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());
} 再看这个类的继承关系，
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 ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
protected RunnableFuture newTaskFor(Runnable runnable, T value) {
return new FutureTask(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 Callable callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter(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 implements Callable {
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/jd...
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/developer...
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
- 用try-catch 捕捉，一般都是用这种
-
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; - 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
- https://www.jcp.org/en/jsr/de...
- https://www.ibm.com/developer...
- http://ifeve.com/%E6%B7%B1%E5...