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抓住异常，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可运行的包装成一个未来
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;
}
}
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);
}
}
提交的方式
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();
}
}
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After Execute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();
}
}
执行的方式
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线程#DispatchUncoghtException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