merge File content function added

Author: VamshikShetty

modules/openapi-generator/src/main/java/org/openapitools/codegen/AbstractGenerator.java

@@ -48,6 +48,31 @@ public File writeToFile(String filename, String contents) throws IOException {
         return output;
     }
 
+    public String mergeFileContent(String fileName, String content) throws IOException {
+
+          File f1 = new File(fileName);
+
+          if(!f1.exists()){
+             return content;
+         }
+
+          FileReader fileReader = new FileReader(f1);
+         BufferedReader bufferedReader = new BufferedReader(fileReader);
+         String line,existingContent = "";
+
+          while((line = bufferedReader.readLine()) != null) {
+             if(existingContent=="")
+                 existingContent = line;
+             else
+                 existingContent += System.lineSeparator() + line;
+         }
+
+          if(!existingContent.isEmpty()) {
+             content = existingContent + System.lineSeparator() + content;
+         }
+         return content;
+    }
+
     public String readTemplate(String name) {
         try {
             Reader reader = getTemplateReader(name);

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultGenerator.java

@@ -712,7 +712,11 @@ public Reader getTemplate(String name) {
                                 .defaultValue("")
                                 .compile(template);
 
-                        writeToFile(outputFilename, tmpl.execute(bundle));
+                        String contents = tmpl.execute(bundle);
+                        if(support.merge)
+                             contents = mergeFileContent(outputFilename, contents);
+                         writeToFile(outputFilename, contents);
+
                         File written = new File(outputFilename);
                         files.add(written);
                         if (config.isEnablePostProcessFile()) {