java并发编程学习: ThreadLocal使用及原理

多线程应用中,如果希望一个变量隔离在某个线程内,即:该变量只能由某个线程本身可见,其它线程无法访问,那么ThreadLocal可以很方便的帮你做到这一点。 

先来看一下示例:

package yjmyzz.test;

public class ThreadLocalTest1 {

    public static class MyRunnable implements Runnable {

        private ThreadLocal<Integer> threadLocal = new ThreadLocal<Integer>();

        @Override
        public void run() {
            threadLocal.set((int) (Math.random() * 100D));
            System.out.println(Thread.currentThread().getName() + ":" + threadLocal.get());
        }
    }


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

运行结果:

B:48
A:32

即:线程A与线程B中ThreadLocal保存的整型变量是各自独立的,互不相干,只要在每个线程内部使用set方法赋值,然后在线程内部使用get就能取到对应的值。

 

把这个示例稍微变化一下:

package yjmyzz.test;


public class ThreadLocalTest2 {

    public static class MyRunnable implements Runnable {

        private ThreadLocal<Integer> threadLocal = new ThreadLocal<Integer>();

        public MyRunnable(){
            threadLocal.set((int) (Math.random() * 100D));
            System.out.println(Thread.currentThread().getName() + ":" + threadLocal.get());
        }

        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName() + ":" + threadLocal.get());
        }
    }


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

把ThreadLocal赋值的地方放在了MyRunnable的构造函数中,然后在run方法中读取该值,看下结果:

main:1
main:47
A:null
B:null

思考一下:为什么会这样? MyRunnable的构造函数是由main主线程调用的,所以TheadLocal的set方法,实际上是在main主线程的环境中完成的,因此也只能在main主线程中get到,而run方法运行的上下文是子线程本身,由于run方法中并没有使用set方法赋值,因此get到的是默认空值null.

 

ThreadLocal还有一个派生的子类:InheritableThreadLocal ,可以允许线程及该线程创建的子线程均可以访问同一个变量(有些OOP中的proteced的意味),这么解释可能理解起来比较费劲,还是直接看代码吧:

package yjmyzz.test;


public class ThreadLocalTest3 {

    private static InheritableThreadLocal<Integer> threadLocal = new InheritableThreadLocal<Integer>();

    public static class MyRunnable implements Runnable {

        private String _name = "";

        public MyRunnable(String name) {
            _name = name;
            System.out.println(name + " => " + Thread.currentThread().getName() + ":" + threadLocal.get());
        }

        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName() + ":" + threadLocal.get());
        }
    }


    public static void main(String[] args) {
        threadLocal.set(1);

        System.out.println(Thread.currentThread().getName() + ":" + threadLocal.get());
        Thread t1 = new Thread(new MyRunnable("R-A"), "A");
        Thread t2 = new Thread(new MyRunnable("R-B"), "B");

        t1.start();
        t2.start();
    }
}

main:1
R-A => main:1
R-B => main:1
A:1
B:1

观察下结果,在主线程main中设置了一个InheritableThreadLocal实例,并在main主线程中设置了值1,然后main主线程及二个子线程t1,t2均正常get到了该值。 

 

实现原理:

可以观察下ThreadLocal及Thread的源码,大致了解其实现原理:

ThreadLocal类的get方法

    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null)
                return (T)e.value;
        }
        return setInitialValue();
    }

从代码上看,主要思路如下:

1.取当前线程

2.取得ThreadLocalMap类(先不管这个的实现,从命名上看,理解成一个Map<K,V>容器即可)

3.如果Map容器不为空,则根据ThreadLocal自身的HashCode(见后面的继续分析)取得对应的Entry(即Map里的k-v元素对)

4.如果entry不为空,则返回值

5.如果Map容器为空,则设置初始值

 

继续顺藤摸瓜:

ThreadLocal的getMap及ThreadLocalMap的getEntry方法

    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

可以发现getMap其实取的是Thread实例t上的一个属性,继续看Thread的代码:

    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

    /*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

说明每个Thread内部都维护着二个ThreadLocalMap,一个应对threadLocals(即:一个Thread内部可以有多个ThreadLocal实例),另一个对应着 inheritableThreadLocals,再看ThreadLocal.ThreadLocalMap的getEntry方法

        private Entry getEntry(ThreadLocal key) {
            int i = key.threadLocalHashCode & (table.length - 1);
            Entry e = table[i];
            if (e != null && e.get() == key)
                return e;
            else
                return getEntryAfterMiss(key, i, e);
        }

从这里看,ThreadLocalMap的key是基于ThreadLocal的Hashcode与内部table的长度-1做位运算的整数值,只要有个印象,threadLocalMap的key跟ThreadLocal实例的hashcode有关即可。

最后看看ThreadLocal的setInitialValue方法:

    private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }

先根据当前线程实例t,找到内部维护的ThreadLocalMap容器,如果容器为空,则创建Map实例,否则直接把值放进去(Key跟ThreadLocal实例本身的hashCode相关)

 

根据以上分析,对于ThreadLocal的内部实现,其主要思路总结如下:

1. 每个Thread实例内部,有二个ThreadLocalMap的K-V容器实例(分别对应threadLocals及inheritableThreadLocals), 容器的元素数量,即为Thread实例里的ThreadLocal实例个数

2. ThreadLocalMap里的每个Entry的Key与ThreadLocal实例的HashCode相关(这样,多个ThreadLocal实例就不会搞混)

3. 每个ThreadLocal实例使用set赋值时,实际上是在ThreadLocalMap容器里,添加(或更新)一条Entry信息

4. 每个ThreadLocal实例使用get取值时,从ThreadLocalMap里根据key取出value 

 

参考文章:

http://qifuguang.me/2015/09/02/[Java并发包学习七]解密ThreadLocal/

http://ifeve.com/java-threadlocal%e7%9a%84%e4%bd%bf%e7%94%a8/

posted @ 2015-10-08 10:41 菩提树下的杨过 阅读(...) 评论(...) 编辑 收藏