How to retrieve Jenkins build parameters using the Groovy API?

Regarding parameters:

See this answer first. To get a list of all the builds for a project (obtained as per that answer):

project.builds

When you find your particular build, you need to get all actions of type ParametersAction with build.getActions(hudson.model.ParametersAction). You then query the returned object for your specific parameters.

Regarding p4.change: I suspect that it is also stored as an action. In Jenkins Groovy console get all actions for a build that contains p4.change and examine them - it will give you an idea what to look for in your code.

For resolving a single parameter (I guess what's most commonly needed), this is the simplest I found:

build.buildVariableResolver.resolve("myparameter")

in your Groovy System script build step.

I've just got this working, so specifically, using the Groovy Postbuild plugin, you can do the following:

def paramText
def actionList = manager.build.getActions(hudson.model.ParametersAction)
if (actionList.size() != 0)
{
  def pA = actionList.get(0)
  paramText = pA.createVariableResolver(manager.build).resolve("MY_PARAM_NAME")
}

Update: Jenkins 2.x solution:

With Jenkins 2 pipeline dsl, you can directly access any parameter with the trivial syntax based on the params (Map) built-in:

echo " FOOBAR value: ${params.'FOOBAR'}"

The returned value will be a String or a boolean depending on the Parameter type itself. The syntax is the same for scripted or declarative syntax. More info at: https://jenkins.io/doc/book/pipeline/jenkinsfile/#handling-parameters

If your parameter name is itself in a variable:

def paramName = "FOOBAR"
def paramValue = params.get(paramName) // or: params."${paramName}"
echo """ FOOBAR value: ${paramValue}"

Original Answer for Jenkins 1.x:

For Jenkins 1.x, the syntax is based on the build.buildVariableResolver built-ins:

// ... or if you want the parameter by name ...
def hardcoded_param = "FOOBAR"
def resolver = build.buildVariableResolver
def hardcoded_param_value = resolver.resolve(hardcoded_param)

Please note the official Jenkins Wiki page covers this in more details as well, especially how to iterate upon the build parameters: https://wiki.jenkins-ci.org/display/JENKINS/Parameterized+System+Groovy+script

The salient part is reproduced below:

// get parameters
def parameters = build?.actions.find{ it instanceof ParametersAction }?.parameters
parameters.each {
   println "parameter ${it.name}:"
   println it.dump()
}