当线程出现未捕获异常（即抛出一个异常）

1. JVM将调用Thread.dispatchUncaughtException();

/**
 * 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);
}

2.getUncaughtExceptionHandler()，判断自己独属于CurrentThread的uncaughtExceptionHandler是否为null，不为null则返回返回UncaughtExceptionHandler，为null 则返回group (extends UncaughtExceptionHandler)

public UncaughtExceptionHandler getUncaughtExceptionHandler() {
        return uncaughtExceptionHandler != null ?
            uncaughtExceptionHandler : group;
    }

3.调用 uncaughtException()

如果返回是ThreadGroup的话，默认会一直找到顶层ThreadGroup（假如中间的ThreadGroup不修改），然后会找 Thread类共用的 defaultUncaughtExceptionHandler ，如果存在则调用，如果不存在，则 打印线程名字和异常

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);
            }
        }
    }