【开发笔记】 Java 线程池 之 被“吃掉”的线程异常(附源码分析和解决方法)

Richard_Yi 2019年04月24日 205次浏览

前言

今天遇到了一个bug,现象是,一个任务放入线程池中,似乎“没有被执行”,日志也没有打。

经过本地代码调试之后,发现在任务逻辑的前半段,抛出了NPE,但是代码外层没有try-catch,导致这个异常被吃掉。

这个问题解决起来是很简单的,外层加个try-catch就好了,但是这个异常如果没有被catch,线程池内部逻辑是怎么处理这个异常的呢?这个异常最后会跑到哪里呢?

带着疑问和好奇心,我研究了一下线程池那一块的源码,并且做了以下的总结。

源码分析

项目中出问题的代码差不多就是下面这个样子

ExecutorService threadPool = Executors.newFixedThreadPool(3);

threadPool.submit(() -> {
    String pennyStr = null;
    Double penny = Double.valueOf(pennyStr);
    ...
})

先进到newFixedThreadPool这个工厂方法中看生成的具体实现类,发现是ThreadPoolExecutor

public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }

再看这个类的继承关系,

image

再进到submit方法,这个方法在ExecutorService接口中约定,其实是在AbstractExectorService中实现,ThreadPoolExecutor并没有override这个方法。

 public Future<?> submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<Void> ftask = newTaskFor(task, null);
        execute(ftask);
        return ftask;
    }

protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
        return new FutureTask<T>(runnable, value);
    }

对应的FutureTask对象的构造方法

public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // state由volatile 修饰 保证多线程下的可见性
    }

对应Callable 对象的构造方法

public static <T> Callable<T> callable(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter<T>(task, result);
    }

对应RunnableAdapter 对象的构造方法

 /**
     * A callable that runs given task and returns given result
     * 一个能执行所给任务并且返回结果的Callable对象
     */
    static final class RunnableAdapter<T> implements Callable<T> {
        final Runnable task;
        final T result;
        RunnableAdapter(Runnable task, T result) {
            this.task = task;
            this.result = result;
        }
        public T call() {
            task.run();
            return result;
        }
    }

总结上面的,newTaskFor就是把我们提交的Runnable 对象包装成了一个Future

接下来就是会把任务提交到队列中给线程池调度处理:

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
    
        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);
    }

因为主要关心的是这个线程怎么执行,异常的抛出和处理,所以我们暂时不解析多余的逻辑。很容易发现,如果任务要被执行,肯定是进到了addWorker方法当中,所以我们再进去看,鉴于addWorker方法的很长,不想列太多的代码,我就摘了关键代码段:

private boolean addWorker(Runnable firstTask, boolean core) {

   ...
   boolean workerStarted = false;
   boolean workerAdded = false;
   Worker w = null;
   try {
      // 实例化一个worker对象
      w = new Worker(firstTask);
      final Thread t = w.thread;
      if (t != null) {
          final ReentrantLock mainLock = this.mainLock;
          mainLock.lock();
          try {
            
              int rs = runStateOf(ctl.get());

              if (rs < SHUTDOWN ||
                  (rs == SHUTDOWN && firstTask == null)) {
                  if (t.isAlive()) // precheck that t is startable
                      throw new IllegalThreadStateException();
                  workers.add(w);
                  int s = workers.size();
                  if (s > largestPoolSize)
                      largestPoolSize = s;
                  workerAdded = true;
              }
          } finally {
              mainLock.unlock();
          }
          if (workerAdded) {
              // 从Worker对象的构造方法看,当这个thread对象start之后,
              // 之后实际上就是调用Worker对象的run()
              t.start();
              workerStarted = true;
          }
      }
   } finally {
      if (! workerStarted)
          addWorkerFailed(w);
   }
   return workerStarted;
}

// Worker的构造方法
  Worker(Runnable firstTask) {
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);
        }
 

我们再看这个ThreadPoolExecutor的内部类Worker对象:

private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable
   {
        ...

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

      ...
   }

看来真正执行任务的是在这个外部的runWorker当中,让我们再看看这个方法是怎么消费Worker线程的。

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 ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            try {
                beforeExecute(wt, task);
                Throwable thrown = null;
                // ==== 关键代码 start ====
                try {
                    // 很简洁明了,调用了任务的run方法
                    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);
                }
                 // ==== 关键代码 end ====
            } finally {
                task = null;
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);
    }
}

终于走到底了,可以看到关键代码中的try-catch block代码块中,调用了本次执行任务的run方法。

// ==== 关键代码 start ====
try {
  // 很简洁明了,调用了任务的run方法
  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);
}
// ==== 关键代码 end ====

可以看到捕捉了异常之后,会再向外抛出,只不过再finally block 中有个afterExecute()方法,似乎在这里是可以处理这个异常信息的,进去看看

protected void afterExecute(Runnable r, Throwable t) { }

可以看到ThreadPoolExecutor#afterExecute()方法中,是什么都没做的,看来是让使用者通过override这个方法来定制化任务执行之后的逻辑,其中可以包括异常处理。

那么这个异常到底是抛到哪里去了呢。我在一个大佬的文章找到了hotSpot JVM处理线程异常的逻辑,

if (!destroy_vm || JDK_Version::is_jdk12x_version()) {
    // JSR-166: change call from from ThreadGroup.uncaughtException to
    // java.lang.Thread.dispatchUncaughtException
    if (uncaught_exception.not_null()) {
      //如果有未捕获的异常
      Handle group(this, java_lang_Thread::threadGroup(threadObj()));
      {
        KlassHandle recvrKlass(THREAD, threadObj->klass());
        CallInfo callinfo;
        KlassHandle thread_klass(THREAD, SystemDictionary::Thread_klass());
        /*  
         这里类似一个方法表,实际就会去调用Thread#dispatchUncaughtException方法
         template(dispatchUncaughtException_name,            "dispatchUncaughtException")                
        */
        LinkResolver::resolve_virtual_call(callinfo, threadObj, recvrKlass, thread_klass,
                                           vmSymbols::dispatchUncaughtException_name(),
                                           vmSymbols::throwable_void_signature(),
                                           KlassHandle(), false, false, THREAD);
        CLEAR_PENDING_EXCEPTION;
        methodHandle method = callinfo.selected_method();
        if (method.not_null()) {
          JavaValue result(T_VOID);
          JavaCalls::call_virtual(&result,
                                  threadObj, thread_klass,
                                  vmSymbols::dispatchUncaughtException_name(),
                                  vmSymbols::throwable_void_signature(),
                                  uncaught_exception,
                                  THREAD);
        } else {
          KlassHandle thread_group(THREAD, SystemDictionary::ThreadGroup_klass());
          JavaValue result(T_VOID);
          JavaCalls::call_virtual(&result,
                                  group, thread_group,
                                  vmSymbols::uncaughtException_name(),
                                  vmSymbols::thread_throwable_void_signature(),
                                  threadObj,           // Arg 1
                                  uncaught_exception,  // Arg 2
                                  THREAD);
        }
        if (HAS_PENDING_EXCEPTION) {
          ResourceMark rm(this);
          jio_fprintf(defaultStream::error_stream(),
                "\nException: %s thrown from the UncaughtExceptionHandler"
                " in thread \"%s\"\n",
                pending_exception()->klass()->external_name(),
                get_thread_name());
          CLEAR_PENDING_EXCEPTION;
        }
      }
    }

代码是C写的,有兴趣可以去全文,根据英文注释能稍微看懂一点

http://hg.openjdk.java.net/jdk7/jdk7/hotspot/file/tip/src/share/vm/runtime/thread.cpp

可以看到这里最终会去调用Thread#dispatchUncaughtException方法:

/**
     * Dispatch an uncaught exception to the handler. This method is
     * intended to be called only by the JVM.
     */
    private void dispatchUncaughtException(Throwable e) {
        getUncaughtExceptionHandler().uncaughtException(this, e);
    }

/**
 * Called by the Java Virtual Machine when a thread in this
 * thread group stops because of an uncaught exception, and the thread
 * does not have a specific {@link Thread.UncaughtExceptionHandler}
 * installed.
 *
 */
public void uncaughtException(Thread t, Throwable e) {
        if (parent != null) {
            parent.uncaughtException(t, e);
        } else {
            Thread.UncaughtExceptionHandler ueh =
                Thread.getDefaultUncaughtExceptionHandler();
            if (ueh != null) {
                ueh.uncaughtException(t, e);
            } else if (!(e instanceof ThreadDeath)) {
               //可以看到会打到System.err里面
                System.err.print("Exception in thread \""
                                 + t.getName() + "\" ");
                e.printStackTrace(System.err);
            }
        }
    }

jdk的注释也说明的很清楚了,当一个线程抛出了一个未捕获的异常,JVM会去调用这个方法。如果当前线程没有声明UncaughtExceptionHandler成员变量并且重写uncaughtException方法的时候,就会看线程所属的线程组(如果有线程组的话)有没有这个类,没有就会打到System.err里面。

IBM这篇文章也提倡我们使用ThreadGroup 提供的 uncaughtException 处理程序来在线程异常终止时进行检测。

https://www.ibm.com/developerworks/cn/java/j-jtp0924/index.html

总结 (解决方法)

从上述源码分析中可以看到,对于本篇的异常“被吃掉”的问题,有以下几种方法

  1. 用try-catch 捕捉,一般都是用这种

  2. 线程或者线程组对象设置UncaughtExceptionHandler成员变量

      Thread t = new Thread(r);
                t.setUncaughtExceptionHandler(
                    (t1, e) -> LOGGER.error(t1 + " throws exception: " + e));
                return t;
    
  3. override 线程池的afterExecute方法。

本篇虽然是提出问题的解决方法,但主旨还是分析源码,了解了整个过程中异常的经过的流程,希望能对您产生帮助。

参考

  1. https://www.jcp.org/en/jsr/detail?id=166
  2. https://www.ibm.com/developerworks/cn/java/j-jtp0924/index.html
  3. http://ifeve.com/%E6%B7%B1%E5%BA%A6%E8%A7%A3%E6%9E%90java%E7%BA%BF%E7%A8%8B%E6%B1%A0%E7%9A%84%E5%BC%82%E5%B8%B8%E5%A4%84%E7%90%86%E6%9C%BA%E5%88%B6/