package io.binghe.concurrency.example.singleton;
/**
 * @author binghe
 * @version 1.0.0
 * @description 懶漢模式，單例實(shí)例在第一次使用的時(shí)候進(jìn)行創(chuàng)建，這個(gè)類是線程不安全的
 */
public class SingletonExample1 {

    private SingletonExample1(){}

    private static SingletonExample1 instance = null;

    public static SingletonExample1 getInstance(){
        //多個(gè)線程同時(shí)調(diào)用，可能會(huì)創(chuàng)建多個(gè)對(duì)象
        if (instance == null){
            instance = new SingletonExample1();
        }
        return instance;
    }
}

package io.binghe.concurrency.example.singleton;
/**
 * @author binghe
 * @version 1.0.0
 * @description 餓漢模式，單例實(shí)例在類裝載的時(shí)候進(jìn)行創(chuàng)建，是線程安全的
 */
public class SingletonExample2 {

    private SingletonExample2(){}

    private static SingletonExample2 instance = new SingletonExample2();

    public static SingletonExample2 getInstance(){
        return instance;
    }
}

package io.binghe.concurrency.example.singleton;
/**
 * @author binghe
 * @version 1.0.0
 * @description 懶漢模式，單例實(shí)例在第一次使用的時(shí)候進(jìn)行創(chuàng)建，這個(gè)類是線程安全的，但是這個(gè)寫(xiě)法不推薦
 */
public class SingletonExample3 {

    private SingletonExample3(){}

    private static SingletonExample3 instance = null;

    public static synchronized SingletonExample3 getInstance(){
        if (instance == null){
            instance = new SingletonExample3();
        }
        return instance;
    }
}

package io.binghe.concurrency.example.singleton;
/**
 * @author binghe
 * @version 1.0.0
 * @description 懶漢模式（雙重鎖同步鎖單例模式）
 *              單例實(shí)例在第一次使用的時(shí)候進(jìn)行創(chuàng)建，這個(gè)類不是線程安全的
 */
public class SingletonExample4 {

    private SingletonExample4(){}

    private static SingletonExample4 instance = null;

    //線程不安全
    //當(dāng)執(zhí)行instance = new SingletonExample4();這行代碼時(shí)，CPU會(huì)執(zhí)行如下指令：
    //1.memory = allocate() 分配對(duì)象的內(nèi)存空間
    //2.ctorInstance() 初始化對(duì)象
    //3.instance = memory 設(shè)置instance指向剛分配的內(nèi)存
    //單純執(zhí)行以上三步?jīng)]啥問(wèn)題，但是在多線程情況下，可能會(huì)發(fā)生指令重排序。
    // 指令重排序?qū)尉€程沒(méi)有影響，單線程下CPU可以按照順序執(zhí)行以上三個(gè)步驟，但是在多線程下，如果發(fā)生了指令重排序，則會(huì)打亂上面的三個(gè)步驟。

    //如果發(fā)生了JVM和CPU優(yōu)化，發(fā)生重排序時(shí)，可能會(huì)按照下面的順序執(zhí)行：
    //1.memory = allocate() 分配對(duì)象的內(nèi)存空間
    //3.instance = memory 設(shè)置instance指向剛分配的內(nèi)存
    //2.ctorInstance() 初始化對(duì)象


    //假設(shè)目前有兩個(gè)線程A和B同時(shí)執(zhí)行g(shù)etInstance()方法，A線程執(zhí)行到instance = new SingletonExample4(); B線程剛執(zhí)行到第一個(gè) if (instance == null){處，
    //如果按照1.3.2的順序，假設(shè)線程A執(zhí)行到3.instance = memory 設(shè)置instance指向剛分配的內(nèi)存，此時(shí)，線程B判斷instance已經(jīng)有值，就會(huì)直接return instance;
    //而實(shí)際上，線程A還未執(zhí)行2.ctorInstance() 初始化對(duì)象，也就是說(shuō)線程B拿到的instance對(duì)象還未進(jìn)行初始化，這個(gè)未初始化的instance對(duì)象一旦被線程B使用，就會(huì)出現(xiàn)問(wèn)題。


    public static SingletonExample4 getInstance(){
        if (instance == null){
            synchronized (SingletonExample4.class){
                if(instance == null){
                    instance = new SingletonExample4();
                }
            }
        }
        return instance;
    }
}

package io.binghe.concurrency.example.singleton;
/**
 * @author binghe
 * @version 1.0.0
 * @description 懶漢模式（雙重鎖同步鎖單例模式）
 *              單例實(shí)例在第一次使用的時(shí)候進(jìn)行創(chuàng)建，這個(gè)類是線程安全的
 */
public class SingletonExample5 {

    private SingletonExample5(){}

    //單例對(duì)象  volatile + 雙重檢測(cè)機(jī)制來(lái)禁止指令重排
    private volatile static SingletonExample5 instance = null;

    public static SingletonExample5 getInstance(){
        if (instance == null){
            synchronized (SingletonExample5.class){
                if(instance == null){
                    instance = new SingletonExample5();
                }
            }
        }
        return instance;
    }
}

package io.binghe.concurrency.example.singleton;
/**
 * @author binghe
 * @version 1.0.0
 * @description 餓漢模式，單例實(shí)例在類裝載的時(shí)候進(jìn)行創(chuàng)建，是線程安全的
 */
public class SingletonExample6 {

    private SingletonExample6(){}

    private static SingletonExample6 instance = null;

    static {
        instance = new SingletonExample6();
    }

    public static SingletonExample6 getInstance(){
        return instance;
    }
}

package io.binghe.concurrency.example.singleton;
/**
 * @author binghe
 * @version 1.0.0
 * @description 枚舉方式進(jìn)行實(shí)例化，是線程安全的，此種方式也是線程最安全的
 */
public class SingletonExample7 {

    private SingletonExample7(){}

    public static SingletonExample7 getInstance(){
        return Singleton.INSTANCE.getInstance();
    }

    private enum Singleton{
        INSTANCE;
        private SingletonExample7 singleton;

        //JVM保證這個(gè)方法絕對(duì)只調(diào)用一次
        Singleton(){
            singleton = new SingletonExample7();
        }
        public SingletonExample7 getInstance(){
            return singleton;
        }
    }
}