【原创】自己动手实现JDK动态代理

引言

项目结构如下图所示，maven项目

1、JDK动态代理

先来一段jdk动态代理的demo，
首先创建一个接口，Person

package bean;

public interface Person {

public void eat();

}

然后写一个实现类PersonImpl

package bean;

public class PersonImpl implements Person{

@Override
public void eat() {
// TODO Auto-generated method stub
System.out.println("time to eat ");
}

}

然后写个使用类PersonInvocationHandler

package jdk;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

public class PersonInvocationHandler implements InvocationHandler {

private Object obj;

public PersonInvocationHandler(Object obj) {
this.obj = obj;
}

@Override
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable {
// TODO Auto-generated method stub
System.out.println("before time to eat");
method.invoke(obj, args);
System.out.println("after time to eat");
return null;
}

}

最后 再写个测试类

package jdk;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.lang.reflect.Proxy;

import sun.misc.ProxyGenerator;
import bean.Person;
import bean.PersonImpl;

public class jdkTest {

public static void main(String[] args) throws Exception {
PersonInvocationHandler personInvocationHandler = new PersonInvocationHandler(
new PersonImpl());
Person personProxy = (Person) Proxy.newProxyInstance(
PersonImpl.class.getClassLoader(),
PersonImpl.class.getInterfaces(), personInvocationHandler);
personProxy.eat();
}
}

输出如下

before time to eat
time to eat
after time to eat

接下里我们不使用JDK的API，自己实现一套代理类

2、自定义动态代理

先上测试类的代码，如下图所示，共有(1)(2)(3)处不同

针对(1)，我们有如下代码,先抄袭JDK的InvocationHandler，改个名字成为MyInvocationHandler

package custom;

import java.lang.reflect.Method;

public interface MyInvocationHandler {

public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable;

}

编写一个JAVA类MyPersonInvocationHandler继承MyInvocationHandler，这段代码与PersonInvocationHandler的代码无异，如下所示

package custom;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

public class MyPersonInvocationHandler implements MyInvocationHandler {

private Object obj;

public MyPersonInvocationHandler(Object obj) {
this.obj = obj;
}

@Override
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable {
// TODO Auto-generated method stub
System.out.println("before time to eat");
method.invoke(obj, args);
System.out.println("after time to eat");
return null;
}

}

针对(2),我们实现一个自己的代理生成类MyProxy,其生成java代理类的步骤分为以下5步

1. 生成java源碼
2. 將源码输出到java文件中
3. 将java文件编译成class文件
4. 将class加载进jvm
5. 返回代理类对象

具体代码如下

package custom;

import java.io.File;
import java.io.FileWriter;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;

import javax.tools.JavaCompiler;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;

public class MyProxy {

public static final String ln = "\r\n";

public static Object newProxyInstance(MyClassLoader myClassLoder,
Class<?>[] interfaces, MyInvocationHandler h) {
try{
// 1 java源碼
String src = generateSrc(interfaces);

// 2 將源码输出到java文件中
String filePath = MyProxy.class.getResource("").getPath();
System.out.println(filePath);
File f = new File(filePath + "$Proxy0.java");
FileWriter fw = new FileWriter(f);
fw.write(src);
fw.flush();
fw.close();

//3、将java文件编译成class文件
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager manage = compiler.getStandardFileManager(null,null,null);
Iterable iterable = manage.getJavaFileObjects(f);

JavaCompiler.CompilationTask task = compiler.getTask(null,manage,null,null,null,iterable);
task.call();
manage.close();

//4、将class加载进jvm
Class proxyClass=myClassLoder.findClass("$Proxy0");
f.delete();

//5、返回代理类对象
Constructor constructor = proxyClass.getConstructor(MyInvocationHandler.class);
return constructor.newInstance(h);
}catch(Exception e){
e.printStackTrace();
}
return null;
}

private static String generateSrc(Class<?>[] interfaces) {
// TODO Auto-generated method stub
StringBuffer sb = new StringBuffer();
sb.append("package custom;" + ln);
sb.append("import java.lang.reflect.Method;" + ln);
sb.append("public class $Proxy0 implements " + interfaces[0].getName() + "{" + ln);
sb.append("private MyInvocationHandler h;"+ln);
sb.append("public $Proxy0(MyInvocationHandler h) { " + ln);
sb.append("this.h = h;"+ln);
sb.append("}" + ln);
for (Method m : interfaces[0].getMethods()) {
sb.append("public " + m.getReturnType().getName() + " "
+ m.getName() + "() {" + ln);
sb.append("try{" + ln);
sb.append("Method m = " + interfaces[0].getName()
+ ".class.getMethod(\"" + m.getName()
+ "\",new Class[]{});" + ln);
sb.append("this.h.invoke(this,m,null);" + ln);
sb.append("}catch(Throwable e){" + ln);
sb.append("e.printStackTrace();" + ln);
sb.append("}"+ln);
sb.append("}"+ln);
}
sb.append("}" + ln);
return sb.toString();
}
}

针对(3)，我们继承ClassLoader,实现一套自己的类加载机制MyClassLoader，如下所示,

package custom;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;

public class MyClassLoader extends ClassLoader {

private File classPathfile;

public MyClassLoader() {
String classpth = MyClassLoader.class.getResource("").getPath();
classPathfile = new File(classpth);
}

@Override
public Class<?> findClass(String name) throws ClassNotFoundException {
String className = MyClassLoader.class.getPackage().getName() + "." +name;
if (classPathfile != null) {
File file = new File(classPathfile, name + ".class");
FileInputStream fileInputStream = null;
ByteArrayOutputStream outputStream = null;
try{
fileInputStream = new FileInputStream(file);
outputStream = new ByteArrayOutputStream();
byte[] buff = new byte[1024];
int len;
while((len=fileInputStream.read(buff))!=-1){
outputStream.write(buff, 0, len);
}
return defineClass(className, outputStream.toByteArray(), 0, outputStream.size());
}catch(Exception e){
e.printStackTrace();
}finally{
if(null!=fileInputStream){
try {
fileInputStream.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
if(null!=outputStream){
try {
outputStream.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
return null;
}

}

最后测试类代码如下所示

package custom;

import java.lang.reflect.Proxy;

import custom.MyPersonInvocationHandler;
import bean.Person;
import bean.PersonImpl;

public class CustomTest {

public static void main(String[] args) {
MyPersonInvocationHandler personInvocationHandler = new MyPersonInvocationHandler(
new PersonImpl());
Person personProxy = (Person) MyProxy.newProxyInstance(
new MyClassLoader(), PersonImpl.class.getInterfaces(),
personInvocationHandler);
personProxy.eat();
}
}

输出如何所示

before time to eat
time to eat
after time to eat

至此，我们已完全实现了一套自定义的jdk动态代理类