InheritableThreadLocal

初识InheritableThreadLocal

一直来只知道ThreadLocal,直到最近看slf4j MDC实现代码的时候,才认识了InheritableThreadLocal.

InheritableThreadLocal顾名思义,可继承的ThreadLocal.

看类描述:

This class extends <tt>ThreadLocal</tt> to provide inheritance of values

* from parent thread to child thread: when a child thread is created, the

* child receives initial values for all inheritable thread-local variables

* for which the parent has values.

测试代码:

1 public class Test {

2

3 public static void main(String[] args) {

4 //使用ThreadLocal,父子线程之间,不共享Value

5 final ThreadLocal<String> tl = new ThreadLocal<String>();

6 tl.set("ThreadLocal-VAL");

7 System.out.println("Main-1:" + tl.get());

8 new Thread() {

9 public void run() {

10 System.out.println("Child-1:" + tl.get());

11 };

12 }.start();

13

14 //使用InheritableThreadLocal,父线程Value可让子线程共享

15 final ThreadLocal<String> itl = new InheritableThreadLocal<String>();

16 itl.set("InheritableThreadLocal-VAL");

17 System.out.println("Main-2:" + itl.get());

18 new Thread() {

19 public void run() {

20 System.out.println("Child-2:" + itl.get());

21 };

22 }.start();

23

24 }

25 }

输出内容:

Main-1:ThreadLocal-VAL

Main-2:InheritableThreadLocal-VAL

Child-1:null

Child-2:InheritableThreadLocal-VAL

......分隔符号......

顺带着简单说下MDC.(Mapped Diagnostic Context). 中文直译太恶心了,我理解的意思是,和环境相关的上下文信息.

比如在web应用中,我们可以把用户的ip,访问url等放入到这个上下文中,log打印的时候,就能得到这个信息.

在slf4j BasicMDCAdapter实现中,就是用了InheritableThreadLocal

1 public class BasicMDCAdapter implements MDCAdapter {

2

3 private InheritableThreadLocal inheritableThreadLocal = new InheritableThreadLocal();

4

5 //

.

6

7 }

ThreadLocal有个缺陷，在子线程里无法访问父线程的变量,

InheritableThreadLocal 解决了这个问题，自动会把父线程的变量传递个子线程,

子线程只能用，修改了不会影响父线程的东西

这里仍然需要注意并发实现~

Java代码


public class Test {

private final static InheritableThreadLocal<String> holder = new InheritableThreadLocal<String>();

public static void main(String[] args){

holder.set("aaa");

System.out.println("begin=" + holder.get());

Thread a = new Thread(){

public void run() {

System.out.println("thread-begin=" + holder.get());

holder.set("vvvvvvvvvvvvv");

System.out.println("thread-end=" + holder.get());

}

};

a.start();

try {

Thread.sleep(2000);

} catch (InterruptedException e) {

e.printStackTrace();

}

System.out.println("end=" + holder.get());