Java动态代理Proxy应用和底层源码详细分析
作者：shandongwill
Java动态代理是一种在运行时生成代理类的机制,用于代替手动编写代理类的过程,这篇文章主要给大家介绍了关于Java动态代理Proxy应用和底层源码详细分析的相关资料,需要的朋友可以参考下
前言
Java Proxy主要用于创建动态代理实例，这些实例实现了指定的一组接口，并在调用方法时将调用转发给指定的调用处理器（InvocationHandler）。这种机制常用于实现AOP（面向切面编程）框架、RPC（远程过程调用）框架等，以及任何需要动态改变对象行为的场景。
一、Proxy代码示例
在这个示例中，我们创建了一个实现了 MyInterface 接口的匿名内部类对象 myObject。然后，我们创建了一个 MyInvocationHandler 对象，并将 myObject 作为目标对象传递给它。接下来，我们使用 Proxy.newProxyInstance 方法创建了一个代理对象 proxy，并将 MyInterface 接口和 handler 作为参数传递给它。最后，我们调用代理对象的 doSomething 方法，这个方法调用会转发给 handler 的 invoke 方法，并在其中添加自定义逻辑。
interface MyInterface {
void doSomething();
}
class MyInvocationHandler implements InvocationHandler {
private Object target;
public MyInvocationHandler(Object target) {
this.target = target;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("Before method call");
Object result = method.invoke(target, args);
System.out.println("After method call");
return result;
}
}
public class DynamicProxyExample {
public static void main(String[] args) {
MyInterface myObject = new MyInterface() {
@Override
public void doSomething() {
System.out.println("Doing something");
}
};
MyInvocationHandler handler = new MyInvocationHandler(myObject);
MyInterface proxy = (MyInterface) Proxy.newProxyInstance(
MyInterface.class.getClassLoader(),
new Class<?>[]{MyInterface.class},
handler
);
proxy.doSomething(); // This will call the proxy's doSomething method, which forwards to the handler's invoke method.
}
}二、底层源码分析
- 从newProxyInstance这个入口来看看Proxy的重点方法。
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h) throws IllegalArgumentException {
//将接口clone，之后对此clone类进行操作
final Class<?>[] intfs = interfaces.clone();
//进行权限检查
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
* 根据传入的类加载器和接口类数组，生成相应的代理类
*/
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
//获取构造
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doexposingd(new PrivilegedAction<Void>() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
//返回代理对象
return cons.newInstance(new Object[]{h});
} catch (IllegalAccessException|InstantiationException e) {
throw new InternalError(e.toString(), e);
}
}
- 再来看查找/生成的代理类getProxyClass0
/**
* Generate a proxy class. Must call the checkProxyAccess method
* to perform permission checks before calling this.
*/
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
// 如果由实现给定接口的给定加载器定义的代理类存在，将返回缓存的副本;
// 否则，它将通过ProxyClassFactory创建代理类
return proxyClassCache.get(loader, interfaces);
}
/**
* a cache of proxy classes
* 动态代理类的弱缓存容器
* KeyFactory:根据接口的数量，映射一个最佳的key生成函数,其中表示接口的类对象被弱引用；
* 也就是key对象被弱引用继承自WeakReference（key0、key1、key2、keyX）， 保存接口密钥(hash值)
* ProxyClassFactory：生成动态类的工厂
*/
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
- WeakCache.get(loader, interfaces)
// key 表示缓存键，parameter 表示参数。它的返回值类型为 V，表示缓存值。
public V get(K key, P parameter) {
// 判断parameter不能为空
Objects.requireNonNull(parameter);
expungeStaleEntries();
Object cacheKey = CacheKey.valueOf(key, refQueue);
// 该方法将 key 和 refQueue 作为参数调用 CacheKey.valueOf() 方法，生成一个缓存键 cacheKey，并从缓存中获取与该缓存键相关的 valuesMap。
// 如果 valuesMap 不存在，则创建一个新的 ConcurrentHashMap 对象，并将其加入到缓存中。valuesMap 是一个 ConcurrentMap 对象，它用于存储缓存键下的子键和值的提供者，其中子键由 subKeyFactory 参数生成。
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;
}
}
// 利用Key的工厂类根据key和parameter参数生成键
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
// 根据该键获取到Supplier对象，表示结果的提供者。Supplier接口包含了一个get方法
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;
while (true) {
if (supplier != null) {
// supplier可能是一个工厂类实例，或者是一个CacheValue<V>实例
V value = supplier.get();
if (value != null) {
return value;
}
}
if (factory == null) {
// 核心的创建代理类的原理在Factory类里实现
// Factory类实际上也是Supplier接口的实现类，实现了get方法
factory = new Factory(key, parameter, subKey, valuesMap);
}
if (supplier == null) {
supplier = valuesMap.putIfAbsent(subKey, factory);
if (supplier == null) {
// 将上面的Factory类对象赋值给suppler，在下一次循环时，就可以调用Factory类的get方法来生成代理类
supplier = factory;
}
} else {
if (valuesMap.replace(subKey, supplier, factory)) {
supplier = factory;
} else {
// retry with current supplier
supplier = valuesMap.get(subKey);
}
}
}
}
代码核心逻辑：
factory = new Factory(key, parameter, subKey, valuesMap);：// 创建Factory对象，Factory类是Supplier接口的实现类
supplier = factory; // 当缓存中不存在代理类时，将supplier=factory对象
V value = supplier.get(); // 调用Factory类的get方法来创建代理类
- Factory.get()
public synchronized V get() {
V value = null;
try {
//valueFactory实际上是一个ProxyClassFactory实例，调用的是ProxyClassFactory.apply方法
value = Objects.requireNonNull(valueFactory.apply(key, parameter));
} finally {
if (value == null) {
valuesMap.remove(subKey, this);
}
}
// 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)
if (!valuesMap.replace(subKey, this, cacheValue)) {
throw new AssertionError("Should not reach here");
}
return value;
}
}
- ProxyClassFactory.apply
ProxyClassFactory是Proxy的静态内部类，apply 用于生成代理类的Class对象。
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) {
}
/*
* Verify that the Class object actually represents an interface.
*/
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
}
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;
/*
* Generate the specified proxy class.
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
}
- ProxyGenerator.generateProxyClass
generateProxyClass 生成代理类字节码
private byte[] generateClassFile() {
//生成hashCode方法
this.addProxyMethod(hashCodeMethod, Object.class);
//生成equals方法
this.addProxyMethod(equalsMethod, Object.class);
//生成toString方法
this.addProxyMethod(toStringMethod, Object.class);
Class[] var1 = this.interfaces;
int var2 = var1.length;
int var3;
Class var4;
for(var3 = 0; var3 < var2; ++var3) {
var4 = var1[var3];
Method[] var5 = var4.getMethods();
int var6 = var5.length;
for(int var7 = 0; var7 < var6; ++var7) {
Method var8 = var5[var7];
this.addProxyMethod(var8, var4);
}
}
Iterator var11 = this.proxyMethods.values().iterator();
List var12;
while(var11.hasNext()) {
var12 = (List)var11.next();
checkReturnTypes(var12);
}
Iterator var15;
try {
this.methods.add(this.generateConstructor());
var11 = this.proxyMethods.values().iterator();
while(var11.hasNext()) {
var12 = (List)var11.next();
var15 = var12.iterator();
while(var15.hasNext()) {
ProxyGenerator.ProxyMethod var16 = (ProxyGenerator.ProxyMethod)var15.next();
this.fields.add(new ProxyGenerator.FieldInfo(var16.methodFieldName, "Ljava/lang/reflect/Method;", 10));
this.methods.add(var16.generateMethod());
}
}
this.methods.add(this.generateStaticInitializer());
} catch (IOException var10) {
throw new InternalError("unexpected I/O Exception", var10);
}
if (this.methods.size() > 65535) {
throw new IllegalArgumentException("method limit exceeded");
} else if (this.fields.size() > 65535) {
throw new IllegalArgumentException("field limit exceeded");
} else {
this.cp.getClass(dotToSlash(this.className));
this.cp.getClass("java/lang/reflect/Proxy");
var1 = this.interfaces;
var2 = var1.length;
for(var3 = 0; var3 < var2; ++var3) {
var4 = var1[var3];
this.cp.getClass(dotToSlash(var4.getName()));
}
this.cp.setReadOnly();
ByteArrayOutputStream var13 = new ByteArrayOutputStream();
DataOutputStream var14 = new DataOutputStream(var13);
try {
var14.writeInt(-889275714);
var14.writeShort(0);
var14.writeShort(49);
this.cp.write(var14);
var14.writeShort(this.accessFlags);
var14.writeShort(this.cp.getClass(dotToSlash(this.className)));
var14.writeShort(this.cp.getClass("java/lang/reflect/Proxy"));
var14.writeShort(this.interfaces.length);
Class[] var17 = this.interfaces;
int var18 = var17.length;
for(int var19 = 0; var19 < var18; ++var19) {
Class var22 = var17[var19];
var14.writeShort(this.cp.getClass(dotToSlash(var22.getName())));
}
var14.writeShort(this.fields.size());
var15 = this.fields.iterator();
while(var15.hasNext()) {
ProxyGenerator.FieldInfo var20 = (ProxyGenerator.FieldInfo)var15.next();
var20.write(var14);
}
var14.writeShort(this.methods.size());
var15 = this.methods.iterator();
while(var15.hasNext()) {
ProxyGenerator.MethodInfo var21 = (ProxyGenerator.MethodInfo)var15.next();
var21.write(var14);
}
var14.writeShort(0);
return var13.toByteArray();
} catch (IOException var9) {
throw new InternalError("unexpected I/O Exception", var9);
}
}
}总结
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