Java实现各种类型的单例模式

单例算是最简单的设计模式了，用途自不用多说，但是写好单例也是比较考验一个程序猿功底的，这就是为什么很多面试官乐于让面试者写一写单例的原因，正所谓简约而不简单。废话不多说，下面就用java来实现各种各样的单例吧。

1.饿汉模式

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

2.懒汉模式

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

3.线程安全的单例模式

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

4.双检的线程安全的单例模式

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

5.最牛逼的线程安全的写法

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