Writing a Native Java REST Client

So far in this blog, we have been learning about building RESTful webservices which are server side components. In this post, we will learn to build a RESTful client for consuming the webservices written in previous posts.

I will be re-using the code base written for RESTEasy + JAXB xml example. I will build a pure java API client without using any third party tools.

1) Build the RESTful webservice API

Follow the steps given in RESTEasy + JAXB xml example

For reference, service and model classes are:

UserManagementModule.java

package com.howtodoinjava.service;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;
import javax.ws.rs.Produces;
import javax.ws.rs.core.Response;

import com.howtodoinjava.model.User;

@Path("/user-management")
public class UserManagementModule
{
	@GET
	@Path("/users/{id}")
	@Produces("application/xml")
	public Response getUserById(@PathParam("id") Integer id)
	{
		User user = new User();
		user.setId(id);
		user.setFirstName("Lokesh");
		user.setLastName("Gupta");
		return Response.status(200).entity(user).build();
	}
}

User.java

package com.howtodoinjava.model;
 
import java.io.Serializable;

import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
 
@XmlAccessorType(XmlAccessType.NONE)
@XmlRootElement(name = "user")
public class User implements Serializable {
 
    private static final long serialVersionUID = 1L;
 
    @XmlAttribute(name = "id")
    private int id;
 
    @XmlElement(name = "firstName")
    private String firstName;
 
    @XmlElement(name = "lastName")
    private String lastName;
 
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getFirstName() {
        return firstName;
    }
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }
    public String getLastName() {
        return lastName;
    }
    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
}

2) Create the RESTFul webservices consumer client

Our java client is based on java.net package APIs. I am doing two steps here:

1. Capture the output in string format
2. Unmarshal the model object from xml response

package test;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.StringReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Unmarshaller;

import com.howtodoinjava.model.User;

public class PureJavaClient 
{
	public static void main(String[] args) 
	{
		try 
		{
			URL url = new URL("http://localhost:8080/RESTfulDemoApplication/user-management/users/10");
			HttpURLConnection conn = (HttpURLConnection) url.openConnection();
			conn.setRequestMethod("GET");
			conn.setRequestProperty("Accept", "application/xml");

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

			BufferedReader br = new BufferedReader(new InputStreamReader((conn.getInputStream())));
			String apiOutput = br.readLine();
			System.out.println(apiOutput);
			conn.disconnect();

			JAXBContext jaxbContext = JAXBContext.newInstance(User.class);
			Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
			User user = (User) jaxbUnmarshaller.unmarshal(new StringReader(apiOutput));

			System.out.println(user.getId());
			System.out.println(user.getFirstName());
			System.out.println(user.getLastName());
			
		} catch (MalformedURLException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		} catch (JAXBException e) {
			e.printStackTrace();
		}
	}
}

Output:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?><user id="10"><firstName>Lokesh</firstName><lastName>Gupta</lastName></user>
10
Lokesh
Gupta

Follow the below link for downloading the source code of this example.

Source code download

Happy Learning !!