JDK动态代理案例实现：实现 InvocationHandler 接口重写 invoke 方法，其中包含一个对象变量和提供一个包含对象的构造方法；
public class MyInvocationHandler implements InvocationHandler {
Object target;//目标对象
public MyInvocationHandler(Object target){
this.target=target;
}
/**
* @param proxy 代理对象
* @param method 目标对象的目标方法
* @param args 目标方法的参数
*/
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("log");
return method.invoke(target,args);
}
}
public class MyInvocationHandlerTest {
public static void main(String[] args) {
//参数： 当前类的classLoader（保证MyInvocationHandlerTest当前类可用）
// 接口数组：通过接口反射得到接口里面的方法，对接口里面的所有方法都进行代理
// 实现的InvocationHandler：参数是目标对象
UserDao jdkproxy = (UserDao) Proxy.newProxyInstance(MyInvocationHandlerTest.class.getClassLoader(),
new Class[]{UserDao.class},new MyInvocationHandler(new UserService()));
jdkproxy.query("query");
//-----结果------
//log
//query
}
}
接下来查看 Proxy.newProxyInstance 源码探究它的实现过程：
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
//检查InvocationHandler是否为空，为空抛出空指针异常
Objects.requireNonNull(h);
//克隆拿到接口
final Class<?>[] intfs = interfaces.clone();
//进行安全校验
final SecurityManager sm = System.getSecurityManager();
//检查是否能被代理
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
* 得到代理类
*/
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
//通过构造方法得到代理类的对象
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;
}
});
}
//new得到代理对象
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);
}
}
从上面可以看出 Class<?> cl = getProxyClass0(loader, intfs); 这行代码最重要，它可以得到一个代理对象，下面是 Proxy#getProxyClass0 的源码：
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
//如果接口的代理类已经存在缓存中了，直接从缓存中取出来返回，如果不存在则通过ProxyClassFactory创建一个并放入缓存*下次使用
return proxyClassCache.get(loader, interfaces);
}
proxyClassCache.get() 的实现在 WeakCache#get() 中：
public V get(K key, P parameter) {
Objects.requireNonNull(parameter);
expungeStaleEntries();
//从缓存中取出代理类的key值，因为代理类的生成需要耗时间和性能，所有缓存起来方便下次直接使用
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 获取到接口信息
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;
//开始进来supplier是null，所以执行后面的代码创建一个Factory然后赋值给supplier
while (true) {
if (supplier != null) {
// supplier might be a Factory or a CacheValue<V> instance
//获取到代理类
V value = supplier.get();
if (value != null) {
//返回代理类
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);//通过接口信息和classLoader创建一个工厂
}
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;//将工厂赋值给supplier
} else {
// retry with current supplier
supplier = valuesMap.get(subKey);
}
}
}
}
下面查看 supplier.get()：WeakCache.Factory#get()
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 {
//创建获取到value
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);//根据代理类创建一个缓存对象
// put into reverseMap
reverseMap.put(cacheValue, Boolean.TRUE);//将代理放入缓存中
// 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;
}
接下来查看 Objects.requireNonNull() 中的 apply方法：Proxy.ProxyClassFactory#apply()
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
//获取到所有实现接口
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
//遍历所有接口
for (Class<?> intf : interfaces) {
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
Class<?> interfaceClass = null;
try {
//对接口再次装载，相当classLoader.loadClass
interfaceClass = Class.forName(intf.getName(), false, loader);
} catch (ClassNotFoundException e) {
}
//判断两个接口是不是同一个接口（同一个类加载器加载的）：判断对象是否是同一个
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());
}
}
//代理对象的包名
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();
//判断接口的类型是否是public，如果不是public，将代理类放到目标对象同一目录下
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
//默认定义包名
//public static final String PROXY_PACKAGE = "com.sun.proxy";
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
/*
* Choose a name for the proxy class to generate.
*/
long num = nextUniqueNumber.getAndIncrement();
//标识：private static final String proxyClassNamePrefix = "$Proxy";
//随机数（防止多线程测试相同的类名）：long num = nextUniqueNumber.getAndIncrement();
//代理类的名字：代理包名+ 标识+ 随机数
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
* 产生指定的代理类信息，是二进制信息，可以使用IO流输出代理类的Java文件（可查看前文有介绍）
* ProxyGenerator.generateProxyClass()是一个静态方法，所以可以外部直接调用
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
//产生代理类，返回一个class：将byte字节码转换成class
//defineClass0是一个native方法，由JVM实现的：private static native Class<?> defineClass0();
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());
}
}