Move code snippet to regular source file

Closes gh-9740

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

@@ -342,20 +342,7 @@ code.
 
 [source,java,indent=0]
 ----
-	@SpringBootApplication
+include::{code-examples}/ExitCodeApplication.java[tag=example]
-	public class ExitCodeApplication {
-
-		public static void main(String[] args) {
-			System.exit(SpringApplication.exit(
-					SpringApplication.run(ExitCodeApplication.class, args)));
-		}
-
-		@Bean
-		public ExitCodeGenerator exitCodeGenerator(){
-			return () -> 42;
-		}
-
-	}
 ----
 
 Also, the `ExitCodeGenerator` interface may be implemented by exceptions. When such an

spring-boot-docs/src/main/java/org/springframework/boot/ExitCodeApplication.java

@@ -0,0 +1,47 @@
+/*
+ * 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;
+
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.context.annotation.Bean;
+
+/**
+ * Example configuration that illustrates the use of {@link ExitCodeGenerator}.
+ *
+ * @author Stephane Nicoll
+ */
+// tag::example[]
+@SpringBootApplication
+public class ExitCodeApplication {
+
+	public static void main(String[] args) {
+		System.exit(SpringApplication.exit(
+				SpringApplication.run(ExitCodeApplication.class, args)));
+	}
+
+	@Bean
+	public ExitCodeGenerator exitCodeGenerator() {
+		return new ExitCodeGenerator() {
+			@Override
+			public int getExitCode() {
+				return 42;
+			}
+		};
+	}
+
+}
+// end::example[]