深度解析Java线程池的异常处理机制

Posted 2021-04-06 学掌门

　　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 ( 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:

　　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

文章来源：github

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