JDK 动态代理
JDK动态代理只能代理接口，内部通过实现接口完成代理。
源码分析 java.lang.reflect.Proxy#newProxyInstance
java.lang.reflect.Proxy#newProxyInstance
/**
* loader 被代理类的加载器，用来加载代理类
* interfaces 被代理类继承的接口们
* h 需要增强处理的 Handler，代理类将 被代理类与h结合
**/
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
final Class<?>[] intfs = interfaces.clone();
// 系统安全性校验
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* 1 寻找或者生成代理类.开始分析
*/
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);
}
}
java.lang.reflect.Proxy#getProxyClass0
/**
* 生成代理类
*/
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
// 2.从缓存中获取代理类，接下来分析 proxyClassCache
// 代理类是什么时候放到缓存中的？如果生成的字节码文件？
// 想知道以上问题需要先了解 WeakCache
return proxyClassCache.get(loader, interfaces);
}
/**
* a cache of proxy classes
*/
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
// 上述代码调用了 proxyClassCache.get(loader, interfaces);
即 java.lang.reflect.WeakCache#get方法
/**
* java.lang.reflect.WeakCache#get 方法
* @param key
* @param parameter
* @return
*/
public V get(K key, P parameter) {
Objects.requireNonNull(parameter);
expungeStaleEntries();
//第一级缓存的key
Object cacheKey = java.lang.reflect.WeakCache.CacheKey.valueOf(key, refQueue);
// lazily install the 2nd level valuesMap for the particular cacheKey
// 懒加载第二级缓存
ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
if (valuesMap == null) {
// 防止并发，如果map中已经存在cacheKey对应的valueMap 返回，如果没有 放进去空集合
ConcurrentMap<Object, Supplier<V>> oldValuesMap
= map.putIfAbsent(cacheKey,
valuesMap = new ConcurrentHashMap<>());
//如果cacheKey有对应的oldValuesMap，赋值给valueMap
if (oldValuesMap != null) {
valuesMap = oldValuesMap;
}
}
// create subKey and retrieve the possible Supplier<V> stored by that
// subKey from valuesMap
// 根据subKeyFactory 获取subKey
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
// valuesMap中获取subKey对应value
Supplier<V> supplier = valuesMap.get(subKey);
java.lang.reflect.WeakCache.Factory factory = null;
while (true) {
// 如果subKey 对应value supplier（WeakCache.Factory） 不为null 直接get，调用valueFactory.apply
if (supplier != null) {
// supplier might be a Factory or a CacheValue<V> instance
// 第一次valueMap.subKey对应的value是Factory ，
// 当调用一次valueFactory.apply之后，valuesMap.subKey value会替换为CacheValue
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
// 构建WeakCache.Factory 继承了Supplier
if (factory == null) {
factory = new java.lang.reflect.WeakCache.Factory(key, parameter, subKey, valuesMap);
}
if (supplier == null) {
// 将subKey 对应factory 放入valuesMap中
supplier = valuesMap.putIfAbsent(subKey, factory);
//如果覆盖成功，supplier 返回一定为null，此时将factory赋值给supplier
if (supplier == null) {
// successfully installed Factory
supplier = factory;
}
// else retry with winning supplier
} else {
// supplier不为空，走到这一步说明旧的supplier是无效的，所以用新生成的factory进行替换
// 因为CacheValue是个弱引用，在动态代理中引用的是实际的代理类，如果该代理类不再使用被gc了
if (valuesMap.replace(subKey, supplier, factory)) {
// successfully replaced
// cleared CacheEntry / unsuccessful Factory
// with our Factory
supplier = factory;
} else {
// retry with current supplier
// 替换失败 则再次尝试用原来的supplier
supplier = valuesMap.get(subKey);
}
}
}
}
// 以上代码 看到了 subKey是keyFacotry.apply计算得到的，value呢？
是从哪里获取到的呢？这个需要看下supplier.get 即WeakCache.Factory.get
//com.mashibing.node.design.proxy.WeakCache.Factory
/**
* Factory即是Supplier，factroy.get最终调用valueFactory.apply方法
* 获取到value后通过CacheValue（也是Supplier）包装，
* 然后放到 valuesMap中
**/
private final class Factory implements Supplier<V> {
private final K key;
private final P parameter;
private final Object subKey;
private final ConcurrentMap<Object, Supplier<V>> valuesMap;
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
// 防止在调用过程supplier 被改变
return null;
}
// else still us (supplier == this)
// create new value
V value = null;
try {
//获取实际的value值
value = Objects.requireNonNull(valueFactory.apply(key, parameter));
} finally {
// 如果获取不到，及时移除
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;
// 一定获取到了返回值 则通过cacheValue包装
// wrap value with CacheValue (WeakReference)
java.lang.reflect.WeakCache.CacheValue<V> cacheValue = new java.lang.reflect.WeakCache.CacheValue<>(value);
// put into reverseMap
reverseMap.put(cacheValue, Boolean.TRUE);
// try replacing us with CacheValue (this should always succeed)
// 将valueFactory.apply返回值，实际subKey的value的弱引用放入到valuesMap中
// 注意CacheValue 继承了Value ，Value继承Supplier 所以在下一次
// 在 weekCache.map get出来的valuesMap.get(subKey)中获取的就是 CacheValue
// 所以 supplier might be a Factory or a CacheValue<V> instance
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;
}
}
WeekCache 看完了实现方式之后，回过头来继续看下中KeyFactory与ProxyClassFactory
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
/**
* KeyFactory 非常简单根据interface不同数量创建不同key不展开细说
* 但是可以确定Key extends WeakReference<Class<?>> 弱引用
**/
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);
}
}
}
ProxyClassFactory 相对复杂些,我们分析get方法
@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);
} 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;
/*
* 关注重点 生成字节码文件
* 本部分内容不具体研究，其中 private static final boolean saveGeneratedFiles = (Boolean)AccessController.doPrivileged(new GetBooleanAction("sun.misc.ProxyGenerator.saveGeneratedFiles"));
* System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
* 将二进制流以文件的形式保存下来，就是在这里启用的
* sun.misc.ProxyGenerator#generateProxyClass(java.lang.String, java.lang.Class<?>[], int)
*/
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());
}
}
}
