單例模式

餓漢式 (靜態(tài)常量)

優(yōu)點(diǎn)：寫(xiě)法簡(jiǎn)單，在類裝載時(shí)就完成了實(shí)例化，避免了線程同步問(wèn)題

缺點(diǎn)：在類裝載時(shí)就完成了實(shí)例化，有可能從始至終都用不上這個(gè)實(shí)例，造成內(nèi)存浪費(fèi)

public class Single {
    private Single() {}

    private final static Single single = new Single();

    public static Single getInstance() {
        return single;
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(Single.getInstance() == Single.getInstance());
    }
}

console

true

餓漢式 (靜態(tài)代碼塊)

這種方式實(shí)際與靜態(tài)常量方式類似，都是在類加載時(shí)實(shí)例化。優(yōu)缺點(diǎn)也與靜態(tài)常量方式相同

public class Single2 {
    static {
        single = new Single2();
    }

    private Single2() {}

    private final static Single2 single;

    public static Single2 getInstance() {
        return single;
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(Single.getInstance() == Single.getInstance());
        System.out.println(Single2.getInstance() == Single2.getInstance());
    }
}

console

true
true

懶漢式

有可能會(huì)出現(xiàn)多個(gè)線程同時(shí)搶到了 getInstance 方法的情況，所以線程不安全，慎用

可以通過(guò)添加 synchronized 來(lái)添加線程鎖使線程同步，但是會(huì)造成線程擁堵。導(dǎo)致效率變低

public class Single3 {
    private Single3() {}

    private static Single3 single = null;

    public static Single3 getInstance() {
        if (null == single) single = new Single3();
        return single;
    }
    
//    public static synchronized Single3 getInstance() {
//        if (null == single) single = new Single3();
//        return single;
//    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(Single.getInstance() == Single.getInstance());
        System.out.println(Single2.getInstance() == Single2.getInstance());
        System.out.println(Single3.getInstance() == Single3.getInstance());
    }
}

console

true
true
true

懶漢式（二）

通過(guò)雙重驗(yàn)證，再加上 volatile 關(guān)鍵字對(duì)變量的修飾達(dá)到懶加載/安全并且速度快的單例結(jié)果

volatile 詳解

public class Single4 {
    private Single4() {}

//    private static Single4 single1 = null;

    /**
     * 添加 volatile 關(guān)鍵字
     * 1、volatile保證可見(jiàn)性
     * 2、volatile不保證原子性
     * 3、volatile禁止指令重排
     */
    private static volatile Single4 single = null;

    /* 反面教材 */
//    public static Single4 getInstance() {
//        if (null == single1) {
//            // 方法沒(méi)有加鎖，無(wú)法阻止多線程進(jìn)入 if 方法
//            synchronized (Single4.class) {
//                single1 = new Single4();
//            }
//        }
//        return single1;
//    }

    /**
     * 通過(guò)雙重驗(yàn)證，再加上 volatile 關(guān)鍵字對(duì)變量的修飾達(dá)到懶加載/安全并且速度快的單例結(jié)果
     * @return
     */
    public static Single4 getInstance() {
        if (null == single) {
            synchronized (Single4.class) {
                if (null == single) {
                    single = new Single4();
                }
            }
        }
        return single;
    }
}

console

true
true
true
true
true

靜態(tài)內(nèi)部類(推薦使用)

線程安全

通過(guò) jvm 虛擬機(jī)裝載機(jī)制來(lái)實(shí)現(xiàn)單例模式

1. 當(dāng)外部類在被裝載時(shí)，內(nèi)部類不會(huì)被裝載
2. 當(dāng)外部類使用到內(nèi)部類時(shí)，才會(huì)裝載到內(nèi)部類。jvm 在裝載類時(shí)線程是安全的。并且靜態(tài)內(nèi)部類只會(huì)被加載一次

public class Single5 {
    private Single5() {}

    private static class SingleInstance {
        private static final Single5 single = new Single5();
    }

    public static Single5 getInstance() {
        return SingleInstance.single;
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(Single.getInstance() == Single.getInstance());
        System.out.println(Single2.getInstance() == Single2.getInstance());
        System.out.println(Single3.getInstance() == Single3.getInstance());
        System.out.println(Single4.getInstance() == Single4.getInstance());
        System.out.println(Single5.getInstance() == Single5.getInstance());
    }
}

console

true
true
true
true
true

枚舉 (推薦使用)

通過(guò)枚舉實(shí)現(xiàn)單例還可以防止被反射進(jìn)行多次實(shí)例化。是目前最完美的單例機(jī)制。

public enum Single6 {
    INSTANCE;

    public String method() {
        return "Hello World";
    }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(Single.getInstance() == Single.getInstance());
        System.out.println(Single2.getInstance() == Single2.getInstance());
        System.out.println(Single3.getInstance() == Single3.getInstance());
        System.out.println(Single4.getInstance() == Single4.getInstance());
        System.out.println(Single5.getInstance() == Single5.getInstance());
        System.out.println(Single6.INSTANCE == Single6.INSTANCE);
        System.out.println(Single6.INSTANCE.method());
    }
}

console

true
true
true
true
true
true
Hello World