java: how to declare final a variable that is initialized inside a try - catch block?

问题

I have a variable that is not supposed to change its value after it's been initialized, so I want to define it as a final variable.

the problem is that the variable has to be initialized inside a try block, so I get the following troubles:

I have the following code:

Connection conn = null;
try {
    conn = getConn(prefix);
    [...do some stuff with conn...]
} catch (Exception e) {
    throw new DbHelperException("error opening connection", e);
} finally {
    closeConnection(conn);
}

If I declare the variabale as final, without initializing it to null, I get a 'The local variable conn may not have been initialized' on the finally block. On the other hand, if I declare it final and initialize it to null, I get the error 'The final local variable conn cannot be assigned' in the try block.

EDIT: after lxx answer, I came with this version

try {
    final Connection conn = conn = getConn(prefix);
    try {
        return selectAll(conn, sql, params);
    } catch (Exception e) {
        throw new DbHelperException("error executing query", e);
    } finally {
        closeConnection(conn);  
    }
} catch (Exception e) {
    throw new DbHelperException("error opening connection", e);
}

So this should be the way to do it?

Lesson learned:

I think that the correct answer to the question is the one that lxx gave, but in this case I guess that the cons of declaring the variable final outweights it's benefits...

EDIT: found two questions on stack overflow about when to use final

When should one use final for method parameters and local variables?

Using "final" modifier whenever applicable in java

回答1:

You could handle the Exceptions more accurately. If you get an Exception opening the connection, you don't have to close it in the finally block I guess. If you get an exception after that, in the try block, and handle the exception in a new nested try-catch block you don't need to define the variable outside. Something like:

    try {
        final Connection conn = getConn(prefix);
        try {
            //code using conn
        } catch (Exception e) {

        } finally {
            closeConnection(conn);
        }
    } catch (DbHelperException e) {
        throw new DbHelperException("error opening connection", e);
    }

回答2:

How about this?

Connection temp = null;
try {
    temp = getConn(prefix);
} catch (Exception e) {
    throw new DbHelperException("error opening connection", e);
} finally {
    closeConnection(conn);
}
final Connection conn = temp;

回答3:

Why do you want it final? If you want to pass it to an anonymous inner class, you could do:

Connection conn = null;
try {
    conn = getConn(prefix);
    final Connection finalConn = conn;
    // pass it to inner class here
} catch (Exception e) {
    throw new DbHelperException("error opening connection", e);
} finally {
    closeConnection(conn);
}

The only problem (and quite a big one) with this solution is that you close your connection as soon as you leave this block. So unless you declare and call your anon inner class straight away, this pattern isn't going to work.

Either way, I'd probably rephrase the whole thing if I were you, making prefix final instead and delegating connection handling to the anon inner class.

回答4:

Can you try assigning it in both the catch and finally blocks? Like so:

Connection connTemp = null;
final Connection conn;
try {
    connTemp = getConn(prefix);
} catch (Exception e) {
    throw new DbHelperException("error opening connection", e);
} finally {
    closeConnection(conn);
}
conn = connTemp;

来源：https://stackoverflow.com/questions/9350605/java-how-to-declare-final-a-variable-that-is-initialized-inside-a-try-catch-b

标签

java

try-catch

final