如何将json对象列表转换为包含每个对象中某些字段的字符串pojo列表？

Question

I have a JSON document that describe list of objects, it looks something like this:

[
   {
     "txId": "ffff",
     "sender" : "0xwwwwwww",
     "recepient" : "0xeferfef"
   },
   {
     "txId": "ffff",
     "sender" : "0xwwwwwww",
     "recepient" : "0xeferfef"
   }
   ...

   ...
]

How can I get List<String> that contains txId values from each object using only Jackson API (without converting this JSON to a list of pojo-objects then proceed this list by foreach and create new list of strings)?

Answer 1

You can always read a JSON document as JsonNode object with Jackson API (no need of creating POJO). Next, there are several ways of reading and manipulating the data represented as JsonNode object. One of the most convenient ways available from Java 8+ is to create a java.util.Stream<JsonNode> and collect the final list as a result of a mapping from JsonNode to String , where String is represents a value of node.txId field.

You can create java.util.Stream<JsonNode> with :

java.util.stream.StreamSupport.stream(jsonNode.spliterator(), false)

and then you can call map(node -> node.get("txId").textValue() and finally call collect() to terminate the stream and get your expected result.

Consider following code as an example (requires Java 8+):

import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.IOException;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.StreamSupport;

final class JacksonReadExample {

    public static void main(String[] args) throws IOException {

        final String json = " [\n" +
                "   {\n" +
                "     \"txId\": \"ffff-001\",\n" +
                "     \"sender\" : \"0xwwwwwww\",\n" +
                "     \"recepient\" : \"0xeferfef\"\n" +
                "   },\n" +
                "   {\n" +
                "     \"txId\": \"ffff-002\",\n" +
                "     \"sender\" : \"0xwwwwwww\",\n" +
                "     \"recepient\" : \"0xeferfef\"\n" +
                "   }\n" +
                "]";

        final ObjectMapper mapper = new ObjectMapper();

        final JsonNode jsonNode = mapper.readTree(json);

        final List<String> ids = StreamSupport.stream(jsonNode.spliterator(), false)
                .map(node -> node.get("txId").textValue())
                .collect(Collectors.toList());

        System.out.println(ids);
    }
}

Output:

[ffff-001, ffff-002]