Java中的单例模式

• 多线程中可能会导致对象的多次初始化.

懒汉式的实现

• 之所以成为懒汉,个人认为应该是类的初始化只在需要的时候进行,所以很"懒".

线程不安全

/*懒汉 非线程安全*/
public class SingleTon_01 {
    private static SingleTon_01 singleTon_01 = null;
    private SingleTon_01() {

    }
    public static SingleTon_01 getSingleTon_01() {
        if (singleTon_01 == null) {
            singleTon_01 = new SingleTon_01();
        }
        return singleTon_01;
    }
    public void print() {
        System.out.println("singleton_01");
    }
}

线程安全

/*懒汉 线程安全*/
public class SingleTon_02 {
    private static SingleTon_02 singleTon_02 = null;
    private SingleTon_02() {}
    private static synchronized SingleTon_02 getSingleTon_02() {
        if (singleTon_02 == null) {
            singleTon_02 = new SingleTon_02();
        }
        return singleTon_02;
    }
}

饿汉实现

• 在类加载时就进行初始化,可能会产生垃圾对象,但是不会有多线程的问题出现.

    /*饿汉式*/ 
    public class SingleTon_03 {
        private static SingleTon_03 singleTon_03 = new SingleTon_03();
        private SingleTon_03() {
  
        }
        public static SingleTon_03 getSingleTon_03() {
            return singleTon_03;
        }
    }
  

双重校验锁模式

• 要注意变量需要添加volatile关键字.

  • 假设有两个进程分别调用了getInstance方法,进程A首先将变量复制到CPU0缓存当中,进行初始化操作,如果没有volatile关键字,那么进程B在调用此方法时复制到CPU1缓存中的可能是未被更新的null,此时问题就出现了.

      /*双重校验锁模式*/
      public class SingleTon_04 {
          private static volatile SingleTon_04 singleTon_04 = null;
          private SingleTon_04() {
    
          }
          public static SingleTon_04 getSingleTon_04() {
              if (singleTon_04 == null) {
                  synchronized (SingleTon_04.class) {
                      singleTon_04 = new SingleTon_04();
                  }
              }
              return singleTon_04;
          }
      }
    

静态内部类实现

• 通过classloader机制确保初始化时只有一个线程

    /*静态内部类*/
    public class SingleTon_05 {
        private static class InstanceHolder{
            private static SingleTon_05 singleTon_05 = new SingleTon_05();
        }
        private SingleTon_05() {
  
        }
        public SingleTon_05 getSingleTon_05() {
            return InstanceHolder.singleTon_05;
        }
    }
  

enum实现

public enum SingleTon_06 {
    INSTANCE;
    public void print() {
        System.out.println("singleTon_06");
    }
}