05-单例模式的概念和类型

单例模式的概念

• 在程序运行时，同一个类只有一个实例

单例模式的类型

饿汉式

• public class Singleton {
    private static Singleton instance = new Singleton();
    private Singleton(){}
    public static Singleton getInstance() {
      return instance;
    }
  }

懒汉式

• class Singleton2 { 
    private static Singleton2 instance = null;
    private Singleton2() {}
    public static Singleton2 getInstance() {
      if(instance == null) {
        instance = new Singleton2(); 
      }
      return instance; 
    } 
  }

懒汉加锁

• class Singleton3 {
    private static Singleton3 instance = null; 
    private Singleton3() {} 
    public static synchronized Singleton3 getInstance() 
    { 
      if(instance == null) { 
        instance = new Singleton3(); 
      }
      return instance; 
    } 
  }

懒汉双检锁

• class Singleton4 {
    private static Singleton4 instance = null; 
    private Singleton4() {} 
    public static Singleton4 getInstance() {
      if(instance == null) {
        synchronized(Singleton4.class) {
          if(instance == null) {
            instance = new Singleton4(); 
          } 
        } 
      }
      return instance; 
    } 
  }

静态内部类实现单例

• class Singleton5 {
    private Singleton5() {} 
    public static final Singleton5 getInstance(){
      return SingletonHolder.INSTANCE; 
    }
    private static class SingletonHolder {
      private static final Singleton5 INSTANCE = new Singleton5(); 
    } 
  }

主要记住饿汉式和懒汉式即可