> refQueue) {
return key == null
// null key means we can't weakly reference it,
// so we use a NULL_KEY singleton as cache key
? NULL_KEY
// non-null key requires wrapping with a WeakReference
: new CacheKey<>(key, refQueue);
}
第二部分：构造了一个valuesMap，用于缓存类加载器对象和代理类。
第三部分：subKeyFactory.apply方法通过KeyFactory和接口的数量，返回了某一种Key对象，Key也是一个弱引用。
/**
* A function that maps an array of interfaces to an optimal key where
* Class objects representing interfaces are weakly referenced.
*/
private static final class KeyFactory
implements BiFunction<ClassLoader, Class<?>[], Object>
{
@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);
}
}
}
第四部分：while(true) 循环就是返回缓存的代理类，或者创建新的代理类。
第一圈，先构建一个Factory，作为生成代理类的工厂，并且作为value缓存到了valuesMap中，并赋值给supplier。
第二圈，调用Factory的get方法。
// lazily construct a Factory
if (factory == null) {
factory = new Factory(key, parameter, subKey, valuesMap);
}
接下来就仔细分析一下Factory的实现。
/**
* A factory {@link Supplier} that implements the lazy synchronized
* construction of the value and installment of it into the cache.
*/
private final class Factory implements Supplier<V> {
private final K key; // 类加载器
private final P parameter; // 接口对象列表
private final Object subKey; //接口对象构造的Key
private final ConcurrentMap<Object, Supplier<V>> valuesMap; // <类加载，代理类生成工厂> map
Factory(K key, P parameter, Object subKey,
ConcurrentMap<Object, Supplier<V>> valuesMap) {
this.key = key;
this.parameter = parameter;
this.subKey = subKey;
this.valuesMap = valuesMap;
}
@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
//正常情况下，在外部的while(true) 循环中，已经赋值supplier = factory, 如果这里不相等，就直接返回，外部循环再执行一次
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);
// 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;
}
}
主要实现是先生成代理类对象，并将其封装成CacheValue，保存到valuesMap中。
关键部分就是生成代理类
// 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);
}
}
具体的生成过程是下面的一大段：整体过程比较简单，注释都写在代码中。最后返回生成的代理类对象。
回到上面的方法，代理类对象生成后，封装成CacheValue，同样是弱引用，保存到了valuesMap中。
至此，valuesMap中缓存了：接口对象列表 -> 代理类对象；
全局map缓存了：类加载器 -> Map<接口对象列表 -> 代理类对象>
/**
* A factory function that generates, defines and returns the proxy class given
* the ClassLoader and array of interfaces.
*/
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// prefix for all proxy class names 如：jdk代理生成的类 == 原包名.$Proxy0
private static final String proxyClassNamePrefix = "$Proxy";
// next number to use for generation of unique proxy class names 记录当前生成到几了
private static final AtomicLong nextUniqueNumber = new AtomicLong();
@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
//将所有的接口对象都存到map中，并且不能有重复
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());
}
}
//代理类的包路径
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();
//如果接口是非public的，就必须在同一个包下，非public跨包无法被引用到
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");
}
}
}
// 如果当前接口没有包名，就用com.sun.proxy。而且一定是public的，如果不是public，包名就是""
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();
//构造代理类的类名： com.xxx.xxx.$Proxy0
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
*/
// 生成类的字节码的字节数组
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
// 通过native方法，用这个类加载器加载这个代理类到jvm中
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());
}
}
}
这里需要多解释一些，ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags); 在生成代理类时，内部有一个generateConstructor方法，专门给代理类继承Proxy类，并添加带有InvocationHandler的构造方法。这样在创建代理类的实例对象时，通过InvocationHandler的回调来实现方法的扩展。
private ProxyGenerator.MethodInfo generateConstructor() throws IOException {
ProxyGenerator.MethodInfo var1 = new ProxyGenerator.MethodInfo("<init>", "(Ljava/lang/reflect/InvocationHandler;)V", 1);
DataOutputStream var2 = new DataOutputStream(var1.code);
this.code_aload(0, var2);
this.code_aload(1, var2);
var2.writeByte(183);
var2.writeShort(this.cp.getMethodRef("java/lang/reflect/Proxy", "<init>", "(Ljava/lang/reflect/InvocationHandler;)V"));
var2.writeByte(177);
var1.maxStack = 10;
var1.maxLocals = 2;
var1.declaredExceptions = new short[0];
return var1;
}
回到最上面的newProxyInstance方法，这里的cl就是生成代理类对象。
Class<?> cl = getProxyClass0(loader, intfs);
代理类生成后，通过调用带有InvocationHandler的构造方法，来newInstance对象。
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;
}
});
}
return cons.newInstance(new Object[]{h});
至此，基于JDK的动态代理对象生成过程就此完成。
总结
- JDK的动态代理内部构建了一个大的ConcurrentHashMap，存储的是<类加载器, <接口列表, 代理类>>。整个过程就是在围绕这个大Map来缓存代理类。
- 创建代理类时，校验了各种语法格式，生成了包名+$Proxy+数字的代理类，并且继承了Proxy，补充了一个带有InvocationHandler的构造方法。在创建代理类实例对象时，通过这个InvocationHandler对象进行方法回调，从而实现接口方法的扩展。
- 自定义的扩展就是实现InvocationHandler接口，实现invoke方法扩展，并将这个InvocationHandler对象传给Proxy的构造过程中。
- 可以发现，每次创建代理类对象时，都是通过缓存获取代理类，并反射instance对象，有一定程度的性能影响。
附加：
生成的代理类大概长下面这样：
public final class $Proxy0 extends Proxy implements ISomeInterface {
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 Object doSomething() throws {
try {
return (Object)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("com.xxx.ISomeInterface").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());
}
}
}
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