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 TestThreadLocal { private static ThreadLocal stringItl = new InheritableThreadLocal (){ protected String initialValue() { System.out.println(Thread.currentThread().getName() + " initialize stringItl variable."); return "String init"; } }; private static ThreadLocal stringItl2 = new InheritableThreadLocal (){ protected String initialValue() { System.out.println(Thread.currentThread().getName() + " initialize stringItl2 variable."); return "String2 init"; } }; private static ThreadLocal stringBufferItl = new InheritableThreadLocal (){ protected StringBuffer initialValue() { System.out.println(Thread.currentThread().getName() + " initialize stringBufferItl variable."); return new StringBuffer("StringBuffer init"); } }; private static ThreadLocal stringBufferItl2 = new InheritableThreadLocal (){ 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：
main first get stringItl : Parentmain first get stringBufferItl : ParentBufferThread-0 first get stringItl : ParentThread-0 get after set stringItl : Thread-0ChildThread-0 initialize stringItl2 variable.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-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-3Thread-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-2main 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：