jdk动态代理提供了一种Java动态扩展机制，可以让我们在无须更改目标源码的情况下进行扩展，在这套机制下最重要的两个组件Proxy 和 InvocationHandler 。下面我们看段代码
public interface GoodService {
String sayHello(String name);
}
public class GoodServiceImpl implements GoodService {
@Override
public String sayHello(String name) {
return name + " 你好";
}
}
public class ControllerProxy implements InvocationHandler {
private GoodService goodService;
public ControllerProxy(GoodService goodService) {
this.goodService = goodService;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("before");
// goodService = (GoodService) proxy;
Object value = method.invoke(goodService,args);
System.out.println("after");
return value;
}
}
public class GoodClient {
/**
*
* @param args
*/
public static void main(String[] args){
GoodService goodService = new GoodServiceImpl();
ControllerProxy proxy = new ControllerProxy(goodService);
GoodService gs = (GoodService) Proxy.newProxyInstance(proxy.getClass().getClassLoader(),goodService.getClass().getInterfaces(),proxy);
String dd = gs.sayHello("张三");
System.out.println(dd);
}
}
我们重点看一下ControllerProxy 和 GoodClient 发现整个环节中我们没有发现在哪里调用invoke方法？，而且invoke方法中的proxy貌似有点多余，这里面是否存在什么关系？
通过debug跟踪，发现invoke 方法中的proxy对象是GoodService的实例，但是跟GoodServiceImpl 没有任何关系，进一步验证发现proxy对应的实例对象是com.sun.proxy.$Proxy0，这个类是哪里来的？
通过查看Proxy.newProxyInstance 源码发现端倪
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.
*/
Class cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
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;
}
});
}
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);
}
}
private static void checkNewProxyPermission(Class caller, Class proxyClass) {
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
if (ReflectUtil.isNonPublicProxyClass(proxyClass)) {
ClassLoader ccl = caller.getClassLoader();
ClassLoader pcl = proxyClass.getClassLoader();
// do permission check if the caller is in a different runtime package
// of the proxy class
int n = proxyClass.getName().lastIndexOf('.');
String pkg = (n == -1) ? "" : proxyClass.getName().substring(0, n);
n = caller.getName().lastIndexOf('.');
String callerPkg = (n == -1) ? "" : caller.getName().substring(0, n);
if (pcl != ccl || !pkg.equals(callerPkg)) {
sm.checkPermission(new ReflectPermission("newProxyInPackage." + pkg));
}
}
}
}
其中下面行代码暴露出问题的根源来了
/*
* Look up or generate the designated proxy class.
*/
Class cl = getProxyClass0(loader, intfs);
原来com.sun.proxy.$Proxy0 这个类是通过这个方法动态生成的
/**
* 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);
}
可能有的朋友会说没看到生成类的代码啊，其实秘密就在proxyClassCache这个变量上面，查找一下发现这个变量是静态变量早已经被实例化
/**
* a cache of proxy classes
*/
private static final WeakCache[], Class>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
我们重点看看一下这个ProxyClassFactory这个类
/**
* 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>
{
// 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());
}
}
}
这个类拿到classloader和代理的接口，判断接口的包名以及和修饰符，然后自己组装并生成一个新的类最后defineClass0这个native方法加载到虚拟机中，至此我们找到问题的根源为什么invoke方法中proxy对象是com.sun.proxy.Proxy0 是怎么来的，上面我已经解释来Proxy0类的代码
import com.zdnst.boot.ctl.GoodService;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class Proxy0 extends Proxy implements GoodService {
private static Method m1;
private static Method m3;
private static Method m2;
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})).booleanValue();
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final String sayHello(String var1) throws {
try {
return (String)super.h.invoke(this, m3, 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 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.zdnst.boot.ctl.GoodService").getMethod("sayHello", new Class[]{Class.forName("java.lang.String")});
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());
}
}
}
很明显sayHello 已经在Proxy0 只不过做了一个强制类型转换，通过Proxy0 类的方法,方法调用后在工程的跟路径下会找到class类，然后反编译一下就可以查看源码
public void createClass(Class clazz) throws IOException {
byte[]classFile = ProxyGenerator.generateProxyClass("Proxy0",clazz.getInterfaces());
File file =new File("./Proxy0.class");
FileOutputStream fos =new FileOutputStream(file);
fos.write(classFile);
fos.flush();
fos.close();
}