Java 中ThreadLocal类详解

threadlocal类，代表一个线程局部变量，通过把数据放在threadlocal中，可以让每个线程创建一个该变量的副本。也可以看成是线程同步的另一种方式吧，通过为每个线程创建一个变量的线程本地副本，从而避免并发线程同时读写同一个变量资源时的冲突。

示例如下：

import java.util.random;
import java.util.concurrent.executorservice;
import java.util.concurrent.executors;
import java.util.concurrent.timeunit;

import com.sun.javafx.webkit.accessor;

public class threadlocaltest {
 static class threadlocalvariableholder {
  private static threadlocal<integer> value = new threadlocal<integer>() {
   private random random = new random();
   
   protected synchronized integer initialvalue() {
    return random.nextint(10000);
   }
  };
  
  public static void increment() {
   value.set(value.get() + 1);
  }
  
  public static int get() {
   return value.get();
  }
 }
 
 static class accessor implements runnable{
  private final int id;
  
  public accessor(int id) {
   this.id = id;
  }
  
  @override
  public void run() {
   while (!thread.currentthread().isinterrupted()) {
    threadlocalvariableholder.increment();
    system.out.println(this);
    thread.yield();
   }
  }
  
  @override
  public string tostring() {
   return "#" + id + ": " + threadlocalvariableholder.get();
  }
  
 }
 
 public static void main(string[] args) {
  executorservice executorservice = executors.newcachedthreadpool();
  for (int i = 0; i < 5; i++) {
   executorservice.execute(new accessor(i));
  }
  try {
   timeunit.microseconds.sleep(1);
  } catch (interruptedexception e) {
   e.printstacktrace();
  }
  executorservice.shutdownnow();
 }

}

运行结果：

#1: 9685
#1: 9686
#2: 138
#2: 139
#2: 140
#2: 141
#0: 5255
。。。

由运行结果可知，各线程都用于各自的local变量，并各自读写互不干扰。

threadlocal共提供了三个方法来操作，set,get和remove。

在android 中的looper，即使用了threadlocal来为每个线程都创建各自独立的looper对象。

public final class looper {
 private static final string tag = "looper";

 // sthreadlocal.get() will return null unless you've called prepare().
 static final threadlocal<looper> sthreadlocal = new threadlocal<looper>();

 private static void prepare(boolean quitallowed) {
  if (sthreadlocal.get() != null) {
   throw new runtimeexception("only one looper may be created per thread");
  }
  sthreadlocal.set(new looper(quitallowed));
 }
 
 。。。
}

当某个线程需要自己的looper及消息队列时，就调用looper.prepare()，它会为线程创建属于线程的looper对象及messagequeue，并将looper对象保存在threadlocal中。