SpringBoot執行緒池和Java執行緒池的實現原理

使用預設的執行緒池

方式一：透過@Async註解呼叫

public class AsyncTest {
    @Async
    public void async(String name) throws InterruptedException {
        System.out.println("async" + name + " " + Thread.currentThread().getName());
        Thread.sleep(1000);
    }
}

啟動類上需要新增@EnableAsync註解，否則不會生效。

@SpringBootApplication
//@EnableAsync
public class Test1Application {
   public static void main(String[] args) throws InterruptedException {
      ConfigurableApplicationContext run = SpringApplication.run(Test1Application.class, args);
      AsyncTest bean = run.getBean(AsyncTest.class);
      for(int index = 0; index <= 10; ++index){
         bean.async(String.valueOf(index));
      }
   }
}

方式二：直接注入 ThreadPoolTaskExecutor

需要加上 @EnableAsync註解

@SpringBootTest
class Test1ApplicationTests {

   @Resource
   ThreadPoolTaskExecutor threadPoolTaskExecutor;

   @Test
   void contextLoads() {
      Runnable runnable = () -> {
         System.out.println(Thread.currentThread().getName());
      };

      for(int index = 0; index <= 10; ++index){
         threadPoolTaskExecutor.submit(runnable);
      }
   }

}

執行緒池預設配置資訊

SpringBoot執行緒池的常見配置：

spring:
  task:
    execution:
      pool:
        core-size: 8
        max-size: 16                          # 預設是 Integer.MAX_VALUE
        keep-alive: 60s                       # 當執行緒池中的執行緒數量大於 corePoolSize 時，如果某執行緒空閒時間超過keepAliveTime，執行緒將被終止
        allow-core-thread-timeout: true       # 是否允許核心執行緒超時,預設true
        queue-capacity: 100                   # 執行緒佇列的大小,預設Integer.MAX_VALUE
      shutdown:
        await-termination: false              # 執行緒關閉等待
      thread-name-prefix: task-               # 執行緒名稱的字首

SpringBoot 執行緒池的實現原理

TaskExecutionAutoConfiguration 類中定義了 ThreadPoolTaskExecutor,該類的內部實現也是基於java原生的 ThreadPoolExecutor類。initializeExecutor()方法在其父類中被呼叫，但是在父類中 RejectedExecutionHandler 被定義為了 private RejectedExecutionHandler rejectedExecutionHandler = new ThreadPoolExecutor.AbortPolicy(); ，並透過initialize()方法將AbortPolicy傳入initializeExecutor()中。

注意在TaskExecutionAutoConfiguration 類中，ThreadPoolTaskExecutor類的bean的名稱為： applicationTaskExecutor 和 taskExecutor。

// TaskExecutionAutoConfiguration#applicationTaskExecutor()
@Lazy
@Bean(name = { APPLICATION_TASK_EXECUTOR_BEAN_NAME,
      AsyncAnnotationBeanPostProcessor.DEFAUL
          T_TASK_EXECUTOR_BEAN_NAME })
@ConditionalOnMissingBean(Executor.class)
public ThreadPoolTaskExecutor applicationTaskExecutor(TaskExecutorBuilder builder) {
   return builder.build();
}

// ThreadPoolTaskExecutor#initializeExecutor()
@Override
protected ExecutorService initializeExecutor(
      ThreadFactory threadFactory, RejectedExecutionHandler rejectedExecutionHandler) {

   BlockingQueue<Runnable> queue = createQueue(this.queueCapacity);

   ThreadPoolExecutor executor;
   if (this.taskDecorator != null) {
      executor = new ThreadPoolExecutor(
            this.corePoolSize, this.maxPoolSize, this.keepAliveSeconds, TimeUnit.SECONDS,
            queue, threadFactory, rejectedExecutionHandler) {
         @Override
         public void execute(Runnable command) {
            Runnable decorated = taskDecorator.decorate(command);
            if (decorated != command) {
               decoratedTaskMap.put(decorated, command);
            }
            super.execute(decorated);
         }
      };
   }
   else {
      executor = new ThreadPoolExecutor(
            this.corePoolSize, this.maxPoolSize, this.keepAliveSeconds, TimeUnit.SECONDS,
            queue, threadFactory, rejectedExecutionHandler);

   }

   if (this.allowCoreThreadTimeOut) {
      executor.allowCoreThreadTimeOut(true);
   }

   this.threadPoolExecutor = executor;
   return executor;
}

// ExecutorConfigurationSupport#initialize()
public void initialize() {
   if (logger.isInfoEnabled()) {
      logger.info("Initializing ExecutorService" + (this.beanName != null ? " '" + this.beanName + "'" : ""));
   }
   if (!this.threadNamePrefixSet && this.beanName != null) {
      setThreadNamePrefix(this.beanName + "-");
   }
   this.executor = initializeExecutor(this.threadFactory, this.rejectedExecutionHandler);
}

覆蓋預設的執行緒池

覆蓋預設的 taskExecutor物件，bean的返回型別可以是ThreadPoolTaskExecutor也可以是Executor。

@Configuration
public class ThreadPoolConfiguration {

    @Bean("taskExecutor")
    public ThreadPoolTaskExecutor taskExecutor() {
        ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
        //設定執行緒池引數資訊
        taskExecutor.setCorePoolSize(10);
        taskExecutor.setMaxPoolSize(50);
        taskExecutor.setQueueCapacity(200);
        taskExecutor.setKeepAliveSeconds(60);
        taskExecutor.setThreadNamePrefix("myExecutor--");
        taskExecutor.setWaitForTasksToCompleteOnShutdown(true);
        taskExecutor.setAwaitTerminationSeconds(60);
        //修改拒絕策略為使用當前執行緒執行
        taskExecutor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
        //初始化執行緒池
        taskExecutor.initialize();
        return taskExecutor;
    }
}

管理多個執行緒池

如果出現了多個執行緒池，例如再定義一個執行緒池 taskExecutor2，則直接執行會報錯。此時需要指定bean的名稱即可。

@Bean("taskExecutor2")
public ThreadPoolTaskExecutor taskExecutor2() {
    ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
    //設定執行緒池引數資訊
    taskExecutor.setCorePoolSize(10);
    taskExecutor.setMaxPoolSize(50);
    taskExecutor.setQueueCapacity(200);
    taskExecutor.setKeepAliveSeconds(60);
    taskExecutor.setThreadNamePrefix("myExecutor2--");
    taskExecutor.setWaitForTasksToCompleteOnShutdown(true);
    taskExecutor.setAwaitTerminationSeconds(60);
    //修改拒絕策略為使用當前執行緒執行
    taskExecutor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
    //初始化執行緒池
    taskExecutor.initialize();
    return taskExecutor;
}

引用執行緒池時，需要將變數名更改為bean的名稱，這樣會按照名稱查詢。

@Resource
ThreadPoolTaskExecutor taskExecutor2;

對於使用@Async註解的多執行緒則在註解中指定bean的名字即可。

    @Async("taskExecutor2")
    public void async(String name) throws InterruptedException {
        System.out.println("async" + name + " " + Thread.currentThread().getName());
        Thread.sleep(1000);
    }

執行緒池的四種拒絕策略

JAVA常用的四種執行緒池

ThreadPoolExecutor 類的建構函式如下：

public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit,
                          BlockingQueue<Runnable> workQueue) {
    this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
         Executors.defaultThreadFactory(), defaultHandler);
}

newCachedThreadPool

不限制最大執行緒數（maximumPoolSize=Integer.MAX_VALUE）,如果有空閒的執行緒超過需要，則回收，否則重用已有的執行緒。

new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                              60L, TimeUnit.SECONDS,
                              new SynchronousQueue<Runnable>());

newFixedThreadPool

定長執行緒池，超出執行緒數的任務會在佇列中等待。

return new ThreadPoolExecutor(nThreads, nThreads,
                              0L, TimeUnit.MILLISECONDS,
                              new LinkedBlockingQueue<Runnable>());

newScheduledThreadPool

類似於newCachedThreadPool，執行緒數無上限，但是可以指定corePoolSize。可實現延遲執行、週期執行。

public ScheduledThreadPoolExecutor(int corePoolSize) {
    super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
          new DelayedWorkQueue());
}

週期執行：

ScheduledExecutorService scheduledThreadPool = Executors.newScheduledThreadPool(5);
scheduledThreadPool.scheduleAtFixedRate(()->{
   System.out.println("rate");
}, 1, 1, TimeUnit.SECONDS);

延時執行：

scheduledThreadPool.schedule(()->{
   System.out.println("delay 3 seconds");
}, 3, TimeUnit.SECONDS);

newSingleThreadExecutor

單執行緒執行緒池，可以實現執行緒的順序執行。

public static ExecutorService newSingleThreadExecutor() {
    return new FinalizableDelegatedExecutorService
        (new ThreadPoolExecutor(1, 1,
                                0L, TimeUnit.MILLISECONDS,
                                new LinkedBlockingQueue<Runnable>()));
}

Java 執行緒池中的四種拒絕策略

• CallerRunsPolicy：執行緒池讓呼叫者去執行。

• AbortPolicy：如果執行緒池拒絕了任務，直接報錯。

• DiscardPolicy：如果執行緒池拒絕了任務，直接丟棄。

• DiscardOldestPolicy：如果執行緒池拒絕了任務，直接將執行緒池中最舊的，未執行的任務丟棄，將新任務入隊。

CallerRunsPolicy

直接在主執行緒中執行了run方法。

public static class CallerRunsPolicy implements RejectedExecutionHandler {
 
    public CallerRunsPolicy() { }
 
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        if (!e.isShutdown()) {
            r.run();
        }
    }
}

效果類似於：

Runnable thread = ()->{
   System.out.println(Thread.currentThread().getName());
   try {
      Thread.sleep(0);
   } catch (InterruptedException e) {
      throw new RuntimeException(e);
   }
};

thread.run();

AbortPolicy

直接丟擲RejectedExecutionException異常，並指示任務的資訊，執行緒池的資訊。、

public static class AbortPolicy implements RejectedExecutionHandler {
 
    public AbortPolicy() { }
 
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        throw new RejectedExecutionException("Task " + r.toString() +
                                             " rejected from " +
                                             e.toString());
    }
}

DiscardPolicy

什麼也不做。

public static class DiscardPolicy implements RejectedExecutionHandler {
 
    public DiscardPolicy() { }
 
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
    }
}

DiscardOldestPolicy

• e.getQueue().poll() : 取出佇列最舊的任務。

• e.execute(r) : 當前任務入隊。

public static class DiscardOldestPolicy implements RejectedExecutionHandler {
 
    public DiscardOldestPolicy() { }
 
    public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        if (!e.isShutdown()) {
            e.getQueue().poll();
            e.execute(r);
        }
    }
}

java 執行緒複用的原理

java的執行緒池中儲存的是 java.util.concurrent.ThreadPoolExecutor.Worker 物件，該物件在 被維護在private final HashSet<Worker> workers = new HashSet<Worker>();。workQueue是儲存待執行的任務的佇列，執行緒池中加入新的任務時，會將任務加入到workQueue佇列中。

private final class Worker
    extends AbstractQueuedSynchronizer
    implements Runnable
{
    /**
     * This class will never be serialized, but we provide a
     * serialVersionUID to suppress a javac warning.
     */
    private static final long serialVersionUID = 6138294804551838833L;

    /** Thread this worker is running in.  Null if factory fails. */
    final Thread thread;
    /** Initial task to run.  Possibly null. */
    Runnable firstTask;
    /** Per-thread task counter */
    volatile long completedTasks;

    /**
     * Creates with given first task and thread from ThreadFactory.
     * @param firstTask the first task (null if none)
     */
    Worker(Runnable firstTask) {
        setState(-1); // inhibit interrupts until runWorker
        this.firstTask = firstTask;
        this.thread = getThreadFactory().newThread(this);
    }

    /** Delegates main run loop to outer runWorker  */
    public void run() {
        runWorker(this);
    }

    // Lock methods
    //
    // The value 0 represents the unlocked state.
    // The value 1 represents the locked state.

    protected boolean isHeldExclusively() {
        return getState() != 0;
    }

    protected boolean tryAcquire(int unused) {
        if (compareAndSetState(0, 1)) {
            setExclusiveOwnerThread(Thread.currentThread());
            return true;
        }
        return false;
    }

    protected boolean tryRelease(int unused) {
        setExclusiveOwnerThread(null);
        setState(0);
        return true;
    }

    public void lock()        { acquire(1); }
    public boolean tryLock()  { return tryAcquire(1); }
    public void unlock()      { release(1); }
    public boolean isLocked() { return isHeldExclusively(); }

    void interruptIfStarted() {
        Thread t;
        if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
            try {
                t.interrupt();
            } catch (SecurityException ignore) {
            }
        }
    }
}

work物件的執行依賴於 runWorker()，與我們平時寫的執行緒不同，該執行緒處在一個迴圈中，並不斷地從佇列中獲取新的任務執行。因此執行緒池中的執行緒才可以複用，而不是像我們平常使用的執行緒一樣執行完畢就結束。

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
        while (task != null || (task = getTask()) != null) {
            w.lock();
            // If pool is stopping, ensure thread is interrupted;
            // if not, ensure thread is not interrupted.  This
            // requires a recheck in second case to deal with
            // shutdownNow race while clearing interrupt
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            try {
                beforeExecute(wt, task);
                Throwable thrown = null;
                try {
                    task.run();
                } catch (RuntimeException x) {
                    thrown = x; throw x;
                } catch (Error x) {
                    thrown = x; throw x;
                } catch (Throwable x) {
                    thrown = x; throw new Error(x);
                } finally {
                    afterExecute(task, thrown);
                }
            } finally {
                task = null;
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);
    }
}