线程池存在空闲线程却将任务入队而非复用的原因排查

@PostConstruct
public void postConstruct() {
    threadPool = new ThreadPoolExecutor(corePoolSize != 0 ? corePoolSize : defaultPoolSize, maxPoolSize != 0 ? maxPoolSize : defaultMaxPoolSize, defaultKeepAliveTime, TimeUnit.SECONDS,
            new LinkedBlockingQueue<>(300000));
}

#ThreadPool
corePool.Size=200
maxPool.Size=400
queue.Capacity=999999
KeepAlive=10

public <T> Future<T> submitCall(Callable<T> callable) {
    logger.info("Submitting , total task {}, active count {} completed count {} queue size {}, threads in pool currently {}, largest thread pool {} ",
            threadPool.getTaskCount(), threadPool.getActiveCount(), threadPool.getCompletedTaskCount(),
            threadPool.getQueue().size(), threadPool.getPoolSize(), threadPool.getLargestPoolSize() );
    return threadPool.submit(callable);
}

2024-11-27 13:46:33,963 [INFO ]---[DAExecutorService.java]---[http-nio-9201-exec-2]: Submitting , total task 200, active count 49 completed count 151 queue size 0, threads in pool currently 200, largest thread pool 200 
2024-11-27 13:46:33,963 [INFO ]---[DAExecutorService.java]---[http-nio-9201-exec-2]: Submitting , total task 201, active count 49 completed count 151 queue size 1, threads in pool currently 200, largest thread pool 200 
2024-11-27 13:46:33,964 [INFO ]---[DAExecutorService.java]---[http-nio-9201-exec-2]: Submitting , total task 202, active count 49 completed count 152 queue size 1, threads in pool currently 200, largest thread pool 200 
2024-11-27 13:46:33,965 [INFO ]---[DAExecutorService.java]---[http-nio-9201-exec-2]: Submitting , total task 203, active count 49 completed count 152 queue size 2, threads in pool currently 200, largest thread pool 200 
2024-11-27 13:46:33,966 [INFO ]---[DAExecutorService.java]---[http-nio-9201-exec-2]: Submitting , total task 204, active count 50 completed count 152 queue size 2, threads in pool currently 200, largest thread pool 200 
2024-11-27 13:46:33,967 [INFO ]---[DAExecutorService.java]---[http-nio-9201-exec-2]: Submitting , total task 205, active count 50 completed count 152 queue size 3, threads in pool currently 200, largest thread pool 200 

问题解答 #

这是Java ThreadPoolExecutor 的标准执行逻辑导致的，核心规则如下：

• 当线程池内线程数未达到核心线程数（corePoolSize）时，新任务会直接触发创建新核心线程执行；
• 当线程数已达到核心线程数后，新任务会优先进入任务队列，而非直接分配给空闲核心线程；
• 只有当任务队列已满，且线程数未达到最大线程数（maxPoolSize）时，才会创建非核心线程执行任务。

回到你的场景：

1. 核心线程数已经拉满到200，此时新提交的任务会先进入LinkedBlockingQueue；
2. 日志中看到的队列size增加，是任务刚提交还未被空闲线程取走的瞬时状态——空闲的核心线程会主动从队列中拉取任务执行，从日志里的completed count从151涨到152也能看出，已有空闲线程在处理队列中的任务；
3. 这类毫秒级的队列波动是正常现象，并非线程池没有复用空闲线程，只是任务提交和线程取任务的动作存在极短的时间差，被日志捕捉到了。

内容的提问来源于stack exchange，提问作者Muddassir Rahman