Grails pitfall: Proxies and inheritance

When using inheritance in a lazy fetched association, proxies are a common pitfall in Grails.


You have a domain class hierarchy and want to use the classes in an association. You typically use the base class in a hasMany:

class Domain {
  static hasMany = [others:BaseClass]

Dependent on the actual class in the set others you want to call a different method:

public doSomething(domain) {
  domain.others.each {doThis(it)}

public doThis(ChildClassOne c) {}

public doThis(ChildClassTwo c) {}

Here the proxy mechanism of Hibernate (and Grails) causes a MethodMissingException to be thrown because the proxies are instances of the base class and not the actual child ones.
One way would be to activate eager fetching or you could use…

Solution: A dynamic visitor

Declare a visit method in the base class which takes a closure

def visit(Closure c) {
return c(this)

and make the dispatch in a method for the base class:

public doThis(BaseClass c) {
  return c.visit {doThis(it)}

Find the bug: Groovy and autogenerated equals

Every program has bugs. Even autogenerated code from IDEs. Can you spot this one?

Take this simple Groovy class:

public class TestClass {
    String s

Hitting ‘autogenerate equals and hashCode’ a popular IDE generates the following code:

public class TestClass {

    String s

    boolean equals(o) {
        if ( return true;

        if (!o || getClass() != o.class) return false;

        TestClass testClass = (TestClass) o;

        if (s ? !s.equals(testClass.s) : testClass.s != null) return false;

        return true;

    int hashCode() {
        return (s ? s.hashCode() : 0);

At first this looks unusual but ok, but taking a closer look there is a bug in there, can you spot it? And even better: write a test for it?

Lessons learned:

  • If something looks suspicious write a test for it.
  • Question your assumptions. There is no assumption which is safe from be in question.
  • Don’t try to be too clever.

Grails Web Application Security: XSS prevention

XSS (Cross Site Scripting) became a favored attack method in the last years. Several things are possible using an XSS vulnerability ranging from small annoyances to a complete desaster.
The XSS prevention cheat sheet states 6 rules to prevent XSS attacks. For a complete solution output encoding is needed in addition to input validation.
Here I take a further look on how to use the built in encoding methods in grails applications to prevent XSS.

Take 1: The global option

There exists a global option that specifies how all output is encoded when using ${}. See grails-app/conf/Config.groovy:

// The default codec used to encode data with ${}
grails.views.default.codec="html" // none, html, base64

So every input inside ${} is encoded but beware of the standard scaffolds where fieldValue is used inside ${}. Since fieldValue uses encoding you get a double escaped output – not a security problem, but the output is garbage.
This leaves the tags from the tag libraries to be reviewed for XSS vulnerability. The standard grails tags use all HTML encoding. If you use older versions than grails 1.1: beware of a bug in the renderErrors tag. Default encoding ${} does not help you when you use your custom tags. In this case you should nevertheless encode the output!
But problems arise with other tags like radioGroup like others found out.
So the global option does not result in much protection (only ${}), double escaping and problems with grails tags.

Take 2: Tainted strings

Other languages/frameworks (like Perl, Ruby, PHP,…) use a taint mode. There are some research works for Java.
Generally speaking in gsps three different outputs have to be escaped: ${}, <%%> and the ones from tags/taglibs. If a tainted String appears you can issue a warning and disallow or escape it. The problem in Java/Groovy is that Strings are value objects and since get copied in every operation so the tainted flag needs to be transferred, too. The same tainted flag must also be introduced for GStrings.
Since there isn’t any implementation or plugin for groovy/grails yet, right now you have to take the classic route:

Take 3: Test suites and reviews

Having a decent test suite in e.g. Selenium and reviewing your code for XSS vulnerabilities is still the best option in your grails apps. Maybe the tainted flags can help you in the future to spot places which you didn’t catch in a review.

P.S. A short overview for Java frameworks and their handling of XSS can be found here

Small gaps in the grails docs

Just for reference, if you come across one of the following problems:

Validation only datasource

Looking at the options of dbCreate in Datasource.groovy I only found 3 values: create-drop, create or update. But there is a fourth one: validate!
This one helps a lot when you use schema generation with Autobase or doing your schema updates external.


Controller.redirect has two options for passing an id to the action id and params, but if you specify both which one will be used?

controller.redirect(id:1, params:[id:2])

Trying this out I found the id supersedes the

Thanks to Burt and Alvaro for their hints. I submitted a JIRA issue

Using grails projects in Hudson

Being an agile software development company we use a continuous integration (CI) server like Hudson.
For our grails projects we wrote a simple ant target -call-grails to call the batch or the shell scripts:

    <condition property="grails" value="${grails.home}/bin/grails.bat">
        <os family="windows"/>
    <property name="grails" value="${grails.home}/bin/grails"/>

    <target name="-call-grails">
		<chmod file="${grails}" perm="u+x"/>
        <exec dir="${basedir}" executable="${grails}" failonerror="true">
            <arg value="${grails.task}"/>
            <arg value="${grails.file.path}"/>
            <env key="GRAILS_HOME" value="${grails.home}"/>

Calling it is as easy as calling any ant target:

  <target name="war" description="--> Creates a WAR of a Grails application">
        <antcall target="-call-grails">
            <param name="grails.task" value="war"/>
            <param name="grails.file.path" value="${}/${}"/>

One pitfall exists though, if your target takes no argument(s) after the task you have to use a different call:

	<target name="-call-grails-without-filepath">
		<chmod file="${grails}" perm="u+x"/>
        <exec dir="${basedir}" executable="${grails}" failonerror="true">
            <arg value="${grails.task}"/>
            <env key="GRAILS_HOME" value="${grails.home}"/>

When as Set is not what you want

When you want to filter out duplicates in a list in groovy you normally do something like:

        def list = [2, 1, 2, 3]
        def filtered = list as Set
        assertEquals([1, 2, 3], filtered as List)

This kicks out all duplicates in a one-liner. But what if the list is sorted (e.g. in reverseOrder)?

        def list = [3, 2, 2, 1]
        def filtered = list as Set
        assertEquals([3, 2, 1], filtered as List) // this fails!

One solution would be to use a small closure:

        def list = [3, 2, 2, 1]
        def filteredList = []
        list.each {
            if (!filteredList.contains(it)) {
                filteredList << it
        assertEquals([3, 2, 1], filteredList)

This closures preserves the order and filters out the duplicates.