深度解析Java线程池的异常处理机制 #3

Posted 2020-12-05

前言

今天小伙伴遇到个小问题，线程池提交的任务如果没有catch异常，那么会抛到哪里去，之前倒是没研究过，本着实事求是的原则，看了一下代码。

正文

小问题

考虑下面这段代码，有什么区别呢？你可以猜猜会不会有异常打出呢？如果打出来的话是在哪里？:

        ExecutorService threadPool = Executors.newFixedThreadPool(1);
        threadPool.submit(() -> {
            Object obj = null;
            System.out.println(obj.toString());
        });
        threadPool.execute(() -> {
            Object obj = null;
            System.out.println(obj.toString());
        });

源码解析

我们下面就来看下代码, 其实就是将我们提交过去的Runnable包装成一个Future

    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);
    }
    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // volatile修饰，保证多线程下的可见性,可以看看Java内存模型
    }
    public static <T> Callable<T> callable(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter<T>(task, result);
    }

    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;
        }
    }

接下来就会实际提交到队列中交给线程池调度处理:

    /**
    * 代码还是很清爽的，一个很典型的生产者/消费者模型，
    * 这里暂不纠结这些细节，那么如果提交到workQueue成功的话，消费者是谁呢?
    * 明显在这个newWorker里搞的鬼，同样细节有兴趣可以自己再去研究，这里我们会发现
    * 核心就是Worker这个内部类
    */
    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);
    }

那么接下来看看线程池核心的流程:

private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable{
          /** Delegates main run loop to outer runWorker  */
        public void run() {
            runWorker(this);
        }
}

final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            //getTask()方法会尝试从队列中抓取数据
            while (task != null || (task = getTask()) != null) {
                w.lock();
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                    //可覆写此方法打日志埋点之类的
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        //简单明了，直接调用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);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

submit的方式

那么我们可以这里是直接调用的run方法，先看submit的方式，我们知道最终传递过去的是一个FutureTask,也就是说会调用这里的run方法，我们看看实现:

    public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    //。。。
                    setException(ex);
                }
                if (ran)
                    set(result);
            }
        } finally {
          //省略
    }

      protected void setException(Throwable t) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t; //赋给了这个变量
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }

可以看到其实类似于直接吞掉了，这样的话我们调用get()方法的时候会拿到, 比如我们可以重写afterExecute方法，从而可以得到实际的异常:

protected void afterExecute(Runnable r, Throwable t) {
          super.afterExecute(r, t);
          if (t == null && r instanceof Future<?>) {
            try {
              //get这里会首先检查任务的状态，然后将上面的异常包装成ExecutionException
              Object result = ((Future<?>) r).get();
            } catch (CancellationException ce) {
                t = ce;
            } catch (ExecutionException ee) {
                t = ee.getCause();
            } catch (InterruptedException ie) {
                Thread.currentThread().interrupt(); // ignore/reset
            }
          }
          if (t != null){
            //异常处理
            t.printStackTrace();
          }
        }

execute的方式

那么如果是直接exeture的方式有啥不同呢？这样的话传递过去的就直接是Runnable,因此就会直接抛出:

    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);
    }

那么这里的异常到底会抛出到哪里呢, 我们看看JVM具体是怎么处理的:

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(),
                "
Exception: %s thrown from the UncaughtExceptionHandler"
                " in thread "%s"
",
                pending_exception()->klass()->external_name(),
                get_thread_name());
          CLEAR_PENDING_EXCEPTION;
        }
      }
    }

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

    private void dispatchUncaughtException(Throwable e) {
        //默认会调用ThreadGroup的实现
        getUncaughtExceptionHandler().uncaughtException(this, e);
    }

?

    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);
            }
        }
    }

这里如果环境是tomcat的话最终会打到catalina.out:

总结

对于线程池、包括线程的异常处理推荐一下方式:

1 直接try/catch，个人 基本都是用这种方式

2 线程直接重写整个方法:

          Thread t = new Thread();
          t.setUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {

              public void uncaughtException(Thread t, Throwable e) {
                 LOGGER.error(t + " throws exception: " + e);
              }
           });
        //如果是线程池的模式:
           ExecutorService threadPool = Executors.newFixedThreadPool(1, r -> {
               Thread t = new Thread(r);
               t.setUncaughtExceptionHandler(
                   (t1, e) -> LOGGER.error(t1 + " throws exception: " + e));
               return t;
           });

3 也可以直接重写protected void afterExecute(Runnable r, Throwable t) { }方法