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?
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![](https://img.haomeiwen.com/i14078400/3e25518ff9d9e04e.jpg)
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
publicstaticExecutorServicenewFixedThreadPool(intnThreads){returnnewThreadPoolExecutor(nThreads, nThreads,0L, TimeUnit.MILLISECONDS,newLinkedBlockingQueue()); }
再看这个类的继承关系,
![](https://img.haomeiwen.com/i14078400/c52811e30d320725.jpg!web)
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。
publicFuture submit(Runnabletask) {if(task==null)thrownewNullPointerException(); RunnableFuture ftask = newTaskFor(task,null); execute(ftask);returnftask; }protected RunnableFuture newTaskFor(Runnable runnable, T value) {returnnewFutureTask(runnable, value); }
对应的 FutureTask对象的 构造方法
publicFutureTask(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
publicstatic Callable callable(Runnabletask, T result) {if(task==null)thrownewNullPointerException();returnnewRunnableAdapter(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
*/staticfinalclassRunnableAdapterimplementsCallable {finalRunnabletask;finalT result; RunnableAdapter(Runnabletask, T result) {this.task=task;this.result = result; }publicTcall() {task.run();returnresult; } }
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:
publicvoidexecute(Runnable command){if(command ==null)thrownewNullPointerException();intc = ctl.get();if(workerCountOf(c) < corePoolSize) {if(addWorker(command,true))return; c = ctl.get(); }if(isRunning(c) && workQueue.offer(command)) {intrecheck = ctl.get();if(! isRunning(recheck) &&remove(command)) reject(command);elseif(workerCountOf(recheck)==0) addWorker(null,false); }elseif(!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:
privatebooleanaddWorker(Runnable firstTask,booleancore){ ...booleanworkerStarted =false;booleanworkerAdded =false; Worker w =null;try{// 实例化一个worker对象w =newWorker(firstTask);finalThread t = w.thread;if(t !=null) {finalReentrantLock mainLock =this.mainLock; mainLock.lock();try{intrs = runStateOf(ctl.get());if(rs < SHUTDOWN || (rs == SHUTDOWN && firstTask ==null)) {if(t.isAlive())// precheck that t is startablethrownewIllegalThreadStateException(); workers.add(w);ints = 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); }returnworkerStarted;}// Worker的构造方法Worker(Runnable firstTask) { setState(-1);// inhibit interrupts until runWorkerthis.firstTask = firstTask;this.thread = getThreadFactory().newThread(this); }
我们再看这个 ThreadPoolExecutor 的内部类 Worker 对象:
privatefinalclassWorkerextendsAbstractQueuedSynchronizerimplementsRunnable{ .../** Delegates main run loop to outer runWorker */publicvoidrun(){ 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。
finalvoidrunWorker(Worker w) { Thread wt = Thread.currentThread(); Runnabletask= w.firstTask; w.firstTask =null; w.unlock();// allow interruptsbooleancompletedAbruptly =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;throwx; }catch(Error x) { thrown = x;throwx; }catch(Throwable x) { thrown = x;thrownewError(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;throwx;}catch(Errorx) { thrown = x;throwx;}catch(Throwable x) { thrown = x;thrownewError(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
protectedvoidafterExecute(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.dispatchUncaughtExceptionif(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 1uncaught_exception,// Arg 2THREAD); }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.
*/privatevoiddispatchUncaughtException(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 {@linkThread.UncaughtExceptionHandler} * installed. * */publicvoid 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); }elseif(!(einstanceofThreadDeath)) {//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 =newThread(r); t.setUncaughtExceptionHandler((t1, e)->LOGGER.error(t1 +" throws exception: "+ e));returnt;
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。
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