深度解析Java线程池的异常处理机制

引言
在Java中，使用线程池来异步执行一些耗时任务是非常常见的操作。最初我们一般都是直接使用new Thread().start的方式，但我们知道，线程的创建和销毁都会耗费大量的资源
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(),
"\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:

深度解析Java线程池的异常处理机制

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

深度解析Java线程池的异常处理机制

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