Create rule S6837: Superfluous "@responsebody" annotations should be …

…removed (#3366)

You can preview this rule
[here](https://sonarsource.github.io/rspec/#/rspec/S6837/java) (updated
a few minutes after each push).

## Review

A dedicated reviewer checked the rule description successfully for:

- [ ] logical errors and incorrect information
- [ ] information gaps and missing content
- [ ] text style and tone
- [ ] PR summary and labels follow [the
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---------

Co-authored-by: johann-beleites-sonarsource <[email protected]>
Co-authored-by: Johann Beleites <[email protected]>
Co-authored-by: Johann Beleites <[email protected]>

rules/S6837/java/metadata.json

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+{
+  "title": "Superfluous \"@ResponseBody\" annotations should be removed",
+  "type": "CODE_SMELL",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+    "spring"
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-6837",
+  "sqKey": "S6837",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "targeted",
+  "code": {
+    "impacts": {
+      "MAINTAINABILITY": "LOW"
+    },
+    "attribute": "CLEAR"
+  }
+}

rules/S6837/java/rule.adoc

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+== Why is this an issue?
+
+The Spring framework's `@RestController` annotation is equivalent to using the `@Controller` and `@ResponseBody` annotations together.
+As such, it is redundant to add a `@ResponseBody` annotation when the class is already annotated with `@RestController`.
+
+== How to fix it
+
+Remove the `@ResponseBody` annotation from the class or method.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,java,diff-id=1,diff-type=noncompliant]
+----
+@RestController
+public class MyController {
+  @ResponseBody // Noncompliant, the @RestController annotation already implies @ResponseBody
+  @RequestMapping("/hello")
+  public String hello() {
+    return "Hello World!";
+  }
+}
+----
+
+==== Compliant solution
+
+[source,java,diff-id=1,diff-type=compliant]
+----
+@RestController
+public class MyController {
+  @RequestMapping("/hello")
+  public String hello() {
+    return "Hello World!";
+  }
+}
+----
+
+==== Noncompliant code example
+
+[source,java,diff-id=2,diff-type=noncompliant]
+----
+@RestController
+@ResponseBody // Noncompliant, the @RestController annotation already implies @ResponseBody
+public class MyController {
+  @RequestMapping("/hello")
+  public String hello() {
+    return "Hello World!";
+  }
+}
+----
+
+==== Compliant solution
+
+[source,java,diff-id=2,diff-type=compliant]
+----
+@RestController
+public class MyController {
+  @RequestMapping("/hello")
+  public String hello() {
+    return "Hello World!";
+  }
+}
+----
+
+== Resources
+=== Articles & blog posts
+* Spring Guides - https://spring.io/guides/gs/rest-service/[Building a RESTful Web Service]
+* Baeldung - https://www.baeldung.com/spring-controller-vs-restcontroller[The Spring @Controller and @RestController Annotations]
+* Baeldung - https://www.baeldung.com/spring-request-response-body[Spring's RequestBody and ResponseBody Annotations]

rules/S6837/metadata.json

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+{
+}