线程池中的线程如果抛异常会怎么样

看到一篇不错的文章，让我有思路也写写关于线程池，本文主要关注线程池里的任务抛了异常怎么办

　　一 Thread能捕获异常吗

public class ThreadExceptionTest {
    
    public static class InitialtiveThread implements Runnable
    {
        @Override
        public void run()
        {
            System.out.println(3/2);
            System.out.println(3/0);
            System.out.println(3/1);
        }
    }
    
    public static void main(String[] args) {
        
        InitialtiveThread runner = new InitialtiveThread();
        try {
            Thread thread = new Thread(runner);
            thread.start();
        } catch(Exception e) {
            
        }
        
    }

}

　　执行之后发现，如果run方法里抛出异常，通过try，catch 线程的start方法是没法捕获异常的，那是因为该异常是一个UncaughtException，没法通过用户的代码捕获的。

　　处理方法有两种

　　 1 run方法里面捕获

　　2  thread.setUncaughtExceptionHandler((Thread t, Throwable e) -> {System.out.println(t.getName() + ": " + e.getMessage());}); 

　　方法2就是给线程指定对于UncaughtException的处理器

　　上述的知识作为前提，为了更好地理解下面的内容

　　二 线程池执行的情况

　　　首先要区分execute和submit两种方式，这两种方式如果没看过源码，可能还真以为差不多，其实差多了

　　　先说execute，看代码段

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);

　　对于execute方式，用户提交的Runnable没有经过任何的转换，保留原样

　　现在看Worker的runWorker

try {
            while (task != null || (task = getTask()) != null) {
                w.lock();
                // If pool is stopping, ensure thread is interrupted;
                // if not, ensure thread is not interrupted.  This
                // requires a recheck in second case to deal with
                // shutdownNow race while clearing interrupt
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    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);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }

　　主动throw了异常，经过第一节的知识，一定会抛出来UncaughtException

　　我们先看Finally分支

private void processWorkerExit(Worker w, boolean completedAbruptly) {//抛异常的情况completedAbruptly是false的
        if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
            decrementWorkerCount();

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            completedTaskCount += w.completedTasks;
            workers.remove(w);//移除该worker
        } finally {
            mainLock.unlock();
        }

        tryTerminate();//检查是否要关闭

        int c = ctl.get();
        if (runStateLessThan(c, STOP)) {
            if (!completedAbruptly) {
                int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
                if (min == 0 && ! workQueue.isEmpty())
                    min = 1;
                if (workerCountOf(c) >= min)
                    return; // replacement not needed
            }
            addWorker(null, false);//如果小于核心线程 再次添加
        }
    }

　　对于execute方式，如果抛出异常，就把该线程移除，如果线程池当前的worker数量不到coreSize，就再添加一个

　　那么，如果增加了 setUncaughtExceptionHandler 会怎么样呢？

　　通过跟代码，run会抛出异常，所以还是会走线程移除流程，只不过之前是把异常抛出来，现在是handler里面的处理，但是线程还是会终止的。

　　

　　再来看submit

　　submit就好玩了，这里也不详细的跟代码了，简单点说，submit会把传进来的Runnable或者Callable包成FutureTask，这个FutureTask本身还实现了Runnable接口

　　所以我们看看他的run

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 {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

　　看得出来，这个run里面是有try catch包装的，这样就不会有异常了。

　　不过使用submit还是要注意，用了submit后即使是有 setUncaughtExceptionHandler 也是没用的，因为里面catch了。所以submit方式会吞异常，使用submit方式必须手动执行get(),来检查是否有异常