1.饿汉式

类加载到内存后，就实例化一个单例，JVM保证线程安全，唯一缺点：不管用到与否，类装载时就完成实例化。简单实用，推荐使用！

public class Sin01 {private static final Sin01 INSTANCE=new Sin01();private Sin01(){}public static Sin01 getInstance(){return INSTANCE;}public static void main(String[] args) {Sin01 m1 = Sin01.getInstance();Sin01 m2 = Sin01.getInstance();System.out.println(m1 == m2);}
}

2.跟1是一个意思，但是是在静态代码块中实现

public class Sin02 {private static final Sin02 INSTANCE;static {INSTANCE = new Sin02();}private Sin02() {};public static Sin02 getInstance() {return INSTANCE;}public static void main(String[] args) {Sin02 m1 = Sin02.getInstance();Sin02 m2 = Sin02.getInstance();System.out.println(m1 == m2);}
}

3.懒汉式

虽然达到了按需初始化的目的，但多线程访问时却带来线程不安全的问题，但可以通过synchronized解决，但也带来效率下降

public class Sin04 {private static Sin04 INSTANCE;private Sin04() {}public static synchronized Sin04 getInstance() {if (INSTANCE == null) {try {Thread.sleep(1);} catch (InterruptedException e) {e.printStackTrace();}INSTANCE = new Sin04();}return INSTANCE;}public static void main(String[] args) {for(int i=0; i<100; i++) {new Thread(()->{System.out.println(Sin04.getInstance().hashCode());}).start();}}
}

4.懒汉式-双重检查方法

public class Sin06 {private static volatile Sin06 INSTANCE; private Sin06() {}public static Sin06 getInstance() {if (INSTANCE == null) {//双重检查synchronized (Sin06.class) {if(INSTANCE == null) {try {Thread.sleep(1);} catch (InterruptedException e) {e.printStackTrace();}INSTANCE = new Sin06();}}}return INSTANCE;}public static void main(String[] args) {for(int i=0; i<100; i++) {new Thread(()->{System.out.println(Sin06.getInstance().hashCode());}).start();}}
}

5.完美的方式

静态内部类方式，JVM保证单例且保证线程安全，加载外部类时不会加载内部类，这样可以实现懒加载

public class Sin07 {private Sin07() {}private static class Mgr07Holder {private final static Sin07 INSTANCE = new Sin07();}public static Sin07 getInstance() {return Mgr07Holder.INSTANCE;}public static void main(String[] args) {for(int i=0; i<100; i++) {new Thread(()->{System.out.println(Sin07.getInstance().hashCode());}).start();}}
}

6.枚举单例-完美中的完美方式

不仅可以解决线程同步，还可以防止反序列化。

public enum Sin08 {INSTANCE;public static void main(String[] args) {for(int i=0; i<100; i++) {new Thread(()->{System.out.println(Sin08.INSTANCE.hashCode());}).start();}}
}

推荐使用的是1和5的方式