• 單例模式，最常見的就是饑餓模式和懶漢模式，一個(gè)直接實(shí)例化對(duì)象，一個(gè)是在調(diào)用方法時(shí)示例化對(duì)象
• 在高并發(fā)的情況下饑餓模式和懶漢模式并不適用，推薦使用double check instance 模式和static inner class模式，最常用的是static inner class，簡單，對(duì)線程安全友好
• static inner class模式

public class Singletion {
    
    private static class InnerSingletion {
        private static Singletion single = new Singletion();
    }
    
    public static Singletion getInstance(){
        return InnerSingletion.single;
    }
    
}

• double check instance 模式

public class DubbleSingleton {

    private static DubbleSingleton ds;
    
    public  static DubbleSingleton getDs(){
        if(ds == null){
            try {
                //模擬初始化對(duì)象的準(zhǔn)備時(shí)間...
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
                        //類級(jí)鎖，別的線程不允許訪問這個(gè)class
            synchronized (DubbleSingleton.class) {
                if(ds == null){
                    ds = new DubbleSingleton();
                }
            }
        }
        return ds;
    }
    
    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        },"t1");
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        },"t2");
        Thread t3 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        },"t3");
        
        t1.start();
        t2.start();
        t3.start();
    }
    
}