Java中单例模式的几种实现

1. 饿汉式单例

public class Single{
  private static Single mInstance = new Single();
  private Single(){}
  public static Single getInstance(){
    return mInstance;
  }
}

优点是线程安全，缺点时没有办法实现懒加载。

2. 懒汉式+同步方法单例

public class Single{
  private static Signle mInstance;
  private Single(){}
  public static synchronized Single getInstance(){
    if(mInstance == null){
        mInstance = new Single();
    }
    return mInstance;
  }
}

优点是线程安全，懒加载
缺点是效率不高

3. volatile+Double Check方式

public class Single{
  private volatile static Single mInstance;
  private Single(){}
  private static Single getInstance(){
    if(mInstance == null){
      synchronized(Single.class){
        if(mInstance == null){
          mInstance = new Single();
        }
      }
    }
    return mInstance;
  }
}

优点是线程安全，效率还可以

4. Holder模式

public class Single{
  private Single(){}
  private static class Holder{
    private static Single instance = new Single();
  }
  public static Single getInstance(){
    return Holder.instance;
  }
}

优点是线程安全，懒加载，效率高。

5. 枚举方式

public enum Single{
  //单例变量，也是枚举变量
  INSTANCE;
  public static getInstance(){
    return INSTANCE;
  }
}

优点，线程安全
缺点，没有懒加载

6. 枚举+Holder

public class Single{
  private Single(){}
  //这个枚举类型的Holder主要是为了确保单例
  private enum EnumHolder{
    INSTANCE;
    //单例的枚举型中包含一个Single类型的变量
    private Single instance;
    EnumHolder(){
      //在构造函数中延时初始化
      this.instance = new Single();
    }
    //提供方法返回单例
    private Single getInstance(){
        return instance;
    }
  }
  //通过枚举类型返回单例实例
  public static Single getInstance(){
    return EnumHolder.INSTANCE.getInstance();
  }
}