前言
今天小夥伴遇到個小問題，執行緒池提交的任務如果沒有catch異常，那麼會拋到哪裡去，之前倒是沒研究過，本著實事求是的原則，看了一下程式碼。
正文
小問題
考慮下面這段程式碼，有什麼區別呢？你可以猜猜會不會有異常打出呢？如果打出來的話是在哪裡？:
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());
});複製程式碼原始碼解析
我們下面就來看下程式碼, 其實就是將我們提交過去的Runnable包裝成一個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修飾，保證多執行緒下的可見性,可以看看Java記憶體模型
}
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;
}
}複製程式碼接下來就會實際提交到佇列中交給執行緒池排程處理:
/**
* 程式碼還是很清爽的，一個很典型的生產者/消費者模型，
* 這裡暫不糾結這些細節，那麼如果提交到workQueue成功的話，消費者是誰呢?
* 明顯在這個newWorker裡搞的鬼，同樣細節有興趣可以自己再去研究，這裡我們會發現
* 核心就是Worker這個內部類
*/
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);
}複製程式碼那麼接下來看看執行緒池核心的流程:
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()方法會嘗試從佇列中抓取資料
while (task != null || (task = getTask()) != null) {
w.lock();
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
//可覆寫此方法打日誌埋點之類的
beforeExecute(wt, task);
Throwable thrown = null;
try {
//簡單明瞭，直接呼叫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);
}
}複製程式碼submit的方式
那麼我們可以這裡是直接呼叫的run方法，先看submit的方式，我們知道最終傳遞過去的是一個FutureTask,也就是說會呼叫這裡的run方法，我們看看實現:
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; //賦給了這個變數
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}複製程式碼可以看到其實類似於直接吞掉了，這樣的話我們呼叫get()方法的時候會拿到, 比如我們可以重寫afterExecute方法，從而可以得到實際的異常:
protected void afterExecute(Runnable r, Throwable t) {
super.afterExecute(r, t);
if (t == null && r instanceof Future<?>) {
try {
//get這裡會首先檢查任務的狀態，然後將上面的異常包裝成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){
//異常處理
t.printStackTrace();
}
}複製程式碼execute的方式
那麼如果是直接exeture的方式有啥不同呢？這樣的話傳遞過去的就直接是Runnable,因此就會直接丟擲:
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);
}複製程式碼那麼這裡的異常到底會丟擲到哪裡呢, 我們看看JVM具體是怎麼處理的:
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()) {
//如果有未捕獲的異常
Handle group(this, java_lang_Thread::threadGroup(threadObj()));
{
KlassHandle recvrKlass(THREAD, threadObj->klass());
CallInfo callinfo;
KlassHandle thread_klass(THREAD, SystemDictionary::Thread_klass());
/*
這裡類似一個方法表，實際就會去呼叫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;
}
}
}複製程式碼可以看到這裡最終會去呼叫Thread#dispatchUncaughtException方法:
private void dispatchUncaughtException(Throwable e) {
//預設會呼叫ThreadGroup的實現
getUncaughtExceptionHandler().uncaughtException(this, e);
}複製程式碼
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)) {
//可以看到會打到System.err裡面
System.err.print("Exception in thread \""
+ t.getName() + "\" ");
e.printStackTrace(System.err);
}
}
}複製程式碼這裡如果環境是tomcat的話最終會打到catalina.out:
總結
對於執行緒池、包括執行緒的異常處理推薦一下方式:
1 直接try/catch，個人 基本都是用這種方式
2 執行緒直接重寫整個方法:
Thread t = new Thread();
t.setUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
public void uncaughtException(Thread t, Throwable e) {
LOGGER.error(t + " throws exception: " + e);
}
});
//如果是執行緒池的模式:
ExecutorService threadPool = Executors.newFixedThreadPool(1, r -> {
Thread t = new Thread(r);
t.setUncaughtExceptionHandler(
(t1, e) -> LOGGER.error(t1 + " throws exception: " + e));
return t;
});複製程式碼3 也可以直接重寫protected void afterExecute(Runnable r, Throwable t) { }方法