Only run task if another isn't UP-TO-DATE in gradle

Having faced the same problem I found a very clean solution. In my case I want an eclipse project setup to be generated when the build is run, but only at the times when a new jar is generated. No project setup should be executed when the jar is up to date. Here is how one can accomplish that:

tasks.eclipse {
  onlyIf {
    !jar.state.upToDate
  }
}

build {
  dependsOn tasks.eclipse
}

I ended up writing to a 'failure file' and making that an input on the serverRun task:

File serverTrigger = project.file("${buildDir}/trigger") 

project.gradle.taskGraph.whenReady { TaskExecutionGraph taskGraph ->
  // make the serverRun task have the same inputs/outputs + extra trigger
  serverRun.inputs.files(funcTestTask.inputs.files, serverTrigger)
  serverRun.outputs.files(funcTestTask.outputs.files)
}

project.gradle.taskGraph.afterTask { Task task, TaskState state ->
  if (task.name == "funcTestTask" && state.failure) {
    serverRun.trigger << new Date()
  }
}

With information from an answer to my question on the Gradle forums : http://forums.gradle.org/gradle/topics/how-can-i-start-a-server-conditionally-before-a-functionaltestrun