大家都知道，JDK动态代理只能为接口创建代理实例，所以CGLib就自然而然的有了用武之地。至于为什么JDK只能为接口创建代理实例，后面在说。要用到JDK动态代理，那InvocationHandler和Proxy这对神雕侠侣肯定是要到场的。
先写一个简单的JDK动态代理例子。
1. 一个UserService接口：
public interface UserService {
void service();
}
2. 一个UserService接口的实现类UserServiceImpl：
public class UserServiceImpl implements UserService {
public void service() {
System.out.println("service() is running....");
}
}
3. 自定义一个MyInvocationHandler：
public class MyInvocationHandler implements InvocationHandler {
//目标对象
Object target;
//通过构造函数传入希望被代理的目标对象并实例化
public MyInvocationHandler(Object target){
this.target = target;
}
//利用InvocationHandler的invoke(Object proxy, Method method, Object[] args)方法将横切逻辑与业务逻辑编织在一起
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//模拟横切逻辑
System.out.println("Start recording........");
//业务逻辑, 并传入目标实例
Object object = method.invoke(target, args);
//模拟横切逻辑
System.out.println("End recording........");
//最后返回
return object;
}
}
4. 通过Proxy结合MyInvocationHandler来创建代理实例。
public class myProxyTest {
@Test
public void test(){
//希望被代理的目标对象
UserService userService = new UserServiceImpl();
//创建InvocationHandler实例, 将横切逻辑与业务逻辑编织在一起
MyInvocationHandler myInvocationHandler = new MyInvocationHandler(userService);
//根据编织了横切逻辑与业务逻辑的InvocationHandler实例,创建代理实例
UserService proxy = (UserService) Proxy.newProxyInstance(
userService.getClass().getClassLoader(),
userService.getClass().getInterfaces(),
myInvocationHandler);
//调用代理实例,和调用接口实例的方式一样
proxy.service();
}
}
测试结果为：
Start recording........ service() is running.... End recording........
用起来不难，但是要理解。之前说到，为什么JDK只能为接口创建代理实例，这里通过Proxy.newProxyInstance()的第2个参数可以看到Class[] interfaces,只能是接口。
看看创建代理实例的源码：
a. 先看newProxyInstance()
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. 查找或者生成指定的代理对象。
*/
Class cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
//调用代理类的构造函数
final Constructor cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doPrivileged(new PrivilegedAction() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
//得到代理类的实例并传入InvocationHandler的实例
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);
}
}
b.接着看getProxyClass0，这里非常清楚地写着如果proxy class已经存在，则直接从缓存中返回，否则通过ProxyClassFactory创建。
/**
* 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");
}
// 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);
}
c. 接着看返回的proxyClassCache，以下是初始化。创建proxy class需看ProxyClassFactory();
/**
* a cache of proxy classes
*/
private static final WeakCache[], Class>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
d.接下来看ProxyClassFactory，重点看看关键部分。
注释
/**
* A factory function that generates, defines and returns the proxy class given
* the ClassLoader and array of interfaces.
*/
private static final class ProxyClassFactory
implements BiFunction[], Class>{...}
实现方法
public Class apply(ClassLoader loader, Class[] interfaces) {
Map, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
//省略部分代码.......
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.定义类名
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.这里生成ProxyClass的字节码文件。
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
//这里通过defineClass0, 利用proxy class字节码文件生成并返回代理类实例.
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());
}
}
}
在进一步看
ProxyGenerator.generateProxyClass(proxyName, interfaces,accessFlags);
public static byte[] generateProxyClass(final String var0, Class[] var1, int var2) {
//三个参数分别是var0: proxyName, var1: interfaces, var2: accessFlags
ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
//动态生成代理类的字节码
final byte[] var4 = var3.generateClassFile();
if(saveGeneratedFiles) {
AccessController.doPrivileged(new PrivilegedAction() {
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), new String[0]);
Files.createDirectories(var3, new FileAttribute[0]);
var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
} else {
var2 = Paths.get(var0 + ".class", new String[0]);
}
Files.write(var2, var4, new OpenOption[0]);
return null;
} catch (IOException var4x) {
throw new InternalError("I/O exception saving generated file: " + var4x);
}
}
});
}
//返回代理类字节码
return var4;
}
以上就是代理类如何动态生成的，包括代理类字节码在什么时候生成。下面我们要得到这个字节码，进行反编译来看看里面到底有何神秘。
5.写个类来得到生成的代理类字节码
public class GetGenerateClassFile {
public static void getGenerateClassFile(String path){
//根据希望被代理的目标对象生成一个名为$Proxy1的代理类字节码，代理类名字格式是这样的： proxyClassNamePrefix = "$Proxy";
byte[] proxyClassFile = ProxyGenerator.generateProxyClass("$Proxy1", UserServiceImpl.class.getInterfaces());
FileOutputStream fileOutputStream = null;
try {
fileOutputStream = new FileOutputStream(path);
fileOutputStream.write(proxyClassFile);
fileOutputStream.flush();
fileOutputStream.close();
} catch (Exception e) {
e.printStackTrace();
}
}
}
然后在myProxyTest中调用上面的静态方法，得到代理类字节码文件。
public void GenerateProxyClassFile(){
GetGenerateClassFile.getGenerateClassFile("C:/$Proxy1.class");
}
6.进行反编译，直接用IDEA打开这个字节码文件，得到如下的类：
public final class $Proxy1 extends Proxy implements UserService {
private static Method m1;
private static Method m2;
private static Method m3;
private static Method m0;
//构造函数 传入InvocationHandler，实际上就是myInvocationHandler
public $Proxy1(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 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);
}
}
//这里就是调用invoke(Object proxy, Method method, Object[] args)的地方。
public final void service() 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)).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")});
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
m3 = Class.forName("com.smart.myProxy.UserService").getMethod("service", 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());
}
}
}
其实这里也看得出来代理类是继承的Proxy,由于JAVA没有多重继承，所以JDK的动态代理只能用于接口。这里真正说明了为什么只能为接口创建代理实例。
public final class $Proxy1 extends Proxy implements UserService