InheritableThreadLocal变量的可见性
来源:互联网 发布:linux下oracle安装教程 编辑:程序博客网 时间:2024/05/16 14:01
转载自:http://stevex.blog.51cto.com/4300375/1289499
InheritableThreadLocal类继承于ThreadLocal类,所以它具有ThreadLocal类的特性,但又是一种特殊的ThreadLocal,其特殊性在于InheritableThreadLocal变量值会自动传递给所有子线程,而普通ThreadLocal变量不行。那么子线程是否可以修改InheritableThreadLocal变量值然后反向传递给主线程了?我们先来看一组测试代码和结果:
import java.util.concurrent.TimeUnit;public class TestMain {private static ThreadLocal<String> stringItl = new InheritableThreadLocal<String>(){ protected String initialValue() { System.out.println(Thread.currentThread().getName() + " initialize stringItl variable."); return "String init"; } }; private static ThreadLocal<String> stringItl2 = new InheritableThreadLocal<String>(){ protected String initialValue() { System.out.println(Thread.currentThread().getName() + " initialize stringItl2 variable."); return "String2 init"; } }; private static ThreadLocal<StringBuffer> stringBufferItl = new InheritableThreadLocal<StringBuffer>(){ protected StringBuffer initialValue() { System.out.println(Thread.currentThread().getName() + " initialize stringBufferItl variable."); return new StringBuffer("StringBuffer init"); } }; private static ThreadLocal<StringBuffer> stringBufferItl2 = new InheritableThreadLocal<StringBuffer>(){ protected StringBuffer initialValue() { System.out.println(Thread.currentThread().getName() + " initialize stringBufferItl2 variable."); return new StringBuffer("StringBuffer2 init"); } }; public static void main(String[] args) throws InterruptedException { System.out.println(Thread.currentThread().getName() + " first get stringItl : " + stringItl.get()); System.out.println(Thread.currentThread().getName() + " first get stringBufferItl : " + stringBufferItl.get().toString()); stringItl.set("Parent"); stringBufferItl.set(new StringBuffer("ParentBuffer")); System.out.println(Thread.currentThread().getName() + " get after set stringItl : " + stringItl.get()); System.out.println(Thread.currentThread().getName() + " get after set stringBufferItl : " + stringBufferItl.get().toString()); for(int i=0; i<2; i++){ new Thread(){ public void run(){ System.out.println(Thread.currentThread().getName() + " first get stringItl : " + stringItl.get()); stringItl.set(Thread.currentThread().getName() + "Child"); System.out.println(Thread.currentThread().getName() + " get after set stringItl : " + stringItl.get()); System.out.println(Thread.currentThread().getName() + " first get stringItl2 : " + stringItl2.get()); stringItl2.set(Thread.currentThread().getName() + "Child"); System.out.println(Thread.currentThread().getName() + " get after set stringItl2 : " + stringItl2.get()); System.out.println(Thread.currentThread().getName() + " first get stringBufferItl : " + stringBufferItl.get().toString()); stringBufferItl.get().append(Thread.currentThread().getName()); System.out.println(Thread.currentThread().getName() + " get after set stringBufferItl : " + stringBufferItl.get().toString()); System.out.println(Thread.currentThread().getName() + " first get stringBufferIt2 : " + stringBufferItl2.get().toString()); stringBufferItl2.get().append(Thread.currentThread().getName()); System.out.println(Thread.currentThread().getName() + " get after set stringBufferItl2 : " + stringBufferItl2.get().toString()); } }.start(); } for(int i=0; i<2; i++){ new Thread(){ public void run(){ System.out.println(Thread.currentThread().getName() + " first get stringItl : " + stringItl.get()); stringItl.set(Thread.currentThread().getName() + "Child"); System.out.println(Thread.currentThread().getName() + " get after set stringItl : " + stringItl.get()); System.out.println(Thread.currentThread().getName() + " first get stringItl2 : " + stringItl2.get()); stringItl2.set(Thread.currentThread().getName() + "Child"); System.out.println(Thread.currentThread().getName() + " get after set stringItl2 : " + stringItl2.get()); System.out.println(Thread.currentThread().getName() + " first get stringBufferItl : " + stringBufferItl.get().toString()); stringBufferItl.set(new StringBuffer(Thread.currentThread().getName() + "Buffer")); System.out.println(Thread.currentThread().getName() + " get after set stringBufferItl : " + stringBufferItl.get().toString()); System.out.println(Thread.currentThread().getName() + " first get stringBufferIt2 : " + stringBufferItl2.get().toString()); stringBufferItl2.get().append(Thread.currentThread().getName()); System.out.println(Thread.currentThread().getName() + " get after set stringBufferItl2 : " + stringBufferItl2.get().toString()); } }.start(); } TimeUnit.SECONDS.sleep(2);//let children threads run first System.out.println(Thread.currentThread().getName() + " second get stringItl : " + stringItl.get()); System.out.println(Thread.currentThread().getName() + " first get stringItl2 : " + stringItl2.get()); System.out.println(Thread.currentThread().getName() + " second get stringBufferItl : " + stringBufferItl.get().toString()); System.out.println(Thread.currentThread().getName() + " first get stringBufferItl2 : " + stringBufferItl2.get().toString()); } }
代码运行结果:
main initialize stringItl variable.main first get stringItl : String initmain initialize stringBufferItl variable.main first get stringBufferItl : StringBuffer initmain get after set stringItl : Parentmain get after set stringBufferItl : ParentBuffer//此时 stringItl->"Parent" stringBufferItl->"ParentBuffer"Thread-0 first get stringItl : Parent//Thread-0不用初始化stringItlThread-0 get after set stringItl : Thread-0ChildThread-0 initialize stringItl2 variable.//父线程未初始化的Thread-0 first get stringItl2 : String2 initThread-0 get after set stringItl2 : Thread-0ChildThread-0 first get stringBufferItl : ParentBufferThread-0 get after set stringBufferItl : ParentBufferThread-0Thread-0 initialize stringBufferItl2 variable.Thread-0 first get stringBufferIt2 : StringBuffer2 initThread-0 get after set stringBufferItl2 : StringBuffer2 initThread-0Thread-1 first get stringItl : ParentThread-1 get after set stringItl : Thread-1ChildThread-1 initialize stringItl2 variable.Thread-1 first get stringItl2 : String2 initThread-1 get after set stringItl2 : Thread-1ChildThread-1 first get stringBufferItl : ParentBufferThread-0Thread-1 get after set stringBufferItl : ParentBufferThread-0Thread-1Thread-1 initialize stringBufferItl2 variable.Thread-1 first get stringBufferIt2 : StringBuffer2 initThread-1 get after set stringBufferItl2 : StringBuffer2 initThread-1Thread-3 first get stringItl : ParentThread-3 get after set stringItl : Thread-3ChildThread-3 initialize stringItl2 variable.Thread-3 first get stringItl2 : String2 initThread-2 first get stringItl : ParentThread-2 get after set stringItl : Thread-2ChildThread-2 initialize stringItl2 variable.Thread-2 first get stringItl2 : String2 initThread-2 get after set stringItl2 : Thread-2ChildThread-2 first get stringBufferItl : ParentBufferThread-0Thread-1Thread-2 get after set stringBufferItl : Thread-2BufferThread-2 initialize stringBufferItl2 variable.Thread-2 first get stringBufferIt2 : StringBuffer2 initThread-2 get after set stringBufferItl2 : StringBuffer2 initThread-2Thread-3 get after set stringItl2 : Thread-3ChildThread-3 first get stringBufferItl : ParentBufferThread-0Thread-1Thread-3 get after set stringBufferItl : Thread-3BufferThread-3 initialize stringBufferItl2 variable.Thread-3 first get stringBufferIt2 : StringBuffer2 initThread-3 get after set stringBufferItl2 : StringBuffer2 initThread-3main second get stringItl : Parentmain initialize stringItl2 variable.main first get stringItl2 : String2 initmain second get stringBufferItl : ParentBufferThread-0Thread-1main initialize stringBufferItl2 variable.main first get stringBufferItl2 : StringBuffer2 init
从运行结果可以看出:
如果InheritableThreadLocal存储的是不变性(immutable)的对象,如String,对于主线程设置的值子线程可以通过get函数获取,但子线程调用set函数设置新值后,对主线程没有影响,对其它子线程也没有影响,只对自己可见,(请参考代码例子中的stringItl变量);如果主线程还没有获取(get)或者设置(set)过ThreadLocal变量,而子线程先获取(get)或者设置(set)了ThreadLocal变量,那么这个份值只属于那个子线程,对主线程和其它子线程都不可见,(请参考代码例子中的stringIt2变量).
如果InheritableThreadLocal存储的是可变性(mutable)的对象,如StringBuffer,对于主线程设置的值子线程可以通过get函数获取,但子线程调用set函数设置新值后,对主线程没有影响,对其它子线程也没有影响,只对自己可见,但如果子线程先get获取再修改对象的属性,那么这个修改对主线程和其它子线程是可见的,即他们还是共享一个引用(请参考代码例子中的stringBufferItl变量);如果主线程还没有获取(get)或者设置(set)过ThreadLocal变量,而子线程先获取(get)或者设置(set)了ThreadLocal变量,那么这份值只属于那个子线程,对主线程和其它子线程都不可见,(请参考代码例子中的stringBufferItl2变量).
所以子线程只能通过修改可变性(Mutable)对象对主线程才是可见的,即才能将修改传递给主线程,但这不是一种好的实践,不建议使用,为了保护线程的安全性,一般建议只传递不可变(Immuable)对象,即没有状态的对象。
关于ThreadLocal,大家可以参考以下两篇文章:
http://www.ibm.com/developerworks/cn/java/j-threads/index3.html
http://geekexplains.blogspot.sg/2009/02/threadlocal-inheritablethreadlocal-in.html
本文出自 “力量来源于赤诚的爱!” 博客,请务必保留此出处http://stevex.blog.51cto.com/4300375/1289499
个人理解:
对于InheritableThreadLocal来说,分2种情况:
1、存储Immuable对象,此时等价于ThreadLocal+可见性(如果对象被main thread设置了),即对修改互不相干,但是子线程能获取到父线程已设置的值。
2、存储Mutable对象时,与Immuable的区别是:对于StringBuffer,其实存储的是引用,所以子线程append后,主线程也能看到。但是不能被set,set会产生新的引用,就会各自独立了。
- InheritableThreadLocal变量的可见性
- C++变量的可见性
- 变量的生存期和可见性
- 趣谈Java变量的可见性问题
- 关于Java变量的可见性问题
- 变量的生存期与可见性
- Java Volatile 变量的可见性
- 共享变量可见性
- 共享变量可见性
- ABAP变量的可见范围
- ABAP变量的可见范围
- ABAP变量的可见范围
- InheritableThreadLocal的使用问题
- InheritableThreadLocal-- 特殊的ThreadLocal
- 类InheritableThreadLocal的使用
- [Java 多线程] 变量可见性
- 变量可见性和Volatile
- InheritableThreadLocal
- OC4_点语法实质
- Clone使用方法详解
- 新建工作空间后需要做的事情
- LeetCode Valid Sudoku
- 全志Android设备增加Usb host ADK 功能
- InheritableThreadLocal变量的可见性
- 我的人生根蒂
- 抽象数据类型标准库
- C++中memset的用法注意事项
- iOS 中方法动作的使用方法
- 错误:Unsupported major.minor version 51.0(jdk版本错误)
- php连接mysql时报错:SQLSTATE[HY000] [2002] No such file or directory
- autoit3修改hosts文件的方式
- wcf并发和吞吐量