浅说动态代理
关于java的代理模式，此处不过多讲解。所谓代理模式是指客户端并不直接调用实际的对象，而是通过调用代理，来间接的调用实际的对象。动态代理指被代理者委托代理者完成相应的功能，是拦截器的一种实现方式，其用于拦截类或接口，内部可通过判断实现对某个方法的拦截。
日常使用中可能经常需要在方法调用时进行拦截，如调用前记录一下调用开始时间，调用结束后记录结束时间，就可以很方便的计算出调用方法的业务逻辑处理耗时。
动态代理使用
简单的看下最简单的使用：
- 编写一个接口：
package my.java.reflect.test;
public interface Animal {
void sayHello();
}
- 委托类
public class Dog implements Animal {
public void sayHello() {
System.out.println("wang wang!");
}
}
- 拦截器
package my.java.reflect.test;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
public class MyInvocationHandler implements InvocationHandler {
private Object target;
public Object bind(Object realObj) {
this.target = realObj;
Class<?>[] interfaces = target.getClass().getInterfaces();
ClassLoader classLoader = this.getClass().getClassLoader();
return Proxy.newProxyInstance(classLoader, interfaces, this);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("proxy method");
return method.invoke(target, args);
}
}
- 测试类
package my.java.reflect.test;
import org.junit.Test;
public class ProxyTest {
@Test
public void testNewProxyInstance() {
Dog dog = new Dog();
Animal proxy = (Animal) new MyInvocationHandler().bind(dog);
proxy.sayHello();
}
}
- 输出
proxy method
wang wang!
动态代理原理总结
之所以将原理先总结了，因为希望把原理先用最简洁的语言说清楚，再来深入分析，否则在深入分析阶段粘贴过多的源码可能会导致阅读兴趣下降。
- 通过
Proxy#newProxyInstance方法得到代理对象实例； - 这个代理对象有着和委托类一模一样的方法；
- 当调用代理对象实例的方法时，这个实例会调用你实现
InvocationHandler里的invoke方法。
而这里，最复杂的显然是得到代理对象实例了，怎么得到的呢？来，看看源码！
动态代理原理
要了解java动态代理的原理只要从Proxy#newProxyInstance入手即可。
- 先看newProxyInstance方法，关注有中文注释的地方即可
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
if (h == null) {// 没有实现InvocationHandler，直接失败
throw new NullPointerException();
}
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.
* 查找或者生成代理类的Class对象
*/
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
// 拿到代理对象的构造方法
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
if (sm != null && ProxyAccessHelper.needsNewInstanceCheck(cl)) {
// create proxy instance with doPrivilege as the proxy class may
// implement non-public interfaces that requires a special permission
return AccessController.doPrivileged(new PrivilegedAction<Object>() {
public Object run() {
return newInstance(cons, ih);
}
});
} else {
// 构造出代理对象实例
return newInstance(cons, ih);
}
} catch (NoSuchMethodException e) {
throw new InternalError(e.toString());
}
}
// 利用反射调用构造方法，产生代理实例，没什么好说的
private static Object newInstance(Constructor<?> cons, InvocationHandler h) {
try {
return cons.newInstance(new Object[] {h} );
} catch (IllegalAccessException | InstantiationException e) {
throw new InternalError(e.toString());
} catch (InvocationTargetException e) {
Throwable t = e.getCause();
if (t instanceof RuntimeException) {
throw (RuntimeException) t;
} else {
throw new InternalError(t.toString());
}
}
}
以上这段代码的核心在
/*
* Look up or generate the designated proxy class.
* 查找或者生成代理类的Class对象
*/
Class<?> cl = getProxyClass0(loader, intfs);
- 查看
getProxyClass0方法
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
if (interfaces.length > 65535) {// 你的类如果实现了超过65535个接口，这个方法疯了。
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
// 如果委托类的接口已经存在于缓存中，则返回，否则利用ProxyClassFactory产生一个新的代理类的Class对象
return proxyClassCache.get(loader, interfaces);
}
这段话的核心很显然就是proxyClassCache.get(loader, interfaces)，其中proxyClassCache是一个缓存：
/**
* a cache of proxy classes
*/
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
- 继续看
proxyClassCache.get(loader, interfaces)，对应的是java.lang.reflect.WeakCache<K, P, V>#get，一大段代码就为了返回一个代理类的Class对象，关注中文注释处即可：
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
// 关键点在这里
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);
}
}
}
}
这段代码的核心就在V value = supplier.get();，通过这段代码，代理类的Class的对象就出来了。
- 查看
subKeyFactory.apply(key, parameter)，这段代码还是在WeakCache里：
@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 {
// 核心点在这里
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;
}
回看proxyClassCache可以知道valueFacotry对应的是ProxyClassFactory类，这是java.reflect.Proxy的内部类：
@Override
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);// 看下接口对应的类文件有没有被ClassLoader加载
} 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()) {// 看下是不是都是接口
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* Verify that this interface is not a duplicate.
*/
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {// 接口不能重复
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
String proxyPkg = null; // package to define proxy class in
/*
* 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)) {// 接口不是public的，截取包名，保证代理类跟委托类同一个包下
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) {// 是public的接口，拼接成com.sun.proxy$Proxy，再拼接一个数字
// if no non-public proxy interfaces, use com.sun.proxy package
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
/*
* Choose a name for the proxy class to generate.
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
*/
// 核心，生成代理类的字节码
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces);
try {
// 根据字节码生成代理类Class对象，native方法，看过类加载器的童鞋应该不陌生
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
/*
* A ClassFormatError here means that (barring bugs in the
* proxy class generation code) there was some other
* invalid aspect of the arguments supplied to the proxy
* class creation (such as virtual machine limitations
* exceeded).
*/
throw new IllegalArgumentException(e.toString());
}
}
这段代码前面一大段就是检查、校验接口，然后利用ProxyGenerator生成代理类的字节码数组，接着将字节码封装成Class对象，就是代理类的Class对象了。生成字节码数组的代码就不详细说了。
- 将字节码数组写到文件里查看一下：
byte[] data = ProxyGenerator.generateProxyClass(“Animal$Proxy”, new Class[] { Animal.class });
FileOutputStream out = new FileOutputStream("Animal$Proxy.class");
out.write(data);
- 利用jd-gui反编译工具可以看看代理类的源码：
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class Animal$Proxy extends Proxy implements my.java.reflect.test.Animal {
private static Method m3;
private static Method m1;
private static Method m0;
private static Method m2;
public Animal$Proxy(InvocationHandler paramInvocationHandler) {
super(paramInvocationHandler);
}
public final void sayHello() {
try {
this.h.invoke(this, m3, null);
return;
} catch (Error | RuntimeException localError) {
throw localError;
} catch (Throwable localThrowable) {
throw new UndeclaredThrowableException(localThrowable);
}
}
public final boolean equals(Object paramObject) {
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 int hashCode() {
try {
return ((Integer) this.h.invoke(this, m0, null)).intValue();
} catch (Error | RuntimeException localError) {
throw localError;
} catch (Throwable localThrowable) {
throw new UndeclaredThrowableException(localThrowable);
}
}
public final String toString() {
try {
return (String) this.h.invoke(this, m2, null);
} catch (Error | RuntimeException localError) {
throw localError;
} catch (Throwable localThrowable) {
throw new UndeclaredThrowableException(localThrowable);
}
}
static {
try {
m3 = Class.forName("my.java.reflect.test.Animal").getMethod("sayHello", new Class[0]);
m1 = Class.forName("java.lang.Object").getMethod("equals",
new Class[] { Class.forName("java.lang.Object") });
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
return;
} catch (NoSuchMethodException localNoSuchMethodException) {
throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
} catch (ClassNotFoundException localClassNotFoundException) {
throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
}
}
}
可以看到生成的代理类有一个构造方法，参数是InvocationHandler，然后回看我们第一步分析的时候，得到代理类的Class对象后，会通过反射得到代理类的构造方法，接着调用构造方法，参数就是InvocationHandler。
在代理类的源码里，最值得注意的就是m3，这个对应的是Animal的sayHello方法，当我们通过MyInvocationHandler#bind方法得到代理对象实例后，调用代理对象Animal$Proxy的sayHello方法，就会执行：
public final void sayHello() {
try {
this.h.invoke(this, m3, null);
return;
} catch (Error | RuntimeException localError) {
throw localError;
} catch (Throwable localThrowable) {
throw new UndeclaredThrowableException(localThrowable);
}
}
正好是我们在MyInvocationHandler实现的invoke方法，这样就完成了代理的功能。
用一个总结收尾
之所以将原理先总结了，因为希望把原理先用最简洁的语言说清楚，再来深入分析，否则在深入分析阶段粘贴过多的源码可能会导致阅读兴趣下降。
- 通过
Proxy#newProxyInstance方法得到代理对象实例； - 这个代理对象有着和委托类一模一样的方法；
- 当调用代理对象实例的方法时，这个实例会调用你实现
InvocationHandler里的invoke方法。
这里面无疑第一步是最复杂的，这里大概经历了：
- 利用参数中的接口，通过缓存或者利用
ProxyGenerator生成字节码并生成代理类的Class对象； - 通过反射得到代理对象的构造方法；
- 通过构造方法和
InvocationHandler参数通过反射实例化出代理对象实例。
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