Create rule S6814: Optional REST parameters should have an object type (

#3250)

rules/S6814/java/metadata.json

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+{
+  "title": "Optional REST parameters should have an object type",
+  "type": "CODE_SMELL",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-6814",
+  "sqKey": "S6814",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown",
+  "code": {
+    "impacts": {
+      "RELIABILITY": "HIGH"
+    },
+    "attribute": "CONVENTIONAL"
+  }
+}

rules/S6814/java/rule.adoc

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+== Why is this an issue?
+
+Spring provides two options to mark a REST parameter as optional:
+
+1. Use `required = false` in the `@PathVariable` or `@RequestParam` annotation of the respective method parameter or
+2. Use type `java.util.Optional<T>` for the method parameter
+
+When using 1., the absence of the parameter, when the REST function is called, is encoded by `null`, which can only be used for object types.
+If `required = false` is used for a parameter with a primitive and the REST function is called without the parameter, a runtime exception occurs because the Spring data mapper cannot map the `null` value to the parameter.
+
+== How to fix it
+
+Replace primitive types, such as `boolean`, `char`, `int`, with the corresponding wrapper type, such as `Boolean`, `Character`, `Integer`.
+
+Alternatively, you might choose to remove `required = false` from the annotation and use an `Optional<T>` type for the parameter, such as `Optional<Boolean>` or `Optional<String>`, which automatically makes the REST parameter optional.
+This is the preferred approach because it enforces the proper handling of `null` in the method implementation.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,java,diff-id=1,diff-type=noncompliant]
+----
+@RequestMapping(value = {"/article", "/article/{id}"})
+public Article getArticle(@PathVariable(required = false) int articleId) { // Noncompliant, null cannot be mapped to int
+   //...
+}
+----
+
+==== Compliant solution
+
+[source,java,diff-id=1,diff-type=compliant]
+----
+@RequestMapping(value = {"/article", "/article/{id}"})
+public Article getArticle(@PathVariable(required = false) Integer articleId) { // Compliant
+   //...
+}
+----
+
+==== Noncompliant code example
+
+[source,java,diff-id=2,diff-type=noncompliant]
+----
+@RequestMapping(value = {"/article", "/article/{id}"})
+public Article getArticle(@PathVariable(required = false) int articleId) { // Noncompliant, null cannot be mapped to int
+   //...
+}
+----
+
+==== Compliant solution
+
+[source,java,diff-id=2,diff-type=compliant]
+----
+@RequestMapping(value = {"/article", "/article/{id}"})
+public Article getArticle(@PathVariable Optional<Integer> articleId) { // Compliant and preferred approach
+   //...
+}
+----
+
+== Resources
+
+=== Documentation
+
+- https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/web/bind/annotation/PathVariable.html[Spring Framework API - Annotation Interface PathVariable]
+
+=== Articles & blog posts
+
+- https://www.baeldung.com/spring-optional-path-variables[Baeldung - Spring Optional Path Variables]

rules/S6814/metadata.json

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+{
+}