谁实际处理 main 方法中抛出的异常？

Question

如果我们在方法 main 中抛出异常并且不处理它，它将正常工作。实际上

public static void main(String[] args) throws IOException {
    throw new IOException(); //OK
}

但是 Java 要求在程序中处理任何已检查的异常，因此应该处理 IOException。在这种情况下，谁实际处理 IOException？

请注意，Java 语言规范定义了如果异常包含在包含 catch 子句的 try 块中，则该异常将被处理，该类型是异常的超类型。

Answer 1

JVM 本身处理从 main() 抛出的异常。

Answer 2

如果您自己没有采取任何特殊操作来捕获异常，则执行 ThreadGroup 的默认值 uncaughtException。

这是在 JLS Chapter 11.3 中指定的。

If no catch clause that can handle an exception can be found, then the current thread (the thread that encountered the exception) is terminated. Before termination, all finally clauses are executed and the uncaught exception is handled according to the following rules:

1. If the current thread has an uncaught exception handler set, then that handler is executed.

2. Otherwise, the method uncaughtException is invoked for the ThreadGroup that is the parent of the current thread. If the ThreadGroup and its parent ThreadGroups do not override uncaughtException, then the default handler's uncaughtException method is invoked.

此外 ThreadGroup.uncaughtException 的 javadoc 内容如下：

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 Thread.UncaughtExceptionHandler installed.

The uncaughtException method of ThreadGroup does the following:

• If this thread group has a parent thread group, the uncaughtException method of that parent is called with the same two arguments.
• Otherwise, this method checks to see if there is a default uncaught exception handler installed, and if so, its uncaughtException method is called with the same two arguments.
• Otherwise, this method determines if the Throwable argument is an instance of ThreadDeath. If so, nothing special is done. Otherwise, a message containing the thread's name, as returned from the thread's getName method, and a stack backtrace, using the Throwable's printStackTrace method, is printed to the standard error stream.

Answer 3

如果未捕获到异常，则调用线程或线程组的未捕获异常处理程序，然后线程终止。

JLS chapter 11 指出：

If no catch clause that can handle an exception can be found, then the current thread (the thread that encountered the exception) is terminated. Before termination, all finally clauses are executed and the uncaught exception is handled according to the following rules:

If the current thread has an uncaught exception handler set, then that handler is executed.

Otherwise, the method uncaughtException is invoked for the ThreadGroup that is the parent of the current thread. If the ThreadGroup and its parent ThreadGroups do not override uncaughtException, then the default handler's uncaughtException method is invoked.