Generally I would recommend the simpler idiom:

    JarFile backupPlugin = new JarFile(backup);
    try {
        // …use it…
    } finally {
        backupPlugin.close();
    }

Not only does this save a null check, it means that a failure to close the file will be reported, which you probably want. (Very likely no `IOException` is ever really thrown here—but if it were, that would be valuable information for someone debugging a weird problem.)

When we can depend on Java 7, then the try-with-resources construct will of course make the code even simpler, and properly manage “suppressed exceptions”:

    try (JarFile backupPlugin = new JarFile(backup)) {
        // …use it…
    }