Spring boot, how to read particular object from json file, what annotation do I need?

Question

I'm studying Spring Boot and I know how to read JSON file from resources directory but I want to get particular data, not whole data. like localhost:8080/user returns user name.

below is my current code

package com.example;


import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.*;
import org.springframework.boot.autoconfigure.*;
import org.springframework.core.io.Resource;
import org.springframework.web.bind.annotation.*;



@RestController
@EnableAutoConfiguration
public class HelloWorld {

    @Value("classpath:json/test.json")
    private Resource resourceFile;
    @RequestMapping("/")
    Resource home() {
        return resourceFile;
    }


    public static void main(String[] args) throws Exception {
        SpringApplication.run(HelloWorld.class, args);
    }
}

I want to read particular data in the test.json file. please give me some advice or steps. Thanks

When you already read json from the file, just convert it to jsonNode or your pojo object and get any property you want. https://www.baeldung.com/jackson-json-to-jsonnode — Huy Nguyen, Oct 22 '18 at 14:08

score 3 · Accepted Answer · answered Oct 22 '18 at 14:20

As always, there are several possible approaches.

Besides a manual parsing and extracting approach (How do I load a resource and use its contents as a string in Spring) you can also try a more advanced approach and use a library like jackson-databind (https://github.com/FasterXML/jackson-databind).

Assuming this json object in your resource:

{
  "foo" : {
    "bar" : 42
  }
}

and the Jackson ObjectMapper already Injected:

@Autowired
private ObjectMapper objectMapper;

Option 1: Use a generic approach with JsonNode

    @Autowired
    ObjectMapper objectMapper;

    @RequestMapping("/")
    JsonNode home() throws IOException {
        JsonNode jsonNode = objectMapper.readTree(resourceFile.getFile());
        return jsonNode.get("foo").get("bar");
    }

Option 2: https://github.com/FasterXML/jackson-databind#1-minute-tutorial-pojos-to-json-and-back

score 0 · Answer 2 · answered Oct 22 '18 at 17:30

This is just a variation of the @ibexit answer with some suggestions.

Use Option 2 in the ibexit answer (use a pojo) unless you really need a JsonNode (The likelyhood that you need a JsonNode rounds to 0% likely).
Create a POJO to represent your Json as an object. See below for an example.
I recommend that you use the @JsonIgnoreProperties(ignoreUnknown = true) annotation. Do a google search.
In your example, it is not required to use the @JsonProperty annotation, but I like to use it.

There are other ways to set the "ignoreUnknown" value, the Jackson documentation is a good and valuable read.

Example POJOs

@JsonIgnoreProperties(ignoreUnknown = true)
public class TopClass
{
    @JsonProperty("foo") // This is optional in your example.
    private Foo foo;
}


@JsonIgnoreProperties(ignoreUnknown = true)
public class Foo
{
    @JsonProperty("bar")
    private int bar;
}

Example Code to read the POJO

private TopClass topClassVariableName;

topClassVariableName = objectMapper.readValue(JSON HERE, TopClass.class);

Spring boot, how to read particular object from json file, what annotation do I need?

2 Answers2