前言

在阅读线程相关知识的时候发现线程吞掉异常的知识，故记录下来。

代码示例

ExecutorService executorService = Executors.newFixedThreadPool(1);
        try {
            executorService.submit(() -> {
                Object obj = null;
                System.out.println(obj.toString());
            });
        } catch (Exception e) {
            System.out.println("catch Exception");
            e.printStackTrace();
        }
        try {
            executorService.execute(() -> {
                Object obj = null;
                System.out.println(obj.toString());
            });
        } catch (Exception e) {
            System.out.println("catch Exception");
            e.printStackTrace();
        }

运行结果：

Exception in thread "pool-1-thread-1" java.lang.NullPointerException
    at my.jdk.concurrent.ThreadPoolTest.lambda$uncaughtExceptionTest$2(ThreadPoolTest.java:68)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

catch肯定不会获取到信息，同时submit报错信息也没有打印出来，只打印的execute报错信息

问题

• 为什么不能抛出到外部线程捕获
• submit为什么不能打印报错信息
• execute怎么使用logger打印报错新信

原因

为什么不能抛出到外部线程捕获？

jvm会在线程即将死掉的时候捕获所有未捕获的异常进行处理。

submit为什么不能打印报错信息

submit源码实现：

public Future<?> submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<Void> ftask = newTaskFor(task, null);//创建FutureTask类
        execute(ftask);
        return ftask;
    }

查看FutureTask.run方法实现

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) {
                    //这里捕获了所有异常调用setException
                    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);
        }
    }

接着查看setException实现

//这个方法就是这事线程状态为completing -> exceptional
//同时用outcome保存异常信息。
protected void setException(Throwable t) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t;
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }

然后我们继续追踪outcome的使用

//report会抛出exception信息
private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL)
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }

//get会调用report方法
public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

所有知道了如果想获取sumbit的异常信息需要调用get方法，如下：

@Test
    public void caughtSumbitException() {
        ExecutorService executorService = Executors.newFixedThreadPool(1);
        Future future = executorService.submit(() -> {
            Object obj = null;
            System.out.println(obj.toString());
        });
        try {
            future.get();
        } catch (Exception e) {
            System.out.println("catch NullPointException");
        }
    }

输出：

catch NullPointException

execute怎么输入logger日志

查看execute代码实现

final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        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) {
                        //这里直接抛出所有异常
                        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);
        }
    }

这里抛出的异常在哪里处理呢？
接下来处理是交由jvm处理，由于本人对jvm源码不了解，只知道jvm调用Thread. dispatchUncaughtException来处理所有未捕获的异常

/**
     * Dispatch an uncaught exception to the handler. This method is
     * intended to be called only by the JVM.
     */
    private void dispatchUncaughtException(Throwable e) {
        getUncaughtExceptionHandler().uncaughtException(this, e);
    }

这里可以根据该方法注释解释，意思就是这个方法只用于JVM调用，处理线程未捕获的异常。
继续查看getUncaughtExceptionHandler()方法

    public interface UncaughtExceptionHandler {
    
        void uncaughtException(Thread t, Throwable e);
    }

    // 处理类
    private volatile UncaughtExceptionHandler uncaughtExceptionHandler;

    // 默认处理类
    private static volatile UncaughtExceptionHandler defaultUncaughtExceptionHandler;

    /**
    * 设置默认的处理类，注意是静态方法，作用域为所有线程设置默认的处理类
    **/
    public static void setDefaultUncaughtExceptionHandler(UncaughtExceptionHandler eh) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(
                new RuntimePermission("setDefaultUncaughtExceptionHandler")
                    );
        }

         defaultUncaughtExceptionHandler = eh;
     }
    //获取默认处理类
    public static UncaughtExceptionHandler getDefaultUncaughtExceptionHandler(){
        return defaultUncaughtExceptionHandler;
    }
    //获取处理类，注意不是静态方法，只作用域该线程
    //处理类为空使用ThreadGroup
    public UncaughtExceptionHandler getUncaughtExceptionHandler() {
        return uncaughtExceptionHandler != null ?
            uncaughtExceptionHandler : group;
    }
    //设置处理类
    public void setUncaughtExceptionHandler(UncaughtExceptionHandler eh) {
        checkAccess();
        uncaughtExceptionHandler = eh;
    }

    /**
     * Dispatch an uncaught exception to the handler. This method is
     * intended to be called only by the JVM.
     */
    private void dispatchUncaughtException(Throwable e) {
        //获取处理类型进行异常处理
        getUncaughtExceptionHandler().uncaughtException(this, e);
    }

如果线程处理器为空则threadGroup处理器
查看threadGroup

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)) {
                System.err.print("Exception in thread \""
                                 + t.getName() + "\" ");
                e.printStackTrace(System.err);
            }
        }
    }

获取默认处理器进行线程处理
默认处理器为空则在Sytem.err就行错误信息输出
到这里有两个方法可以实现用logger输出

• thread定义uncaughtExceptionHandler
• Thread定义defaultUncaughtExceptionHandler
  示例代码：

@Test
    public void setUncaughtExceptionHandler() throws InterruptedException {
        Thread t1 = new Thread(() -> {
            for (;;) {
                throw new RuntimeException("soming Exception");
            }
        }, "t1");
        t1.setUncaughtExceptionHandler(new MyUncaughtExceptionHandler());
        t1.start();
        new Thread(() -> {
            for (;;) {
                throw new RuntimeException("soming Exception");
            }
        }, "t2").start();
        Thread.sleep(1000);
    }

class MyUncaughtExceptionHandler implements Thread.UncaughtExceptionHandler {
        private Thread.UncaughtExceptionHandler defaultHandler;

        public MyUncaughtExceptionHandler() {
            this.defaultHandler = Thread.getDefaultUncaughtExceptionHandler();
        }

        @Override
        public void uncaughtException(Thread t, Throwable e) {
            logger.info("logger println exception info, threadName:{}", t.getName());
        }
    }

输出：

Exception in thread "t2" java.lang.RuntimeException: soming Exception
    at my.jdk.concurrent.ThreadTest.lambda$setUncaughtExceptionHandler$1(ThreadTest.java:27)
    at java.lang.Thread.run(Thread.java:745)
[no tracer] 2018-06-05 19:01:55.230 [t1] INFO  my.jdk.concurrent.ThreadTest - logger println exception info, threadName:t1

可以看到setUncaughtExceptionHandler只作用与t1,t2还是system.err输出

@Test
    public void defaultUncaughtExceptionHandler() throws InterruptedException {
        Thread.setDefaultUncaughtExceptionHandler(new MyUncaughtExceptionHandler());
        new Thread(() -> {
            for (;;) {
                throw new RuntimeException("soming Exception");
            }
        }, "t1").start();
        new Thread(() -> {
            for (;;) {
                throw new RuntimeException("soming Exception");
            }
        }, "t2").start();
        Thread.sleep(1000);

    }
class MyUncaughtExceptionHandler implements Thread.UncaughtExceptionHandler {
        private Thread.UncaughtExceptionHandler defaultHandler;

        public MyUncaughtExceptionHandler() {
            this.defaultHandler = Thread.getDefaultUncaughtExceptionHandler();
        }

        @Override
        public void uncaughtException(Thread t, Throwable e) {
            logger.info("logger println exception info, threadName:{}", t.getName());
        }
    }

输出：

[no tracer] 2018-06-05 19:03:37.292 [t2] INFO  my.jdk.concurrent.ThreadTest - logger println exception info, threadName:t2
[no tracer] 2018-06-05 19:03:37.292 [t1] INFO  my.jdk.concurrent.ThreadTest - logger println exception info, threadName:t1

作用域为所有线程