用 IDEA 新建了两个测试项目，一个 JDK8 ，另一个 JDK17

测试代码如下：

public class Task implements Runnable { private int no; private int size; public Task(int no, int size) { this.no = no; this.size = size; } @Override public void run() { try { System.out.println("执行中, Task: " + no + ", Thread: " + Thread.currentThread().getName() + ", queue.size: " + size); Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } } } 

public class Main { private static ExecutorService pool; public static void main(String[] args) { BlockingQueue<Runnable> queue = new ArrayBlockingQueue<>(8); pool = new ThreadPoolExecutor( 2, 5, 1000, TimeUnit.MILLISECONDS, queue, Executors.defaultThreadFactory(), new RejectedExecutionHandler() { @Override public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) { System.out.println("拒绝中, " + r.toString() + ", queue.size: " + queue.size()); } }); for (int i = 0; i < 15; i++) { pool.execute(new Task(i, queue.size())); } pool.shutdown(); } } 

JDK 8 环境下的运行结果：

执行中, Task: 1, Thread: pool-1-thread-2, queue.size: 0 执行中, Task: 10, Thread: pool-1-thread-3, queue.size: 8 执行中, Task: 11, Thread: pool-1-thread-4, queue.size: 8 执行中, Task: 12, Thread: pool-1-thread-5, queue.size: 8 执行中, Task: 0, Thread: pool-1-thread-1, queue.size: 0 拒绝中, com.example.demo.Task@6d6f6e28, queue.size: 8 拒绝中, com.example.demo.Task@135fbaa4, queue.size: 8 执行中, Task: 2, Thread: pool-1-thread-4, queue.size: 0 执行中, Task: 3, Thread: pool-1-thread-3, queue.size: 1 执行中, Task: 4, Thread: pool-1-thread-1, queue.size: 2 执行中, Task: 5, Thread: pool-1-thread-2, queue.size: 3 执行中, Task: 6, Thread: pool-1-thread-5, queue.size: 4 执行中, Task: 7, Thread: pool-1-thread-4, queue.size: 5 执行中, Task: 9, Thread: pool-1-thread-3, queue.size: 7 执行中, Task: 8, Thread: pool-1-thread-1, queue.size: 6 

JDK 17 环境下的运行结果：

拒绝中, demo.Task@2d98a335, queue.size: 8 拒绝中, demo.Task@4e50df2e, queue.size: 8 执行中, Task: 11, Thread: pool-1-thread-4, queue.size: 8 执行中, Task: 10, Thread: pool-1-thread-3, queue.size: 8 执行中, Task: 12, Thread: pool-1-thread-5, queue.size: 8 执行中, Task: 0, Thread: pool-1-thread-1, queue.size: 0 执行中, Task: 1, Thread: pool-1-thread-2, queue.size: 0 执行中, Task: 3, Thread: pool-1-thread-4, queue.size: 1 执行中, Task: 2, Thread: pool-1-thread-3, queue.size: 0 执行中, Task: 4, Thread: pool-1-thread-5, queue.size: 2 执行中, Task: 5, Thread: pool-1-thread-2, queue.size: 3 执行中, Task: 6, Thread: pool-1-thread-1, queue.size: 4 执行中, Task: 7, Thread: pool-1-thread-4, queue.size: 5 执行中, Task: 8, Thread: pool-1-thread-3, queue.size: 6 执行中, Task: 9, Thread: pool-1-thread-2, queue.size: 7 

结论

可见 JDK17 的 ThreadPoolExecutor ，在通过 execut 提交 runnable 后，不会立即执行被提交的 runnable ，而是等待一段时间。如果在这段等待时间内没有新的 runnable 提交，才开始执行。