Support expressing application args in @SpringBootTest

Add `args` property to the `@SpringBootTest` annotation so tests
can easily supply application arguments to pass to their app under test.

See gh-14823

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

@@ -6752,6 +6752,17 @@ NOTE: If you directly use `@ComponentScan` (that is, not through
 `@SpringBootApplication`) you need to register the `TypeExcludeFilter` with it. See
 {dc-spring-boot}/context/TypeExcludeFilter.{dc-ext}[the Javadoc] for details.
 
+[[boot-features-testing-spring-boot-applications-arguments]]
+==== Testing with Application Arguments
+
+If your application expects <<boot-features-application-arguments,arguments>>, you can
+have `@SpringBootTest` inject them using the `args` field.
+
+[source,java,indent=0]
+----
+include::{test-examples}/context/ApplicationArgumentsExampleTests.java[tag=args]
+}
+----
 
 [[boot-features-testing-spring-boot-applications-testing-with-mock-environment]]
 ==== Testing with a mock environment

spring-boot-project/spring-boot-docs/src/test/java/org/springframework/boot/docs/context/ApplicationArgumentsExampleTests.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2012-2018 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.docs.context;
+
+// tag::args[]
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.ApplicationArguments;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.test.context.junit4.SpringRunner;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+@RunWith(SpringRunner.class)
+@SpringBootTest(args = { "--foo=bar" })
+public class ApplicationArgumentsExampleTests {
+
+	@Autowired
+	private ApplicationArguments args;
+
+	@Test
+	public void applicationArgumentsPopulated() {
+		assertThat(this.args.getOptionNames()).contains("foo");
+		assertThat(this.args.getOptionValues("foo")).contains("bar");
+	}
+
+	// end::args[]
+	@Configuration
+	protected static class Config {
+
+	}
+
+}