关于ThreadLocal的使用

package cn.itcast.heima2;

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 myData = new MyThreadScopeData();
					myData.setName("name" + data);
					myData.setAge(data);
					myThreadScopeData.set(myData);*/
					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.get();;
			System.out.println("A from " + Thread.currentThread().getName() 
					+ " getMyData: " + myData.getName() + "," +
					myData.getAge());*/
			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 MyThreadScopeData(){}
	public static /*synchronized*/ MyThreadScopeData getThreadInstance(){
		MyThreadScopeData instance = map.get();
		if(instance == null){
			instance = new MyThreadScopeData();
			map.set(instance);
		}
		return instance;
	}
	//private static MyThreadScopeData instance = null;//new MyThreadScopeData();
	private static ThreadLocal<MyThreadScopeData> map = new ThreadLocal<MyThreadScopeData>();
	
	private String name;
	private int age;
	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;
	}
}

首先，ThreadLocal 不是用来解决共享对象的多线程访问问题的，一般情况下，通过ThreadLocal.set() 到线程中的对象是该线程自己使用的对象，其他线程是不需要访问的，也访问不到的。各个线程中访问的是不同的对象。 

另外，说ThreadLocal使得各线程能够保持各自独立的一个对象，并不是通过ThreadLocal.set()来实现的，而是通过每个线程中的new 对象 的操作来创建的对象，每个线程创建一个，不是什么对象的拷贝或副本。通过ThreadLocal.set()将这个新创建的对象的引用保存到各线程的自己的一个map中，每个线程都有这样一个map，执行ThreadLocal.get()时，各线程从自己的map中取出放进去的对象，因此取出来的是各自自己线程中的对象，ThreadLocal实例是作为map的key来使用的。 

如果ThreadLocal.set()进去的东西本来就是多个线程共享的同一个对象，那么多个线程的ThreadLocal.get()取得的还是这个共享对象本身，还是有并发访问问题。