Inject dependencies in Grails Spock Specification test

I need to get the dependencies injected in my domain objects in my tests.

This tests are placed in the test/integration directory and extends from spock.lang.Specification.

How can I achieve this?

Note: I've seen this post How to inject spring beans into spock test, but it is not related with grails.

Edit:

The dependency I want to get injected is springSecurityService in my SecUser subclass called Player. The method that is failing is the encodePassword(), which is called in the beforeInsert().

I can mock this encodePassword() method in some tests, but when I want to test my controllers method save(), I can't mock the Player that is being created because it all happens inside the controllers method.

After changing to extend IntegrationSpec, this is my test code:

package intertigre.test.domain
import intertigre.domain.Fecha;
import intertigre.test.util.DomainFactoryTestService
import grails.plugin.spock.IntegrationSpec
import grails.test.mixin.TestFor

    @TestFor(Fecha)
    class FechaSpec extends IntegrationSpec{

    DomainFactoryTestService domainFactoryTestService = new DomainFactoryTestService()

    def 'test'(){
        given:
            def fecha = new Fecha()
        when:
            fecha.save()
        then:
            Fecha.get(1) == fecha
    }

}

I'm getting this exception when running grails test-app :spock:

java.lang.NullPointerException: Cannot get property 'mainContext' on null object
    at grails.plugin.spock.IntegrationSpec.$spock_initializeSharedFields(IntegrationSpec.groovy)

And this one when I run the test alone:

| Failure:  intertigre.test.domain.FechaSpec
|  java.lang.NullPointerException: Cannot get property 'autowireCapableBeanFactory' on null object
    at grails.plugin.spock.IntegrationSpec.setupSpec(IntegrationSpec.groovy:47)
| Failure:  intertigre.test.domain.FechaSpec
|  java.lang.NullPointerException: Cannot invoke method isActive() on null object
    at grails.test.mixin.support.GrailsUnitTestMixin.shutdownApplicationContext(GrailsUnitTestMixin.groovy:232)
    at org.spockframework.util.ReflectionUtil.invokeMethod(ReflectionUtil.java:176)
    at org.spockframework.runtime.extension.builtin.JUnitFixtureMethodsExtension$FixtureType$FixtureMethodInterceptor.intercept(JUnitFixtureMethodsExtension.java:145)
    at org.spockframework.runtime.extension.MethodInvocation.proceed(MethodInvocation.java:84)
    at org.spockframework.util.ReflectionUtil.invokeMethod(ReflectionUtil.java:176)