为什么java线程池的submit的不抛出异常
【摘要】
前言
大家好，这里是经典鸡翅，最近有人问我线程池的execute和submit，有的抛出异常，有的不抛出异常，这里鸡翅老哥给大家整理下。通过源码跟踪跟大家讲解。
两个方法
我们先用最普通的方式定义一个线程池。
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecut...
前言
大家好，这里是经典鸡翅，最近有人问我线程池的execute和submit，有的抛出异常，有的不抛出异常，这里鸡翅老哥给大家整理下。通过源码跟踪跟大家讲解。
两个方法
我们先用最普通的方式定义一个线程池。
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(5, 10, 5, TimeUnit.SECONDS, new LinkedBlockingDeque<>(1024), new ThreadPoolExecutor.AbortPolicy());
线程池有两种执行任务的方式，一种是execute方法，一种是submit方法。
我们引入一个最简单的例子。执行一个有异常的方法。
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@Test
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public void test() throws ExecutionException, InterruptedException {
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ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(5, 10, 5, TimeUnit.SECONDS, new LinkedBlockingDeque<>(1024), new ThreadPoolExecutor.AbortPolicy());
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threadPoolExecutor.execute(() -> {
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testThread();
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});
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}
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private int testThread() {
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int i = 1 / 0;
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return i;
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}
如上的一段代码，由于1/0的存在，必定是会出现异常的，我们执行后，看到结果，是有异常的。符合我们的预期。
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Exception in thread "pool-1-thread-1" java.lang.ArithmeticException: / by zero
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at com.hq.hqframe.textUtil.TextUtilTest.testThread(TextUtilTest.java:104)
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at com.hq.hqframe.textUtil.TextUtilTest.lambda$test$5(TextUtilTest.java:99)
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at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
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at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
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at java.lang.Thread.run(Thread.java:748)
如果我们使用submit提交任务呢？
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@Test
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public void test() throws ExecutionException, InterruptedException {
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ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(5, 10, 5, TimeUnit.SECONDS, new LinkedBlockingDeque<>(1024), new ThreadPoolExecutor.AbortPolicy());
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threadPoolExecutor.submit(() -> {
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testThread();
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});
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}
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private int testThread() {
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int i = 1 / 0;
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return i;
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}
执行代码我们发现，控制台并没有打印出任何异常。异常被吞了。这就是execute和submit的第一个区别。
为什么submit不会抛出异常呢？
我们开始跟踪一下，为什么submit不会抛出异常。先看submit的第一个方法。
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public Future<?> submit(Runnable task) {
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if (task == null) throw new NullPointerException();
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RunnableFuture<Void> ftask = newTaskFor(task, null);
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execute(ftask);
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return ftask;
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}
将task进行包装为FutureTask。这里面的task是runnable的，我们将其包装成callable形式的。通过runnableadapter。
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static final class RunnableAdapter<T> implements Callable<T> {
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final Runnable task;
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final T result;
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RunnableAdapter(Runnable task, T result) {
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this.task = task;
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this.result = result;
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}
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public T call() {
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task.run();
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return result;
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}
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}
包装后，执行这个task。
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public void execute(Runnable command) {
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if (command == null)
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throw new NullPointerException();
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/*
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* Proceed in 3 steps:
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*
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* 1. If fewer than corePoolSize threads are running, try to
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* start a new thread with the given command as its first
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* task. The call to addWorker atomically checks runState and
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* workerCount, and so prevents false alarms that would add
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* threads when it shouldn't, by returning false.
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*
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* 2. If a task can be successfully queued, then we still need
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* to double-check whether we should have added a thread
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* (because existing ones died since last checking) or that
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* the pool shut down since entry into this method. So we
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* recheck state and if necessary roll back the enqueuing if
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* stopped, or start a new thread if there are none.
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*
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* 3. If we cannot queue task, then we try to add a new
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* thread. If it fails, we know we are shut down or saturated
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* and so reject the task.
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*/
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int c = ctl.get();
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if (workerCountOf(c) < corePoolSize) {
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if (addWorker(command, true))
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return;
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c = ctl.get();
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}
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if (isRunning(c) && workQueue.offer(command)) {
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int recheck = ctl.get();
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if (! isRunning(recheck) && remove(command))
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reject(command);
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else if (workerCountOf(recheck) == 0)
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addWorker(null, false);
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}
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else if (!addWorker(command, false))
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reject(command);
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}
我们直接将目光聚焦到addWorker，这里面干了一个什么事呢？重点建造了一个Worker类。把当前的task传入进去。Worker类里面把自己的task作为一个线程赋值。
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Worker(Runnable firstTask) {
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setState(-1); // inhibit interrupts until runWorker
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this.firstTask = firstTask;
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this.thread = getThreadFactory().newThread(this);
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}
回到addWorker处，我们可以看到start方法，这个方法，实际调用的就是线程的run方法。也就是我们worker类里面自己实现的run方法。
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public void run() {
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runWorker(this);
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}
再看下runworker方法，我们发现里面继续调用了task.run方法。
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final void runWorker(Worker w) {
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Thread wt = Thread.currentThread();
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Runnable task = w.firstTask;
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w.firstTask = null;
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w.unlock(); // allow interrupts
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boolean completedAbruptly = true;
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try {
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while (task != null || (task = getTask()) != null) {
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w.lock();
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// If pool is stopping, ensure thread is interrupted;
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// if not, ensure thread is not interrupted. This
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// requires a recheck in second case to deal with
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// shutdownNow race while clearing interrupt
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if ((runStateAtLeast(ctl.get(), STOP) ||
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(Thread.interrupted() &&
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runStateAtLeast(ctl.get(), STOP))) &&
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!wt.isInterrupted())
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wt.interrupt();
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try {
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beforeExecute(wt, task);
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Throwable thrown = null;
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try {
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task.run();
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} catch (RuntimeException x) {
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thrown = x; throw x;
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} catch (Error x) {
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thrown = x; throw x;
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} catch (Throwable x) {
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thrown = x; throw new Error(x);
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} finally {
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afterExecute(task, thrown);
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}
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} finally {
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task = null;
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w.completedTasks++;
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w.unlock();
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}
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}
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completedAbruptly = false;
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} finally {
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processWorkerExit(w, completedAbruptly);
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}
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}
我们就可以重新追踪到futureTask。
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public void run() {
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if (state != NEW ||
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!UNSAFE.compareAndSwapObject(this, runnerOffset,
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null, Thread.currentThread()))
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return;
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try {
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Callable<V> c = callable;
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if (c != null && state == NEW) {
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V result;
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boolean ran;
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try {
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result = c.call();
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ran = true;
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} catch (Throwable ex) {
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result = null;
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ran = false;
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setException(ex);
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}
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if (ran)
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set(result);
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}
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} finally {
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// runner must be non-null until state is settled to
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// prevent concurrent calls to run()
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runner = null;
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// state must be re-read after nulling runner to prevent
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// leaked interrupts
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int s = state;
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if (s >= INTERRUPTING)
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handlePossibleCancellationInterrupt(s);
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}
此时我们发现，异常已经被捕获了，并且调用了setexception方法。把我们的异常信息给了outcome。
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protected void setException(Throwable t) {
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if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
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outcome = t;
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UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
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finishCompletion();
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}
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}
到此我们就明白了为什么submit没有抛出异常了！
文章来源: blog.csdn.net，作者：经典鸡翅，版权归原作者所有，如需转载，请联系作者。
原文链接：blog.csdn.net/hanqing456/article/details/122293430
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