JDK动态代理的过程
JDK动态代理采用字节重组，重新生成对象来替代原始对象，以达到动态代理的目的。
JDK中有一个规范，在ClassPath下只要是$开头的.class文件，一般都是自动生成的。
要实现JDK动态代理生成对象，首先得弄清楚JDK动态代理的过程。
1.获取被代理对象的引用，并且使用反射获取它的所有接口。
2.JDK动态代理类重新生成一个新的类，同时新的类要实现被代理类实现的所有接口。
3.动态生成Java代码，新添加的业务逻辑方法由一定的逻辑代码调用。
4.编译新生成的Java代码(.class文件)。
2.重新加载到VM中运行。
手写实现JDK动态代理
JDK动态代理功能非常强大， 接下来就模仿JDK动态代理实现一个属于自己的动态代理。
创建MyInvocationHandler接口
public interface MyInvocationHandler {
Object invoke(Object proxy, Method method, Object[] args) throws Throwable;
}
创建MyClassLoader类加载器
public class MyClassLoader extends ClassLoader {
private File classPathFile;
public MyClassLoader() {
String classPath = MyClassLoader.class.getResource("").getPath();
this.classPathFile = new File(classPath);
}
@Override
protected Class<?> findClass(String name) {
String className = MyClassLoader.class.getPackage().getName() + "." + name;
if (classPathFile != null) {
File classFile = new File(classPathFile, name.replaceAll("\\.", "/") + ".class");
if (classFile.exists()) {
FileInputStream in = null;
ByteArrayOutputStream out = null;
try {
in = new FileInputStream(classFile);
out = new ByteArrayOutputStream();
byte[] buff = new byte[1024];
int len;
while ((len = in.read(buff)) != -1) {
out.write(buff, 0, len);
}
return defineClass(className, out.toByteArray(), 0, out.size());
} catch (Exception e) {
e.printStackTrace();
}
}
}
return null;
}
}
创建代理类
创建的代理类是整个JDK动态代理的核心
public class MyProxy {
// 回车、换行符
public static final String ln = "\r\n";
/**
* 重新生成一个新的类,并实现被代理类实现的所有接口
*
* @param classLoader 类加载器
* @param interfaces 被代理类实现的所有接口
* @param invocationHandler
* @return 返回字节码重组以后的新的代理对象
*/
public static Object newProxyInstance(MyClassLoader classLoader, Class<?>[] interfaces, MyInvocationHandler invocationHandler) {
try {
// 动态生成源代码.java文件
String sourceCode = generateSourceCode(interfaces);
// 将源代码写入到磁盘中
String filePath = MyProxy.class.getResource("").getPath();
File f = new File(filePath + "$Proxy0.java");
FileWriter fw = new FileWriter(f);
fw.write(sourceCode);
fw.flush();
fw.close();
// 把生成的.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();
// 编译生成的.class文件加载到JVM中来
Class proxyClass = classLoader.findClass("$Proxy0");
Constructor c = proxyClass.getConstructor(MyInvocationHandler.class);
//删除生成的.java文件
f.delete();
// 返回字节码重组以后的新的代理对象
return c.newInstance(invocationHandler);
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
/**
* 动态生成源代码.java文件
*
* @param interfaces 被代理类实现的所有接口
* @return .java文件的源代码
*/
private static String generateSourceCode(Class<?>[] interfaces) {
StringBuffer sb = new StringBuffer();
sb.append(MyProxy.class.getPackage() + ";" + ln);
sb.append("import " + interfaces[0].getName() + ";" + ln);
sb.append("import java.lang.reflect.*;" + ln);
sb.append("public class $Proxy0 implements " + interfaces[0].getName() + "{" + ln);
sb.append("MyInvocationHandler invocationHandler;" + ln);
sb.append("public $Proxy0(MyInvocationHandler invocationHandler) { " + ln);
sb.append("this.invocationHandler = invocationHandler;");
sb.append("}" + ln);
for (Method m : interfaces[0].getMethods()) {
Class<?>[] params = m.getParameterTypes();
StringBuffer paramNames = new StringBuffer();
StringBuffer paramValues = new StringBuffer();
StringBuffer paramClasses = new StringBuffer();
for (int i = 0; i < params.length; i++) {
Class clazz = params[i];
String type = clazz.getName();
String paramName = toLowerFirstCase(clazz.getSimpleName());
paramNames.append(type + " " + paramName);
paramValues.append(paramName);
paramClasses.append(clazz.getName() + ".class");
if (i > 0 && i < params.length - 1) {
paramNames.append(",");
paramClasses.append(",");
paramValues.append(",");
}
}
sb.append("public " + m.getReturnType().getName() + " " + m.getName() + "(" + paramNames + ") {" + ln);
sb.append("try{" + ln);
sb.append("Method m = " + interfaces[0].getName() + ".class.getMethod(\"" + m.getName() + "\",new Class[]{" + paramClasses + "});" + ln);
sb.append((hasReturnValue(m.getReturnType()) ? "return " : "") + getCaseCode("this.invocationHandler.invoke(this,m,new Object[]{" + paramClasses + "})", m.getReturnType()) + ";" + ln);
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();
}
/**
* 定义返回类型
*/
private static Map<Class, Class> mappings = new HashMap<Class, Class>();
/**
* 初始化一些返回类型
*/
static {
mappings.put(int.class, Integer.class);
mappings.put(Integer.class, Integer.class);
mappings.put(double.class, Double.class);
mappings.put(Double.class, Double.class);
}
private static String getReturnEmptyCode(Class<?> returnClass) {
if (mappings.containsKey(returnClass)) {
if (returnClass.equals(int.class) || returnClass.equals(Integer.class)) {
return "return 0;";
} else if (returnClass.equals(double.class) || returnClass.equals(Double.class)) {
return "return 0.0;";
} else {
return "return 0;";
}
} else if (returnClass == void.class) {
return "";
} else {
return "return null;";
}
}
/**
* 判断返回值类型
*
* @param code
* @param returnClass
* @return
*/
private static String getCaseCode(String code, Class<?> returnClass) {
if (mappings.containsKey(returnClass)) {
// ((java.lang.Double) this.invocationHandler.invoke(this, m, new Object[]{})).doubleValue();
String re = "((" + mappings.get(returnClass).getName() + ")" + code + ")." + returnClass.getSimpleName().toLowerCase() + "Value()";
return re;
}
return code;
}
/**
* 判断代理接口的方法的返回值是否为void
*
* @param clazz 方法的返回值类型
* @return
*/
private static boolean hasReturnValue(Class<?> clazz) {
return clazz != void.class;
}
/**
* 参数首字母小写
*
* @param src
* @return
*/
private static String toLowerFirstCase(String src) {
char[] chars = src.toCharArray();
if (chars[0] >= 'A' && chars[0] <= 'Z') {
chars[0] += 32;
}
return String.valueOf(chars);
}
/**
* 首字母大写
*
* @param src
* @return
*/
private static String toUpperFirstCase(String src) {
char[] chars = src.toCharArray();
if (chars[0] >= 'a' && chars[0] <= 'z') {
chars[0] -= 32;
}
return String.valueOf(chars);
}
}
使用自定义动态代理类
创建接口
public interface IUser {
void shopping();
Double expenses();
}
创建被代理接口
public class User implements IUser {
@Override
public void shopping() {
System.out.println("user shopping....");
}
@Override
public Double expenses() {
return 50.5;
}
}
创建代理接口
public class UseProxy implements MyInvocationHandler {
private Object target;
public Object myJDKProxy(Object target){
this.target = target;
Class<?> clazz = target.getClass();
return MyProxy.newProxyInstance(new MyClassLoader(),clazz.getInterfaces(),this);
}
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("代理user，执行shopping()开始...");
Object result = method.invoke(this.target, args);
System.out.println("代理user，执行shopping()结束...");
return result;
}
}
客户端调用
public static void main(String[] args) {
UseProxy useProxy = new UseProxy();
IUser user = (IUser) useProxy.myJDKProxy(new User());
user.shopping();
System.out.println(user.expenses());
}
执行结果
代理user，执行shopping()开始...
user shopping....
代理user，执行shopping()结束...
--------------------------------
代理user，执行shopping()开始...
代理user，执行shopping()结束...
--------------------------------
50.5
生成源代码
查看生产的Java文件源代码
package cn.ybzy.demo.proxy.proxy;
import cn.ybzy.demo.proxy.client.IUser;
import java.lang.reflect.*;
public class $Proxy0 implements cn.ybzy.demo.proxy.client.IUser{
MyInvocationHandler invocationHandler;
public $Proxy0(MyInvocationHandler invocationHandler) {
this.invocationHandler = invocationHandler;}
public java.lang.Double expenses() {
try{
Method m = cn.ybzy.demo.proxy.client.IUser.class.getMethod("expenses",new Class[]{});
return ((java.lang.Double)this.invocationHandler.invoke(this,m,new Object[]{})).doubleValue();
}catch(Error ex) { }catch(Throwable e){
throw new UndeclaredThrowableException(e);
}return 0.0;}public void shopping() {
try{
Method m = cn.ybzy.demo.proxy.client.IUser.class.getMethod("shopping",new Class[]{});
this.invocationHandler.invoke(this,m,new Object[]{});
}catch(Error ex) { }catch(Throwable e){
throw new UndeclaredThrowableException(e);
}}}