[MNG-6434] Cannot specify empty string for configuration value

[jira-importer]

Chris Egerton opened MNG-6434 and commented

We have a MOJO class that involves several user-configured parameters. Some of these parameters have default values that we'd like to use in the event that the user doesn't specify a value for them; however, we'd also like the user to be able to explicitly specify that the parameter should not have a value. The approach we wanted to take was to specify default values in the parameter's annotation:

@Parameter(property = "foo.bar", defaultValue = "${project.scm.url}")
private String foo;

And then detect empty strings as the user's way of saying "I know there's a default value for this parameter but I'm sure that I'd actually like it to be empty anyways.":

<configuration>
  <foo></foo>
</configuration>

or

<configuration>
  <foo />
</configuration>

 

However, there's been some trouble as we haven't found a way yet to distinguish between an intentionally-empty string and one that hasn't been specified at all; the value for foo is always null regardless.

Is there a friendly and/or intuitive way for the user to pass an empty string as a parameter value?

---

Affects: 3.5.4

Issue Links:

• JCRVLT-651 validate-package: Allow to exclude main artifact
  ("causes")

0 votes, 6 watchers

[jira-importer]

Chris Egerton commented

A workaround we're thinking of is allowing the user to specify "none" as a value when they really want it to be null/empty, and then checking for that special case during execution. However, this only works for String parameters and not for Booleans or Integers.

[jira-importer]

Karl Heinz Marbaise commented

I would really hard reconsider my design if a user needs to define an empty value as a valid value? Apart from that have you tried to put xml:space="preserve" at your configuration elements..like this:

<configuration>
  <foo xml:space="preserve"> </foo>
</configuration>

And check if you get the space?

https://maven.apache.org/ref/3.5.4/maven-model/maven.html

[jira-importer]

Chris Egerton commented

Thanks for your help! However, the issue isn't that whitespace-only values can't be specified, it's that empty values can't be specified ("" vs " "). And, beneath that, it's that there's no good way at the moment for a user to override a default value for a configuration with something that says "I know this has a default value but I really don't want it included". We could just recommend a value for the configuration and remove the defaults altogether, but that would make it harder on our users and be a non-backwards-compatible change that we'd like to avoid if possible.

Trying with xml:space="preserve" and values of " " and "" still resulted in the default value being used for the parameter.

[jira-importer]

Chris Egerton commented

It seems like the behavior for the API here doesn't really align with the expectations a user would have when configuring a Maven plugin. A default value is what should be used in the event that a parameter is not specified; if the user goes out of their way to specify a value, even an empty/null one, why would they still want it overridden by the default?

[jira-importer]

Konrad Windszus commented

The problem is that the Xpp3DomBuilder trims all XML element values by default (https://github.com/codehaus-plexus/plexus-utils/blob/3896620b4fbf031b1b02b4d46b328c5839a231da/src/main/java/org/codehaus/plexus/util/xml/Xpp3DomBuilder.java#L209) and Sisu.Plexus injects the configuration value only if its length is greater than 0 (https://github.com/eclipse/sisu.plexus/blob/27a29dc633e6b03753a3c8d29a033648630c8831/org.eclipse.sisu.plexus/src/org/codehaus/plexus/component/configurator/converters/AbstractConfigurationConverter.java#L49) and the parameter has a property value.
However, using the xml:space attribute with value preserve should disable the trim logic (https://github.com/codehaus-plexus/plexus-utils/blob/3896620b4fbf031b1b02b4d46b328c5839a231da/src/main/java/org/codehaus/plexus/util/xml/Xpp3DomBuilder.java#L195). Still the blank value does not kick in usually, as the XML is merged via Xpp3DomUtils to consider the property attribute of the parameter which disregards blank values due to the trim logic in https://github.com/codehaus-plexus/plexus-utils/blob/3896620b4fbf031b1b02b4d46b328c5839a231da/src/main/java/org/codehaus/plexus/util/xml/Xpp3DomUtils.java#L285.
The logic merges the configuration from the pom.xml and the plugin descriptor (https://maven.apache.org/ref/3.8.6/maven-plugin-api/plugin.html#paramname), where the plugin descriptor wins in case the configuration value from the pom.xml is blank.

[jira-importer]

Konrad Windszus commented

I raised codehaus-plexus/plexus-utils#212 to clarify the merging behaviour in plexus-utils with regards to blank values.

[jira-importer]

Michael Osipov commented

Pure crap: apache/maven-site#354 (comment)

Konrad Windszus, to what extend can we fix this for 3.9 or 4.0?

[jira-importer]

Konrad Windszus commented

This requires a fix in Sisu Plexus: https://github.com/eclipse/sisu.plexus/blob/27a29dc633e6b03753a3c8d29a033648630c8831/org.eclipse.sisu.plexus/src/org/codehaus/plexus/component/configurator/converters/AbstractConfigurationConverter.java#L49. Currently default values always overwrite empty explicit configuration values.

[jira-importer]

Michael Osipov commented

Ah OK, then we need to raise this with Stuart as well. Can you open up an issue with Sisu Plexus? I consider "" to be a valid value at least for string types.

[jira-importer]

Konrad Windszus commented

Done in https://github.com/eclipse/sisu.plexus/issues/29.