Java代理2 动态代理的实现原理分析
上篇Java代理（一）：代理和动态代理的基础与使用中得知通过调用静态方法Proxy.newProxyInstance()可以创建动态代理，之后会调用InvocationHandler接口的一个实现DynamicProxyHandler对象的invoker方法。 故这里我们主要探寻这两个方法的源码实现。 本篇所有源码基于JDK1.8.
proxy的class文件生成方式
Proxy.newProxyInstance
上篇中也提到该方法存在三个参数
- ClassLoader loader：一个类加载器，用于定义代理类的类加载器，通常可以从已经被加载的对象获取其类加载器，然后传递给它。
- Class<?>[] interfaces：代理类要实现的接口列表
- InvocationHandler h： 关联的调用处理器引用，即InvocationHandler接口的一个实现，如DynamicProxyHandler，代表的是当我这个动态代理对象在调用方法的时候，会关联到哪一个InvocationHandler对象上
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
// 检测指定的对象引用不为空（即h不为空），若为空则抛出NullPointerException异常。
Objects.requireNonNull(h);
// 拷贝代理类要实现的接口列表,之后的操作均是使用该拷贝intfs，而不会涉及原列表interfaces。
final Class<?>[] intfs = interfaces.clone();
// 获取一个安全管理器对象security，这个对象所属的目录为java.lang.SecurityManager
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
// 检测创建代理类需要的权限
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
// 关键代码1：获得与传入的指定类装载器（loader）和接口列表（intfs）相关的代理类类型对象
/*
* Look up or generate the designated proxy class.
*/
Class<?> cl = getProxyClass0(loader, intfs);
// 通过反射获取构造函数对象并生成代理类cl的实例
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
// 检测权限
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
// 关键代码2：通过反射获取该代理类的构造函数
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;
}
});
}
// 关键代码3：返回这个新的代理类的一个实例
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);
}
}
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checkProxyAccess
检测创建代理类需要的权限,此处涉及到的SecurityManager安全管理器内容，后期单开篇细究。
private static void checkProxyAccess(Class<?> caller,
ClassLoader loader,
Class<?>... interfaces)
{
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
ClassLoader ccl = caller.getClassLoader();
if (VM.isSystemDomainLoader(loader) && !VM.isSystemDomainLoader(ccl)) {
sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
}
ReflectUtil.checkProxyPackageAccess(ccl, interfaces);
}
}
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Class<?> cl = getProxyClass0(loader, intfs);
关键代码1，用于从缓存获取或重新生成所需要这个代理类。 通过下面具体实现可见，这方法会根据类加载器与接口类型到缓存中寻找一个代理类的Class对象，如果没有就通过ProxyClassFactory创建一个新的。 由此可见，在传入loader参数的时候，需要跟传入的interface相关，所以比较常见的做法就是用接口或其实现类getClass().getClassLoader()方法（或如上篇的Animal.class.getClassLoader()）获得一个类加载器
/**
* Generate a proxy class. Must call the checkProxyAccess method
* to perform permission checks before calling this.
*/
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
// 校验接口数量
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
// 从缓存中获取，如果没有就通过ProxyClassFactory创建
// 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
return proxyClassCache.get(loader, interfaces);
}
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proxyClassCache.get(loader, interfaces);
继续跟入proxyClassCache.get(loader, interfaces); 该方法是java.lang.reflect.WeakCache的get方法。 此处不深究里面提到的缓存，后期单开篇细究。 方法中最终通过 V value = supplier.get();获取动态代理类，其中supplier是Factory,这个类定义在WeakCach的内部。
public V get(K key, P parameter) {
Objects.requireNonNull(parameter);
expungeStaleEntries();
// 通过类加载器classLoader生成以及一级缓存key
Object cacheKey = CacheKey.valueOf(key, refQueue);
// lazily install the 2nd level valuesMap for the particular cacheKey
// 获取二级缓存，不存在则新建
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
// 生成二级缓存key
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
// 通过key获取二级缓存value,即缓存的代理类。不存在则新建代理类并加入缓存。
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;
while (true) {
if (supplier != null) {
// supplier might be a Factory or a CacheValue<V> instance
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
if (factory == null) {
factory = new Factory(key, parameter, subKey, valuesMap);
}
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;
} else {
// retry with current supplier
supplier = valuesMap.get(subKey);
}
}
}
}
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supplier.get()
现在进入上面supplier.get()内部一探究竟。 发现内部关键语句 value = Objects.requireNonNull(valueFactory.apply(key, parameter)); 其中，valueFactory是ProxyClassFactory类型
@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
return null;
}
// else still us (supplier == this)
// create new value
V value = null;
try {
// 这里又通过valueFactory.apply(key, parameter)得到value进行返回
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)
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;
}
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valueFactory.apply(key, parameter)
进入java.lang.reflect.Proxy.ProxyClassFactory#apply 在这里面可以找到生成字节码（即代理类）的语句。
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
for (Class<?> intf : interfaces) {
// 确保该loader加载的此类（intf）
/*
* 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");
}
// 确保是一个接口
/*
* Verify that the Class object actually represents an
* interface.
*/
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
// 确保接口没重复
/*
* 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;
// 验证所有非公共的接口在同一个包内；公共的就无需处理.
/*
* 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 + ".";
}
// 为代理类生成一个名字，防止重复
/*
* Choose a name for the proxy class to generate.
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
// 生成指定代理类
/*
* Generate the specified proxy class.
*/
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());
}
}
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关键语句有：
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
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ProxyGenerator.generateProxyClass( proxyName, interfaces, accessFlags)
进入sun.misc.ProxyGenerator#generateProxyClass(java.lang.String, java.lang.Class<?>[])。 这里面便是得到class文件byte内容的逻辑。
public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
//动态生成代理类的字节码,非常复杂
final byte[] var4 = var3.generateClassFile();
//根据配置，判断class文件是否存储在本地
if (saveGeneratedFiles) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
try {
int var1 = var0.lastIndexOf(46);
Path var2;
if (var1 > 0) {
Path var3 = Paths.get(var0.substring(0, var1).replace('.', File.separatorChar));
Files.createDirectories(var3);
var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
} else {
var2 = Paths.get(var0 + ".class");
}
Files.write(var2, var4, new OpenOption[0]);
return null;
} catch (IOException var4x) {
throw new InternalError("I/O exception saving generated file: " + var4x);
}
}
});
}
//返回代理类的字节码
return var4;
}
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proxy的class文件内容
可以自定义存储硬盘位置 调用sun.misc.ProxyGenerator#generateProxyClass(java.lang.String, java.lang.Class<?>[])函数即可
public static void showProxyClass() {
String path = "./$Proxy0.class";
byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0",
Dog.class.getInterfaces());
FileOutputStream out = null;
try {
out = new FileOutputStream(path);
out.write(classFile);
out.flush();
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
out.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
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在mian函数中调用，即可获取到生成的$Proxy0.class类
public final class $Proxy0 extends Proxy implements Animal {
private static Method m1;
private static Method m3;
private static Method m2;
private static Method m4;
private static Method m0;
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
public final boolean equals(Object var1) throws {
try {
return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final void doBark() throws {
try {
super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final String toString() throws {
try {
return (String)super.h.invoke(this, m2, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final void somethingElse(String var1) throws {
try {
super.h.invoke(this, m4, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final int hashCode() throws {
try {
return (Integer)super.h.invoke(this, m0, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m3 = Class.forName("Others.base.SimpleProxy.Animal").getMethod("doBark");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m4 = Class.forName("Others.base.SimpleProxy.Animal").getMethod("somethingElse", Class.forName("java.lang.String"));
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
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proxy类对象生成方式
可以看最上面java.lang.reflect.Proxy#newProxyInstance中的详情，里面便是生成方式。 简单说是生成了代理对象的字节码文件后,根据构造函数生成对象
//生成字节码，获取类文件
Class<?> cl = getProxyClass0(loader, intfs);
//省略
//反射，根据构造函数，生成对象，构造参数为InvocationHandler h
return cons.newInstance(new Object[]{h});
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对应的$Proxy0构造函数为：
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
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其会调用父级的，即：
protected Proxy(InvocationHandler h) {
Objects.requireNonNull(h);
this.h = h;
}
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此处的h即为之前传入的InvocationHandler实现类DynamicProxyHandler
invoke方法触发
由方法doBark()分析可见，代理类实现了委托类的方法，也就是说 代理对象触发doBark方法时，调用InvocationHandler的m3方法
public final void doBark() throws {
try {
super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
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其m3的定义在下面
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m3 = Class.forName("Others.base.SimpleProxy.Animal").getMethod("doBark");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m4 = Class.forName("Others.base.SimpleProxy.Animal").getMethod("somethingElse", Class.forName("java.lang.String"));
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
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可见m3是通过反射获取到的Animal的doBark方法。
m3 = Class.forName("Others.base.SimpleProxy.Animal").getMethod("doBark");
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然后看我们自定义的实现Others.base.SimpleProxy.DynamicProxyDemo.DynamicProxyHandler#invoke 三个参数分别对应：
- Object proxy 代理类
- Method method 方法
- Object[] args 方法method的参数
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("*** proxy: " + proxy.getClass() + " , method: " + method + " , args: " + args);
if (args != null){
for (Object arg: args){
PrintUtill.println(" " + arg);
}
}
//通过反射，把proxy的代理类 转交给 实现类上
return method.invoke(proxied, args);
}
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代理类调用doBark方法，实际是利用父类InvocationHandler属性h,传入doBark这个method到h实现类DynamicProxyHandler的invoke中， 在自定义的InvocationHandler即DynamicProxyHandler中，再利用反射，利用实现类Dog完成doBark操作，最终完成了代理类 到 实现类的调用。
参考资料
动态代理
java动态代理作用及源码分析 Java JDK 动态代理使用及实现原理分析 代理 3 动态代理
System.getSecurityManager()
java之jvm学习笔记四(安全管理器) 打开Java中的SecurityManager
Reflection.getCallerClass()
AccessController.doPrivileged
关于AccessController.doPrivileged 对AccessController.doPrivileged一点了解
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