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 init
main initialize stringBufferItl variable.
main first get stringBufferItl : StringBuffer init
main get after set stringItl : Parent
main get after set stringBufferItl : ParentBuffer
//此时 stringItl->"Parent" stringBufferItl->"ParentBuffer"
Thread-0 first get stringItl : Parent//Thread-0不用初始化stringItl
Thread-0 get after set stringItl : Thread-0Child
Thread-0 initialize stringItl2 variable.//父线程未初始化的
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 variable.
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 variable.
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 variable.
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 variable.
Thread-3 first get stringItl2 : String2 init
Thread-2 first get stringItl : Parent
Thread-2 get after set stringItl : Thread-2Child
Thread-2 initialize stringItl2 variable.
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 variable.
Thread-2 first get stringBufferIt2 : StringBuffer2 init
Thread-2 get after set stringBufferItl2 : StringBuffer2 initThread-2
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 variable.
Thread-3 first get stringBufferIt2 : StringBuffer2 init
Thread-3 get after set stringBufferItl2 : StringBuffer2 initThread-3
main second get stringItl : Parent
main initialize stringItl2 variable.
main first get stringItl2 : String2 init
main second get stringBufferItl : ParentBufferThread-0Thread-1
main 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。