Document how to structure configurations for efficient slice tests

Closes gh-16088

spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/testing.adoc

@@ -845,6 +845,7 @@ include::{docs-java}/features/testing/springbootapplications/userconfigurationan
 
 NOTE: Depending on the complexity of your application, you may either have a single `@Configuration` class for your customizations or one class per domain area.
 The latter approach lets you enable it in one of your tests, if necessary, with the `@Import` annotation.
+See <<howto#howto.testing.slice-tests,this how-to section>> for more details on when you might want to enable specific `@Configuration` classes for slice tests.
 
 Test slices exclude `@Configuration` classes from scanning.
 For example, for a `@WebMvcTest`, the following configuration will not include the given `WebMvcConfigurer` bean in the application context loaded by the test slice:

spring-boot-project/spring-boot-docs/src/docs/asciidoc/howto/testing.adoc

@@ -45,3 +45,36 @@ include::{docs-java}/howto/testing/testcontainers/dynamicproperties/MyIntegratio
 ----
 
 The above configuration allows Neo4j-related beans in the application to communicate with Neo4j running inside the Testcontainers-managed Docker container.
+
+
+
+[[howto.testing.slice-tests]]
+=== Structure `@Configuration` classes for inclusion in slice tests
+Slice tests work by restricting Spring Framework's component scanning to a limited set of components based on their type.
+For any beans that are not created via component scanning, for example, beans that are created using the `@Bean` annotation, slice tests will not be able to include/exclude them from the application context.
+Consider this example:
+
+[source,java,indent=0,subs="verbatim"]
+----
+include::{docs-java}/howto/testing/slicetests/MyConfiguration.java[]
+----
+
+For a `@WebMvcTest` for an application with the above `@Configuration` class, you might expect to have the `SecurityFilterChain` bean in the application context so that you can test if your controller endpoints are secured properly.
+However, `MyConfiguration` is not picked up by @WebMvcTest's component scanning filter because it doesn't match any of the types specified by the filter.
+You can include the configuration explicitly by annotating the test class with `@Import(MySecurityConfiguration.class)`.
+This will load all the beans in `MyConfiguration` including the `BasicDataSource` bean which isn't required when testing the web tier.
+Splitting the configuration class into two will enable importing just the security configuration.
+
+[source,java,indent=0,subs="verbatim"]
+----
+include::{docs-java}/howto/testing/slicetests/MySecurityConfiguration.java[]
+----
+
+[source,java,indent=0,subs="verbatim"]
+----
+include::{docs-java}/howto/testing/slicetests/MyDatasourceConfiguration.java[]
+----
+
+Having a single configuration class can be inefficient when beans of a certain domain needed to be included in slice tests.
+Instead, structuring the application's configuration as multiple granular classes with beans for a specific domain can enable importing them only for specific slice tests.
+

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/howto/testing/slicetests/MyConfiguration.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2012-2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.howto.testing.slicetests;
+
+import org.apache.commons.dbcp2.BasicDataSource;
+
+import org.springframework.boot.context.properties.ConfigurationProperties;
+import org.springframework.boot.jdbc.DataSourceBuilder;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.security.config.annotation.web.builders.HttpSecurity;
+import org.springframework.security.web.SecurityFilterChain;
+
+@Configuration(proxyBeanMethods = false)
+public class MyConfiguration {
+
+	@Bean
+	public SecurityFilterChain securityFilterChain(HttpSecurity http) throws Exception {
+		http.authorizeRequests().anyRequest().authenticated();
+		return http.build();
+	}
+
+	@Bean
+	@ConfigurationProperties("app.datasource.second")
+	public BasicDataSource secondDataSource() {
+		return DataSourceBuilder.create().type(BasicDataSource.class).build();
+	}
+
+}

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/howto/testing/slicetests/MyDatasourceConfiguration.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2012-2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.howto.testing.slicetests;
+
+import org.apache.commons.dbcp2.BasicDataSource;
+
+import org.springframework.boot.context.properties.ConfigurationProperties;
+import org.springframework.boot.jdbc.DataSourceBuilder;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+@Configuration(proxyBeanMethods = false)
+class MyDatasourceConfiguration {
+
+	@Bean
+	@ConfigurationProperties("app.datasource.second")
+	public BasicDataSource secondDataSource() {
+		return DataSourceBuilder.create().type(BasicDataSource.class).build();
+	}
+
+}

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/howto/testing/slicetests/MySecurityConfiguration.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2012-2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.docs.howto.testing.slicetests;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.security.config.annotation.web.builders.HttpSecurity;
+import org.springframework.security.web.SecurityFilterChain;
+
+@Configuration(proxyBeanMethods = false)
+class MySecurityConfiguration {
+
+	@Bean
+	public SecurityFilterChain securityFilterChain(HttpSecurity http) throws Exception {
+		http.authorizeRequests().anyRequest().authenticated();
+		return http.build();
+	}
+
+}