jdk动态代理的使用及实现原理
已经毕业两个月啦，在找工作的过程中经常有人问起spring相关的知识，尤其是spring核心aop是必不可少的，回答时总会回答spring的aop是由动态代理实现的，但是具体使用方法和原理有些模糊。spring提供了jdk动态代理，那么什么是jdk动态代理呢，jdk动态代理是java本身提供的一种代理实现，提供了一些实现jdk动态代理的重要类或接口在java.lang.reflect包下，如proxy类和InvocationHandler接口，我们先使用一下jdk动态代理，了解如何使用。jdk动态代理需要提供一个接口和接口的实现类，使用Proxy类的newProxyInstance方法得到生成的代理类，然后就可以调用这个代理类的方法啦。我的代码结构
//接口提供了一个HelloReflect方法
public interface HelloReflect { void helloReflect(); }
//接口的实现
public class HelloReflectImpl implements HelloReflect{ public void helloReflect() { System.out.println("hello reflect"); } }
JDKProxy 类继承了InvocationHandler接口，因为在Porxy类生产代理类的newProxyInstance方法中，需要三个参数，一个是类加载器，一个是实现接口的Class数组，还有一个就是InvocationHandler
的实现，重写了InvocationHandler接口的invoke方法，生成的代理类调用方法时最终会调用这个我们重写的invoke方法，在JDKProxy中还提供了一个getProxy方法里面使用newProxyInstance来获得代理类
public class JDKProxy implements InvocationHandler {
private Object target;
public JDKProxy(Object target) {
this.target=target;
}
public <T> T getProxy(){
return (T) Proxy.newProxyInstance(target.getClass().getClassLoader(),target.getClass().getInterfaces(),this);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("输出hello reflect之前");
Object result=method.invoke(target,args);
System.out.println("输出hello reflect之后");
return result;
}
}
下面是我们测试的主方法
public class TestProxy {
public static void main(String[] args) {
System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true");//将生成的代理类的字节码文件保存到本地，后面分析原理会用到
HelloReflect helloReflect1=new HelloReflectImpl();
Class<?>[] arr=helloReflect1.getClass().getInterfaces();
for(Class clazz:arr){
System.out.println(clazz.getName());
}
HelloReflect helloReflect2=new JDKProxy(helloReflect1).getProxy();
helloReflect2.helloReflect();
}
}
结果输出
这就是jdk动态代理的使用过程，下面进行一下原理的分析
在上面我们可以知道，我们是通过Proxy类的newProxyInstance来获取生产的代理类的，我们看一下
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);//检查InvocationHandler是否为空
final Class<?>[] intfs = interfaces.clone();//拷贝实现类实现的接口
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); //获取代理类的Class对象，在这个过程中，生成代理类的字节码，并通过类加载器加载到JVN中
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
final Constructor<?> cons = cl.getConstructor(constructorParams); //通过Class对象获取构造器
final InvocationHandler ih = h;
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
return cons.newInstance(new Object[]{h}); //返回实例化的代理类，并且将我们实现的InvocationHandler作为参数传入其中
} 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);
}
}
从上面可以看出来，通过getProxyClass0方法得到了代理类的Class对象，有了Class对象，我们就可以通过反射初始化代理类。因此在newProxyInstance方法中主要做了两件事
1.得到代理类的Class对象
2.通过反射得到代理类的构造器并生成代理对象，然后返回
下面我们查看得到Class对象的getProxyClass0方法
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
return proxyClassCache.get(loader, interfaces);//查看缓存中是否已经有代理类的Class对象，如果没有，进行创建
}
proxyClassCache.get方法中会查看缓存中是否已经存在，如果不存在，则会调用Proxy的内部类ProxyClassFactory的apply方法进行创建
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// prefix for all proxy class names
private static final String proxyClassNamePrefix = "$Proxy";//代理类的名字前缀
// 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.
*/
Class<?> interfaceClass = null;
try {
interfaceClass = Class.forName(intf.getName(), false, loader);
} catch (ClassNotFoundException e) {
}
if (interfaceClass != intf) { //对比是否加载的接口是否正确
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 + "."; //ReflectUtil.PROXY_PACKAGE值为com.sun.proxy
}
/*
* Choose a name for the proxy class to generate.
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;//创建出来代理类的名称
/*
* Generate the specified proxy class.
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags); //生成代理类字节码的地方
try {
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length); //类加载加载到JVM中
} catch (ClassFormatError e) {
throw new IllegalArgumentException(e.toString());
}
}
}
可以看到在这类中生成了代理类的class字节码文件，并通过类加载器加载到了JVM中，可以设置将生成的字节码文件保存下来，在com.sun.proxy包下。generateProxyClass是具体生成字节码的地方，因此我们在测试的main函数中设置了System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true");这样就能将生成的class文件保存下来
public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
final byte[] var4 = var3.generateClassFile(); //生成字节码
if (saveGeneratedFiles) { //如果设置了saveGeneratedFiles为true，则保存生成的class文件在var0下，也就是com.sun.proxy加上class文件的名称
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);
}
}
});
}
return var4; //返回生成的字节码
}
看生成的Proxy0.class文件
public final class $Proxy0 extends Proxy implements HelloReflect {
private static Method m1;
private static Method m2;
private static Method m3;
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 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 helloReflect() 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 { //静态代码块，默认重写了hashcode，equals，toString方法，再加上接口的方法
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("com.jinz.reflect.HelloReflect").getMethod("helloReflect");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
可以看到我们在调用代理类的helloReflect方法时，实际调用了我们写的InvocationHandler的invoke方法
整个jdk动态代理的过程就是这样了
