先举个简单的栗子：
- 创建一个接口
public interface Animal {
void say();
}
- 创建接口实现
public class Cat implements Animal {
@Override
public void say() {
System.out.println("喵呜");
}
}
- 创建invocationHandler实现类，并用Proxy创建代理
public class JDKDynamicProxy implements InvocationHandler {
/**
* 要代理的目标
*/
private Object target;
public JDKDynamicProxy(Object target) {
if (target == null) {
throw new NullPointerException("target should not be null");
}
this.target = target;
}
// 代理方法，实现方法的增强
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("before invoke");
Object result = method.invoke(target, args);
System.out.println("after invoke");
return result;
}
/**
* 用jdk的Proxy类创建代理，参数为classLoader, 目标对象的接口，以及InvocationHandler的实现类
* @return
*/
public Object getProxy() {
return Proxy.newProxyInstance(
target.getClass().getClassLoader(),
target.getClass().getInterfaces(),
this);
}
}
然后测试一下
public class App {
public static void main(String[] args) {
Animal c = new Cat();
JDKDynamicProxy jdp = new JDKDynamicProxy(c);
Animal proxy = (Animal) jdp.getProxy();
proxy.say();
}
}
运行结果:
before invoke
喵呜
after invoke
栗子很简单，下面我们看一下Proxy是如何创建代理的。
我们进入newProxyInstance方法。
@CallerSensitive
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.
*/
// 通过classLoader和传入的interfaces获取Class
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
// 获取构造方法
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;
}
});
}
// 创建代理对象
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);
}
}
可以看到，主要就是获取class，然后通过它的构造方法创建代理对象。我们主要看它怎么创建类的，进入getProxyClass0方法。
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);
}
proxyClassCache的定义:
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
大概就是根据classloader和interfaces去缓存中找已生成的class，如果没有，就用ProxyClassFactory生成，并放到缓存里。
我们继续来看一下ProxyClassFactory。
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], 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<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.
* 验证类加载器是否将此接口的名称解析为同一Class对象。
*/
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.
* 验证它是接口
*/
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* Verify that this interface is not a duplicate.
* 验证是否重复
*/
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.
* 拿一个非公有的接口的包名，如果存在多个，则抛出异常
*/
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");
}
}
}
// 如果为空，则使用默认包名com.sun.proxy
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.
* 生成一个类名
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
* 生成类的字节数组
*/
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());
}
}
}
看apply方法，他先验证传入的接口是否是该类加载器加载的，是否真的是接口，是否重复。然后获取包名、类名，接着生成代理类的字节码，最后加载类。
以上就是动态代理的过程，最后我们看看ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags)生成的类长什么样吧。
public static void main(String[] args) {
testProxyGenerator();
}
public static void testProxyGenerator() {
Animal cat = new Cat();
// 根据cat的接口(即Animal)生成一个"com.lnstark.$ProxyCat的类的字节码
byte[] bytes = ProxyGenerator.generateProxyClass("com.lnstark.$ProxyCat", cat.getClass().getInterfaces());
// 转成class文件放到桌面
byte2File(bytes, "C:\\Users\\98353\\Desktop", "$ProxyCat.class");
}
/**
* byte 转file
*/
public static File byte2File(byte[] buf, String filePath, String fileName){
BufferedOutputStream bos = null;
FileOutputStream fos = null;
File file = null;
try{
File dir = new File(filePath);
if (!dir.exists() && dir.isDirectory()){
dir.mkdirs();
}
file = new File(filePath + File.separator + fileName);
fos = new FileOutputStream(file);
bos = new BufferedOutputStream(fos);
bos.write(buf);
}catch (Exception e){
e.printStackTrace();
}
finally{
if (bos != null){
try{
bos.close();
}catch (IOException e){
e.printStackTrace();
}
}
if (fos != null){
try{
fos.close();
}catch (IOException e){
e.printStackTrace();
}
}
}
return file;
}
运行后在桌面生成$ProxyCat.class文件，把他丢到idea里可以看到
package com.lnstark;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import lnstark.testproxy.Animal;
public final class $ProxyCat extends Proxy implements Animal {
private static Method m1;
private static Method m2;
private static Method m3;
private static Method m0;
public $ProxyCat(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 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 say() throws {
try {
super.h.invoke(this, m3, (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);
} 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"));
m2 = Class.forName("java.lang.Object").getMethod("toString");
m3 = Class.forName("lnstark.testproxy.Animal").getMethod("say");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
我们可以看到这是一个继承了Proxy，实现了我们传入的Interfaces的类。在构造方法里传入我们实现的InvocationHandler。最下面的静态代码块里获取到Object的equals、toString和hashCode方法，以及我们接口的say方法。然后在该代理类的方法中调用了InvocationHandler.invoke()方法，这正是我们重写的增强方法。
以上就是个人理解的jdk动态代理的原理了。
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