Java-单例设计模式

单例模式

单线程

饿汉式：

  public Class Singleton{
  //类被加载的时候就实例化一个对象
        private static Singleton singleton1 = new Singleton();
  //私有的构造方法
        private Singleton(){}
  //以自己实例为返回值的静态的公有方法
        public static Singleton getSingleton(){
              return singleton1;
        }
  }

懒汉式：

  public Class Singleton{
        private static Singleton singleton2;
        private Singleton(){}
        public static Singleton getSingleton(){
              if(singleton2 == null){
                    singleton2 = new Singleton();
              }
              return singleton2;
        }
  }

多线程

饿汉式天生线程安全

懒汉式保证线程安全的四种方法：

• synchronized方法

  public Class Singleton{
        private static Singleton singleton2;
        private Singleton(){}
        public static synchronized Singleton getSingleton(){
              if(singleton2 == null){
                    singleton2 = new Singleton();
              }
              return singleton2;
        }
  }
  

• synchronized代码块

  public Class Singleton{
        private static Singleton singleton2;
        private Singleton(){}
        public static Singleton getSingleton(){
              synchronized（Singleton.class）{
                    if(singleton2 == null){
                          singleton2 = new Singleton();
                    }
                    
              }
              return singleton2;
        }
  }
  

• 内部类

  public Class Singleton{
        private Singleton(){}
        private static Singleton Holder(){
              private static singleton2 = new Singleton() ;
        }
        public static Singleton getSingleton(){
              return Holder.singleton2;
        }
  }
  

• 双重延迟检测

  public Class Singleton{
        private static volatile Singleton singleton2;
        private Singleton(){}
        public static Singleton getSingleton(){
              if(singleton2 == null){
              //只在第一次new实例的时候同步
                    synchronized（Singleton.class）{
                          singleton2 = new Singleton();
                    }
                    
              }
              return singleton2;
        }
  }
  

• ThreadLocal

  public Class Singleton{
        private static Singleton singleton2;
        private static ThreadLocal<Singleton> threadLocal = new ThreadLocal<>();
        private Singleton(){}
        private static Singleton CreateSingleton(){
              if (singleton4 == null) {          // 第二次检查：该单例是否被创建
                  singleton4 = new Singleton4();
              }
              threadLocal.set(singleton4);
        }
        public static Singleton getSingleton(){
              if (threadLocal.get() == null) {        // 第一次检查：该线程是否第一次访问
                    createSingleton();
              }
              return singleton2;
        }
  }