JDK动态代理，通过实现被代理类的所有接口，生成一个字节码文件后构造一个代理对象，通过持有反射构造被代理类的一个实例，再通过invoke反射调用被代理类实例的方法，来实现代理。
代码入口Proxy.newProxyInstance()：
public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException {
//handler 对象不能是空
Objects.requireNonNull(h);
//被代理对象实现的所有接口
final Class<?>[] intfs = interfaces.clone();
//安全控制，可以不用管
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
*/
//生成指定的代理对象的类，这个才是重点方法
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;
//如果代理对象的类或者修饰符不是 public 的， 执行 PrivilegedAction.run 方法。这里设置当前构造函数为访问权限
if (!Modifier.isPublic(cl.getModifiers())) {
//启用特权，执行指定的 PrivilegedAction。
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
//使用 InvocationHandler 作为参数调用构造方法来获得代理类的实例
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);
}
}
getProxyClass0() 源码：
private static Class<?> getProxyClass0(ClassLoader loader, Class<?>... interfaces) {
//被代理对象接口数量不能大于 65535
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
//class loader 实现的接口代理类定义已经存在，那么直接返回；否则通过 ProxyClassFactory 生成proxy class。（可以自己翻译）
return proxyClassCache.get(loader, interfaces);
}创建proxyClassCache, 是直接new 的，指定 KeyFactory 和指定 ProxyClassFactory
//两个参数要说一下：分别对应 Key工厂和代理类工厂，看名字也知道生成key 和 生成代理类class对象的
private static final WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
注：所以下面的 WeakCache 泛型对应：<ClassLoader, Class<?>[], Class<?>>
proxyClassCache.get() 源码
final class WeakCache<K, P, V> {
//
private final ReferenceQueue<K> refQueue = new ReferenceQueue<>();
//Map<类加载器，Map<接口数组对象key，代理类工厂Factory或代理类包装对象LookupValue>， 类加载器，已经加载的类class
//可以把 Supplier<V> 看成生成类的class，调用get 方法就返回class
private final ConcurrentMap<Object, ConcurrentMap<Object, Supplier<V>>> map = new ConcurrentHashMap<>();
//当前key所对应的Supplier<V> ，是否已经生成了class
private final ConcurrentMap<Supplier<V>, Boolean> reverseMap = new ConcurrentHashMap<>();
//创建 WeakCache 时指定的 KeyFactoryt 和 ProxyClassFactory
private final BiFunction<K, P, ?> subKeyFactory;
private final BiFunction<K, P, V> valueFactory;
//参数：key：class loader, paramer：接口class 数组
public V get(K key, P parameter) {
//判断不是null
Objects.requireNonNull(parameter);
//去除老的元素
expungeStaleEntries();
//缓存的key，hash 值计算方式为：System.identityHashCode(key)
Object cacheKey = CacheKey.valueOf(key, refQueue);
// lazily install the 2nd level valuesMap for the particular cacheKey
//类加载器，已经加载的类
ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
if (valuesMap == null) {
//map中不存在 cacheKey，new 一个ConcurrentHashMap 放入到 map中，并返回
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
//根据 key 生成 subKey，并在valuesMap 中查找Supplier<V>
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;
while (true) {
//如果supplier 在缓存中已存在，则返回。 supplier 是一个Factory 或者是 CacheValue<V>，这两个都是 Supplier 的子类。
if (supplier != null) {
// supplier might be a Factory or a CacheValue<V> instance
V value = supplier.get();
if (value != null) {
return value;
}
}
//如果supplier 在缓存中不存在，则生成，并放到缓存(valuesMap)中
// 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) {
//创建一个工厂，用来生成 supplier
factory = new Factory(key, parameter, subKey, valuesMap);
}
if (supplier == null) {
//valuesMap 是ConcurrentMap，所以在这里放入之后要再检查一遍，防止并发问题。
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
把 valuesMap 中的 supplier 替换为 factory
supplier = factory;
} else {
// retry with current supplier
//重新从valuesMap 获取值
supplier = valuesMap.get(subKey);
}
}
}
}
//省略代码.....
}上面代码只是生成了动态代理类class的载体 Supplier<V>，获取动态代理类的class，还要看方法supplier.get()，
第一次生成动态代理类的时候 Supplier<V> 就是 Factory 对象，所以就看 Factory.get() 方法
private final class Factory implements Supplier<V> {
//class loader
private final K key;
//Class[]
private final P parameter;
//根据key 生成的 subkey，是 valuesMap的key
private final Object subKey;
private final ConcurrentMap<Object, Supplier<V>> valuesMap;
//省略构造函数
@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
//返回nul 是为了在外层(WeakCache.get()方法) 循环调用，可能会因为并发造成 supplier != this(这个Factory 是new 的)
return null;
}
// else still us (supplier == this)
// create new value
V value = null;
try {
//这里才调用 valueFactory 生成动态代理类class
//在这里又回到了 创建proxyClassCache的地方，创建proxyClassCache时指定的 ProxyClassFactory，前面做的工作都是缓存操作等。
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)
//缓存value 值，这里缓存的就是 CacheValue<V>
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;
}
}
ProxyClassFactory.apply() 方法是如何生成动态代理类的class
//ProxyClassFactory类是 Proxy 内部类
private static final class ProxyClassFactory implements BiFunction<ClassLoader, Class<?>[], Class<?>>{
// prefix for all proxy class names
//动态代理类类名前缀
private static final String proxyClassNamePrefix = "$Proxy";
// next number to use for generation of unique proxy class names
//动态代理类类名前缀数字
private static final AtomicLong nextUniqueNumber = new AtomicLong();
@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 {
//加载intf的 class 对象
interfaceClass = Class.forName(intf.getName(), false, loader);
} catch (ClassNotFoundException e) {
}
//接口的class 不是同一个class loader 加载的
if (interfaceClass != intf) {
throw new IllegalArgumentException(
intf + " is not visible from class loader");
}
/*
* Verify that the Class object actually represents an
* interface.
*/
//加载intf的 不是一个接口
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* Verify that this interface is not a duplicate.
*/
//有重复的 interfaceClass
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
//动态代理类的包名
String proxyPkg = null; // package to define proxy class in
//动态代理类的签名
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.
*/
//生成动态代理类的包名和签名
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");
}
}
}
//生成动态代理类的包名是空的，使用默认的包名
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.
*/
//生成动态代理类的类名，如：XX$Proxy1 XX$Proxy2
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
*/
//生成动态代理类的字节数组
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);
try {
//根据类字节数组，加载类
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());
}
}
}生成动态代理类的字节数组 ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags) ，生成class 实现接口所有的方法，生成一个构造方法，构造方法参数就是InvocationHandler。再继续看吧