InheritableThreadLocal变量的可见性

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：

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import  java.util.concurrent.TimeUnit; public  class  TestThreadLocal {      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 {          stringItl.set( "Parent" );          stringBufferItl.set( new  StringBuffer( "ParentBuffer" ));                                                                                                                                                                                                                System.out.println(Thread.currentThread().getName() +  " first get stringItl : "  + stringItl.get());          System.out.println(Thread.currentThread().getName() +  " first get 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：

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main first  get  stringItl : Parent main first  get  stringBufferItl : ParentBuffer Thread- 0  first  get  stringItl : Parent Thread- 0  get  after  set  stringItl : Thread-0Child Thread- 0  initialize stringItl2  var iable. Thread- 0  first  get  stringItl2 : String2 init Thread- 0  get  after  set  stringItl2 : Thread-0Child Thread- 0  first  get  stringBufferItl : ParentBuffer Thread- 0  get  after  set  stringBufferItl : ParentBufferThread- 0 Thread- 0  initialize stringBufferItl2  var iable. Thread- 0  first  get  stringBufferIt2 : StringBuffer2 init Thread- 0  get  after  set  stringBufferItl2 : StringBuffer2 initThread- 0 Thread- 1  first  get  stringItl : Parent Thread- 1  get  after  set  stringItl : Thread-1Child Thread- 1  initialize stringItl2  var iable. Thread- 1  first  get  stringItl2 : String2 init Thread- 1  get  after  set  stringItl2 : Thread-1Child Thread- 1  first  get  stringBufferItl : ParentBufferThread- 0 Thread- 1  get  after  set  stringBufferItl : ParentBufferThread-0Thread- 1 Thread- 1  initialize stringBufferItl2  var iable. Thread- 1  first  get  stringBufferIt2 : StringBuffer2 init Thread- 1  get  after  set  stringBufferItl2 : StringBuffer2 initThread- 1 Thread- 3  first  get  stringItl : Parent Thread- 3  get  after  set  stringItl : Thread-3Child Thread- 3  initialize stringItl2  var iable. Thread- 3  first  get  stringItl2 : String2 init Thread- 3  get  after  set  stringItl2 : Thread-3Child Thread- 3  first  get  stringBufferItl : ParentBufferThread-0Thread- 1 Thread- 3  get  after  set  stringBufferItl : Thread-3Buffer Thread- 3  initialize stringBufferItl2  var iable. Thread- 3  first  get  stringBufferIt2 : StringBuffer2 init Thread- 3  get  after  set  stringBufferItl2 : StringBuffer2 initThread- 3 Thread- 2  first  get  stringItl : Parent Thread- 2  get  after  set  stringItl : Thread-2Child Thread- 2  initialize stringItl2  var iable. Thread- 2  first  get  stringItl2 : String2 init Thread- 2  get  after  set  stringItl2 : Thread-2Child Thread- 2  first  get  stringBufferItl : ParentBufferThread-0Thread- 1 Thread- 2  get  after  set  stringBufferItl : Thread-2Buffer Thread- 2  initialize stringBufferItl2  var iable. Thread- 2  first  get  stringBufferIt2 : StringBuffer2 init Thread- 2  get  after  set  stringBufferItl2 : StringBuffer2 initThread- 2 main second  get  stringItl : Parent main initialize stringItl2  var iable. main first  get  stringItl2 : String2 init main second  get  stringBufferItl : ParentBufferThread-0Thread- 1 main initialize stringBufferItl2  var iable. main first  get  stringBufferItl2 : StringBuffer2 init

从运行结果可以看出：

如果ThreadLocal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ThreadLocal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

     本文转自sarchitect 51CTO博客，原文链接：，http://blog.51cto.com/stevex/1289499如需转载请自行联系原作者