单例：在虚拟机里保证类对象只有一个。比如：计数器。
1.直接初始化
2.你用我的时候我再初始化：
问题：多线程的时候会出现多个
3.加锁 synchronized
4.加锁加强版
5.防止指令重排序 volatile
1.直接初始化
Public calss Mgr01
{private static final Mgr01 INSTANCE =new Mgr01();private Mgr01(){}public static Mgr01 getInstance(){return INSTANCE;}public void m(){System.out.println("m");}public static void main(){Mgr01 m1=Mgr01.getInstance();Mgr01 m2=Mgr01.getInstance();System.out.println(m1==m2);}
}
2.你用我的时候我再初始化
问题：多线程的时候会出现多个
Public calss Mgr02
{ private static final Mgr02 INSTANCE;//1.实例变量private Mgr02(){} //2.构造函数public static Mgr02 getInstance() //3.得到实例{if(INSTANCE==null) //两个线程同时访问，会new两个INSTANCE。{ INSTANCE=new Mgr02();}return INSTANCE;}//匿名的线程构造函数// thread().start();/*thread(()->).start();*/public void m(){ System.out.println("m");}public static void main(){for(int i=0;i<100;i++){new Thread(()->System.out.println(Mgr02.getInstance().hashCode())).start();}}
}
3.加锁 synchronized
Public calss Mgr03
{ private static final Mgr03 INSTANCE;private Mgr03(){} public static synchronized Mgr03 getInstance() //加锁synchronized{if(INSTANCE==null) { INSTANCE=new Mgr03();}return INSTANCE;}public void m(){ System.out.println("m");}public static void main(){for(int i=0;i<100;i++){new Thread(()->System.out.println(Mgr03.getInstance().hashCode())).start();}}
}
4.加锁加强版
5.防止指令重排序 volatile
Public calss Mgr05
{ private static final Mgr05 INSTANCE;private Mgr05(){} public static volatile Mgr05 getInstance() //volatile {if(INSTANCE==null) //如果为空{ // 1.如果两个线程同时运行到此 那么会顺序的运行两次newsynchronized(Mgr05.class){if(INSTANCE==null) //2.所以必须有这句{ INSTANCE=new Mgr03();}}}return INSTANCE;}public void m(){ System.out.println("m");}public static void main(){for(int i=0;i<100;i++){new Thread(()->System.out.println(Mgr05.getInstance().hashCode())).start();}}
}