架构师养成记--6.单例和多线程、ThreadLocal

一、ThreadLocal

使用wait/notify方式实现的线程安全，性能将受到很大影响。解决方案是用空间换时间，不用锁也能实现线程安全。

来看一个小例子，在线程内的set、get就是threadLocal

public class ConnThreadLocal {

    public static ThreadLocal<String> th = new ThreadLocal<String>();
    
    public void setTh(String value){
        th.set(value);
    }
    public void getTh(){
        System.out.println(Thread.currentThread().getName() + ":" + this.th.get());
    }
    
    public static void main(String[] args) throws InterruptedException {
        
        final ConnThreadLocal ct = new ConnThreadLocal();
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                ct.setTh("张三");
                ct.getTh();
            }
        }, "t1");
        
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                    ct.getTh();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "t2");
        
        t1.start();
        t2.start();
    }
    
}

二、单例和多线程

单例常见的有饥饿模式和懒汉模式，但是这两个模式在多线程情况下是不行的。在多线程中考虑到新能和线程安全的问题一般考虑double check instance 和 static inner class这两种方式实现单例模式。

static inner class方式：

public class InnerSingleton{
  private static class Singleton{
      private static Singleton single = new Singleton();
  }      
  public Singleton getInstance(){
      return Singleton.single;    
   }
}    

double check instance 方式：

public class DubbleSingleton {

    private static DubbleSingleton ds;
    
    public  static DubbleSingleton getDs(){
        if(ds == null){
            try {
                //模拟初始化对象的准备时间...
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized (DubbleSingleton.class) {
                if(ds == null){
                    ds = new DubbleSingleton();
                }
            }
        }
        return ds;
    }
    
    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        },"t1");
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        },"t2");
        Thread t3 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println(DubbleSingleton.getDs().hashCode());
            }
        },"t3");
        
        t1.start();
        t2.start();
        t3.start();
    }
    
}