前言

今天小夥伴遇到個小問題，執行緒池提交的任務如果沒有catch異常，那麼會拋到哪裡去，之前倒是沒研究過，本著實事求是的原則，看了一下程式碼。

正文

小問題

考慮下面這段程式碼，有什麼區別呢？你可以猜猜會不會有異常打出呢？如果打出來的話是在哪裡？:

        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());
        });

原始碼解析

我們下面就來看下程式碼, 其實就是將我們提交過去的Runnable包裝成一個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修飾，保證多執行緒下的可見性,可以看看Java記憶體模型
    }
    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;
        }
    }

接下來就會實際提交到佇列中交給執行緒池排程處理:

	/**
	* 程式碼還是很清爽的，一個很典型的生產者/消費者模型，
	* 這裡暫不糾結這些細節，那麼如果提交到workQueue成功的話，消費者是誰呢?
	* 明顯在這個newWorker裡搞的鬼，同樣細節有興趣可以自己再去研究，這裡我們會發現
	* 核心就是Worker這個內部類
	*/
	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);
    }

那麼接下來看看執行緒池核心的流程:

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()方法會嘗試從佇列中抓取資料
            while (task != null || (task = getTask()) != null) {
                w.lock();
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                  	//可覆寫此方法打日誌埋點之類的
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        //簡單明瞭，直接呼叫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

那麼我們可以這裡是直接呼叫的run方法，先看submit的方式，我們知道最終傳遞過去的是一個FutureTask,也就是說會呼叫這裡的run方法，我們看看實現:

	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; //賦給了這個變數
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }

可以看到其實類似於直接吞掉了，這樣的話我們呼叫get()方法的時候會拿到, 比如我們可以重寫afterExecute方法，從而可以得到實際的異常:

protected void afterExecute(Runnable r, Throwable t) {
          super.afterExecute(r, t);
          if (t == null && r instanceof Future<?>) {
            try {
              //get這裡會首先檢查任務的狀態，然後將上面的異常包裝成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){
            //異常處理
            t.printStackTrace();
          }
        }

execute的方式

那麼如果是直接exeture的方式有啥不同呢？這樣的話傳遞過去的就直接是Runnable,因此就會直接丟擲:

    try {
        task.run();
    } catch (RuntimeException x) {
        thrown = x; throw x;
    } catch ( x) {
        thrown = x; throw x;
    } catch (Throwable x) {
        thrown = x; throw new Error(x);
    } finally {
        afterExecute(task, thrown);
    }

那麼這裡的異常到底會丟擲到哪裡呢, 我們看看JVM具體是怎麼處理的:

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()) {
      //如果有未捕獲的異常
      Handle group(this, java_lang_Thread::threadGroup(threadObj()));
      {
        KlassHandle recvrKlass(THREAD, threadObj->klass());
        CallInfo callinfo;
        KlassHandle thread_klass(THREAD, SystemDictionary::Thread_klass());
        /*	
        	這裡類似一個方法表，實際就會去呼叫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;
        }
      }
    }

可以看到這裡最終會去呼叫Thread#dispatchUncaughtException方法:

    private void dispatchUncaughtException(Throwable e) {
      	//預設會呼叫ThreadGroup的實現
        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)) {
              	//可以看到會打到System.err裡面
                System.err.print("Exception in thread \""
                                 + t.getName() + "\" ");
                e.printStackTrace(System.err);
            }
        }
    }

這裡如果環境是tomcat的話最終會打到catalina.out:

總結

對於執行緒池、包括執行緒的異常處理推薦一下方式:

1 直接try/catch，個人 基本都是用這種方式

2 執行緒直接重寫整個方法:

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

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