Springboot 線程池配置

最近的項目里要手動維護線程池，然后看到一起開發的小伙伴直接用Java了，我堅信Springboot不可能沒這功能，於是查了些資料，果然有，這里給一下。

首先我們都知道@Async標簽能讓方法異步執行，但是這個標簽用的是Springboot默認的線程池，想自己實現線程池就要在項目里創建一個TaskExecutor或它的子類的Bean，像這樣：

    @Bean
    public AsyncTaskExecutor threadPoolTaskExecutor(){
        ThreadPoolTaskExecutor threadPoolTaskExecutor=new ThreadPoolTaskExecutor();
        //加入此頭后此線程池成為系統線程池
        threadPoolTaskExecutor.setThreadNamePrefix("Anno-Executor");
        threadPoolTaskExecutor.setCorePoolSize(corePoolSize);
        threadPoolTaskExecutor.setMaxPoolSize(maxPoolSize);
        threadPoolTaskExecutor.setQueueCapacity(queueCapacity);
        threadPoolTaskExecutor.setKeepAliveSeconds(keepAliveSeconds);
        threadPoolTaskExecutor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
        return threadPoolTaskExecutor;
    }

其中拒絕策略可以改為手動編寫，像下面這樣：

        threadPoolTaskExecutor.setRejectedExecutionHandler(new RejectedExecutionHandler() {
            @Override
            public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
                
            }
        });

JDK里提供了四種默認的策略，非常粗暴：

    public static class CallerRunsPolicy implements RejectedExecutionHandler {
        /**
         * Creates a {@code CallerRunsPolicy}.
         */
        public CallerRunsPolicy() { }

        /**
         * Executes task r in the caller's thread, unless the executor
         * has been shut down, in which case the task is discarded.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            if (!e.isShutdown()) {
                r.run();
            }
        }
    }

    /**
     * A handler for rejected tasks that throws a
     * {@code RejectedExecutionException}.
     */
    public static class AbortPolicy implements RejectedExecutionHandler {
        /**
         * Creates an {@code AbortPolicy}.
         */
        public AbortPolicy() { }

        /**
         * Always throws RejectedExecutionException.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         * @throws RejectedExecutionException always
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            throw new RejectedExecutionException("Task " + r.toString() +
                                                 " rejected from " +
                                                 e.toString());
        }
    }

    /**
     * A handler for rejected tasks that silently discards the
     * rejected task.
     */
    public static class DiscardPolicy implements RejectedExecutionHandler {
        /**
         * Creates a {@code DiscardPolicy}.
         */
        public DiscardPolicy() { }

        /**
         * Does nothing, which has the effect of discarding task r.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        }
    }

    /**
     * A handler for rejected tasks that discards the oldest unhandled
     * request and then retries {@code execute}, unless the executor
     * is shut down, in which case the task is discarded.
     */
    public static class DiscardOldestPolicy implements RejectedExecutionHandler {
        /**
         * Creates a {@code DiscardOldestPolicy} for the given executor.
         */
        public DiscardOldestPolicy() { }

        /**
         * Obtains and ignores the next task that the executor
         * would otherwise execute, if one is immediately available,
         * and then retries execution of task r, unless the executor
         * is shut down, in which case task r is instead discarded.
         *
         * @param r the runnable task requested to be executed
         * @param e the executor attempting to execute this task
         */
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            if (!e.isShutdown()) {
                e.getQueue().poll();
                e.execute(r);
            }
        }
    }

• CallerRunsPolicy是讓調用方法的線程自己運行。
• AbortPolicy丟棄任務拋出異常。
• DiscardPolicy：丟棄任務不報錯。
• DiscardOldestPolicy：丟棄隊列最前面任務，之后嘗試執行。

順便說一下，查源碼可知，默認的策略是AbortPolicy，也就是最粗暴的那個，不過考慮到通常是不要讓拒絕發生的，這里用粗暴的方案問題不大。為了實現方便配置，可以用yml對其進行配置：

@Component
@Data
@ConfigurationProperties("thread-pool-factory")
public class ThreadPoolFactory {
    private int corePoolSize;
    private int maxPoolSize;
    private int queueCapacity;
    private int keepAliveSeconds;
    @Bean
    public AsyncTaskExecutor threadPoolTaskExecutor(){
        //......
    }
}

這里@Data標簽是lambok的標簽，快速生成getter和setter用。以上Component構建好后，可以直接配置：

thread-pool-factory:
    #IO密集型應用，線程數為2N+1
    corePoolSize: 9
    maxPoolSize: 18
    queueCapacity: 100
    keepAliveSeconds: 120

以上。