之前的文章 JVM 如何处理未捕获异常 我们介绍了JVM如何处理未捕获异常,今天我们研究一个更加有意思的问题,就是在JVM中如果发生了未捕获异常,会导致JVM进程退出么。
关于什么是未捕获异常,我们在之前的文章已经介绍过,这里不再赘述,如欲了解,请阅读 JVM 如何处理未捕获异常
//In Utils.java file public static void causeNPE() { String s = null; s.length(); }
//In Utils.java file public static void makeThreadSleep(long durationInMillSeconds) { try { Thread.sleep(durationInMillSeconds); } catch (InterruptedException e) { System.out.println("makeThreadSleep interrupted"); e.printStackTrace(); } }
使用该方法的目的主要有
//In Utils.java file public static void dumpAllThreadsInfo() { Set<Thread> threadSet = Thread.getAllStackTraces().keySet(); for(Thread thread: threadSet) { System.out.println("dumpAllThreadsInfo thread.name=" + thread.getName() + ";thread.state=" + thread.getState() + ";thread.isAlive=" + thread.isAlive() + ";group=" + thread.getThreadGroup() ); } }
//输出结果类似 16:55:55 public static String getTimeForDebug() { SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss"); return sdf.format(new Date()); }
这里的验证我们按照表现来区分,我们将验证以下场景
同时上面的场景,在通用的JVM和Android上表现有一些差异,我们也都会进行覆盖研究。
我们使用下面的代码,模拟一个在子线程中出现未捕获异常的场景。
private static void startErrorThread() { new Thread(new Runnable(){ @Override public void run() { System.out.println("startErrorThread currentThread.name=" + Thread.currentThread().getName() + "; happened at " + Utils.getTimeForDebug()); Utils.causeNPE(); } }).start(); Utils.makeThreadSleep(10 * 1000); System.out.println("Thread main sleepFinished at " + Utils.getTimeForDebug()); Utils.dumpAllThreadsInfo(); }
我们期待的输出结果是
//异常发生 输出线程名称和发生异常的时间 startErrorThread currentThread.name=Thread-0; happened at 16:59:04 //异常崩溃的信息 Exception in thread "Thread-0" java.lang.NullPointerException at Utils.causeNPE(Utils.java:35) at Main$3.run(Main.java:115) at java.lang.Thread.run(Thread.java:748) //主线程睡眠结束(对比时间,确定差为10秒) Thread main sleepFinished at 16:59:14 //主线程不受影响,继续执行操作 dumpAllThreadsInfo thread.name=Attach Listener;thread.state=RUNNABLE;thread.isAlive=true;group=java.lang.ThreadGroup[name=system,maxpri=10] dumpAllThreadsInfo thread.name=Reference Handler;thread.state=WAITING;thread.isAlive=true;group=java.lang.ThreadGroup[name=system,maxpri=10] dumpAllThreadsInfo thread.name=Monitor Ctrl-Break;thread.state=RUNNABLE;thread.isAlive=true;group=java.lang.ThreadGroup[name=main,maxpri=10] dumpAllThreadsInfo thread.name=Signal Dispatcher;thread.state=RUNNABLE;thread.isAlive=true;group=java.lang.ThreadGroup[name=system,maxpri=10] dumpAllThreadsInfo thread.name=Finalizer;thread.state=WAITING;thread.isAlive=true;group=java.lang.ThreadGroup[name=system,maxpri=10] dumpAllThreadsInfo thread.name=main;thread.state=RUNNABLE;thread.isAlive=true;group=java.lang.ThreadGroup[name=main,maxpri=10] //进程结束 Process finished with exit code 0
看起来,子线程发生未捕获的异常不会导致进程的退出(也不会影响其他的线程)。
这个时候可能做Android开发的同学可能会站起来。
提问:不对啊,我把你的代码放到Android项目中执行,会出现应用已停止的对话框,然后我的进程怎么就退出了呢,老哥,你的结论不对吧。
回答:哈哈,这个问题是一个好问题,想要回答这个问题,就需要了解JVM如何处理未捕获异常的。这也是我们之前文章 JVM 如何处理未捕获异常 介绍的。
这里简单概括一下就是,当JVM发现异常后
所以,我们按照这个流程扒了一下 RuntimeInit.java 发现了这样的代码。
/** * Use this to log a message when a thread exits due to an uncaught * exception. The framework catches these for the main threads, so * this should only matter for threads created by applications. */ private static class UncaughtHandler implements Thread.UncaughtExceptionHandler { public void uncaughtException(Thread t, Throwable e) { try { // Don't re-enter -- avoid infinite loops if crash-reporting crashes. if (mCrashing) return; mCrashing = true; if (mApplicationObject == null) { Slog.e(TAG, "*** FATAL EXCEPTION IN SYSTEM PROCESS: " + t.getName(), e); } else { Slog.e(TAG, "FATAL EXCEPTION: " + t.getName(), e); } // 展示 应用已停止的 对话框 // Bring up crash dialog, wait for it to be dismissed ActivityManagerNative.getDefault().handleApplicationCrash( mApplicationObject, new ApplicationErrorReport.CrashInfo(e)); } catch (Throwable t2) { try { Slog.e(TAG, "Error reporting crash", t2); } catch (Throwable t3) { // Even Slog.e() fails! Oh well. } } finally { //杀掉进程 // Try everything to make sure this process goes away. Process.killProcess(Process.myPid()); System.exit(10); } } }
上述代码会执行两个主要的操作
Android系统会在进程启动后,通过下面的代码为所有的线程设置默认的UncaughtExceptionHandler
/* set default handler; this applies to all threads in the VM */ Thread.setDefaultUncaughtExceptionHandler(new UncaughtHandler());
同时由于如下原因
Thread.setUncaughtExceptionHandler uncaughtException
所以出现未捕获的异常,默认就会走到了Android系统默认设置的所有线程共用的处理者。
前面说的都是子线程,那么如果主线程出现未捕获异常,进程应该会退出吧。
private static void uncaughtExceptionInMainThread() { Utils.causeNPE(); }
执行上面的代码,得到进程退出的日志
Exception in thread "main" java.lang.NullPointerException at Utils.causeNPE(Utils.java:35) at Main.uncaughtExceptionInMainThread(Main.java:28) at Main.main(Main.java:14) Process finished with exit code 1
可是当我们执行下面的这份代码(启动另一个线程并休眠20秒),结果却是不一样的
private static void uncaughtExceptionInMainThreadNotLastUserThread() { new Thread(new Runnable() { @Override public void run() { Utils.makeThreadSleep(20 * 1000); System.out.println("uncaughtExceptionInMainThreadNotLastUserThread time=" + Utils.getTimeForDebug() + ";thread=" + Thread.currentThread().getName() ); Utils.dumpAllThreadsInfo(); } }).start(); Utils.makeThreadSleep(5 * 1000); System.out.println("uncaughtExceptionInMainThreadNotLastUserThread mainThread time=" + Utils.getTimeForDebug()); Utils.causeNPE(); }
得到的日志输出是
uncaughtExceptionInMainThreadNotLastUserThread mainThread time=20:48:09 // 异常发生 Exception in thread "main" java.lang.NullPointerException at Utils.causeNPE(Utils.java:35) at Main.uncaughtExceptionInMainThreadNotLastUserThread(Main.java:44) at Main.main(Main.java:15) //Thread-0 线程休眠结束 uncaughtExceptionInMainThreadNotLastUserThread time=20:48:24;thread=Thread-0 // 打印此时的全部线程信息 dumpAllThreadsInfo thread.name=Signal Dispatcher;thread.state=RUNNABLE;thread.isAlive=true;group=java.lang.ThreadGroup[name=system,maxpri=10] dumpAllThreadsInfo thread.name=DestroyJavaVM;thread.state=RUNNABLE;thread.isAlive=true;group=java.lang.ThreadGroup[name=main,maxpri=10] dumpAllThreadsInfo thread.name=Thread-0;thread.state=RUNNABLE;thread.isAlive=true;group=java.lang.ThreadGroup[name=main,maxpri=10] dumpAllThreadsInfo thread.name=Monitor Ctrl-Break;thread.state=RUNNABLE;thread.isAlive=true;group=java.lang.ThreadGroup[name=main,maxpri=10] dumpAllThreadsInfo thread.name=Reference Handler;thread.state=WAITING;thread.isAlive=true;group=java.lang.ThreadGroup[name=system,maxpri=10] dumpAllThreadsInfo thread.name=Attach Listener;thread.state=RUNNABLE;thread.isAlive=true;group=java.lang.ThreadGroup[name=system,maxpri=10] dumpAllThreadsInfo thread.name=Finalizer;thread.state=WAITING;thread.isAlive=true;group=java.lang.ThreadGroup[name=system,maxpri=10] //进程退出 Process finished with exit code 1
进程并没有随着主线程中出现未捕获异常而理解退出,而是等到我们启动的Thread-0结束之后才退出的。
那么这是为什么呢,看过我之前文章JVM 中的守护线程的朋友应该了解
JVM退出通常有两种情况
因此不难得出结论
所以未捕获异常只会导致所属线程销毁,并不会导致JVM退出。这里我还找到一份官方API文档作为佐证。
Uncaught exceptions are handled in shutdown hooks just as in any other thread, by invoking the uncaughtException method of the thread’s ThreadGroup object. The default implementation of this method prints the exception’s stack trace to System.err and terminates the thread; it does not cause the virtual machine to exit or halt.
上面的内容来自 Runtime.addShutdownHook