Always <tt>close()</tt> in a <tt>finally</tt> block

Here’s one for my fellow Java programmers, but it’s really generally applicable.

When you call close() on I/O streams, readers, writers, network sockets, database connections, etc., it’s easy to forgot the most appropriate idiom. I just spent a few hours fixing some examples of misuse in otherwise very good Java code.

What’s wrong the following code?

    
public void writeContentToFile(String content, String fileName) throws Exception {
    File output = new File(fileName);
    OutputStreamWriter writer = new OutputStreamWriter(new FileOutputStream(output), "UTF-8");
    writer.write(content);
    writer.close();
}
    

It doesn’t look all that bad. It tells it’s story. It’s easy to understand.

However, it’s quite likely that you won’t get to the last line, which closes the writer, from time to time. File and network I/O errors are common. For example, what if you can’t actually write to the location specified by fileName? So, we have to be more defensive. We want to be sure we always clean up.

The correct idiom is to use a try … finally … block.

    
public void writeContentToFile(String content, String fileName) throws Exception {
    File output = new File(getFileSystemPath() + contentFilename);
    OutputStreamWriter writer = null;
    try {
        writer = new OutputStreamWriter(new FileOutputStream(output), "UTF-8");
        writer.write(content);
    } finally {
        if (writer != null)
            writer.close();
    }
}
    

Now, no matter what happens, the writer will be closed, if it’s not null, even if writing the output was unsuccessful.

Note that we don’t necessarily need a catch block, because in this case we’re willing to let any Exceptions propagate up the stack (notice the throws clause). A lot of developers don’t realize that there are times when you need a try block, but not necessarily a catch block. This is one of those times.

So, anytime you need to clean up or otherwise release resources, use a finally block to ensure that the clean up happens, no matter what.