What are your favorite Grails debugging tricks?


Grails can be a bit of a bear to debug with its long stack dumps. Getting to the source of the problem can be tricky. I've gotten burned a few times in the BootStrap.groovy doing "def foo = new Foo(a: a, b: b).save()", for example. What are your favorite tricks for debugging Grails apps?

Best Solution

Some general tips:

  • Clear stacktrace.log, do grails run-app, then open stacktrace.log in a viewer (I prefer less stacktrace.log on linux)... once in your viewer, search for .groovy and .gsp... that generally brings you to what you actually care about.
  • When a stacktrace refers to a line number in a GSP file, you should open that view in a browser with ?showSource in the query string, i.e. http://localhost:8080/myProject/myController/myAction?showSource... this shows the compiled GSP source, and all GSP line numbers in the stacktrace refer to the compiled GSP, not the actual GSP source
  • Always, always, always surround your saves with at least some minimal error handling.


try {
    if(!someDomainObject.save()) {
        throw new Exception ("Save failed")
} catch(Exception e) {
    println e.toString()
    // This will at least tell you what is wrong with
    // the instance you are trying to save
    someDomainObject.errors.allErrors.each {error ->
        println error.toString()

Beyond that, a lot of it just comes down to recognizing stacktraces and error messages... a lot of the time, Grails is incredibly unhelpful in the error messages it gives you, but you can learn to recognize patterns, like the following:

  • Some of the hardest errors to make sense of are because you didn't run grails clean or grails upgrade... to avoid these problems, I always use the following on the command line to run grails: grails clean; yes | grails upgrade; grails run-app
  • If the error has to do with duplicate definitions of a class, make sure that you declare the package the class belongs to at the top of the class's file
  • If the error has to do with schema metadata, connection, socket, or anything like that, make sure your database connector is in lib/, make sure your permissions are correct both in DataSource.groovy and in the database for username, password, and host, and make sure that you know the ins and outs of your connector's version (i.e. mysql connector version 5.1.X has a weird issue with aliases that may require you to set useOldAliasMetadataBehavior=true on the url in DataSource.groovy)

And so on. There are a lot of patterns to learn to recognize.

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