Thursday, May 18, 2017

File.listFiles Gotcha

I had another WTF!? moment trying to get to the bottom of a NullPointerException we've been seeing. It turns out File.listFiles returns null when you're trying to list something that's not actually a directory, or when an I/O error occurs!
 * Returns an array of abstract pathnames denoting the files in the
 * directory denoted by this abstract pathname.


 * @return  An array of abstract pathnames denoting the files and
 *          directories in the directory denoted by this abstract pathname.
 *          The array will be empty if the directory is empty.  Returns
 *          {@code null} if this abstract pathname does not denote a
 *          directory, or if an I/O error occurs.
 * @throws  SecurityException
 *          If a security manager exists and its {@link
 *          SecurityManager#checkRead(String)} method denies read access to
 *          the directory
 * @since  1.2
public File[] listFiles() {
Good luck trying to find out which I/O error occurred! I'm guessing just throwing an IOException would have been too easy. Luckily Java 8 provides Files.list as an alternative which does throw an IOException if needed.

Friday, November 25, 2016

Java 8 Update of Bitemporal Framework

Just to push this out to the world a bit more: I've updated the "com.ervacon.bitemporal" framework to bring it up-to-date with current-day Java:
  • Use Java 8's new time classes instead of JodaTime
  • Leverage Java 8's goodies where relevant: streams, diamond operator, ...
  • Switched to Hibernate 5
  • Use JUnit 4
Just as before, "com.ervacon.bitemporal" provides a good starting point for those that need to tackle temporal issues in their applications. For more information, check the GitHub page:

Monday, May 23, 2016

Java Bean Validation Gotcha

A well known trick for doing cross field validation using Java Bean Validation (JSR-303) is to simply put an @AssertTrue annotation on a method that actually does the validation, like so:
public class Document {

   private Status status;   
   private Date signingDate;

   @AssertTrue(message = "A signed document should have a signing date")
   public boolean isASignedDocumentHasASigningDate() {
      return status != SIGNED || signingDate != null;
The fact that the assertion method actually returns a boolean also makes unit testing easy. So far so good!

However, there is a little known gotcha related to this: the annotated validation method actually has to follow the naming conventions of a boolean bean property getter, i.e. getBla() or isBla(). In other words the following would not work:

   @AssertTrue(message = "A signed document should have a signing date")
   public boolean aSignedDocumentHasASigningDate() {
      return status != SIGNED || signingDate != null;
That's quite unfortunate, and certainly doesn't follow the principle of least astonishment: we're explicitly annotating the method so there is no reason why the bean validator should not pick it up! Even worse, unless you're thoroughly testing all you bean validation annotations, it's easy to miss the fact that the validation is not actually happening. Something to keep in mind!

Friday, December 11, 2015

The Benefits of Building on Multiple Platforms

Organizational policy at a client of mine recently saw me switching my development setup from an Ubuntu Linux based machine to Windows 7 Enterprise. Before this switch everything was Linux based: development was done in Linux, continuous integration ran on Linux, and we were ultimately also deploying to Linux, albeit another distribution.

Making the switch to Windows unearthed a few subtle coding errors, mainly related to resource management. It's no secret Windows is a lot stricter when it comes to file manipulation. For instance, consider the following:

File f = new File("test.tmp");
try (FileOutputStream fout = new FileOutputStream(f)) {
 Files.move(f.toPath(), new File("test.txt").toPath());
Using Java 8, this runs fine on Linux. However, on Windows you get an error:
java.nio.file.FileSystemException: test.tmp -> test.txt: The process cannot access the file because it is being used by another process.
Looking at the code again you can see that the file is being moved inside the try-with-resources block. In other words, we're attempting to move the file before we've closed the output stream!

In general I would advise different developers in your development team to use different platforms. That will highlight these kind of subtle errors early and will generally improve the quality of your system. It of course comes as little surprise that Juergen Hoeller (of Spring fame) told me on several occasions that he's still developing on Windows for exactly this reason! :-)

Friday, November 20, 2015

Bizar replaceAll tricks

Image you want to replace all asterisk (*) characters in an input string with \*. In other words you want to escape them. One way of doing this in Java is using the String.replaceAll() method:
"foo*bar".replaceAll("\\*", "\\\\*");
To understand what's going on here, let's remind ourselves of what replaceAll() actually does:
 * Replaces each substring of this string that matches the given regular expression
 * with the given replacement.
 * ...
public String replaceAll(String regex, String replacement) {
So that already explains why the first argument to replaceAll() is "\\*": for it to be a valid regular expression we need to escape the asterisk (which of course means zero or more times in a regular expression) using a backslash, and we all know that a backslash character in a Java String needs to be escaped.

But what about the second argument? Shouldn't that just be "\\*" also: a backslash followed by an asterisk? It turns out replaceAll() doesn't treat the replacement as a simple string literal. The Javadoc states the following:

* Note that backslashes (\) and dollar signs ($) in the
* replacement string may cause the results to be different than if it were
* being treated as a literal replacement string
So just having "\\*" as the replacement string would mean we have a backslash in there which we again need to escape! Hence the "\\\\*". It's interesting to note that in a funny twist of fate this actually makes the code less bizar. If the replacement string would have been a simple literal the code would have been "foo*bar".replaceAll("\\*", "\\*");. Imagine coming across that gem when trying to maintain some old piece of code... :-)

Friday, September 4, 2015

Java Exception Fun

Look at this:
public class Test {

  public void f() {

  public void g() {
    try {
    } catch (Exception e) {
      throw e;
Does this compile? Notice that method g() is throwing an exception object declared to be of type Exception but does not actually mention this in its throws clause. In Java 6 this indeed does not compile: unreported exception java.lang.Exception; must be caught or declared to be thrown
      throw e;
1 error
But it came as somewhat of a surprise to me that this compiles just fine in both Java 7 and Java 8! Thinking back this was of course part of the exception improvements made in Java 7. Goes to show that you can still come across little fun nuggets like this after all these years :-).

Saturday, April 25, 2015

Ubuntu 15.04 on Lenovo X1 Carbon 3rd Generation

Following up on my previous post, where I talked about installing Ubuntu 14.10 on my Lenovo X1 Carbon 3rd generation, I've now gone ahead and reinstalled the machine from scratch using the recently released Ubuntu 15.04.

I'm happy to tell you installation is now a complete breeze:

  • No more USB startup disk problems.
  • The brightness function keys work out-of-the-box.
  • The trackpoint buttons work out-of-the-box.
  • No more graphics glitches (mainly text rendering) in Unity.

It even seems that the default font sizes are better suited for a WQHD display, although I can't actually confirm anything has changed here compared to 14.10. In the end, the only things I did was setting the "Scale for menu and title bars" to 1,25 in the display settings and changing the "Page zoom" to 125% in Google Chrome.

In summary: Ubuntu 15.10 just works on the 3rd generation Lenovo X1 Carbon. No tweaking required really. I highly recommend this combination for those looking for a new Linux based laptop!