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 tl =  new ThreadLocal();
 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 itl =  new InheritableThreadLocal();
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~ 

 public class Test {

  private final static InheritableThreadLocal holder = new InheritableThreadLocal();
  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());