关于抛异常

　　今天早上，同事拿了一段代码来问原因，我们讨论了一下：

    @Test
    public void testThrow(){
        try{
            throw new RuntimeException("123");
        } catch (Exception e){
            System.out.println(e.getCause().getLocalizedMessage());
        }
    }

　　代码很简单，不过很有意思，抛出了NullPointerException，什么情况下会出这种事呢，e肯定不是null，那原因就一定是null了：

    /**
     * Returns the cause of this throwable or {@code null} if the
     * cause is nonexistent or unknown.  (The cause is the throwable that
     * caused this throwable to get thrown.)
     *
     * <p>This implementation returns the cause that was supplied via one of
     * the constructors requiring a {@code Throwable}, or that was set after
     * creation with the {@link #initCause(Throwable)} method.  While it is
     * typically unnecessary to override this method, a subclass can override
     * it to return a cause set by some other means.  This is appropriate for
     * a "legacy chained throwable" that predates the addition of chained
     * exceptions to {@code Throwable}.  Note that it is <i>not</i>
     * necessary to override any of the {@code PrintStackTrace} methods,
     * all of which invoke the {@code getCause} method to determine the
     * cause of a throwable.
     *
     * @return  the cause of this throwable or {@code null} if the
     *          cause is nonexistent or unknown.
     * @since 1.4
     */
    public synchronized Throwable getCause() {
        return (cause==this ? null : cause);
    }

　　它判断了下如果原因就是异常本身，那就把原因置为null，因为它认为原因是未知的。这个有点意思，异常的原因就是异常怎么产生的，但是当它发现和自己一样的时候，就说明不知道是什么，所以就是null了:>。

　　那怎么办呢，传个原因给它呗：

    @Test
    public void testThrow(){
        try{
            throw new RuntimeException("123", new Throwable("cause:aaa"));
        } catch (Exception e){
            System.out.println(e.getCause().getLocalizedMessage());
        }
    }

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咱最近用的github：https://github.com/saaavsaaa

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