通过字节重组，重新生成对象来代替原始对象，以达到代理的目的。
字节码重组的基本步骤如下：
1.获取被代理对象的引用，利用反射获取到它的所有接口。
2.JDK动态代理类Proxy重新生成一个新的类，此类要实现刚才获取到的所有接口。
3.动态生成新类的Java代码。
4.编译.java文件成.class文件。
5.加载编译好的.class文件。
创建Person接口：
public interface Person {
void sing();
}
创建Customer类：
public class Customer implements Person{
@Override
public void sing() {
System.out.println("唱的很好听");
}
}创建MyInvocationHandler接口：
import java.lang.reflect.Method;
public interface MyInvocationHandler {
Object invoke(Object proxy, Method method, Object[] args) throws Throwable;
}
自定义类加载器MyClassLoader:
import java.io.*;
public class MyClassLoader extends ClassLoader {
private File classPathFile;
public MyClassLoader() {
//class文件路径
String classPath = MyClassLoader.class.getResource("").getPath();
this.classPathFile = new File(classPath);
}
/**
* 寻找需要加载的class文件并返回类对象
*
* @param name 类名称
* @return 找到Class对象
* @throws ClassNotFoundException
*/
protected Class<?> findClass(String name) throws ClassNotFoundException {
String className = MyClassLoader.class.getPackage().getName() + "." + name;
if (null != classPathFile) {
File classFile = new File(classPathFile, name + ".class");
if (classFile.exists()) {
FileInputStream fis = null;
ByteArrayOutputStream baos = null;
try {
fis = new FileInputStream(classFile);
baos = new ByteArrayOutputStream();
byte[] buff = new byte[1024];
int len;
while ((len = fis.read(buff)) != -1){
baos.write(buff,0,len);
}
return defineClass(className,baos.toByteArray(),0,baos.size());
} catch (Exception e) {
e.printStackTrace();
} finally {
if (null != fis) {
try {
fis.close();
if (null != baos) {
baos.close();
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
}
return null;
}
}自定义Proxy类MyProxy:
import javax.tools.JavaCompiler;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;
import java.io.File;
import java.io.FileWriter;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;
import java.util.HashMap;
import java.util.Map;
public class MyProxy {
public static final String ln = "\r\n";
public static Object newMyProxyInstance(MyClassLoader classLoader, Class<?>[] interfaces, MyInvocationHandler handler) {
try {
String src = generateSrc(interfaces);
//将Java文件输出到磁盘上
String filePath = MyProxy.class.getResource("").getPath() + "$Proxy0.java";
File file = new File(filePath);
FileWriter fw = new FileWriter(file);
fw.write(src);
fw.flush();
fw.close();
//编译Java文件成class文件
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager manager = compiler.getStandardFileManager(null, null, null);
Iterable iterable = manager.getJavaFileObjects(file);
JavaCompiler.CompilationTask task = compiler.getTask(null, manager, null, null, null, iterable);
task.call();
manager.close();
//加载类
Class proxyClass = classLoader.findClass("$Proxy0");
Constructor constructor = proxyClass.getConstructor(MyInvocationHandler.class);
file.delete();
return constructor.newInstance(handler);
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
private static String generateSrc(Class<?>[] interfaces) {
StringBuffer sb = new StringBuffer();
sb.append("package com.study.demo.proxy2;" + ln);
sb.append("import com.study.demo.proxy2.Person;" + ln);
sb.append("import java.lang.reflect.*;" + ln);
sb.append("public class $Proxy0 implements " + interfaces[0].getName() + "{" + ln);
sb.append("MyInvocationHandler handler;" + ln);
sb.append("public $Proxy0(MyInvocationHandler handler) {" + ln);
sb.append("this.handler = handler;");
sb.append("}" + ln);
for (Method m : interfaces[0].getMethods()) {
//获得每个方法的形式参数
Class<?>[] params = m.getParameterTypes();
StringBuffer paramNames = new StringBuffer();
StringBuffer paramValus = new StringBuffer();
StringBuffer paramClasses = new StringBuffer();
for (int i = 0; i < params.length; i++) {
Class clazz = params[i];
String type = clazz.getName();
//将类写名称第一个字母小写order
String paramName = toLowerFirstCase(clazz.getSimpleName());
paramNames.append(type + " " + paramName);
paramValus.append(paramName);
//cn.xupt.design_pattern.structure.proxy.handwriting_dynamic_proxy.Order.class
paramClasses.append(clazz.getName() + ".class");
if (i > 0 && i < params.length - 1) {
paramNames.append(",");
paramClasses.append(",");
paramValus.append(",");
}
}
//构造各个方法
sb.append("public " + m.getReturnType().getName() + " " + m.getName() + " " + "(" +
paramNames.toString() + "){" + ln);
sb.append("try{" + ln);
sb.append("Method method = " + interfaces[0].getName() + ".class.getMethod(\"" + m.getName()
+ "\", new Class[]{" + paramClasses.toString() + "});" + ln);
sb.append((hasReturnValue(m.getReturnType()) ? "return " : "") + getCaseCode(
"this.handler.invoke(this, method, new Object[]{" + paramValus + "})", m.getReturnType()) + ";");
sb.append("}catch(Error _ex){}");
sb.append("catch(Throwable e){" + ln);
sb.append("throw new UndeclaredThrowableException(e);" + ln);
sb.append("}");
sb.append(getReturnEmptyCode(m.getReturnType()));
sb.append("}");
}
sb.append("}" + ln);
return sb.toString();
}
private static Map<Class, Class> mapping = new HashMap<Class, Class>();
static {
mapping.put(int.class, Integer.class);
}
private static String getReturnEmptyCode(Class<?> returnClass) {
if (mapping.containsKey(returnClass)) {
return "0";
} else if (returnClass == void.class) {
return "";
} else {
return "return null";
}
}
private static String getCaseCode(String code, Class<?> returnClass) {
if (mapping.containsKey(returnClass)) {
return "((" + mapping.get(returnClass).getName() + ")" + code + ")." + returnClass.getSimpleName() + "Value()";
}
return code;
}
private static boolean hasReturnValue(Class<?> clazz) {
return clazz != void.class;
}
private static String toLowerFirstCase(String src) {
char[] chars = src.toCharArray();
chars[0] += 32;
return String.valueOf(chars);
}
}创建歌手类Singer:
import java.lang.reflect.Method;
public class Singer implements MyInvocationHandler{
private Object target;
public Object getInstance(Object target) {
this.target = target;
Class<?> aClass = target.getClass();
return MyProxy.newMyProxyInstance(new MyClassLoader(), aClass.getInterfaces(), this);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
before();
Object invoke = method.invoke(this.target, args);
after();
return invoke;
}
private void before() {
System.out.println("我是歌手");
}
private void after() {
System.out.println("休息！");
}
}创建Client类
public class Test {
public static void main(String[] args) {
Person person = (Person) new Singer().getInstance(new Customer());
person.sing();
}
}