JDK动态代理详解
java动态代理类
Java动态代理类位于java.lang.reflect包下，一般主要涉及到以下两个类：
InvocationHandler
该类是个接口，仅定义了一个方法
public interface InvocationHandler {
/**
* Processes a method invocation on a proxy instance and returns
* the result. This method will be invoked on an invocation handler
* when a method is invoked on a proxy instance that it is
* associated with.
*
* @param proxy the proxy instance that the method was invoked on
*
* @param method the {@code Method} instance corresponding to
* the interface method invoked on the proxy instance. The declaring
* class of the {@code Method} object will be the interface that
* the method was declared in, which may be a superinterface of the
* proxy interface that the proxy class inherits the method through.
*
* @param args an array of objects containing the values of the
* arguments passed in the method invocation on the proxy instance,
* or {@code null} if interface method takes no arguments.
* Arguments of primitive types are wrapped in instances of the
* appropriate primitive wrapper class, such as
* {@code java.lang.Integer} or {@code java.lang.Boolean}.
*
* @return the value to return from the method invocation on the
* proxy instance. If the declared return type of the interface
* method is a primitive type, then the value returned by
* this method must be an instance of the corresponding primitive
* wrapper class; otherwise, it must be a type assignable to the
* declared return type. If the value returned by this method is
* {@code null} and the interface method's return type is
* primitive, then a {@code NullPointerException} will be
* thrown by the method invocation on the proxy instance. If the
* value returned by this method is otherwise not compatible with
* the interface method's declared return type as described above,
* a {@code ClassCastException} will be thrown by the method
* invocation on the proxy instance.
*
* @throws Throwable the exception to throw from the method
* invocation on the proxy instance. The exception's type must be
* assignable either to any of the exception types declared in the
* {@code throws} clause of the interface method or to the
* unchecked exception types {@code java.lang.RuntimeException}
* or {@code java.lang.Error}. If a checked exception is
* thrown by this method that is not assignable to any of the
* exception types declared in the {@code throws} clause of
* the interface method, then an
* {@link UndeclaredThrowableException} containing the
* exception that was thrown by this method will be thrown by the
* method invocation on the proxy instance.
*
* @see UndeclaredThrowableException
*/
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable;
}
在实际使用时，第一个参数obj一般是指代理类，method是被代理的方法，args为该方法的参数数组。第一个参数基本上不会用到。
Proxy
该类即为动态代理类，其中主要包含以下内容
- protected Proxy(InvocationHandler h)：构造函数，用于给内部的h赋值
- static Class getProxyClass (ClassLoaderloader, Class[] interfaces)：获得一个代理类，其中loader是类装载器，interfaces是真实类所拥有的全部接口的数组
- static Object newProxyInstance(ClassLoaderloader, Class[] interfaces, InvocationHandler h)：返回代理类的一个实例，返回后的代理类可以当作被代理类使用(可使用被代理类的在Subject接口中声明过的方法)
在使用动态代理类时，我们必须实现InvocationHandler接口
动态代理步骤
- 创建一个实现接口InvocationHandler的类，它必须实现invoke方法
- 创建被代理的类以及接口
- 通过Proxy的静态方法
newProxyInstance(ClassLoaderloader, Class[] interfaces, InvocationHandler h)创建一个代理
- 通过代理调用方法
使用
- 需要动态代理的接口
/**
* @author Date: 2017/5/16 Time: 10:30.
*/
public interface Student {
void study();
}
- 需要代理的实际对象
/**
* @author Date: 2017/5/16 Time: 10:39.
*/
public class GoodStudent implements Student {
public void study() {
System.out.println("study hard");
}
}
- 调用处理器实现类
/**
* @author Date: 2017/5/16 Time: 10:37.
*/
public class ProxyHandler implements InvocationHandler {
private Object student;
public ProxyHandler(Object student) {
this.student = student;
}
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("hello before");
Object result = method.invoke(student,args);
System.out.println("bye after");
return result;
}
}
该调用处理器实现类构造函数接收一个Object对象。在invoke方法中method.invoke(student,args)是对被代理对象方法的调用，在该调用前后分别输出了语句。
- 测试
/**
* @author Date: 2017/5/16 Time: 10:31.
*/
public class ProxyTest {
public static void main(String[] args) {
GoodStudent goodStudent = new GoodStudent();
Student proxyStudent =
(Student) Proxy.newProxyInstance(Student.class.getClassLoader(),
new Class[] {Student.class}, new ProxyHandler(goodStudent));
proxyStudent.study();
}
}
输出
hello before
study hard
bye after
可以看到study hard是由goodStudy对象的study方法输出的，而前后的输出则是调用处理器实现类中增加的。
原理
从代码中可以看出关键点在于以下这段代码
Student proxyStudent =
(Student) Proxy.newProxyInstance(Student.class.getClassLoader(),
new Class[] {Student.class}, new ProxyHandler(goodStudent));
来看看newProxyInstance的源码
public static Object newProxyInstance(ClassLoader loader,
Class[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
final Class[] intfs = interfaces.clone();
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
*/
//重点是cl怎么来的
Class cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
//cons是cl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() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
//最后返回的实例是由cons构造函数构造出来的
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);
}
}
可以看到该方法返回的实例是cl的构造函数构造出来的，那我们重点看看cl是怎么来的。
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
return proxyClassCache.get(loader, interfaces);
}
可以看到这里对传入的interfaces数组长度有限制，不能超过65535.最后的数据都是从proxyClassCache缓存中获取的，来看看这个缓存的定义。
private static final WeakCache[], Class>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
缓存中的对象则是由ProxyClassFactory构造的。
private static final class ProxyClassFactory
implements BiFunction[], Class>
{
// prefix for all proxy class names
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, 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) {
}
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();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* 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());
}
}
}
重点在于这一句
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
这一句生成了代理类的字节码。接下来我们着重分析该方法做了什么
generateProxyClass
我们可以将ProxyGenerator为我们生成的字节码保存在磁盘中，然后通过反编译看看其实现。代码如下：
/**
* @author Date: 2017/5/16 Time: 10:31.
*/
public class ProxyTest {
public static void main(String[] args) {
createProxyClassFile();
}
public static void createProxyClassFile() {
String name = "ProxyStudent";
byte[] data = ProxyGenerator.generateProxyClass(name, new Class[] {Student.class});
try {
FileOutputStream out = new FileOutputStream(name + ".class");
out.write(data);
out.close();
} catch (Exception e) {
e.printStackTrace();
}
}
}
上述代码会替我们生成一个Student的代理类，并保存在ProxyStudent.class文件中，类名为ProxyStudent。
来看看反编译后的结果
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
public final class ProxyStudent extends Proxy implements Student {
private static Method m1;
private static Method m3;
private static Method m2;
private static Method m0;
public ProxyStudent(InvocationHandler var1) throws {
super(var1);
}
public final boolean equals(Object var1) throws {
try {
return ((Boolean)super.h.invoke(this, m1, new Object[]{var1})).booleanValue();
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final void study() 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 int hashCode() throws {
try {
return ((Integer)super.h.invoke(this, m0, (Object[])null)).intValue();
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[]{Class.forName("java.lang.Object")});
m3 = Class.forName("com.test.Student").getMethod("study", new Class[0]);
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
可以看到该类继承了Proxy类，并实现了Student接口。这就是为什么我们能将其实例转化为代理接口对象。该类包含了4个Method对象，并在静态代码快中初始化了这四个对象。其中三个是继承自Object的方法：
- m1：equals
- m2：toString
- m0：hashCode
m3才是我们自定义接口中的方法。
该类的构造函数接收一个InvocationHandler对象，并将其传递给了父类Proxy。而该类中所有方法的调用都直接扔给了Proxy类中的InvocationHandler对象。现在可以知道我们实现的InvocationHandler接口类的实例的作用了。
流程
梳理下流程：
- 利用ProxyGenerator.generateProxyClass为被代理的类（接口）生成代理类（Proxy）
- 将实现的InvocationHandler对象作为代理类的构造函数参数传递进去，得到代理类实例
- 使用代理类实例完成对被代理对象的代理
Mybatis中的应用
mybatis中mapper的实现就利用了jdk动态代理。
核心类
public class MapperProxyFactory {
private final Class mapperInterface;
private final Map methodCache = new ConcurrentHashMap();
public MapperProxyFactory(Class mapperInterface) {
this.mapperInterface = mapperInterface;
}
public Class getMapperInterface() {
return mapperInterface;
}
public Map getMethodCache() {
return methodCache;
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
final MapperProxy mapperProxy = new MapperProxy(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
}
所有对Mapper的方法调用最终都代理给了MapperProxy。该类的核心代码如下：
public class MapperProxy implements InvocationHandler, Serializable {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else if (isDefaultMethod(method)) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
}
注意到该类正是实现了InvocationHandler接口。而自定义的方法最终都由mapperMethod来执行了，接着mapperMethod又交给SqlSession来执行了，细节请自行阅读Mybatis源码。