ThreadLocal 实例

java.lang 
Class ThreadLocal

java.lang.Object
  java.lang.ThreadLocal

Direct Known Subclasses:

InheritableThreadLocal

---

public class ThreadLocal

extends Object

This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its get or set method) has its own, independently initialized copy of the variable. ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread (e.g., a user ID or Transaction ID).

For example, the class below generates unique identifiers local to each thread. A thread's id is assigned the first time it invokesUniqueThreadIdGenerator.getCurrentThreadId() and remains unchanged on subsequent calls.

 import java.util.concurrent.atomic.AtomicInteger;

 public class UniqueThreadIdGenerator {

     private static final AtomicInteger uniqueId = new AtomicInteger(0);

     private static final ThreadLocal < Integer > uniqueNum = 
         new ThreadLocal < Integer > () {
             @Override protected Integer initialValue() {
                 return uniqueId.getAndIncrement();
         }
     };
 
     public static int getCurrentThreadId() {
         return uniqueId.get();
     }
 } // UniqueThreadIdGenerator
 

Each thread holds an implicit reference to its copy of a thread-local variable as long as the thread is alive and the ThreadLocal instance is accessible; after a thread goes away, all of its copies of thread-local instances are subject to garbage collection (unless other references to these copies exist).

 public static int getCurrentThreadId() {
         return uniqueId.get();
     }

应该是uniqueNum.get();

给出一个全面的例子：

package org.ouc.michael.test;

import java.util.concurrent.atomic.AtomicInteger;

public class ThreadLocalTest implements Runnable{
	
	private static final AtomicInteger uniqueId =new AtomicInteger(0);
	
	private static final ThreadLocal  uniqueNum=
		new ThreadLocal  (){
		@Override protected Integer initialValue(){
			return uniqueId.getAndIncrement();
		}
	};
	
	public static int getCurrentThreadId(){
		return uniqueNum.get();
	}
	
	@Override
	public void run() {
		for (int i = 0; i < 100; i++) {
			int id=ThreadLocalTest.getCurrentThreadId();
			System.out.println("Thread:"+getCurrentThreadId());
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
	}
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		ThreadLocalTest th1=new ThreadLocalTest();
		
		ThreadLocalTest th2=new ThreadLocalTest();
		
		ThreadLocalTest th3=new ThreadLocalTest();
		
		ThreadLocalTest th4=new ThreadLocalTest();
		
		ThreadLocalTest th5=new ThreadLocalTest();
		
		
		Thread run1=new Thread(th1);
		
		run1.start();
		
		Thread run2=new Thread(th1);
		
		run2.start();
		
		Thread run3=new Thread(th1);
		
		run3.start();
		
		Thread run4=new Thread(th1);
		
		run4.start();
		
		Thread run5=new Thread(th1);
		
		run5.start();
		
		
	}
}