让人困惑的ThreadLocal 博客分类: j2ee ThreadLocaljava多线程
程序员文章站
2024-02-17 09:57:58
...
目前网上介绍ThreadLocal类用法的博文很多,看过后我们基本可以得出以下结论
ThreadLocal的作用和目的:
用于实现线程内的数据共享,即对于相同的程序代码,多个模块在同一个线程中运行时要共享一份数据,而在另外线程中运行时又共享另外一份数据。
好了,至此ThreadLocal的概念我们弄清了,它是每个线程的菊部变量(该死的输入法)。但是java有了类的局部变量,这个ThreadLocal是否显得多余,我们为什么或者在什么情况下用这个东西?
先看以下两段代码
代码1.使用ThreadLocal
public class ThreadLocalDemo implements Runnable { private final static ThreadLocal<Integer> tLocal = new ThreadLocal<Integer>(); public static void main(String[] agrs) { ThreadLocalDemo td = new ThreadLocalDemo(); Thread t1 = new Thread(td, "t1"); Thread t2 = new Thread(td, "t2"); t1.start(); t2.start(); } /** * 示例业务方法 */ public void test() { while (true) { try { Integer b = (Integer) tLocal.get(); if (b == null) { b = 0; tLocal.set(b); } String currentThreadName = Thread.currentThread().getName(); System.out.println("thread " + currentThreadName + " set value to:" + b++); tLocal.set(b); Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } } @Override public void run() { test(); } }
运行结果:
thread t2 set value to:0 thread t1 set value to:0 thread t1 set value to:1 thread t2 set value to:1 thread t1 set value to:2 thread t2 set value to:2 thread t1 set value to:3 thread t2 set value to:3
代码2.使用局部变量
public class ThreadLocalDemoMutli implements Runnable { private int a = 0; public static void main(String[] agrs) { ThreadLocalDemoMutli td = new ThreadLocalDemoMutli(); ThreadLocalDemoMutli td2 = new ThreadLocalDemoMutli(); Thread t1 = new Thread(td, "t1"); Thread t2 = new Thread(td2, "t2"); t1.start(); t2.start(); } /** * 示例业务方法,用来测 */ public void test() { while (true) { try { String currentThreadName = Thread.currentThread().getName(); System.out.println("thread " + currentThreadName + " set age to:" + a++); Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } } @Override public void run() { test(); } }
运行结果
thread t1 set age to:0 thread t2 set age to:0 thread t1 set age to:1 thread t2 set age to:1 thread t2 set age to:2 thread t1 set age to:2 thread t1 set age to:3 thread t2 set age to:3
感觉用不用结果都一样嘛?再看下面代码
代码3
public class ThreadLocalDemoSingle implements Runnable { private int a = 0; public static void main(String[] agrs) { ThreadLocalDemoSingle td = new ThreadLocalDemoSingle(); Thread t1 = new Thread(td, "t1"); Thread t2 = new Thread(td, "t2"); t1.start(); t2.start(); } /** * 示例业务方法,用来测 */ public void test() { while (true) { try { String currentThreadName = Thread.currentThread().getName(); System.out.println("thread " + currentThreadName + " set age to:" + a++); Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } } @Override public void run() { test(); } }
运行结果:
thread t1 set age to:0 thread t2 set age to:1 thread t1 set age to:2 thread t2 set age to:3 thread t1 set age to:4 thread t2 set age to:5 thread t1 set age to:6 thread t2 set age to:7 thread t1 set age to:8 thread t2 set age to:9 thread t2 set age to:10 thread t1 set age to:11
这回不一样了,对比代码2与代码3的区别就在于创建线程的时候
代码2里
ThreadLocalDemoMutli td = new ThreadLocalDemoMutli(); ThreadLocalDemoMutli td2 = new ThreadLocalDemoMutli(); Thread t1 = new Thread(td, "t1"); Thread t2 = new Thread(td2, "t2");
代码3里
ThreadLocalDemoSingle td = new ThreadLocalDemoSingle(); Thread t1 = new Thread(td, "t1"); Thread t2 = new Thread(td, "t2");
只不过多new了一个对象,原因也一眼看的清楚,但是始终没回答为什么要用ThreadLocal
看来只能从框架里找找原因了,下面搬出hibernate源码(我已经修改了很多)
代码4
public class HibernateUtil { //创建线程局部变量session,用来保存Hibernate的Session public static final ThreadLocal<Integer> session = new ThreadLocal<Integer>(); public static Integer s2=0; /** * 获取当前线程中的Session * @return Session * @throws HibernateException */ public static Integer currentSession() { Integer s = session.get(); if (s == null) { s = 0; session.set(s); } s++; session.set(s); return s; } /** * 获取当前线程中的Session * @return Session * @throws HibernateException */ public static Integer currentSessionError() { return s2++; } }
代码5
public class THibernate implements Runnable { @Override public void run() { while (true) { try { String currentThreadName = Thread.currentThread().getName(); System.out.println("thread " + currentThreadName +"|tlocal:"+HibernateUtil.currentSession()+"|common:"+HibernateUtil.currentSessionError()); Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } } public static void main(String[] args) { THibernate t=new THibernate(); Thread t1 = new Thread(t, "t1"); Thread t2 = new Thread(t, "t2"); t1.start(); t2.start(); } }
运行结果:
thread t1|tlocal:1|common:0 thread t2|tlocal:1|common:1 thread t1|tlocal:2|common:2 thread t2|tlocal:2|common:3 thread t1|tlocal:3|common:4 thread t2|tlocal:3|common:5 thread t1|tlocal:4|common:6 thread t2|tlocal:4|common:7 thread t1|tlocal:5|common:8 thread t2|tlocal:5|common:9
HibernateUtil是一个可供多线程访问的工具类,从运行结果来看ThreadLocal做到了变量的线程隔离,而普通的类变量无法做到这点。
那我把代码4里的
public static Integer s2=0;
改成
public Integer s2=0;
不就也完成了ThreadLocal做的事情吗
但是,你的方法是static的,用到的变量也需要是static的,在这种情况下,还真的必须用ThreadLocal,才能做到变量的线程隔离.
原创文章,转载请声名出处 http://spjich.iteye.com/blog/2264457