Java线程是一项非常基本和重要的技术，在偏底层和偏技术的Java程序中不可避免地要使用到Java多线程技术，那么数据的共享也就是我们必须考虑的问题之一，自然我也就会想到ThreadLocal和synchronized。

       ThreadLocal使用场合主要解决多线程中数据因并发产生不一致问题。ThreadLocal为每个线程中并发访问的数据提供一个副本，通过访问副本来运行业务，这样的结果虽然耗费一些内存，但可以大大降低线程同步所带来性能消耗，同时也减少了线程并发控制的复杂度。synchronized是利用锁的机制，使变量或代码块在某一时该只能被一个线程访问。二者虽然都是解决多线程并发访问的问题，但是区别很大。Synchronized用于线程间的数据共享，而ThreadLocal则用于线程间的数据隔离。下面是一个小例子，使用ThreadLocal时我们可以参照一下它的写法：

import java.util.HashMap;
import java.util.Map;
import java.util.Random;

public class ThreadLocalTest {

	private static ThreadLocal<Integer> x = new ThreadLocal<Integer>();
	private static ThreadLocal<MyThreadScopeData> myThreadScopeData = new ThreadLocal<MyThreadScopeData>();
	public static void main(String[] args) {
		for(int i=0;i<2;i++){
			new Thread(new Runnable(){
				@Override
				public void run() {
					int data = new Random().nextInt();
					System.out.println(Thread.currentThread().getName() 
							+ " has put data :" + data);
					x.set(data);
					MyThreadScopeData.getThreadInstance().setName("name" + data);
					MyThreadScopeData.getThreadInstance().setAge(data);
					new A().get();
					new B().get();
				}
			}).start();
		}
	}
	
	static class A{
		public void get(){
			int data = x.get();
			System.out.println("A from " + Thread.currentThread().getName() 
					+ " get data :" + data);
			MyThreadScopeData myData = MyThreadScopeData.getThreadInstance();
			System.out.println("A from " + Thread.currentThread().getName() 
					+ " getMyData: " + myData.getName() + "," +
					myData.getAge());
		}
	}
	
	static class B{
		public void get(){
			int data = x.get();			
			System.out.println("B from " + Thread.currentThread().getName() 
					+ " get data :" + data);
			MyThreadScopeData myData = MyThreadScopeData.getThreadInstance();
			System.out.println("B from " + Thread.currentThread().getName() 
					+ " getMyData: " + myData.getName() + "," +
					myData.getAge());			
		}		
	}
}

class MyThreadScopeData{
	
	private static ThreadLocal<MyThreadScopeData> map = new ThreadLocal<MyThreadScopeData>();
	
	private String name;
	private int age;

	private MyThreadScopeData(){}
	public static /*synchronized*/ MyThreadScopeData getThreadInstance(){
		MyThreadScopeData instance = map.get();
		if(instance == null){
			instance = new MyThreadScopeData();
			map.set(instance);
		}
		return instance;
	}

	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
}

ThreadLocal类及使用技巧