ThreadLocal把变量绑定到线程

最新推荐文章于 2024-01-09 20:50:44 发布

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最新推荐文章于 2024-01-09 20:50:44 发布

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本文链接：https://blog.youkuaiyun.com/liangwenrong/article/details/106567476

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本文探讨ThreadLocal的工作原理，解释其如何将变量绑定到线程，确保线程安全而不依赖`synchronized`关键字。ThreadLocal内部维护了每个线程自己的threadLocalMap，通过get和set方法在对应线程的map中存取值，避免锁的开销，提供了一种高效的线程隔离策略。

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import java.util.Random;

public class ThreadLocalTest {

    static ThreadLocal<Integer> threadLocal = ThreadLocal.withInitial(() -> 123);

    public static void main(String[] args) {
        Thread thread1 = new Thread(() -> {
            Random random = new Random();
            while (true) {
                int nextInt = random.nextInt();
                threadLocal.set(nextInt);
                System.out.println("nextInt:" + nextInt + "=" + threadLocal.get());
                try {
                    Thread.currentThread().sleep(200);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        Thread thread2 = new Thread(() -> {
            while (true) {
                Integer integer = threadLocal.get();
                System.out.println("++++++++++++++++++++++++++++++++++++ " + integer);
                threadLocal.set(++integer);
                try {
                    Thread.currentThread().sleep(200);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        thread1.start();
        thread2.start();
    }
}

解析:

看到static ThreadLocal<Integer> threadLocal = ThreadLocal.withInitial(() -> 123);是定义了一个静态变量threadLocal
thread1不断地通过threadLocal.set(nextInt);设置一个随机整数给静态变量threadLocal
thread2的Integer integer = threadLocal.get();不断地拿到静态变量threadLocal中的值

结果：

++++++++++++++++++++++++++++++++++++ 123
nextInt:-1877256441=-1877256441
++++++++++++++++++++++++++++++++++++ 124
nextInt:607964023=607964023
++++++++++++++++++++++++++++++++++++ 125
nextInt:1946708981=1946708981
++++++++++++++++++++++++++++++++++++ 126
nextInt:1558534812=1558534812
++++++++++++++++++++++++++++++++++++ 127
nextInt:97463914=97463914

从结果可知，不管线程1如何给同一个静态变量threadLocal赋值，线程2都不会拿到线程1设置的值。
线程2可以拿到自己线程设置的值。

由此可知threadLocal内部的值是绑定到线程的，

对于需要保证线程安全，也就是变量不被其他线程改变，可以使用threadLocal来做同步。

与synchronized不同的是，使用synchronized关键字会导致锁等待造成时间上的开销。
使用ThreadLocal使用的是内存空间的开销。

补充

原理解析：
1、每个存在的线程都维护自己的一个threadLocalMap：

public class Thread {

    Map threadLocalMap = new HashMap();

}

2、哪个线程调用ThreadLocal的get、set方法，就从哪个线程内部的threadLocalMap保存或者获取值：

public class ThreadLocal<T> {

    public void set(T value) {
        Thread currentThread = Thread.getCurrentThread();
        currentThread.threadLocalMap.put(this, value);
    }

    public T get() {
        Thread currentThread = Thread.getCurrentThread();
        return (T) currentThread.threadLocalMap.get(this);
    }
}

测试例子：

public class Test{
    public static void main(String[] args) {
        Thread t1 = null;
        Thread t2 = null;

        ThreadLocal<Integer> threadLocal_1 = new ThreadLocal<Integer>();
        ThreadLocal threadLocal_2 = new ThreadLocal();

        threadLocal_1.set(1);
        threadLocal_2.set(2);

        Integer value_1 = threadLocal_1.get();//1
        Object value_2 = threadLocal_2.get();//2
    }

}