JVM 如何处理未捕获异常

继之前的文章 详解JVM如何处理异常,今天再次发布一篇比较关联的文章,如题目可知,今天聊一聊在JVM中线程遇到未捕获异常的问题,其中涉及到线程如何处理未捕获异常和一些内容介绍。

什么是未捕获异常

未捕获异常指的是我们在方法体中没有使用try-catch捕获的异常,比如下面的例子

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private static void testUncaughtException(String arg) {
 try {
 System.out.println(1 / arg.length());
 } catch (ArithmeticException e) {
 e.printStackTrace();
 }
}

上面的代码很有可能发生如下情况

  • 如果方法参数arg传递null,会出现NullPointerException
  • 如果参数arg传递内容为空的字符串(“”),会出现ArithmeticException

对于上面的问题,我们不难发现

  • 上面可能出现的NullPointerException和ArithmeticException都属于Unchecked Exceptions
  • 而ArithmeticException被我们人为try-catch捕获了,它不符合本文对于未捕获异常的定义
  • NullPointerException 由于我们没有catch住,就变成了我们要聊的未捕获异常
  • 另外,未捕获异常实际是Unchecked Exceptions的子集

UncaughtExceptionHandler 是什么

  • 它是线程遇到未捕获异常的一个处理者接口
  • 它包含一个方法void uncaughtException(Thread t, Throwable e); 用来处理接收处理异常发生后的操作,比如收集崩溃信息并上报等
  • 可以通过 实例方法 Thread.setUncaughtExceptionHandler 为某一个Thread实例设置未捕获异常处理者
  • 也可以通过 静态方法 Thread.setDefaultUncaughtExceptionHandler 设置所有Thread实例的未捕获异常处理者

ThreadGroup 是什么

  • ThreadGroup 是线程的集合
  • ThreadGroup 也可以包含子ThreadGroup
  • 除了初始的ThreadGroup 之外,每个ThreadGroup都有一个父 ThreadGroup
  • ThreadGroup 自身实现了Thread.UncaughtExceptionHandler,用来相应处理其内部的线程和ThreadGroup发生未捕获异常。

未捕获异常处理者 设置指南

how to set uncaught exception handler

线程发生了未捕获异常,JVM怎么处理

分发Throwable实例

当线程A中出现了未捕获异常时,JVM会调用线程A的dispatchUncaughtException(Throwable)方法

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/**
 * 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);
}

获取未捕获异常处理者

  • 每个线程会有一个变量(uncaughtExceptionHandler)来保存未捕获异常的处理者
  • 在线程需要确定Throwable分发目标的处理者时,优先获取当前线程中uncaughtExceptionHandler变量
  • 如果出问题线程的uncaughtExceptionHandler为null(即没有显式设置异常处理者),则使用自己所在的ThreadGroup来作为未捕获异常处理者。
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/**
 * Returns the handler invoked when this thread abruptly terminates
 * due to an uncaught exception. If this thread has not had an
 * uncaught exception handler explicitly set then this thread's
 * <tt>ThreadGroup</tt> object is returned, unless this thread
 * has terminated, in which case <tt>null</tt> is returned.
 * @since 1.5
 * @return the uncaught exception handler for this thread
 */
public UncaughtExceptionHandler getUncaughtExceptionHandler() {
 return uncaughtExceptionHandler != null ?
 uncaughtExceptionHandler : group;
}

如果Throwable分发给ThreadGroup

  • ThreadGroup会尝试转给它的父ThreadGroup(如果存在的话)
  • 如果上面没有找到对应的ThreadGroup,则尝试获取Thread.getDefaultUncaughtExceptionHandler()并分发
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/**
 * Called by the Java Virtual Machine when a thread in this
 * thread group stops because of an uncaught exception, and the thread
 * does not have a specific {@link Thread.UncaughtExceptionHandler}
 * installed.
 * <p>
 * The <code>uncaughtException</code> method of
 * <code>ThreadGroup</code> does the following:
 * <ul>
 * <li>If this thread group has a parent thread group, the
 * <code>uncaughtException</code> method of that parent is called
 * with the same two arguments.
 * <li>Otherwise, this method checks to see if there is a
 * {@linkplain Thread#getDefaultUncaughtExceptionHandler default
 * uncaught exception handler} installed, and if so, its
 * <code>uncaughtException</code> method is called with the same
 * two arguments.
 * <li>Otherwise, this method determines if the <code>Throwable</code>
 * argument is an instance of {@link ThreadDeath}. If so, nothing
 * special is done. Otherwise, a message containing the
 * thread's name, as returned from the thread's {@link
 * Thread#getName getName} method, and a stack backtrace,
 * using the <code>Throwable</code>'s {@link
 * Throwable#printStackTrace printStackTrace} method, is
 * printed to the {@linkplain System#err standard error stream}.
 * </ul>
 * <p>
 * Applications can override this method in subclasses of
 * <code>ThreadGroup</code> to provide alternative handling of
 * uncaught exceptions.
 *
 * @param t the thread that is about to exit.
 * @param e the uncaught exception.
 * @since JDK1.0
 */
 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);
 }
 }
 }

将上面的处理流程做成图的形式,就是下图所示

Uncaught exception process

注:上述图片来自https://www.javamex.com/tutorials/exceptions/exceptions_uncaught_handler.shtml

Questions

初始的ThreadGroup是什么

上面提到了初始的ThreadGroup没有父ThreadGroup,是主线程所在的ThreadGroup么?

这个问题,我们可以通过这样一段代码验证

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private static void dumpThreadGroups() {
 ThreadGroup threadGroup = Thread.currentThread().getThreadGroup();
 while(threadGroup != null) {
 System.out.println("dumpThreadGroups threadGroup=" + threadGroup.getName());
 threadGroup = threadGroup.getParent();
 }
}

执行该方法对应的输出是

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dumpThreadGroups threadGroup=main
dumpThreadGroups threadGroup=system

因此我们可以发现,初始的ThreadGroup是一个叫做system的ThreadGroup,而不是main ThreadGroup

setDefaultUncaughtExceptionHandler 设置的一定会被调用到么

这其实是一个很好的问题,答案是不一定会被调用,因为可能存在以下的情况

  • 出问题的线程设置了对应的UncaughtExcpetionHandler,优先响应分发到这个Handler
  • 出问题的线程所在的ThreadGroup包括其祖先ThreadGroup 重写了uncaughtException 也可能造成线程默认的Handler无法被调用
  • 出问题的线程重写了dispatchUncaughtException 可能性较小
  • 出问题的线程重写了getUncaughtExceptionHandler 可能性较小

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