【GOF23設計模式】_單例模式JAVA226-228

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一、S03E226_01【GOF23設計模式】_單例模式、應用場景、餓漢式、懶漢式

1、GOF23設計模式

2、單例模式

3、餓漢式

package com.test.singleton;
/**
 * 測試餓漢式單例模式
 */
public class SingletonDemo01 {

    //類初始化時，立即加載這個對象(沒有延時加載的優勢)。加載類時，天然的是線程安全的！
    private static SingletonDemo01 instance = new SingletonDemo01();

    private SingletonDemo01(){//私有化構造器
    }

    //方法不用同步，調用效率高！
    public static SingletonDemo01 getInstance(){
        return instance;
    }
}

4、懶漢式

package com.test.singleton;
/**
 * 測試懶漢式單例模式
 */
public class SingletonDemo02 {

    //類初始化時，不初始化這個對象(延時加載，真正用的時候再創建)
    private static SingletonDemo02 instance;

    private SingletonDemo02(){//私有化構造器
    }

    //方法同步，調用效率低！
    public static synchronized SingletonDemo02 getInstance(){
        if(null==instance){
            instance = new SingletonDemo02();
        }
        return instance;
    }
}

二、S03E227_01【GOF23設計模式】_單例模式、雙重檢查鎖式、靜態內部類式、枚舉式、UML類圖

雙重檢測鎖實現

package com.test.singleton;
/**
 * 測試雙重檢測鎖式單例模式
 */
public class SingletonDemo03 {

    //類初始化時，不初始化這個對象(延時加載，真正用的時候再創建)
    private static SingletonDemo03 instance;

    private SingletonDemo03(){//私有化構造器
    }

    //調用效率低！
    public static SingletonDemo03 getInstance(){
        if(null==instance){
            SingletonDemo03 sc;
            synchronized(SingletonDemo03.class){
                sc = instance;
                if(null==sc){
                    synchronized (SingletonDemo03.class) {
                        if(null==sc){
                            sc = new SingletonDemo03();
                        }
                    }
                    instance = sc;
                }
            }
        }
        return instance;
    }
}

靜態內部類實現

package com.test.singleton;
/**
 * 測試靜態內部類實現單例模式
 * 這種方式：線程安全，調用效率高，並且實現了延時加載！
 */
public class SingletonDemo04 {

    private static class SingletonClassInstance {
        private static final SingletonDemo04 instance = new SingletonDemo04();
    }

    private SingletonDemo04(){//私有化構造器
    }

    //方法沒有同步，調用效率高！
    public static SingletonDemo04 getInstance(){
        return SingletonClassInstance.instance;
    }
}

枚舉實現

package com.test.singleton;
/**
 * 測試枚舉式實現單例模式(沒有延時加載)
 */
public enum SingletonDemo05 {

    //這個枚舉元素，本身就是單例對象！
    INSTANCE;

    //添加自己需要的操作！
    public void singletonOperation(){
    }
}

package com.test.singleton;

public class Client {
    public static void main(String[] args) {
        SingletonDemo01 s1 = SingletonDemo01.getInstance();
        SingletonDemo01 s2 = SingletonDemo01.getInstance();
        System.out.println(s1==s2);//true

        SingletonDemo03 s3 = SingletonDemo03.getInstance();
        SingletonDemo03 s4 = SingletonDemo03.getInstance();
        System.out.println(s3==s4);//true

        System.out.println(SingletonDemo05.INSTANCE==SingletonDemo05.INSTANCE);//true
    }
}

五種單例模式實現

三、S03E228_01【GOF23設計模式】_單例模式、反射和反序列化漏洞和解決方案、多線程環境測試、CountDownLatch同步類的使用

package com.test.singleton;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Constructor;

/**
 * 測試反射和反序列化破解單例模式
 */
public class Client2 {
    public static void main(String[] args) throws Exception {
        SingletonDemo06 s1 = SingletonDemo06.getInstance();
        SingletonDemo06 s2 = SingletonDemo06.getInstance();
        System.out.println(s1);
        System.out.println(s2);

        //通過反射的方式直接調用私有構造器
/*      Class<SingletonDemo06> clazz = (Class<SingletonDemo06>) Class.forName("com.test.singleton.SingletonDemo06");
        Constructor<SingletonDemo06> c = clazz.getDeclaredConstructor(null);
        c.setAccessible(true);//訪問私有，破解單例
        SingletonDemo06 s3 = c.newInstance();
        SingletonDemo06 s4 = c.newInstance();
        System.out.println(s3);//SingletonDemo06的私有構造器中加了代碼防止破解
        System.out.println(s4);*/

        //通過反序列化的方式構造多個對象
        FileOutputStream fos = new FileOutputStream("g:/java/test/a.txt");
        ObjectOutputStream oos = new ObjectOutputStream(fos);
        oos.writeObject(s1);
        oos.close();
        fos.close();

        ObjectInputStream ois = new ObjectInputStream(new FileInputStream("g:/java/test/a.txt"));
        SingletonDemo06 s3 = (SingletonDemo06) ois.readObject();
        System.out.println(s3);//SingletonDemo06定義了readResolve()防止破解
    }
}

package com.test.singleton;

import java.io.ObjectStreamException;
import java.io.Serializable;

/**
 * 測試懶漢式單例模式(如何防止反射和反序列化漏洞)
 */
public class SingletonDemo06 implements Serializable{

    //類初始化時，不初始化這個對象(延時加載，真正用的時候再創建)
    private static SingletonDemo06 instance;

    private SingletonDemo06(){//私有化構造器
        if(instance!=null){//防止反射破解單例(對象.setAccessible(true);)
            throw new RuntimeException();
        }
    }

    //方法同步，調用效率低！
    public static synchronized SingletonDemo06 getInstance(){
        if(null==instance){
            instance = new SingletonDemo06();
        }
        return instance;
    }

    //反序列化時，如果定義了readResolve()則直接返回此方法指定的對象。而不需要單獨再創建新對象！
    private Object readResolve() throws ObjectStreamException{
        return instance;
    }
}

package com.test.singleton;

import java.util.concurrent.CountDownLatch;

/**
 * 測試多線程環境下五種創建單例模式的效率
 */
public class Client3 {
    public static void main(String[] args) throws Exception{

        int threadNum = 10;
        final CountDownLatch countDownLatch = new CountDownLatch(threadNum); 

        long start = System.currentTimeMillis();
        for(int i=0;i<10;i++){
            new Thread(new Runnable(){
                @Override
                public void run() {
                    for (int i = 0; i < 1000000; i++) {
                        Object o = SingletonDemo01.getInstance();//分別測試幾種單例模式的效率
                        //Object o = SingletonDemo05.INSTANCE;
                    }

                    countDownLatch.countDown();//計數器減1
                }
            }).start();
        }

        countDownLatch.await();//main線程阻塞，直到計數器變爲0，纔會繼續往下執行！

        long end = System.currentTimeMillis();
        System.out.println("總耗時：" + (end-start));
    }
}