使用两次试捕？

Question

What is the benefit of having two try-catch as seen below? Taken from the book Beginning Hibernate.

protected void rollback() {
        try {
            getSession().getTransaction().rollback();
        } catch (HibernateException e) {
            // TODO change to log
            System.out.println(e);
        }

        try {
            getSession().close();
        } catch (HibernateException e) {
            // TODO change to log
            System.out.println(e);
        }
    }

Answer 1

它保证即使rollback()抛出异常，也将调用close() 。

Answer 2

The initial purpose was to close the session even if the rollback() method throws an exception but this solutions is not good enough.

If roolback throws a RuntimeException the close code will never be called

You should do the following:

protected void rollback() {
        try {
            getSession().getTransaction().rollback();
        } catch (HibernateException e) {
            // TODO change to log
            System.out.println(e);
        } finally {
            try {
                getSession().close();
            } catch (HibernateException e) {
                // TODO change to log
                System.out.println(e);
            }
        }
    }

This ensures that the close code will be called no matter what.

Answer 3

None, really. If you replace that code with

protected void rollback() {
        try {
            getSession().getTransaction().rollback();
            getSession().close();
        } catch (HibernateException e) {
            // TODO change to log
            System.out.println(e);
        }
    }

you still get pretty much the same info. There are some minute differences:

in the first case, the line getSession().close(); will still be called even if getSession().getTransaction().rollback(); throws an exception, while in my example it will not. The proper way to handle this however is to use a finally block if you want that .close() line to be called no matter what.

Answer 4

The reason is independence between these parts. Both parts may fail independently. (And handled independently as well). A fail of 'rollback' invocation won't prevent execution of 'close' statement, as opposite to a single try-catch block approach for this case.