内容简介: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。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,比如下面的例子上面的代码很有可能发生如下情况
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
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
void uncaughtException(Thread t, Throwable e); Thread.setUncaughtExceptionHandler Thread.setDefaultUncaughtExceptionHandler
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
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
/** * 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。
/** * 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
Thread.getDefaultUncaughtExceptionHandler()
/** * 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
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
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
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
参考声明
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