Add "testing your auto-configuration" doc section

Closes gh-10011

spring-boot-project/spring-boot-docs/src/test/java/org/springframework/boot/autoconfigure/UserServiceAutoConfigurationTests.java

@@ -0,0 +1,85 @@
+/*
+ * Copyright 2012-2017 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
+ *
+ *      http://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.autoconfigure;
+
+import org.junit.Test;
+
+import org.springframework.boot.test.context.FilteredClassLoader;
+import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link UserServiceAutoConfiguration}.
+ *
+ * @author Stephane Nicoll
+ */
+public class UserServiceAutoConfigurationTests {
+
+	// tag::runner[]
+	private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
+			.withConfiguration(AutoConfigurations.of(UserServiceAutoConfiguration.class));
+	// end::runner[]
+
+	// tag::test-env[]
+	@Test
+	public void serviceNameCanBeConfigured() {
+		this.contextRunner.withPropertyValues("user.name=test123")
+				.run((context) -> {
+					assertThat(context).hasSingleBean(UserService.class);
+					assertThat(context.getBean(UserService.class).getName())
+							.isEqualTo("test123");
+				});
+	}
+	// end::test-env[]
+
+	// tag::test-classloader[]
+	@Test
+	public void serviceIsIgnoredIfLibraryIsNotPresent() {
+		this.contextRunner.withClassLoader(new FilteredClassLoader(UserService.class))
+				.run((context) -> {
+					assertThat(context).doesNotHaveBean("userService");
+				});
+	}
+	// end::test-classloader[]
+
+
+	// tag::test-user-config[]
+	@Test
+	public void defaultServiceBacksOff() {
+		this.contextRunner.withUserConfiguration(UserConfiguration.class)
+				.run((context) -> {
+					assertThat(context).hasSingleBean(UserService.class);
+					assertThat(context.getBean(UserService.class)).isSameAs(
+							context.getBean(UserConfiguration.class).myUserService());
+				});
+	}
+
+	@Configuration
+	static class UserConfiguration {
+
+		@Bean
+		public UserService myUserService() {
+			return new UserService("mine");
+		}
+
+	}
+	// end::test-user-config[]
+
+}