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();
     }

 }