如何使用Spring Boot Rest Java EE项目正确管理对文件的写入？

Question

I am trying to update a JSON file on my server resources folder in response to a form submission to my api endpoint.

I want to update my QueryMap.json from the controller QueryMapController.java

     @CrossOrigin
   @RequestMapping("/newQuery")
    public String newQuery(@RequestParam("qid") String qid, @RequestParam("query") String query) throws Exception{

    String fileName="QueryMap.json";

     File file = new ClassPathResource(fileName).getFile();




      JSONParser parser = new JSONParser();
      FileReader fr = new FileReader(file);
     Object ob = parser.parse(fr);
     fr.close();


     JSONArray arr = (JSONArray)ob;

     JSONObject newob = new JSONObject();
     newob.put("Query_id",qid);
     newob.put("Query",query);

     arr.add(newob);


     FileWriter fw = new FileWriter(file.getAbsoluteFile(),false);
     BufferedWriter out = new BufferedWriter(fw);
     out.write(arr.toJSONString());
     out.flush();
     out.close();
     fw.close();

   return arr.toJSONString();

}

The above code is the Controller Method I want to use to update my .json file. Apparently Everything seems to work. It works without errors and the program executes as if the file is updated but the actual file is not changed. As I restart the server the changes are gone. It seems the file is cached somewhere internally during runtime. How should I handle this.

Pardon my Custom Data Connection and writing code like this which doesn't strictly follow MVC guidelines. It has reasons behind to code this way.

Answer 1

tl;dr Place files you want to modify outside of your classpath because they are overwritten with the ones in your <projectDir>/src/main/resources directory when you build/package your project. If you want to see code for this check the end of the answer.

---

The reason this happens is most likely because when you restart your server, your project is re-build. During this process every class is re-compiled and the compiled classes are placed in a separate directory, along with all the resources ( <projectDir>/target/classes/ when using maven). This replaces your changed version of the json file with the original one from your <projectDir>/src/main/resources directory.

You can verify this by debugging your newQuery Method and following these steps:

1. Check the location of your json file with File.getAbsolutePath() . For me it is <projectDir>/target/classes/QueryMap.json .
2. Check the content of the file, it should be in its original state.
3. Let your code do the changes, then re-check the file content. The changes should be there.
4. Stop the server and re-build your project (With maven mvn clean package ).
5. The file content should be in its original state again, because the build process replaced <projectDir>/target/classes/QueryMap.json with <projectDir>/src/main/resources/QueryMap.json .

One solution to this would be to use the json file on your class path as a default and copy it to the current working directory where you can change it any way you want:

File editableQueryMap = new File("./QueryMap.json");

    if (!editableQueryMap.exists()) {
      File defaultQueryMap = new ClassPathResource("QueryMap.json").getFile();

      try (OutputStream os = Files.newOutputStream(editableQueryMap.toPath())) {
        Files.copy(defaultQueryMap.toPath(), os);
      }
    }

    //do your changes with editableQueryMap here

For me, this puts the editable QueryMap.json file in the root of my project directory. If you use a jar file, it should place the file in the same directory as the jar (unless the working directory is changed).