Question

Findbugs bugs me about a method which opens two Closeable instances, but I can’t understand why.

Source

public static void sourceXmlToBeautifiedXml(File input, File output)
        throws TransformerException, IOException, JAXBException {

    FileReader fileReader = new FileReader(input);
    FileWriter fileWriter = new FileWriter(output);

    try {
        // may throw something
        sourceXmlToBeautifiedXml(fileReader, fileWriter);
    } finally {
        try {
            fileReader.close();
        } finally {
            fileWriter.close();
        }
    }
}

Findbugs analysis

Findbugs tells me

Method [...] may fail to clean up java.io.Reader [...]

and points to the line with FileReader fileReader = ...

Question

Who is wrong: me or Findbugs?

Answer 1

FindBugs is correct: If the FileWriter’s constructor throws an exception, the file reader will not be closed. To verify this, try passing an invalid filename for output.

I’d do it as follows:

    FileReader fileReader = new FileReader(input);

    try {
        FileWriter fileWriter = new FileWriter(output);
        try {
            // may throw something
            sourceXmlToBeautifiedXml(fileReader, fileWriter);
        } finally {
            fileWriter.close();
        }
    } finally {
        fileReader.close();
    }

Note that the handling of exception thrown when closing could be improved, since leaving a finally-block by throwing an exception will cause the try-statement to terminate by throwing that exception, swallowing any exception thrown in the try-block, which generally would be more useful for debugging. See duffymo’s answer for a simple way on how to avoid this.

Edit: Since Java 7, we can use the try-with-resources statement, which permits correct and concicse handling of these corner cases:

try (
    FileReader fileReader = new FileReader(input); 
    FileWriter fileWriter = new FileWriter(output)
) {
    // may throw something
    sourceXmlToBeautifiedXml(fileReader, fileWriter);
}