add java8 and async options to springboot codegen

modules/swagger-codegen/src/main/resources/JavaSpringBoot/apiController.mustache

@@ -1,7 +1,6 @@
 package {{apiPackage}};
 
-import {{modelPackage}}.*;
-
+{{^java8}}
 {{#imports}}import {{import}};
 {{/imports}}
 
@@ -9,7 +8,9 @@ import io.swagger.annotations.*;
 
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
+{{/java8}}
 import org.springframework.stereotype.Controller;
+{{^java8}}
 import org.springframework.web.bind.annotation.PathVariable;
 import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RequestHeader;
@@ -18,18 +19,26 @@ import org.springframework.web.bind.annotation.RequestPart;
 import org.springframework.web.multipart.MultipartFile;
 
 import java.util.List;
+{{#async}}
+import java.util.concurrent.Callable;
+{{/async}}{{/java8}}
 
-@Controller
 {{>generatedAnnotation}}
+@Controller
 {{#operations}}
 public class {{classname}}Controller implements {{classname}} {
-  {{#operation}}
-    public ResponseEntity<{{>returnTypes}}> {{operationId}}({{#allParams}}{{>queryParams}}{{>pathParams}}{{>headerParams}}{{>bodyParams}}{{>formParams}}{{#hasMore}},
-      {{/hasMore}}{{/allParams}}) {
-      // do some magic!
-      return new ResponseEntity<{{>returnTypes}}>(HttpStatus.OK);
-  }
-
-  {{/operation}}
+{{^java8}}{{#operation}}
+    public {{#async}}Callable<{{/async}}ResponseEntity<{{>returnTypes}}>{{#async}}>{{/async}} {{operationId}}({{#allParams}}{{>queryParams}}{{>pathParams}}{{>headerParams}}{{>bodyParams}}{{>formParams}}{{#hasMore}},
+        {{/hasMore}}{{/allParams}}) {
+        // do some magic!{{^async}}
+        return new ResponseEntity<{{>returnTypes}}>(HttpStatus.OK);{{/async}}{{#async}}
+        return new Callable<ResponseEntity<{{>returnTypes}}>>() {
+            @Override
+            public ResponseEntity<{{>returnTypes}}> call() throws Exception {
+                return new ResponseEntity<{{>returnTypes}}>(HttpStatus.OK);
+            }
+        };{{/async}}
+    }
+{{/operation}}{{/java8}}
 }
 {{/operations}}