简单总结单例模式的4种写法

/**
 * 饿汉式:
 * 不适用new创建对象而是使用静态的getInstance()方法创建对象
 * jvm保证线程安全
 */
public class SingleTest{
    public static void main(String[] args) {
        Single single1 = Single.getInstance();
        Single single2 = Single.getInstance();
        System.out.println(single1.hashCode());
        System.out.println(single2.hashCode());
        System.out.println(single1 == single2);
    }
}

class Single {
    final static Single INSTANCE = new Single();
    public static Single getInstance(){return INSTANCE;}
}

//懒汉式的线程安全的写法又称为双检索懒汉模式
public class SingleTest {
    public static void main(String[] args) {
        for(int i = 0;i < 10; i++){
            new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println(Single.getInstance().hashCode());
                }
            }).start();
        }
    }
}

class Single{
    private static Single instance = null;
    Single(){}
    public static Single getInstance(){
      if (instance == null){
          synchronized (Single.class) {
              if(instance == null){
                  try{
                     Thread.sleep(1);
                  }catch (Exception e){
                     e.printStackTrace();
                  }
                 instance = new Single();
              }
          }
      }
      return instance;
    }
}

/**
 * 静态内部类:完美写法
 */
public class SingleTest {
    public static void main(String[] args) {
        Single single1 = Single.getInstance();
        Single single2 = Single.getInstance();
        System.out.println(single1.hashCode());
        System.out.println(single2.hashCode());
        System.out.println(single1 == single2);
    }
}

class Single{
    private static Single instance;

    public static Single getInstance(){
        return InnerSingle.INSTANCE;
    }

    private static class InnerSingle{
        private final static Single INSTANCE = new Single();
    }
}

//枚举：完美写法
public class SingleTest {
    public static void main(String[] args) {
        Single single1 = Single.INSTANCE;
        Single single2 = Single.INSTANCE;
        System.out.println(single1.hashCode());
        System.out.println(single2.hashCode());
        System.out.println(single1 == single2);
    }
}

enum Single{
    INSTANCE;
}