大家好👋，我是极客涛😎，上一篇中我们对代理模式有两大类，静态代理和动态代理，对于静态代理相信大家都信手拈来。对于动态代理还有两种实现，一种是java原生的Jdk代理，一种是Cglib方式。因为涉及到源码解读，所以我也将分两期完成，本期主要讲讲JDK动态代理的实现方式
先举个小例子，创建接口
public interface Father {
void eat();
}
创建实现类
public class Son implements Father{
@Override
public void eat() {
System.out.println("吃饭");
}
}
测试
public class ProxyTest {
public static void main(String[] args) {
ProxyTest test = new ProxyTest();
test.jdkProxy();
}
private void jdkProxy(){
Father son = new Son();
Father proxySon = (Father) Proxy.newProxyInstance(son.getClass().getClassLoader(), son.getClass().getInterfaces(), new InvocationHandler() {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("做饭");
Object invoke = method.invoke(son, args);
System.out.println("洗碗");
return invoke;
}
});
proxySon.eat();
}
输出结果
做饭
吃饭
洗碗
如上，是一个使用JDK动态代理的简单例子，通过Proxy类的静态方法newProxyInstance可以生成目标类的代理类，这里有几个疑问：
我们带着这几个问题看看源码
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
// 克隆被代理类的接口Class对象
final Class<?>[] intfs = interfaces.clone();
// 使用Java安全管理器校验程序，防止恶心代码
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
* 查找或生成指定的代理类。这里会使用缓存，缓存里没有就创建代理类，放放到缓存中
* 接下来我们进入这个方法看看
*/
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
* 使用指定的调用处理程序调用其构造函数。
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
// 获取代理类的构造器，这里会生成一个入参为InvocationHandler.class的构造器
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;
}
});
}
// 通过反射使用构造方法（带有InvocationHandler入参的构造方法）创建代理类的实例对象
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);
}
}
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
// 这里为什么是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
/**
* 如果给定加载器定义的代理类实现,给定的接口存在，这将简单地返回缓存的副本；否则，它将通过 ProxyClassFactory 创建代理类
* 在进入这个方法看看
/
return proxyClassCache.get(loader, interfaces);
}
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
// 其它代码先不关注，这里的apply方法会调用$ProxyClassFactory的apply方法
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;
...
}
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// prefix for all proxy class names
// 代理类名前缀
private static final String proxyClassNamePrefix = "$Proxy";
// next number to use for generation of unique proxy class names
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 {
// 循环获取被代理类的接口结合的Class对象
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();
// com.sun.proxy. + $Proxy + 0
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
* 生成指定的代理类文件，并判断是否需要持久化，这里只是普通的文件字节数组，jvm并不认识
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
// 定义代理类，加载到jvm中，生成真正可以使用的运行时代理类的Class对象，
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
/*
* A ClassFormatError here means that (barring bugs in the
* proxy class generation code) there was some other
* invalid aspect of the arguments supplied to the proxy
* class creation (such as virtual machine limitations
* exceeded).
*/
throw new IllegalArgumentException(e.toString());
}
}
}
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);这块代码可以生成代理类的字节数组，那么我们是不是可以看看生成的代理类到底长什么样呢？写个测试方法看看
private void write(){
Father father = new Son();
byte[] proxyArr = ProxyGenerator.generateProxyClass("$Proxy0", father.getClass().getInterfaces());
try {
Files.write(Paths.get("C:\\Users\\wxt\\Desktop\\test.class"), proxyArr);
} catch (IOException e) {
e.printStackTrace();
}
}
文件内容
// 代理类默认继承了Proxy类，实现了被代理类的接口
public final class $Proxy0 extends Proxy implements Father {
private static Method m1;
private static Method m3;
private static Method m2;
private static Method m0;
static {
try {
// 这里初始化4个成员变量
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m3 = Class.forName("com.wangxt.wxt.design.patterns.proxy.dynamic.Father").getMethod("eat");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
public $Proxy0(InvocationHandler var1) throws {
// 这里会调用父类的构造，并把InvocationHandler传递
super(var1);
}
// 这里是我们自定义的方法，其它方法道理相同
public final void eat() throws {
try {
// 可以看到，当我们调用代理类的方法时，实际上会调用父类的h(InvocationHandler)的invoke方法
// @Override
// public Object invoke(Object proxy, Method method, Object[] args) throws Throwable
// 所以我们重写InvocationHandler的invoke方法时传入的就是这几个参数
super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
...
}
好了，到这里基本信息都看完了，我们做个总结
准备代理类的描述数据
创建代理类（实现接口）的字节码文件
通过ClassLoader将代理类的字节数组加载到JVM中
创建代理类的实例对象，执行对象的目标方法
我们回过头来，在想想最开始的3个问题：
因为JDK动态代理生成的代理对象默认继承了Proxy类，又因为Java中是单继承多实现，所以导致了JDK动态代理无法代理实现类，只能代理接口；而且我们通过观察Proxy类，维护了InvocationHandler h成员变量并提供了相应的方法，然后通过子类对InvocationHandler进行透传，Proxy对其进行方法执行。其实理论上只要我们把InvocationHandler提出来，不由Proxy进行维护，也就不需要继承Proxy类，就可以对实现类进行代理，可能作者基于面向接口开发的实际场景，以及抽象思维才这么进行设计的吧。