为什么 slf4j 记录器有时会打印出父线程，即使代码应该由子线程执行？答案

【问题标题】：Why is slf4j logger sometimes print out parent thread even though the code is suppose to be executed by child thread?为什么 slf4j 记录器有时会打印出父线程，即使代码应该由子线程执行？
【发布时间】：2018-06-14 01:23:47
【问题描述】：

考虑这个例子

@Test
public void test2() throws InterruptedException {
    int MAX_ENTRIES = 2;
    Map cache = new LinkedHashMap(MAX_ENTRIES+1, .75F, true) {
        public boolean removeEldestEntry(Map.Entry eldest) {
            return size() > MAX_ENTRIES;
        }
    };

    MyBufferService testService = new MyBufferService("test2");

    for (int i = 0; i < 100000; i++) {
        testService.putBufferTask(Integer.toString(i));
    }

    TimeUnit.SECONDS.sleep(1);
}


public class MyBufferService {

    private ThreadPoolExecutor executor;

    private final Logger LOGGER = LoggerFactory.getLogger(MyBufferService.class);
    private final Map cache;
    final int MAX_ENTRIES = 1;

    public MyBufferService(String buffName) {
        executor = new ThreadPoolExecutor(1, // corePoolSize
                1, // maximumPoolSize
                60, TimeUnit.SECONDS, // keepAlive
                new LinkedBlockingQueue<>(10000), // workQueue
                new ThreadFactoryBuilder().setNameFormat(buffName + "-MyBufferService-thread-%d").build(), // factory
                new ThreadPoolExecutor.CallerRunsPolicy() // rejected execution handler
        );

        this.cache = new LinkedHashMap(MAX_ENTRIES+1, .75F, true) {
            public boolean removeEldestEntry(Map.Entry eldest) {
                return size() > MAX_ENTRIES;
            }
        };
    }


    private class BufferTask implements Runnable {

        private final String mystring;
        private final Map cache;

        BufferTask(String mystring, Map cache) throws NullPointerException {
            this.mystring = mystring;
            this.cache = cache;
        }
        @Override
        public void run() {
            try {
                synchronized (this.cache) {
                    this.cache.put(this.mystring, "hi");
                    if (this.cache.size() > 0) {
                        LOGGER.info("this is size {}", this.cache.size() );
                    }
                }
            } catch (Throwable t) {

            }
        }
    }

    public void putBufferTask(
            String mystring) throws RejectedExecutionException, NullPointerException {
        executor.submit(new BufferTask(mystring, this.cache));
    }

}

并考虑该行的以下输出 sn-p

LOGGER.info("this is size {}", this.cache.size() );


2018-06-13 18:19:46,760 [INFO] [main] c.t.u.w.$MyBufferService - this is size 1
2018-06-13 18:19:46,761 [INFO] [main] c.t.u.w.$MyBufferService - this is size 1
2018-06-13 18:19:46,761 [INFO] [main] c.t.u.w.$MyBufferService - this is size 1
2018-06-13 18:19:46,761 [INFO] [main] c.t.u.w.$MyBufferService - this is size 1
2018-06-13 18:19:46,761 [INFO] [main] c.t.u.w.$MyBufferService - this is size 1
2018-06-13 18:19:46,761 [INFO] [test2-MyBufferService-thread-0] c.t.u.w.$MyBufferService - this is size 1
2018-06-13 18:19:46,761 [INFO] [test2-MyBufferService-thread-0] c.t.u.w.$MyBufferService - this is size 1
2018-06-13 18:19:46,761 [INFO] [test2-MyBufferService-thread-0] c.t.u.w.$MyBufferService - this is size 1
2018-06-13 18:19:46,761 [INFO] [test2-MyBufferService-thread-0] c.t.u.w.$MyBufferService - this is size 1

我们看到BufferTask 中的代码由两个线程运行，main 线程和main 线程中的子线程MyBufferService-thread-0

我原以为只有子线程正在执行任务，但似乎父线程也在执行任务。

为什么会这样？我做错什么了吗？

【问题讨论】：

标签： java multithreading concurrency parallel-processing threadpoolexecutor

【解决方案1】：

您将拒绝的excecution policy 设置为让调用者运行任务。这意味着如果任务在 #execute 中被拒绝，则调用线程（即您的情况下的主线程）将运行该任务，这就是您所看到的。更改该策略或处理 #execute 方法上的异常。

【讨论】：