software tools: JML and Daikon

Posted in Hacking, programmer productivity at 12:40 pm by ducky

From the name, I had thought that the Java Modeling Language (JML) was going to be some specialized variant of UML. I haven’t worked with UML, but what I see other people doing with it is drawing pictures.

Instead, JML turns out to be a specification for putting assertions about the code’s behaviour in comments. In that way, it is much more similar to Javadoc than to UML.

With both Javadoc and JML, the author of a method makes promises about what the method will do and what the method requires. In Javadoc, for example, @return int is a promise that the method will return an int, and @param foo int says that the method needs a parameter foo of type int.

With Javadoc, however, the promises are pretty minimal and the requirements are all stated elsewhere (like in the method definition). With JML, the promises about post-conditions and requirements on the pre-conditions can be much more elaborate. The programmer can promise that after the method finishes, for example:

  • a particular instance variable will be less than and/or greater than some value
  • an output sequence will be sorted in ascending order, or
  • variable foo will be bigger than when the method started.

The programmer can also state very detailed input requirements, like that an instance variable can’t be null, that an input must be in a certain range, or that the sum of foo and bar must be less than baz.

This rigorous definition of pre- and post-conditions is useful for documentation. The next programmer doesn’t have to read through the entire method to figure out that foo can’t be null, for example.

Additionally, the JML spec is rigorous enough that it can be used for with a variety of interesting and useful tools. With a special compiler (jmlc), pre- and post-condition checks can get compiled in to the code. Thus if someone calls a method with a parameter outside the allowed bounds, the code can assert an error. (The assertions can also be turned off for production code if so desired.)

But wait, there’s more! The specs are rigorous enough that a lot of checking can be done at compile time. If method A() promises that its output will be greater than 3, and method B() says that it requires the output to be greater than 5, then B(A()) would give a warning: A can give output (between 3 and 5) that B would gag on. See ESC/Java2.

But wait, there’s more! The JML annotations can be used to create unit and tests. The jmlunit writes tests that wiggle the input parameters over the legal inputs to a method, and checks to see if the outputs are correct.

There’s the small problem that it’s a pain to write all the pre-and post-conditions. Fortunately, there’s a tool (Daikon) which will help you with that. Daikon watches as you run a program and sees what changes and what doesn’t change, and from that, generates promises/requirements. Note that those might not be correct. If there are bugs in your program or if that execution of your program didn’t hit all the corner cases, then those won’t be correct. However, it will give you a good start, and I find that it is easier to spot mistakes in somebody else’s stuff than it is to spot omissions in things that I did.

This is all way cool.


  1. Best Webfoot Forward » comparative programming linguistics said,

    May 21, 2007 at 8:42 pm

    […] Sometimes the various devotees will give a nod to the richness of their language’s libraries, or to the robustness of their compiler, but rarely. Recently, I’ve been working on a hobby project in PHP while reading up on tools like odb, JML, Daikon, Esc/Java2, javaspider, and EmmaECL. The contrast is stark. […]

  2. Best Webfoot Forward » VanDev talk summary said,

    February 7, 2008 at 10:54 am

    […] flagged. It sounds like it would be really tedious to generate all those promises, but the tool Daikon can help. Daikon can generate promises based on actual run data; if something changes to violate […]

Leave a Comment

Powered by WP Hashcash

Comment moderation is enabled. Your comment may take some time to appear.