diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
index b42858a24945..8b987c863a1b 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
@@ -915,6 +915,11 @@ public String toEnumVarName(String value, String datatype) {
     }
 
     public boolean specVersionGreaterThanOrEqualTo310(OpenAPI openAPI) {
+        String originalSpecVersion = getOriginalSpecVersion(openAPI);
+        return getMajorSpecVersion(originalSpecVersion) == 3 && getMinorSpecVersion(originalSpecVersion) >= 1;
+    }
+
+    private String getOriginalSpecVersion(OpenAPI openAPI) {
         String originalSpecVersion;
         String xOriginalSwaggerVersion = "x-original-swagger-version";
         if (openAPI.getExtensions() != null && !openAPI.getExtensions().isEmpty() && openAPI.getExtensions().containsValue(xOriginalSwaggerVersion)) {
@@ -922,9 +927,15 @@ public boolean specVersionGreaterThanOrEqualTo310(OpenAPI openAPI) {
         } else {
             originalSpecVersion = openAPI.getOpenapi();
         }
-        Integer specMajorVersion = Integer.parseInt(originalSpecVersion.substring(0, 1));
-        Integer specMinorVersion = Integer.parseInt(originalSpecVersion.substring(2, 3));
-        return specMajorVersion == 3 && specMinorVersion >= 1;
+        return originalSpecVersion;
+    }
+
+    private Integer getMajorSpecVersion(String originalSpecVersion) {
+        return Integer.parseInt(originalSpecVersion.substring(0, 1));
+    }
+
+    private Integer getMinorSpecVersion(String originalSpecVersion) {
+        return Integer.parseInt(originalSpecVersion.substring(2, 3));
     }
 
     /**
@@ -5201,16 +5212,22 @@ public CodegenParameter fromParameter(Parameter parameter, Set<String> imports)
             parameterSchema = null;
         }
 
+        String defaultV3Style = null;
+
         if (parameter instanceof QueryParameter || "query".equalsIgnoreCase(parameter.getIn())) {
             codegenParameter.isQueryParam = true;
             codegenParameter.isAllowEmptyValue = parameter.getAllowEmptyValue() != null && parameter.getAllowEmptyValue();
+            defaultV3Style = Parameter.StyleEnum.FORM.toString();
         } else if (parameter instanceof PathParameter || "path".equalsIgnoreCase(parameter.getIn())) {
             codegenParameter.required = true;
             codegenParameter.isPathParam = true;
+            defaultV3Style = Parameter.StyleEnum.SIMPLE.toString();
         } else if (parameter instanceof HeaderParameter || "header".equalsIgnoreCase(parameter.getIn())) {
             codegenParameter.isHeaderParam = true;
+            defaultV3Style = Parameter.StyleEnum.SIMPLE.toString();
         } else if (parameter instanceof CookieParameter || "cookie".equalsIgnoreCase(parameter.getIn())) {
             codegenParameter.isCookieParam = true;
+            defaultV3Style = Parameter.StyleEnum.FORM.toString();
         } else {
             LOGGER.warn("Unknown parameter type: {}", parameter.getName());
         }
@@ -5247,11 +5264,15 @@ public CodegenParameter fromParameter(Parameter parameter, Set<String> imports)
             codegenParameter.style = parameter.getStyle().toString();
             codegenParameter.isDeepObject = Parameter.StyleEnum.DEEPOBJECT == parameter.getStyle();
             codegenParameter.isMatrix = Parameter.StyleEnum.MATRIX == parameter.getStyle();
+        } else if (openAPI != null && getMajorSpecVersion(getOriginalSpecVersion(openAPI)) >= 3) {
+            // Properly sets the default style for oas V3
+            codegenParameter.style = defaultV3Style;
         }
 
-        // the default value is false
+        // the default value is true if style is set to form, false otherwise
         // https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#user-content-parameterexplode
-        codegenParameter.isExplode = parameter.getExplode() != null && parameter.getExplode();
+        boolean isFormStyle = Parameter.StyleEnum.FORM.toString().equals(codegenParameter.style);
+        codegenParameter.isExplode = parameter.getExplode() != null ? parameter.getExplode() : isFormStyle;
 
         // TODO revise collectionFormat, default collection format in OAS 3 appears to multi at least for query parameters
         // https://swagger.io/docs/specification/serialization/
@@ -7105,18 +7126,23 @@ public static Set<String> getProducesInfo(final OpenAPI openAPI, final Operation
     }
 
     protected String getCollectionFormat(Parameter parameter) {
-        if (Parameter.StyleEnum.FORM.equals(parameter.getStyle())) {
+        String style = parameter.getStyle() != null ? parameter.getStyle().toString() : null;
+        return getCollectionFormat(style, parameter.getExplode());
+    }
+
+    private String getCollectionFormat(String style, Boolean explode) {
+        if (Parameter.StyleEnum.FORM.toString().equals(style)) {
             // Ref: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.1.md#style-values
-            if (Boolean.TRUE.equals(parameter.getExplode())) { // explode is true (default)
+            if (Boolean.TRUE.equals(explode)) { // explode is true (default)
                 return "multi";
             } else {
                 return "csv";
             }
-        } else if (Parameter.StyleEnum.SIMPLE.equals(parameter.getStyle())) {
+        } else if (Parameter.StyleEnum.SIMPLE.toString().equals(style)) {
             return "csv";
-        } else if (Parameter.StyleEnum.PIPEDELIMITED.equals(parameter.getStyle())) {
+        } else if (Parameter.StyleEnum.PIPEDELIMITED.toString().equals(style)) {
             return "pipes";
-        } else if (Parameter.StyleEnum.SPACEDELIMITED.equals(parameter.getStyle())) {
+        } else if (Parameter.StyleEnum.SPACEDELIMITED.toString().equals(style)) {
             return "ssv";
         } else {
             return null;
@@ -8424,23 +8450,7 @@ protected static boolean isJsonVendorMimeType(String mime) {
      * @return string for a collectionFormat.
      */
     protected String getCollectionFormat(CodegenParameter codegenParameter) {
-        if ("form".equals(codegenParameter.style)) {
-            // Ref: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.1.md#style-values
-            if (codegenParameter.isExplode) {
-                return "multi";
-            } else {
-                return "csv";
-            }
-        } else if ("simple".equals(codegenParameter.style)) {
-            return "csv";
-        } else if ("pipeDelimited".equals(codegenParameter.style)) {
-            return "pipes";
-        } else if ("spaceDelimited".equals(codegenParameter.style)) {
-            return "ssv";
-        } else {
-            // Doesn't map to any of the collectionFormat strings
-            return null;
-        }
+        return getCollectionFormat(codegenParameter.style, codegenParameter.isExplode);
     }
 
     private CodegenComposedSchemas getComposedSchemas(Schema schema) {
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
index 6789b897591a..1fb7f6b4d3ac 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java
@@ -1912,6 +1912,22 @@ public void shouldProperlyExplodeRestTemplateQueryParameters_issue907() {
                                 + " objectParam.getSomeInteger()));");
     }
 
+    @Test
+    public void shouldProperlyExplodeRestTemplateArrayQueryParameters_issue20127() {
+        final Map<String, File> files = generateFromContract(
+                "src/test/resources/3_0/exploded-query-param-array.yaml",
+                RESTTEMPLATE
+        );
+
+        JavaFileAssert.assertThat(files.get("DefaultApi.java"))
+                .printFileContent()
+                .assertMethod("getSomeValueWithHttpInfo")
+                .bodyContainsLines(
+                        "localVarQueryParams.putAll(apiClient.parameterToMultiValueMap("
+                                + "ApiClient.CollectionFormat.valueOf(\"multi\".toUpperCase(Locale.ROOT)),"
+                                + " \"QueryArray\", queryArray));");
+    }
+
     @Test
     public void shouldProperlyExplodeWebClientQueryParameters() {
         final Map<String, File> files = generateFromContract(
@@ -1936,6 +1952,22 @@ public void shouldProperlyExplodeWebClientQueryParameters() {
                                 + " objectParam.getSomeInteger()));");
     }
 
+    @Test
+    public void shouldProperlyExplodeWebClientArrayQueryParameters_issue20127() {
+        final Map<String, File> files = generateFromContract(
+                "src/test/resources/3_0/exploded-query-param-array.yaml",
+                JavaClientCodegen.WEBCLIENT
+        );
+
+        JavaFileAssert.assertThat(files.get("DefaultApi.java"))
+                .printFileContent()
+                .assertMethod("getSomeValueRequestCreation")
+                .bodyContainsLines(
+                        "queryParams.putAll(apiClient.parameterToMultiValueMap("
+                                + "ApiClient.CollectionFormat.valueOf(\"multi\".toUpperCase(Locale.ROOT)),"
+                                + " \"QueryArray\", queryArray));");
+    }
+
     private static Map<String, File> generateFromContract(final String pathToSpecification, final String library) {
         return generateFromContract(pathToSpecification, library, new HashMap<>());
     }
diff --git a/modules/openapi-generator/src/test/resources/3_0/exploded-query-param-array.yaml b/modules/openapi-generator/src/test/resources/3_0/exploded-query-param-array.yaml
index a46d6b753e08..5c064b0476d8 100644
--- a/modules/openapi-generator/src/test/resources/3_0/exploded-query-param-array.yaml
+++ b/modules/openapi-generator/src/test/resources/3_0/exploded-query-param-array.yaml
@@ -21,6 +21,12 @@ paths:
                 type: array
                 items:
                     type: string
+      - name: QueryArray
+        in: query
+        schema:
+          type: array
+          items:
+            type: string
       responses:
         '200':
           description: Some return value