Two problems that may occur in the use of ThreadLocal and thread pool

Get the from the main thread or non thread pool from the direct thread pool ThreadLocal Set the value of the variable

for example

    private static final ThreadPoolExecutor syncAccessPool = new ThreadPoolExecutor(
            50,
            80,
            8000,
            TimeUnit.MILLISECONDS,
            new LinkedBlockingQueue<>(600)
    );
    public static void main(String[] args) {
        ThreadLocal<String> threadLocal = new ThreadLocal<>();
        threadLocal.set("userId115807");
        syncAccessPool.execute(()->{
            System.out.println(threadLocal.get());
        });
    }

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The final print is null

terms of settlement ： In real use, I believe you won't use it like this , But I made a mistake mainly because I used the encapsulation method , The encapsulation method uses ThreadLocal, In this case, we should start from ThreadLocal Get the method in , Then set it to the thread pool

Used in thread pool ThreadLocal The value is set, but it is not removed after use, resulting in a surge in memory or OOM

public class ThreadLocalOOM {
    static class LocalVariable{
        private Long[] a = new Long[1024*1024];
    }
    private static final ThreadPoolExecutor syncAccessPool = new ThreadPoolExecutor(
            50,
            80,
            8000,
            TimeUnit.MILLISECONDS,
            new LinkedBlockingQueue<>(600)
    );
    final static  ThreadLocal<LocalVariable> threadLocal = new ThreadLocal<LocalVariable>();


    public static void main(String[] args) throws InterruptedException {

        Thread.sleep(10000);
        for (int i=0;i<100;i++){
            syncAccessPool.execute(()->{
                threadLocal.set(new LocalVariable());
                System.out.println("use local variable");
            });
            Thread.sleep(1000);
        }
        System.out.println("pool execute over");
    }
}

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This program uses jconsole The memory change observed by the program is

After use remove Subsequent memory changes

   public static void main(String[] args) throws InterruptedException {

        for (int i=0;i<100;i++){
            syncAccessPool.execute(()->{
                threadLocal.set(new LocalVariable());
                System.out.println("use local variable");
                threadLocal.remove();
            });
            Thread.sleep(1000);
        }
        System.out.println("pool execute over");
    }

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Memory is several times lower than before . The reason is that there is no remove, All existing threads in the thread pool will hold this local variable , This led to a huge increase in memory . If you will private Long[] a = new Long[1024*1024]; Expansion may soon be thrown out OOM abnormal