[Spring] Document CucumberContextConfiguration semantics

Cucumber uses Spring Test Context Manager framework. This framework was
written for JUnit and assumes that there is a "test instance".
Cucumber however uses multiple step definition classes and so it has
multiple test instances.

Originally `@CucumberContextConfiguration` was added to signal to Spring
which class should be used to configure the application context from.
But as people also expected mock beans and other features provided by
Springs test execution listeners to work (#2661) the annotated instance
was only instantiated but never initialized by Spring.

This changed the semantics somewhat as now features that depend on the
bean being initialized stopped working (#2886). Unfortunately, there is
little that can be done here. Spring expects that the instance provided
to the Test Context Manager to be an uninitialized bean. The solution
for this is to put the context configuration and step definitions in
different classes.

Cleaning up the examples to follow this pattern should avoid this
problem somewhat in the future. Though I won't go as far as recommending
people do this. Putting everything in one class looks quite nice. And
generally still works.

Closes: #2886

cucumber-spring/README.md

@@ -45,6 +45,13 @@ public class CucumberSpringConfiguration {
 Note: Cucumber Spring uses Spring's `TestContextManager` framework internally.
 As a result, a single Cucumber scenario will mostly behave like a JUnit test.
 
+The class annotated with `@CucumberContextConfiguration` is instantiated but not
+initialized by Spring. Instead, this instance is processed by Springs test
+execution listeners. So features that depend on a test execution listener such
+as mock beans will work on the annotated class - but not on other step definition
+classes. Features that depend on initializing beans - such as AspectJ - will not
+work on the annotated class - but will work on other step definition classes.
+
 For more information configuring Spring tests see:
  - [Spring Framework Documentation - Testing](https://docs.spring.io/spring-framework/docs/current/spring-framework-reference/testing.html)
  - [Spring Boot Features - Testing](https://docs.spring.io/spring-boot/docs/current/reference/html/spring-boot-features.html#boot-features-testing)
@@ -97,7 +104,8 @@ Repeat as needed.
 ## Accessing the application context
 
 Components from the application context can be accessed by autowiring.
-Annotate a field in your step definition class with `@Autowired`. 
+
+Either annotate a field in your step definition class with `@Autowired`
 
 ```java
 package com.example.app;
@@ -117,6 +125,72 @@ public class MyStepDefinitions {
 }
 ```
 
+Or declare a dependency through the constructor:
+
+```java
+package com.example.app;
+
+import io.cucumber.java.en.Given;
+
+public class MyStepDefinitions {
+
+   private final MyService myService;
+
+   public MyStepDefinitions(MyService myService){
+       this.myService = myService;
+   }
+
+   @Given("feed back is requested from my service")
+   public void feed_back_is_requested(){
+      myService.requestFeedBack();
+   }
+}
+```
+
+## Using Mock Beans
+
+To use mock beans, declare a mock bean in the context configuration.
+
+```java
+package com.example.app;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.boot.test.mock.mockito.MockBean;
+
+import io.cucumber.spring.CucumberContextConfiguration;
+
+@CucumberContextConfiguration
+@SpringBootTest(classes = TestConfig.class)
+@MockBean(MyService.class)
+public class CucumberSpringConfiguration {
+
+}
+```
+
+Then in your step definitions, use the mock as you would normally.
+
+```java
+package com.example.app;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import io.cucumber.java.en.Given;
+
+import static org.mockito.Mockito.mockingDetails;
+import static org.springframework.test.util.AssertionErrors.assertTrue;
+
+public class MyStepDefinitions {
+
+    @Autowired
+    private MyService myService;
+
+    @Given("my service is a mock")
+    public void feed_back_is_requested(){
+        assertTrue(mockingDetails(myService).isMock());
+    }
+}
+```
+
 ## Sharing State 
 
 Cucumber Spring creates an application context and uses Spring's

cucumber-spring/src/main/java/io/cucumber/spring/CucumberContextConfiguration.java

@@ -14,14 +14,13 @@
  * {@code @BootstrapWith}. In case of SpringBoot, the configuration class can be
  * annotated as follows:
  * <p>
- * 
  * <pre>
  * &#64;CucumberContextConfiguration
  * &#64;SpringBootTest(classes = TestConfig.class)
  * public class CucumberSpringConfiguration {
  * }
  * </pre>
- * 
+ * <p>
  * Notes:
  * <ul>
  * <li>Only one glue class should be annotated with
@@ -30,6 +29,15 @@
  * <li>Cucumber Spring uses Spring's {@code TestContextManager} framework
  * internally. As a result a single Cucumber scenario will mostly behave like a
  * JUnit test.</li>
+ * <li>The class annotated with {@code CucumberContextConfiguration} is
+ * instantiated but not initialized by Spring. This instance is processed by
+ * Springs {@link org.springframework.test.context.TestExecutionListener
+ * TestExecutionListeners}. So features that depend on a test execution listener
+ * such as mock beans will work on the annotated class - but not on other step
+ * definition classes. Features that depend on initializing beans - such
+ * as AspectJ - will not work on the annotated class - but will work on other
+ * step definition classes.</li>
+ * <li></li>
  * </ul>
  */
 @Retention(RetentionPolicy.RUNTIME)