单例模式

一、

二、

 1.	饿汉式:类初始化时,会立即加载该对象，线程天生安全,调用效率高。
  
 2.	懒汉式: 类初始化时,不会初始化该对象,真正需要使用的时候才会创建该对象,具备懒加载功能

        
        public class TestModel {
        public static void main(String[] args) {
                SingletonDemo01 instance1 = SingletonDemo01.getInstance();
                SingletonDemo01 instance2 = SingletonDemo01.getInstance();
                System.out.println(instance1 == instance2);

                SingletonDemo02 instance3 = SingletonDemo02.getInstance();
                SingletonDemo02 instance4 = SingletonDemo02.getInstance();

                System.out.println(instance3 == instance4);
            }

        }

        // 饿汉式
        class SingletonDemo01{
            // 类初始化时,会立即加载该对象，线程天生安全,调用效率高
            private static SingletonDemo01 singletonDemo01 =  new SingletonDemo01();

            private SingletonDemo01(){
                System.out.println("SingletonDemo01初始化");
            }

            public static SingletonDemo01 getInstance(){
                System.out.println("getInstance");
                return  singletonDemo01;
            }
        }

        // 懒汉式
        class SingletonDemo02 {
            //类初始化时，不会初始化该对象，真正需要使用的时候才会创建该对象。
            private static  SingletonDemo02 singletonDemo02;
            private SingletonDemo02(){
       }

      public synchronized static SingletonDemo02 getInstance(){
          if(singletonDemo02 == null){
              singletonDemo02 = new SingletonDemo02();
          }
              return singletonDemo02;
          }
      }