2. JDK动态代理
JDK 动态代理
相比于静态代理，动态代理避免了开发人员编写各个繁锁的静态代理类，只需简单地指定一组接口及目标类对象就能动态的获得代理对象。
代理模式
使用代理模式必须要让代理类和目标类实现相同的接口，客户端通过代理类来调用目标方法，代理类会将所有的方法调用分派到目标对象上反射执行，还可以在分派过程中添加"前置通知"和后置处理（如在调用目标方法前校验权限，在调用完目标方法后打印日志等）等功能。
demo.
你可以在通过github看到源码。
public interface Animal {
String sound();
}
public class Dog implements Animal {
@Override
public String sound() {
return "Wang, Wang, Wang";
}
}通过实现InvocationHandler接口来自定义自己的InvocationHandler
public class DogProxyHandler implements InvocationHandler {
private Object proxyied; //代理对象
public DogProxyHandler(Object proxyied){
this.proxyied = proxyied; //初始化代理对象
}
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("before invoked...");
System.out.println(method.getName() + method.getModifiers());
Object object = method.invoke(proxyied, args);
System.out.println("after invoked...");
return object;
}
}client调用过程。
public class Test {
public static void main(String[] args) {
Dog dog = new Dog();
Animal animal = (Animal) Proxy.newProxyInstance(
Animal.class.getClassLoader(),
new Class[]{Animal.class},
new DogProxyHandler(dog));
System.out.println(animal.sound());
}
}console 打印输出：
before invoked...
Wang, Wang, Wang
after invoked...其实这里基本上就是AOP的一个简单实现了，在目标对象的方法执行之前和执行之后进行了增强。Spring的AOP实现其实也是用了Proxy和InvocationHandler这两个类来操作的。
###源码分析 1.newProxyInstance(ClassLoader loader,Class[] interfaces,InvocationHandler h)
@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.
*/
//核心在这里，这里生成了最关键的代理类。
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<Void>() {
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);
}
}在这里，我们要关心的是ProxyClassFactory具体做了什么。
2.ProxyClassFactory.apply(ClassLoader loader, Class[] interfaces)
/**
* 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<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.
*/
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.
*/
/*验证你传入的接口中是否有非public接口，只要有一个接口是非public的，那么这些接口都必须在同一包中
这里的接口修饰符直接影响到System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true")所生成
的代理类的路径，往下看！！
*/
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.
*/
//这里是关键的生成字节码文件的地方。
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());
}
}
}我们可以看到，这个方法主要是做了一些验证性工作，做了一些准备工作，生成字节码的工作，主要是交给了ProxyGenerator的generateProxyClass来做。
3.ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);
private static final boolean saveGeneratedFiles =
((Boolean)AccessController.doPrivileged(new GetBooleanAction("sun.misc.ProxyGenerator.saveGeneratedFiles"))).booleanValue();
public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
//真正的生成字节码byte.
final byte[] var4 = var3.generateClassFile();
//这里对应是否保存文件。sun.misc.ProxyGenerator.saveGeneratedFiles == true
if (saveGeneratedFiles) {
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;
}层层调用后，最终generateClassFile才是真正生成代理类字节码文件的方法，注意开头的三个addProxyMethod方法是只将Object的hashcode,equals,toString方法添加到代理方法容器中，代理类除此之外并没有重写其他Object的方法，所以除这三个方法外，代理类调用其他方法的行为与Object调用这些方法的行为一样不通过Invoke.
4.generateClassFile–根源
private byte[] generateClassFile() {
/*addProxyMethod系列方法就是将接口的方法和Object的hashCode,equals,toString方法添加到代理方法容器(proxyMethods),
其中方法签名作为key,proxyMethod作为value*/
/*hashCodeMethod方法位于静态代码块中通过Object对象获得，hashCodeMethod=Object.class.getMethod("hashCode",new Class[0]),
相当于从Object中继承过来了这三个方法equalsMethod,toStringMethod*/
this.addProxyMethod(hashCodeMethod, Object.class);
this.addProxyMethod(equalsMethod, Object.class);
this.addProxyMethod(toStringMethod, Object.class);
//获得所有接口中的所有方法，并将方法添加到代理方法中
Class[] var1 = this.interfaces;
int var2 = var1.length;
int var3;
Class var4;
for(var3 = 0; var3 < var2; ++var3) {
var4 = var1[var3];
Method[] var5 = var4.getMethods();
int var6 = var5.length;
for(int var7 = 0; var7 < var6; ++var7) {
Method var8 = var5[var7];
this.addProxyMethod(var8, var4);
}
}
//验证具有相同方法签名的的方法的返回值类型是否一致，因为不可能有两个方法名相同,参数相同，而返回值却不同的方法
Iterator var11 = this.proxyMethods.values().iterator();
List var12;
while(var11.hasNext()) {
var12 = (List)var11.next();
checkReturnTypes(var12);
}
//接下来就是写代理类文件的步骤了
Iterator var15;
try {
//生成代理类的构造函数
this.methods.add(this.generateConstructor());
var11 = this.proxyMethods.values().iterator();
while(var11.hasNext()) {
var12 = (List)var11.next();
var15 = var12.iterator();
while(var15.hasNext()) {
ProxyGenerator.ProxyMethod var16 = (ProxyGenerator.ProxyMethod)var15.next();
/*将代理字段声明为Method，10为ACC_PRIVATE和ACC_STATAIC的与运算，表示该字段的修饰符为private static
所以代理类的字段都是private static Method */
this.fields.add(new ProxyGenerator.FieldInfo(var16.methodFieldName, "Ljava/lang/reflect/Method;", 10));
//生成代理类的代理方法
this.methods.add(var16.generateMethod());
}
}
//为代理类生成静态代码块，对一些字段进行初始化
this.methods.add(this.generateStaticInitializer());
} catch (IOException var10) {
throw new InternalError("unexpected I/O Exception", var10);
}
if (this.methods.size() > 65535) {
throw new IllegalArgumentException("method limit exceeded");
} else if (this.fields.size() > 65535) {
throw new IllegalArgumentException("field limit exceeded");
} else {
//代理类文件过程。
this.cp.getClass(dotToSlash(this.className));
this.cp.getClass("java/lang/reflect/Proxy");
var1 = this.interfaces;
var2 = var1.length;
for(var3 = 0; var3 < var2; ++var3) {
var4 = var1[var3];
this.cp.getClass(dotToSlash(var4.getName()));
}
this.cp.setReadOnly();
ByteArrayOutputStream var13 = new ByteArrayOutputStream();
DataOutputStream var14 = new DataOutputStream(var13);
try {
var14.writeInt(-889275714);
var14.writeShort(0);
var14.writeShort(49);
this.cp.write(var14);
var14.writeShort(this.accessFlags);
var14.writeShort(this.cp.getClass(dotToSlash(this.className)));
var14.writeShort(this.cp.getClass("java/lang/reflect/Proxy"));
var14.writeShort(this.interfaces.length);
Class[] var17 = this.interfaces;
int var18 = var17.length;
for(int var19 = 0; var19 < var18; ++var19) {
Class var22 = var17[var19];
var14.writeShort(this.cp.getClass(dotToSlash(var22.getName())));
}
var14.writeShort(this.fields.size());
var15 = this.fields.iterator();
while(var15.hasNext()) {
ProxyGenerator.FieldInfo var20 = (ProxyGenerator.FieldInfo)var15.next();
var20.write(var14);
}
var14.writeShort(this.methods.size());
var15 = this.methods.iterator();
while(var15.hasNext()) {
ProxyGenerator.MethodInfo var21 = (ProxyGenerator.MethodInfo)var15.next();
var21.write(var14);
}
var14.writeShort(0);
return var13.toByteArray();
} catch (IOException var9) {
throw new InternalError("unexpected I/O Exception", var9);
}
}
}在动态代理中InvocationHandler是核心，每个代理实例都具有一个关联的调用处理程序InvocationHandler。对代理实例调用方法时，将对方法调用进行编码并将其指派到它的调用处理程序InvocationHandler的 invoke 方法。所以对代理方法的调用都是通InvocationHadler的invoke来实现中，而invoke方法根据传入的代理对象，方法和参数来决定调用代理的哪个方法invoke方法签名：invoke（Object Proxy，Method method，Object[] args）
5.生成的代理类
public final class $Proxy0 extends Proxy implements Animal {
private static Method m1;
private static Method m2;
private static Method m3;
private static Method m0;
//代理类的构造函数，其参数正是是InvocationHandler实例，Proxy.newInstance方法就是通过通过这个构造函数来创建代理实例的
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 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 String sound() throws {
try {
return (String)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", Class.forName("java.lang.Object"));
m2 = Class.forName("java.lang.Object").getMethod("toString");
m3 = Class.forName("com.proxy.com.Animal").getMethod("sound");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}