java动态代理分析及理解
代理设计模式
定义：为其他对象提供一种代理以控制对这个对象的访问。
动态代理使用
java动态代理机制以巧妙的方式实现了代理模式的设计理念。
代理模式示例代码
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public interface subject
{
public void dosomething();
}
public class realsubject implements subject
{
public void dosomething()
{
system.out.println( "call dosomething()" );
}
}
public class proxyhandler implements invocationhandler
{
private object proxied;
public proxyhandler( object proxied )
{
this.proxied = proxied;
}
public object invoke( object proxy, method method, object[] args ) throws throwable
{
//在转调具体目标对象之前，可以执行一些功能处理
//转调具体目标对象的方法
return method.invoke( proxied, args);
//在转调具体目标对象之后，可以执行一些功能处理
}
}
import java.lang.reflect.invocationhandler;
import java.lang.reflect.method;
import java.lang.reflect.proxy;
import sun.misc.proxygenerator;
import java.io.*;
public class dynamicproxy
{
public static void main( string args[] )
{
realsubject real = new realsubject();
subject proxysubject = (subject)proxy.newproxyinstance(subject.class.getclassloader(),
new class[]{subject.class},
new proxyhandler(real));
proxysubject.dosomething();
//write proxysubject class binary data to file
createproxyclassfile();
}
public static void createproxyclassfile()
{
string name = "proxysubject";
byte[] data = proxygenerator.generateproxyclass( name, new class[] { subject.class } );
try
{
fileoutputstream out = new fileoutputstream( name + ".class" );
out.write( data );
out.close();
}
catch( exception e )
{
e.printstacktrace();
}
}
}
动态代理内部实现
首先来看看类proxy的代码实现 proxy的主要静态变量
// 映射表：用于维护类装载器对象到其对应的代理类缓存 private static map loadertocache = new weakhashmap(); // 标记：用于标记一个动态代理类正在被创建中 private static object pendinggenerationmarker = new object(); // 同步表：记录已经被创建的动态代理类类型，主要被方法 isproxyclass 进行相关的判断 private static map proxyclasses = collections.synchronizedmap(new weakhashmap()); // 关联的调用处理器引用 protected invocationhandler h;
proxy的构造方法
// 由于 proxy 内部从不直接调用构造函数，所以 private 类型意味着禁止任何调用
private proxy() {}
// 由于 proxy 内部从不直接调用构造函数，所以 protected 意味着只有子类可以调用
protected proxy(invocationhandler h) {this.h = h;}
proxy静态方法newproxyinstance
public static object newproxyinstance(classloader loader, class<?>[]interfaces,invocationhandler h) throws illegalargumentexception {
// 检查 h 不为空，否则抛异常
if (h == null) {
throw new nullpointerexception();
}
// 获得与指定类装载器和一组接口相关的代理类类型对象
class cl = getproxyclass(loader, interfaces);
// 通过反射获取构造函数对象并生成代理类实例
try {
constructor cons = cl.getconstructor(constructorparams);
return (object) cons.newinstance(new object[] { h });
} catch (nosuchmethodexception e) { throw new internalerror(e.tostring());
} catch (illegalaccessexception e) { throw new internalerror(e.tostring());
} catch (instantiationexception e) { throw new internalerror(e.tostring());
} catch (invocationtargetexception e) { throw new internalerror(e.tostring());
}
}
类proxy的getproxyclass方法调用proxygenerator的 generateproxyclass方法产生proxysubject.class的二进制数据：
public static byte[] generateproxyclass(final string name, class[] interfaces)
我们可以import sun.misc.proxygenerator，调用 generateproxyclass方法产生binary data，然后写入文件，最后通过反编译工具来查看内部实现原理。 反编译后的proxysubject.java proxy静态方法newproxyinstance
import java.lang.reflect.*;
public final class proxysubject extends proxy
implements subject
{
private static method m1;
private static method m0;
private static method m3;
private static method m2;
public proxysubject(invocationhandler invocationhandler)
{
super(invocationhandler);
}
public final boolean equals(object obj)
{
try
{
return ((boolean)super.h.invoke(this, m1, new object[] {
obj
})).booleanvalue();
}
catch(error _ex) { }
catch(throwable throwable)
{
throw new undeclaredthrowableexception(throwable);
}
}
public final int hashcode()
{
try
{
return ((integer)super.h.invoke(this, m0, null)).intvalue();
}
catch(error _ex) { }
catch(throwable throwable)
{
throw new undeclaredthrowableexception(throwable);
}
}
public final void dosomething()
{
try
{
super.h.invoke(this, m3, null);
return;
}
catch(error _ex) { }
catch(throwable throwable)
{
throw new undeclaredthrowableexception(throwable);
}
}
public final string tostring()
{
try
{
return (string)super.h.invoke(this, m2, null);
}
catch(error _ex) { }
catch(throwable throwable)
{
throw new undeclaredthrowableexception(throwable);
}
}
static
{
try
{
m1 = class.forname("java.lang.object").getmethod("equals", new class[] {
class.forname("java.lang.object")
});
m0 = class.forname("java.lang.object").getmethod("hashcode", new class[0]);
m3 = class.forname("subject").getmethod("dosomething", new class[0]);
m2 = class.forname("java.lang.object").getmethod("tostring", new class[0]);
}
catch(nosuchmethodexception nosuchmethodexception)
{
throw new nosuchmethoderror(nosuchmethodexception.getmessage());
}
catch(classnotfoundexception classnotfoundexception)
{
throw new noclassdeffounderror(classnotfoundexception.getmessage());
}
}
}
proxygenerator内部是如何生成class二进制数据，可以参考源代码。
private byte[] generateclassfile() {
/*
* record that proxy methods are needed for the hashcode, equals,
* and tostring methods of java.lang.object. this is done before
* the methods from the proxy interfaces so that the methods from
* java.lang.object take precedence over duplicate methods in the
* proxy interfaces.
*/
addproxymethod(hashcodemethod, object.class);
addproxymethod(equalsmethod, object.class);
addproxymethod(tostringmethod, object.class);
/*
* now record all of the methods from the proxy interfaces, giving
* earlier interfaces precedence over later ones with duplicate
* methods.
*/
for (int i = 0; i < interfaces.length; i++) {
method[] methods = interfaces[i].getmethods();
for (int j = 0; j < methods.length; j++) {
addproxymethod(methods[j], interfaces[i]);
}
}
/*
* for each set of proxy methods with the same signature,
* verify that the methods' return types are compatible.
*/
for (list<proxymethod> sigmethods : proxymethods.values()) {
checkreturntypes(sigmethods);
}
/* ============================================================
* step 2: assemble fieldinfo and methodinfo structs for all of
* fields and methods in the class we are generating.
*/
try {
methods.add(generateconstructor());
for (list<proxymethod> sigmethods : proxymethods.values()) {
for (proxymethod pm : sigmethods) {
// add static field for method's method object
fields.add(new fieldinfo(pm.methodfieldname,
"ljava/lang/reflect/method;",
acc_private | acc_static));
// generate code for proxy method and add it
methods.add(pm.generatemethod());
}
}
methods.add(generatestaticinitializer());
} catch (ioexception e) {
throw new internalerror("unexpected i/o exception");
}
/* ============================================================
* step 3: write the final class file.
*/
/*
* make sure that constant pool indexes are reserved for the
* following items before starting to write the final class file.
*/
cp.getclass(dottoslash(classname));
cp.getclass(superclassname);
for (int i = 0; i < interfaces.length; i++) {
cp.getclass(dottoslash(interfaces[i].getname()));
}
/*
* disallow new constant pool additions beyond this point, since
* we are about to write the final constant pool table.
*/
cp.setreadonly();
bytearrayoutputstream bout = new bytearrayoutputstream();
dataoutputstream dout = new dataoutputstream(bout);
try {
/*
* write all the items of the "classfile" structure.
* see jvms section 4.1.
*/
// u4 magic;
dout.writeint(0xcafebabe);
// u2 minor_version;
dout.writeshort(classfile_minor_version);
// u2 major_version;
dout.writeshort(classfile_major_version);
cp.write(dout); // (write constant pool)
// u2 access_flags;
dout.writeshort(acc_public | acc_final | acc_super);
// u2 this_class;
dout.writeshort(cp.getclass(dottoslash(classname)));
// u2 super_class;
dout.writeshort(cp.getclass(superclassname));
// u2 interfaces_count;
dout.writeshort(interfaces.length);
// u2 interfaces[interfaces_count];
for (int i = 0; i < interfaces.length; i++) {
dout.writeshort(cp.getclass(
dottoslash(interfaces[i].getname())));
}
// u2 fields_count;
dout.writeshort(fields.size());
// field_info fields[fields_count];
for (fieldinfo f : fields) {
f.write(dout);
}
// u2 methods_count;
dout.writeshort(methods.size());
// method_info methods[methods_count];
for (methodinfo m : methods) {
m.write(dout);
}
// u2 attributes_count;
dout.writeshort(0); // (no classfile attributes for proxy classes)
} catch (ioexception e) {
throw new internalerror("unexpected i/o exception");
}
return bout.tobytearray();
总结
一个典型的动态代理创建对象过程可分为以下四个步骤：
1、通过实现invocationhandler接口创建自己的调用处理器 ivocationhandler handler = new invocationhandlerimpl(...);
2、通过为proxy类指定classloader对象和一组interface创建动态代理类
class clazz = proxy.getproxyclass(classloader,new class[]{...});
3、通过反射机制获取动态代理类的构造函数，其参数类型是调用处理器接口类型
constructor constructor = clazz.getconstructor(new class[]{invocationhandler.class});
4、通过构造函数创建代理类实例，此时需将调用处理器对象作为参数被传入
interface proxy = (interface)constructor.newinstance(new object[] (handler));
为了简化对象创建过程，proxy类中的newinstance方法封装了2~4，只需两步即可完成代理对象的创建。
生成的proxysubject继承proxy类实现subject接口，实现的subject的方法实际调用处理器的invoke方法，而invoke方法利用反射调用的是被代理对象的的方法（object result=method.invoke(proxied,args)）
美中不足
诚然，proxy已经设计得非常优美，但是还是有一点点小小的遗憾之处，那就是它始终无法摆脱仅支持interface代理的桎梏，因为它的设计注定了这个遗憾。回想一下那些动态生成的代理类的继承关系图，它们已经注定有一个共同的父类叫proxy。java的继承机制注定了这些动态代理类们无法实现对class的动态代理，原因是多继承在java中本质上就行不通。有很多条理由，人们可以否定对 class代理的必要性，但是同样有一些理由，相信支持class动态代理会更美好。接口和类的划分，本就不是很明显，只是到了java中才变得如此的细化。如果只从方法的声明及是否被定义来考量，有一种两者的混合体，它的名字叫抽象类。实现对抽象类的动态代理，相信也有其内在的价值。此外，还有一些历史遗留的类，它们将因为没有实现任何接口而从此与动态代理永世无缘。如此种种，不得不说是一个小小的遗憾。但是，不完美并不等于不伟大，伟大是一种本质，java动态代理就是佐例。
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