1.靜態(tài)代理

靜態(tài)代理缺點(diǎn)：
①.每次代理都要實(shí)現(xiàn)一個(gè)類，導(dǎo)致項(xiàng)目中代碼很多；
②.代碼都是寫(xiě)死的，一個(gè)代理只能實(shí)現(xiàn)固定的功能，無(wú)法靈活多變

1.繼承方式實(shí)現(xiàn)代理（靜態(tài)代理中的繼承代理）

//目標(biāo)對(duì)象
public class UserImpl {
    public void query(String name) {
        System.out.println("query name = " + name);
    }
}

//代理對(duì)象
public class LogUserProxy extends UserImpl{
    public void query(String name) {
        System.out.println("log ...");
        System.out.println("query name = " + name);
    }
}

//測(cè)試類
public class Main {
    public static void main(String[] args) {
        //父類指向子類
        UserImpl user = new LogUserProxy();
        user.query("張三");
    }
}

代理對(duì)象實(shí)現(xiàn)了對(duì)目標(biāo)對(duì)象的增強(qiáng)，但只能增強(qiáng)一次。

2.聚合方式實(shí)現(xiàn)代理

//抽象對(duì)象
public interface UserService {
    public void query(String name);
}

//目標(biāo)對(duì)象
public class UserImpl implements UserService {

    public void query(String name) {
        System.out.println("query name = " + name);
    }
}

//代理對(duì)象1
public class LogUserProxy implements UserService {

    private UserService userService;

    public LogUserProxy(UserService userService) {
        this.userService = userService;
    }

    public void query(String name) {
        System.out.println("log ...");
        userService.query(name);
    }
}

//代理對(duì)象2
public class TimeUserProxy implements UserService {

    private UserService userService;

    public TimeUserProxy(UserService user) {
        this.userService = user;
    }

    public void query(String name) {
        System.out.println("time ...");
        userService.query(name);
    }
}

public class Main {

    public static void main(String[] args) {
        UserService user = new UserImpl();
        UserService logUser = new LogUserProxy(user);
        UserService timeUser = new TimeUserProxy(logUser);
        timeUser.query("張三");
    }
}

輸出結(jié)果：

time ...
log ...
query name = 張三

代理對(duì)象1和代理對(duì)象2對(duì)目標(biāo)對(duì)象實(shí)現(xiàn)了兩次增強(qiáng)。聚合代理方式可以實(shí)現(xiàn)多次增強(qiáng)。

2.動(dòng)態(tài)代理

??動(dòng)態(tài)代理有JDK的動(dòng)態(tài)代理和cglib的動(dòng)態(tài)代理，例如Spring Boot中就使用了這兩種代理方式，默認(rèn)使用JDK的代理方式，當(dāng)類沒(méi)有實(shí)現(xiàn)接口時(shí)使用cglib的動(dòng)態(tài)代理。
Spring Boot動(dòng)態(tài)代理參考文章 http://www.itdecent.cn/p/8cd3b352ab38

下面介紹JDK的動(dòng)態(tài)代理

??下面代碼類似JDK框架的動(dòng)態(tài)代理，主要作用是生成一個(gè)臨時(shí)實(shí)現(xiàn)類。通過(guò)此代碼可以了解JDK動(dòng)態(tài)代理的思想

package com.tomorrowsg.test.util;

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.net.URL;
import java.net.URLClassLoader;


public class ProxyUtil {
    public static Object newProxyInstance(Object target) throws Exception{
        String content = "";
        String packageContent = "package com.test;";
        Class targetInfo = target.getClass().getInterfaces()[0];
        String targetInfoName = targetInfo.getSimpleName();
        String importContent = "import " + targetInfo.getName() + ";";
        String classContent = "public class $Proxy implements " + targetInfoName + "{";

        String fieldContent = "private " + targetInfoName + " target;";
        String construterContent = "public $Proxy(" + targetInfoName + " target){"
                                        +"this.target = target;}";
        String methodsContent = "";
        Method[] methods = targetInfo.getDeclaredMethods();
        for (Method method : methods) {
            String methodName = method.getName();
            Class returnType = method.getReturnType();
            Class<?>[] parameterTypes = method.getParameterTypes();
            String argsContent = "";
            String argsNames = "";
            int i = 0;
            for (Class<?> parameterType : parameterTypes) {
                String simpleName = parameterType.getSimpleName();
                argsContent+= simpleName + " p" + i + ",";
                argsNames+="p" + i + ",";
                i++;
            }
            if (argsContent.length() > 0) {
                argsContent = argsContent.substring(0, argsContent.lastIndexOf(",")-1);
                argsNames = argsNames.substring(0, argsNames.lastIndexOf(",")-1);
            }
            methodsContent += "public " + returnType + " " + methodName + "(" + argsContent+"){"
                    +"System.out.println(\"log...\");"
                            +"target." + methodName + "(" + argsNames + ");}";

        }
        content += packageContent + importContent + classContent + fieldContent + construterContent + methodsContent + "}";
        File file = new File("D:\\com\\test\\$Proxy.java");
        if (!file.exists()) {
            file.createNewFile();
        }
        FileWriter fileWriter = new FileWriter(file);
        fileWriter.write(content);
        fileWriter.flush();
        fileWriter.close();

        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        StandardJavaFileManager fileManager = compiler.getStandardFileManager(null, null, null);
        Iterable units = fileManager.getJavaFileObjects(file);
        JavaCompiler.CompilationTask task = compiler.getTask(null, fileManager, null, null, null, units);
        task.call();
        fileManager.close();
        URL[] urls = new URL[]{new URL("file:D:\\\\")};
        URLClassLoader urlClassLoader = new URLClassLoader(urls);
        Class clazz = urlClassLoader.loadClass("com.test.$Proxy");
        Constructor constructor = clazz.getConstructor(targetInfo);
        Object proxy = constructor.newInstance(target);
        return proxy;
    }
}

??上面代碼的作用是在磁盤(pán)上生成一個(gè)類似如下代碼的一個(gè)代理類，并自動(dòng)完成編譯、裝載并返回一個(gè)實(shí)例化對(duì)象。此種方式生成的代理類更具靈活性，可以重用。

public class LogUserProxy implements UserService {

    private UserService userService;

    public LogUserProxy(UserService userService) {
        this.userService = userService;
    }

    public void query(String name) {
        System.out.println("log ...");
        userService.query(name);
    }
}

public class Main {

    public static void main(String[] args) throws Exception {
        UserService user = new UserImpl();
        UserService proxy = (UserService) ProxyUtil.newProxyInstance(user);
        proxy.query("張三");
    }
}

輸出結(jié)果如下

log...
query name = 張三