ThreadLocal與線程池使用的問題

感謝博主的這篇分享，見 https://www.cnblogs.com/qifenghao/p/8977378.html

在今天的面試中，突然被考官問了這個問題，當時脫口而出的是 threadlocal容易會有內存泄漏，需要注意remove。其實自己仔細想想，這個回答太過於結果了，沒有思考為何要配合線程池的時候，去remove。

 

注意，這里需要你的jdk版本為1.8及以上，否者清將lambda表達式改為匿名內部類

 

問題的版本

public class ThreadLocalAndPool {

    /**
     * jdk8 的語法
     */
    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();
    }


}

得到的結果

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

 

這個其實就是threadlocal與線程池使用的問題了，因為threadlocal維護是 Map<Thread,T>這個結構，而線程池是對線程進行復用的，如果沒有及時的清理，那么之前對該線程的使用，就會影響到后面的線程了，造成數據不准確。

 

修正的版本，就是加一個remove

public class ThreadLocalAndPool {

    /**
     * jdk8 的語法
     */
    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();
    }


}

上面運行的結果如下（不同機器的threadid會有所不同）

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