让人困惑的ThreadLocal

目前网上介绍ThreadLocal类用法的博文很多，看过后我们基本可以得出以下结论

ThreadLocal的作用和目的：
用于实现线程内的数据共享，即对于相同的程序代码，多个模块在同一个线程中运行时要共享一份数据，而在另外线程中运行时又共享另外一份数据。

好了，至此ThreadLocal的概念我们弄清了，它是每个线程的菊部变量（该死的输入法）。但是java有了类的局部变量，这个ThreadLocal是否显得多余，我们为什么或者在什么情况下用这个东西？

先看以下两段代码

 

代码1.使用ThreadLocal

 

public class ThreadLocalDemo implements Runnable {

	private final static ThreadLocal<Integer> tLocal = new ThreadLocal<Integer>();

	public static void main(String[] agrs) {
		ThreadLocalDemo td = new ThreadLocalDemo();
		Thread t1 = new Thread(td, "t1");
		Thread t2 = new Thread(td, "t2");
		t1.start();
		t2.start();
	}

	/**
	 * 示例业务方法
	 */
	public void test() {
		while (true) {
			try {
				Integer b = (Integer) tLocal.get();
				if (b == null) {
					b = 0;
					tLocal.set(b);
				}
				String currentThreadName = Thread.currentThread().getName();
				System.out.println("thread " + currentThreadName + " set value to:" + b++);
				tLocal.set(b);
				Thread.sleep(1000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}

	}

	@Override
	public void run() {
		test();
	}

}

运行结果： 

thread t2 set value to:0
thread t1 set value to:0
thread t1 set value to:1
thread t2 set value to:1
thread t1 set value to:2
thread t2 set value to:2
thread t1 set value to:3
thread t2 set value to:3

 

代码2.使用局部变量

 

public class ThreadLocalDemoMutli implements Runnable {

	private int a = 0;

	public static void main(String[] agrs) {
		ThreadLocalDemoMutli td = new ThreadLocalDemoMutli();
		ThreadLocalDemoMutli td2 = new ThreadLocalDemoMutli();
		Thread t1 = new Thread(td, "t1");
		Thread t2 = new Thread(td2, "t2");
		t1.start();
		t2.start();
	}

	/**
	 * 示例业务方法，用来测
	 */
	public void test() {
		while (true) {
			try {
				String currentThreadName = Thread.currentThread().getName();
				System.out.println("thread " + currentThreadName + " set age to:" + a++);
				Thread.sleep(1000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}

	}

	@Override
	public void run() {
		test();
	}
}

 运行结果

 

thread t1 set age to:0
thread t2 set age to:0
thread t1 set age to:1
thread t2 set age to:1
thread t2 set age to:2
thread t1 set age to:2
thread t1 set age to:3
thread t2 set age to:3

 

感觉用不用结果都一样嘛？再看下面代码

代码3

public class ThreadLocalDemoSingle implements Runnable {

	private int a = 0;

	public static void main(String[] agrs) {
		ThreadLocalDemoSingle td = new ThreadLocalDemoSingle();
		Thread t1 = new Thread(td, "t1");
		Thread t2 = new Thread(td, "t2");
		t1.start();
		t2.start();
	}

	/**
	 * 示例业务方法，用来测
	 */
	public void test() {
		while (true) {
			try {
				String currentThreadName = Thread.currentThread().getName();
				System.out.println("thread " + currentThreadName + " set age to:" + a++);
				Thread.sleep(1000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}

	}

	@Override
	public void run() {
		test();
	}
}

 运行结果：

thread t1 set age to:0
thread t2 set age to:1
thread t1 set age to:2
thread t2 set age to:3
thread t1 set age to:4
thread t2 set age to:5
thread t1 set age to:6
thread t2 set age to:7
thread t1 set age to:8
thread t2 set age to:9
thread t2 set age to:10
thread t1 set age to:11

 这回不一样了，对比代码2与代码3的区别就在于创建线程的时候

代码2里

ThreadLocalDemoMutli td = new ThreadLocalDemoMutli();
ThreadLocalDemoMutli td2 = new ThreadLocalDemoMutli();
		Thread t1 = new Thread(td, "t1");
		Thread t2 = new Thread(td2, "t2");

 代码3里

ThreadLocalDemoSingle td = new ThreadLocalDemoSingle();
		Thread t1 = new Thread(td, "t1");
		Thread t2 = new Thread(td, "t2");

 只不过多new了一个对象，原因也一眼看的清楚，但是始终没回答为什么要用ThreadLocal

 

 

 

看来只能从框架里找找原因了，下面搬出hibernate源码（我已经修改了很多）

代码4

public class HibernateUtil {

    //创建线程局部变量session，用来保存Hibernate的Session
    public static final ThreadLocal<Integer> session = new ThreadLocal<Integer>();
    
    public static Integer s2=0;
 
    /**
     * 获取当前线程中的Session
     * @return Session
     * @throws HibernateException
     */
    public static Integer currentSession()  {
        Integer s =   session.get();
        if (s == null) {
            s = 0;
            session.set(s);         
        }
        s++;
        session.set(s);         
        return s;
    }
    
    /**
     * 获取当前线程中的Session
     * @return Session
     * @throws HibernateException
     */
    public static Integer currentSessionError()  {
        return s2++;
    }
}

 

 

 代码5

public class THibernate implements Runnable {

	@Override
	public void run() {
		while (true) {
			try {
				String currentThreadName = Thread.currentThread().getName();
				System.out.println("thread " + currentThreadName +"|tlocal:"+HibernateUtil.currentSession()+"|common:"+HibernateUtil.currentSessionError());
				Thread.sleep(1000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}

	public static void main(String[] args) {
		THibernate t=new THibernate();
		Thread t1 = new Thread(t, "t1");
		Thread t2 = new Thread(t, "t2");
		t1.start();
		t2.start();
	}
}

 

 

 

 运行结果：

thread t1|tlocal:1|common:0
thread t2|tlocal:1|common:1
thread t1|tlocal:2|common:2
thread t2|tlocal:2|common:3
thread t1|tlocal:3|common:4
thread t2|tlocal:3|common:5
thread t1|tlocal:4|common:6
thread t2|tlocal:4|common:7
thread t1|tlocal:5|common:8
thread t2|tlocal:5|common:9

 HibernateUtil是一个可供多线程访问的工具类，从运行结果来看ThreadLocal做到了变量的线程隔离，而普通的类变量无法做到这点。

 那我把代码4里的

public static Integer s2=0;

 改成

public Integer s2=0;

 不就也完成了ThreadLocal做的事情吗

但是，你的方法是static的，用到的变量也需要是static的，在这种情况下，还真的必须用ThreadLocal，才能做到变量的线程隔离.

 

 

 

 原创文章，转载请声名出处  http://spjich.iteye.com/blog/2264457