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());
});
复制代码源码解析 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 ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
protected RunnableFuture newTaskFor(Runnable runnable, T value) {
return new FutureTask(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 Callable callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter(task, result);
}
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;
}
}
复制代码 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);
}
复制代码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 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();
}
}
复制代码 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);
}
复制代码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);
}
复制代码
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);
}
}
}
复制代码这里如果环境是tomcat的话最终会打到catalina.out:
_6145c123-4ec7-4856-b106-6c61e6dca285
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;
});
复制代码3 也可以直接重写protected void afterExecute(Runnable r, Throwable t) { }方法