读JDK动态代理源码(Proxy类)

动态代理最重要的实现就是Proxy.newInstance ,那我们直接看这个方法

public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)//这里有三个参数,第一个是传入classloader,一般情况是传入当前的classloader.但是我在上一节模拟实现里传入的是URL loader..第二个参数表示的是接口,第三个是Invocationhandler,除了第一个参数,其他和我在上一节里的一样.JDK的封装的比较好,所以他传入的是Interface的数组,
throws IllegalArgumentException
{
if (h == null) {
throw new NullPointerException();//如果Invocationhandler 为空,抛异常
}

/*
* Look up or generate the designated proxy class.这个方法里最主要的地方在这里,它直接通过调用 getProxyClass方法获取到了自动生成的动态代理类的二进制码.在我上一节的内容里,我们是通过自己生成JAVA文件,然后通过JAVA文件动态编译成对应的class文件,然后通过URLClassLoader.loadClass("com.cjb.proxy.Proxy1");这个方法来获取对应的二进制码的.这个方法下面会继续解释.
*/
Class cl = getProxyClass(loader, interfaces);

/*
* Invoke its constructor with the designated invocation handler.这个方法就是获取对应class的Constructor,然后通过这个Constructor来实例化..
*/
try {
Constructor cons = cl.getConstructor(constructorParams);
return (Object) cons.newInstance(new Object[] { h });
} catch (NoSuchMethodException e) {
throw new InternalError(e.toString());
} catch (IllegalAccessException e) {
throw new InternalError(e.toString());
} catch (InstantiationException e) {
throw new InternalError(e.toString());
} catch (InvocationTargetException e) {
throw new InternalError(e.toString());
}
}

上面的方法,除了最重要的getProxyClass,其他都很容易理解.,那么下面开始读getProxyClass方法

public static Class<?> getProxyClass(ClassLoader loader,
Class<?>... interfaces)
throws IllegalArgumentException
{
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");//JDK想的果然比较到位,连interface传的太多都想到了..~!~
}

Class proxyClass = null;//这个就是最后要生成的二进制码,首先初始化一下

/* collect interface names to use as key for proxy class cache */
String[] interfaceNames = new String[interfaces.length];//这个存放的是对应的Interface的名字..

Set interfaceSet = new HashSet(); //这个HashSet 是为了检测interface重复记录的.

for (int i = 0; i < interfaces.length; i++) {
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
String interfaceName = interfaces[i].getName();
Class interfaceClass = null;
try {
interfaceClass = Class.forName(interfaceName, false, loader);//创建对应接口的二进制码,第二个参数false表示,不需要初始化
} catch (ClassNotFoundException e) {
}
if (interfaceClass != interfaces[i]) {//如果创建出来的二进制码和原来的接口不一样,表示这个接口对于这个classloader来说是不可见的
throw new IllegalArgumentException(
interfaces[i] + " is not visible from class loader");
}

/*
* Verify that the Class object actually represents an
* interface.
*/
if (!interfaceClass.isInterface()) {//如果不是接口,那么就抛异常,这里就规定了,我们必须通过接口来代理..或者说,必须面向接口编程
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}

/*
* Verify that this interface is not a duplicate.前面说,InterfaceSet是拿来判断对应的interface接口是否有重复的.这里的方法是:在循环interfaces的时候,把每个interface都添加到interfaceSet里..当然,在添加之前会判断,当前循环到的接口在InterfaceSet里是否有,如果已经有了,则抛异常,说这个接口重复了..没有,则添加..
*/
if (interfaceSet.contains(interfaceClass)) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
interfaceSet.add(interfaceClass);

interfaceNames[i] = interfaceName;//这句就是把每个接口名放到interfaceNames数组里..
}

/*
* Using string representations of the proxy interfaces as
* keys in the proxy class cache (instead of their Class
* objects) is sufficient because we require the proxy
* interfaces to be resolvable by name through the supplied
* class loader, and it has the advantage that using a string
* representation of a class makes for an implicit weak
* reference to the class.
*/
Object key = Arrays.asList(interfaceNames);//这里把Interface数组转换成list..这里直接写成了Object类型

/*
* Find or create the proxy class cache for the class loader.
*/
Map cache;//放缓存的Map
synchronized (loaderToCache) {//loaderToCache也是一个Map.它的key是classloader,对应的value是对应的缓存,也是一个HashMap.他把对应的不同的classloader放到loaderToCache里,如果下次还要调用这个方法创建代理,并传入的是同一个classloader,那么可以直接从cache里取..增加速度.当然,如果没有,则创建一条记录,放到loaderToCache里
cache = (Map) loaderToCache.get(loader);
if (cache == null) {
cache = new HashMap();
loaderToCache.put(loader, cache);
}
/*
* This mapping will remain valid for the duration of this
* method, without further synchronization, because the mapping
* will only be removed if the class loader becomes unreachable.
*/
}

/*
* Look up the list of interfaces in the proxy class cache using
* the key. This lookup will result in one of three possible
* kinds of values:
* null, if there is currently no proxy class for the list of
* interfaces in the class loader,
* the pendingGenerationMarker object, if a proxy class for the
* list of interfaces is currently being generated,
* or a weak reference to a Class object, if a proxy class for
* the list of interfaces has already been generated.
*/
synchronized (cache) {
/*
* Note that we need not worry about reaping the cache for
* entries with cleared weak references because if a proxy class
* has been garbage collected, its class loader will have been
* garbage collected as well, so the entire cache will be reaped
* from the loaderToCache map.
*/
do {
Object value = cache.get(key);//这里从cache里获取对应的Object..第一次执行的话,明显获取到的是Null..key表示的是用接口名字转换而来的list..这个可以看上面的代码.
if (value instanceof Reference) {
proxyClass = (Class) ((Reference) value).get();//如果已经能获取到了,那么,我们需要的二进制码文件就是这个获取到的
}
if (proxyClass != null) {
// proxy class already generated: return it
return proxyClass;
} else if (value == pendingGenerationMarker) {//这里的pendingGenerationMarker是一个静态常量,表示 new Object().JDK给出的 解释是,如果代理正在创建,那么等待他
// proxy class being generated: wait for it
try {
cache.wait();
} catch (InterruptedException e) {
/*
* The class generation that we are waiting for should
* take a small, bounded time, so we can safely ignore
* thread interrupts here.
*/
}
continue;
} else {
/*
* No proxy class for this list of interfaces has been
* generated or is being generated, so we will go and
* generate it now. Mark it as pending generation.
*/
cache.put(key, pendingGenerationMarker);//如果cache里获取到的对应于key的value是Null ,那么,就创建一个object对象放进去.上面说了,pendingGenerationMarker= new Object();
break;
}
} while (true);
}

try {
String proxyPkg = null; // package to define proxy class in 这个是代理类的包名

/*
* 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 (int i = 0; i < interfaces.length; i++) {
int flags = interfaces[i].getModifiers();//getModifiers()方法返回的是该接口的修饰类型,用Int类型表示
if (!Modifier.isPublic(flags)) {//如果不是public 的接口..

//我们可以这样理解,我们动态创建的代理,他的修饰类型必须是和接口的修饰类型是一样的,我们知道,接口可以是public或者默认,两种修饰类型.这里的判断如果不是public接口,那么,该接口肯定是默认的,如果是默认修饰类型,那么,它只能被同一个包下面的类看到,所以,就必须为该代理类创建一个包名..当然,如果是public的话,就没必要,因为,反正所有的类都能看到..
String name = interfaces[i].getName();
int n = name.lastIndexOf('.');
String pkg = ((n == -1) ? "" : name.substring(0, n + 1));//注意这里的N+1 ,其实是包括 "."的..比如 com.cjb.proxy.Proxy..它返回的就是"com.cjb.proxy."注意最后的那个点

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,这里可以看到,如果是public 的接口,对应代理类的包名就是"",也就是没有包名
proxyPkg = ""; // use the unnamed package
}

{
/*
* Choose a name for the proxy class to generate.
*/
long num;
synchronized (nextUniqueNumberLock) {
num = nextUniqueNumber++;//Num是一个计数器.用处是,创建代理的类名的时候用..我们可以看到,它是初始值是0.然后,每被调用一次,Num++.
}
String proxyName = proxyPkg + proxyClassNamePrefix + num;//proxyPkg是之前生成的包名, proxyClassNamePrefix 是一个静态常量,proxyClassNamePrefix="$Proxy".最后Num是计数器.也就是说,它的代理类的名字是从 $Proxy1 $Proxy2 $Proxy3一直在增长的,这样的话,就避免了重复.
/*
* Verify that the class loader hasn't already
* defined a class with the chosen name.
*/

/*
* Generate the specified proxy class.下面打红字的两个方法是最后生成代理对象的..但是,很悲剧的是,他是用native修饰的,也就是说,它是不是用java来实现的..也就是说,最最关键的地方,不是用java实现的...
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces);
try {
proxyClass = 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());
}
}
// add to set of all generated proxy classes, for isProxyClass
proxyClasses.put(proxyClass, null);

} finally {
/*
* We must clean up the "pending generation" state of the proxy
* class cache entry somehow. If a proxy class was successfully
* generated, store it in the cache (with a weak reference);
* otherwise, remove the reserved entry. In all cases, notify
* all waiters on reserved entries in this cache.
*/
synchronized (cache) {
if (proxyClass != null) {
cache.put(key, new WeakReference(proxyClass));
} else {
cache.remove(key);
}
cache.notifyAll();
}
}
return proxyClass;
}

可以看到,我们想看的,最重要的生成二进制码的方法,是native的..读了这么多源码,都是一些前期处理.关键的地方不知道..当然,虽然没看到关键的地方,但是对于它前面的处理的学习,也是非常有用的..看看JDK是怎么处理重名问题的,怎么处理cache.等等..