线程池的执行过程

迈不过友情╰ 2023-01-19 13:56 3阅读 0赞

线程池的执行过程有很多人都很模糊，今天就通过一张图来理解一下线程池的执行过程。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MTc1MTYyNQ_size_16_color_FFFFFF_t_70][]

附上源码

/**
         * Executes the given task sometime in the future.  The task
         * may execute in a new thread or in an existing pooled thread.
         *
         * If the task cannot be submitted for execution, either because this
         * executor has been shutdown or because its capacity has been reached,
         * the task is handled by the current {@code RejectedExecutionHandler}.
         *
         * @param command the task to execute
         * @throws RejectedExecutionException at discretion of
         *         {@code RejectedExecutionHandler}, if the task
         *         cannot be accepted for execution
         * @throws NullPointerException if {@code command} is null
         */
        public void execute(Runnable command) {
            if (command == null)
                throw new NullPointerException();
            /*
             * Proceed in 3 steps:
             *
             * 1. If fewer than corePoolSize threads are running, try to
             * start a new thread with the given command as its first
             * task.  The call to addWorker atomically checks runState and
             * workerCount, and so prevents false alarms that would add
             * threads when it shouldn't, by returning false.
             *
             * 2. If a task can be successfully queued, then we still need
             * to double-check whether we should have added a thread
             * (because existing ones died since last checking) or that
             * the pool shut down since entry into this method. So we
             * recheck state and if necessary roll back the enqueuing if
             * stopped, or start a new thread if there are none.
             *
             * 3. If we cannot queue task, then we try to add a new
             * thread.  If it fails, we know we are shut down or saturated
             * and so reject the task.
             */
            int c = ctl.get();
            if (workerCountOf(c) < corePoolSize) {
                if (addWorker(command, true))
                    return;
                c = ctl.get();
            }
            if (isRunning(c) && workQueue.offer(command)) {
                int recheck = ctl.get();
                if (! isRunning(recheck) && remove(command))
                    reject(command);
                else if (workerCountOf(recheck) == 0)
                    addWorker(null, false);
            }
            else if (!addWorker(command, false))
                reject(command);
        }

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MTc1MTYyNQ_size_16_color_FFFFFF_t_70]: /images/20221021/768a55fa63a54035878225f70890208d.png