设计模式之---单例模式和多线程

in #steemit7 years ago (edited)


单例模式是23个设计模式中比较简单,也是最常用的模式之一,虽然简单,但在多线程并发访问时如果不注意一些使用细节,会引发意想不到的bug。

单例模式

定义:保证一个类只有一个实例,并且自行实例化并向整个系统提供这个实例。
类图:待完成
优点:

  • 减少内存开支
  • 减少性能开销
  • 避免对资源的多重占用
  • 提供和共享全局访问量
    缺点:
  • 扩展性差
  • 测试不方便
  • 单例模式和单一职责莫设计原则向冲突

单例的两种使用形式

饿汉模式


public class EagerSingleton {
    private static EagerSingleton singleton=new EagerSingleton();
 private EagerSingleton(){

    }
    public static EagerSingleton getSingleton(){
        return singleton;
  }

}

懒汉模式

该代码采用了DCL双锁检测(double checked locking),避免了因为多线程并发下可能出现的异常

  public class LazySingleton {
    private volatile static LazySingleton singleton;
 private LazySingleton(){

    }
    public static LazySingleton getSingleton() throws InterruptedException {
        if (singleton != null) {

        } else {
            Thread.sleep(3000);
 synchronized (LazySingleton.class) {
                if (singleton == null) {
                    singleton = new LazySingleton();
  }
            }
        }
        return singleton;
  }
}

测试类:

  public class MyThread extends Thread{
    @Override
  public void run() {
        super.run();
  System.out.println("EagerSingleton"+EagerSingleton.getSingleton().hashCode());
 try {
            System.out.println("LazySingleton"+LazySingleton.getSingleton().hashCode());
  } catch (InterruptedException e) {
            e.printStackTrace();
  }
    }
}
public class Test {
    public static void main(String[] args) {
        MyThread t1=new MyThread();
  MyThread t2=new MyThread();
  MyThread t3=new MyThread();
  MyThread t4=new MyThread();
  t1.start();
  t2.start();
  t3.start();
  t4.start();
  }

}

运行结果

EagerSingleton908288934
EagerSingleton908288934
EagerSingleton908288934
EagerSingleton908288934
LazySingleton1515217202
LazySingleton1515217202
LazySingleton1515217202
LazySingleton1515217202

可以看到hash值相同,证明了我们的结论,试着把双重检查中的判空代码去掉,再运行下结果,你会发现单例失效了!
懒汉模式的使用比较多,我们可以使用一些技巧,对懒汉模式进行封装
代码如下:

public abstract class Singleton<T> {
    private T mInstance;

    protected abstract T create();

    public final T get() {
        synchronized (this) {
            if (mInstance == null) {
                mInstance = create();
            }
            return mInstance;
        }
    }
}

可以看到,这是一个基于单例的模版类,使用方法也很简单,继承此类,提供约束的泛型,实现create()方法即可

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