/**
     * Indicates that the specified milestone is necessary before executing this initializer.
     *
     * <p>
     * This has the identical purpose as {@link #requires()}, but it's separated to allow better type-safety
     * when using {@link InitMilestone} as a requirement (since enum member definitions need to be constant.)
     */
    InitMilestone after() default STARTED;

    /**
     * Indicates that this initializer is a necessary step before achieving the specified milestone.
     *
     * <p>
     * This has the identical purpose as {@link #attains()}. See {@link #after()} for why there are two things
     * to achieve the same goal.
     */
    InitMilestone before() default COMPLETED;

import hudson.init.InitMilestone; 
import hudson.util.HttpResponses;
import javax.servlet.http.HttpServletResponse;

final Jenkins jenkins = Jenkins.getInstance();

if (jenkins.getInitLevel().compareTo(InitMilestone.COMPLETED) < 0) {
  println "Error: Jenkins initialization has not reached the COMPLETED state. Current state is ${jenkins.getInitLevel()}"
  return HttpResponses.status(HttpServletResponse.SC_SERVICE_UNAVAILABLE);
}