Send JSON data in an HTTP GET request to a REST API from JAVA code

Question

I am making the following curl request successfully to my API:

curl -v -X GET -H "Content-Type: application/json" -d {'"query":"some text","mode":"0"'} http://host.domain.abc.com:23423/api/start-trial-api/

I would like to know how can i make this request from inside JAVA code. I have tried searching through Google and stack overflow for the solution. All i have found is how to send data through a query string or how to send JSON data through a POST request.

Thanks

Answer 1

Using below code you should be able to invoke any rest API.

Make a class called RestClient.java which will have method for get and post

package test;

import java.io.IOException;

import org.codehaus.jackson.JsonParseException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;

import com.javamad.utils.JsonUtils;
import com.sun.jersey.api.client.Client;
import com.sun.jersey.api.client.ClientResponse;
import com.sun.jersey.api.client.WebResource;

public class RestClient {

    public static <T> T post(String url,T data,T t){
        try {
        Client client = Client.create();
        WebResource webResource = client.resource(url);
        ClientResponse response = webResource.type("application/json").post(ClientResponse.class, JsonUtils.javaToJson(data));

        if (response.getStatus() != 200) {
            throw new RuntimeException("Failed : HTTP error code : "
                 + response.getStatus());
        }
        String output = response.getEntity(String.class);
        System.out.println("Response===post="+output);

            t=(T)JsonUtils.jsonToJavaObject(output, t.getClass());
        } catch (JsonParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (JsonMappingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return t;
    }
    public static <T> T get(String url,T t)
    {
         try {
        Client client = Client.create();

        WebResource webResource = client.resource(url);

        ClientResponse response = webResource.accept("application/json").get(ClientResponse.class);

        if (response.getStatus() != 200) {
           throw new RuntimeException("Failed : HTTP error code : "
            + response.getStatus());
        }

        String output = response.getEntity(String.class);
        System.out.println("Response===get="+output);



            t=(T)JsonUtils.jsonToJavaObject(output, t.getClass());
        } catch (JsonParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (JsonMappingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return t;
    }

}

invoke the get and post method

public class QuestionAnswerService {
    static String baseUrl="http://javamad.com/javamad-webservices-1.0";

    //final String baseUrl="http://javamad.com/javamad-webservices-1.0";
    @Test
    public void getQuestions(){
        System.out.println("javamad.baseurl="+baseUrl);
        GetQuestionResponse gqResponse=new GetQuestionResponse();

        gqResponse =RestClient.get(baseUrl+"/v1/questionAnswerService/getQuestions?questionType=2",gqResponse);


        List qList=new ArrayList<QuestionDetails>();
        qList=(List) gqResponse.getQuestionList();

        //System.out.println(gqResponse);

    }

    public void postQuestions(){
        PostQuestionResponse pqResponse=new PostQuestionResponse();
        PostQuestionRequest pqRequest=new PostQuestionRequest();
        pqRequest.setQuestion("maybe working");
        pqRequest.setQuestionType("2");
        pqRequest.setUser_id("2");
        //Map m= new HashMap();
        pqResponse =(PostQuestionResponse) RestClient.post(baseUrl+"/v1/questionAnswerService/postQuestion",pqRequest,pqResponse);

    }

    }

Make your own Request and response class.

for json to java and java to json use below class

package com.javamad.utils;

import java.io.IOException;

import org.apache.log4j.Logger;
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.JsonParseException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;

public class JsonUtils {

    private static Logger logger = Logger.getLogger(JsonUtils.class.getName());


    public static <T> T jsonToJavaObject(String jsonRequest, Class<T> valueType)
            throws JsonParseException, JsonMappingException, IOException {
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.configure(org.codehaus.jackson.map.DeserializationConfig.Feature.UNWRAP_ROOT_VALUE,false);     
        T finalJavaRequest = objectMapper.readValue(jsonRequest, valueType);
        return finalJavaRequest;

    }

    public static String javaToJson(Object o) {
        String jsonString = null;
        try {
            ObjectMapper objectMapper = new ObjectMapper();
            objectMapper.configure(org.codehaus.jackson.map.DeserializationConfig.Feature.UNWRAP_ROOT_VALUE,true);  
            jsonString = objectMapper.writeValueAsString(o);

        } catch (JsonGenerationException e) {
            logger.error(e);
        } catch (JsonMappingException e) {
            logger.error(e);
        } catch (IOException e) {
            logger.error(e);
        }
        return jsonString;
    }

}

I wrote the RestClient.java class , to reuse the get and post methods. similarly you can write other methods like put and delete...

Hope it will help you.

Answer 2

You could use the Jersey client library, if your project is a Maven one just include in your pom.xml the jersey-client and jersey-json artifacts from the com.sun.jersey group id. To connect to a web service you need a WebResource object:

WebResource resource = ClientHelper.createClient().resource(UriBuilder.fromUri(" http://host.domain.abc.com:23423/api/ ").build());

To make a call sending a payload you can model the payload as a POJO, ie

class Payload {
    private String query;
    private int mode;

    ... get and set methods
}

and then call the call using the resource object:

Payload payload = new Payload();

payload.setQuery("some text"); payload.setMode(0);

ResultType result = service  
    .path("start-trial-api").  
    .type(MediaType.APPLICATION_JSON)  
    .accept(MediaType.APPLICATION_JSON)  
    .get(ResultType.class, payload);

where ResultType is the Java mapped return type of the called service, in case it's a JSON object, otherwise you can remove the accept call and just put String.class as the get parameter and assign the return value to a plain string.

Answer 3

Spring's RESTTemplate is also useful for sending all REST requests ie GET , PUT , POST , DELETE

By usingSpring REST template , You can pass JSON request with POST like below,

You can pass JSON representation serialized into java Object using JSON serializer such as jackson

RestTemplate restTemplate = new RestTemplate();
List<HttpMessageConverter<?>> list = new ArrayList<HttpMessageConverter<?>>();
list.add(new MappingJacksonHttpMessageConverter());
restTemplate.setMessageConverters(list);
Person person = new Person();
String url = "http://localhost:8080/add";
HttpEntity<Person> entity = new HttpEntity<Person>(person);

// Call to Restful web services with person serialized as object using jackson
ResponseEntity<Person> response = restTemplate.postForEntity(  url, entity, Person.class);
Person person = response.getBody();

Answer 4

Struggled on the same question and found a nice solution using the gson .

The code:

// this method is based on gson (see below) and is used to parse Strings to json objects
public static JsonParser jsonParser = new JsonParser();

public static void getJsonFromAPI() {
    // the protocol is important
    String urlString = "http://localhost:8082/v1.0/Datastreams"
    StringBuilder result = new StringBuilder();

    try {
        URL url = new URL(urlString);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        BufferedReader rd = new BufferedReader(new InputStreamReader(conn.getInputStream()));
        String line;  // reading the lines into the result
        while ((line = rd.readLine()) != null) {
            result.append(line);
        }
        rd.close();
        // parse the String to a jsonElement
        JsonElement jsonObject = jsonParser.parse(result.toString());  
        System.out.println(jsonObject.getAsJsonObject()  // object is a mapping
                .get("value").getAsJsonArray()  // value is an array
                .get(3).getAsJsonObject()  // the fourth item is a mapping 
                .get("name").getAsString());  // name is a String

    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

The packages:

import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParser;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.Properties;

My pom.xml :

<!--    https://mvnrepository.com/artifact/com.google.code.gson/gson -->
    <dependency>
        <groupId>com.google.code.gson</groupId>
        <artifactId>gson</artifactId>
        <version>2.8.5</version>
    </dependency>

I hope this helps you!