Sonar violation: “Method may fail to close stream

Question

I have this method:

 private void unZipElementsTo(String inputZipFileName, String destPath) throws FileNotFoundException, IOException {

        OutputStream out = null;
        InputStream in = null;
        ZipFile zf = null;

        try {
            zf = new ZipFile(inputZipFileName);

            for (Enumeration<? extends ZipEntry> em = zf.entries(); em.hasMoreElements();) {
                ZipEntry entry = em.nextElement();
                String targetFile = destPath + FILE_SEPARATOR + entry.toString().replace("/", FILE_SEPARATOR);
                File temp = new File(targetFile);

                if (!temp.getParentFile().exists()) {
                    temp.getParentFile().mkdirs();
                }

                in = zf.getInputStream(entry);

                out = new FileOutputStream(targetFile);
                byte[] buf = new byte[4096];
                int len;
                while ((len = in.read(buf)) > 0) {
                    out.write(buf, 0, len);
                }
                out.flush();
                out.close();
                in.close();
            }
        }
        finally
        {
            if (out!=null) out.close();
            if (zf!=null) zf.close();
            if (in!=null) in.close();
        }
    }

For this method Sonar give me this Violation:

Bad practice - Method may fail to close stream on exception unZipElementsTo(String, String) may fail to close stream on exception

But, I don't see any violations there. Maybe, it is just a False-positive ?

Answer 1

If out.close() or zf.close() in the finally block throw an exception then the other closes won't be executed.

Answer 2

Alternatively if you're using Java 7 or better, you can use the new try-with-resources mechanism, which handles the close for you. See: http://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html for details on this new mechanism.

Note that try-with-resources also works with multiple objects that are opened and closed and still preserves the guarantee that objects will be closed in the reverse order of their construction. Quoth that same page:

Note that the close methods of resources are called in the opposite order of their creation.

Answer 3

To avoid masking an exception with an exception during the close of the streams It's often recommended to "hide" any io exception in the finally.

To fix use the org.apache.commons.io.IOUtils.closeQuietly(...) or guava Closeables.html#closeQuietly(java.io.Closeable) in the finally close

more on exception handling issues :
http://mestachs.wordpress.com/2012/10/10/through-the-eyes-of-sonar-exception-handling/

Answer 4

That's right. The OutputStream.close() method can itself throw an exception. If this happens, e.g. on the 1st line of your finally{} block, then the other streams will be left open.