ThreadLocal与线程池使用的问题

感谢博主的这篇分享，见 https://www.cnblogs.com/qifenghao/p/8977378.html

在今天的面试中，突然被考官问了这个问题，当时脱口而出的是 threadlocal容易会有内存泄漏，需要注意remove。其实自己仔细想想，这个回答太过于结果了，没有思考为何要配合线程池的时候，去remove。

注意，这里需要你的jdk版本为1.8及以上，否者清将lambda表达式改为匿名内部类

问题的版本

public class ThreadLocalAndPool {

    /**
     * jdk8 的语法
     */
    private static ThreadLocal<Integer> variableLocal = ThreadLocal.withInitial(() -> 0);

    public static int get() {
        return variableLocal.get();
    }

    public static void remove() {
        variableLocal.remove();
    }

    public static void increment() {
        variableLocal.set(variableLocal.get() + 1);
    }

    public static void main(String[] args) {
        ExecutorService executorService = new ThreadPoolExecutor(2,2,0, TimeUnit.SECONDS,new LinkedBlockingDeque<>(12));

        for(int i=0;i<5;i++){
            executorService.execute(()->{
                long threadId = Thread.currentThread().getId();

                int before = get();
                increment();
                int after = get();
                System.out.println("threadid " + threadId +"  before " + before + ", after " + after);
            });
        }

        executorService.shutdown();
    }


}

得到的结果

threadid 12 before 0, after 1
threadid 13 before 0, after 1
threadid 12 before 1, after 2
threadid 13 before 1, after 2
threadid 12 before 2, after 3

这个其实就是threadlocal与线程池使用的问题了，因为threadlocal维护是 Map<Thread,T>这个结构，而线程池是对线程进行复用的，如果没有及时的清理，那么之前对该线程的使用，就会影响到后面的线程了，造成数据不准确。

修正的版本，就是加一个remove

public class ThreadLocalAndPool {

    /**
     * jdk8 的语法
     */
    private static ThreadLocal<Integer> variableLocal = ThreadLocal.withInitial(() -> 0);

    public static int get() {
        return variableLocal.get();
    }

    public static void remove() {
        variableLocal.remove();
    }

    public static void increment() {
        variableLocal.set(variableLocal.get() + 1);
    }

    public static void main(String[] args) {
        ExecutorService executorService = new ThreadPoolExecutor(2,2,0, TimeUnit.SECONDS,new LinkedBlockingDeque<>(12));

        for(int i=0;i<5;i++){
            executorService.execute(()->{
                try {
                    long threadId = Thread.currentThread().getId();

                    int before = get();
                    increment();
                    int after = get();
                    System.out.println("threadid " + threadId +"  before " + before + ", after " + after);
                }
                finally {
                    remove();
                }
            });
        }

        executorService.shutdown();
    }


}

上面运行的结果如下（不同机器的threadid会有所不同）

threadid 12 before 0, after 1
threadid 13 before 0, after 1
threadid 12 before 0, after 1
threadid 13 before 0, after 1
threadid 13 before 0, after 1