Today most of the mobile applications that used to communicate to some server use REST services. These services are also common practice to use with JavaScript or jQuery. Right now I know 2 ways to create client for REST service in java and in this article I will try to demonstrate both the ways I know hoping that it will help someone in some way.

1. Using Apache HttpClient

The Apache HttpClient library simplifies handling HTTP requests. To use this library you have to download the binaries with dependencies from their website.
Here is the code for HTTP GET method:
 
 

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
public class Test {
 public static void main(String[] args) throws ClientProtocolException, IOException {
 HttpClient client = new DefaultHttpClient();
 HttpGet request = new HttpGet('http://restUrl');
 HttpResponse response = client.execute(request);
 BufferedReader rd = new BufferedReader (new InputStreamReader(response.getEntity().getContent()));
 String line = '';
 while ((line = rd.readLine()) != null) {
 System.out.println(line);
 }
 }
}

And for Post method; for sending simple string in post:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.DefaultHttpClient;
public class Test {
 public static void main(String[] args) throws ClientProtocolException, IOException {
 HttpClient client = new DefaultHttpClient();
 HttpPost post = new HttpPost('http://restUrl');
 StringEntity input = new StringEntity('product');
 post.setEntity(input);
 HttpResponse response = client.execute(post);
 BufferedReader rd = new BufferedReader(new InputStreamReader(response.getEntity().getContent()));
 String line = '';
 while ((line = rd.readLine()) != null) {
 System.out.println(line);
 }
 }
}

You can also send full JSON or XML of a POJO by putting String representing JSON or XML as a parameter of StringEntity and then set the input content type. Something like this:

StringEntity input = new StringEntity('{\'name1\':\'value1\',\'name2\':\'value2\'}'); //here instead of JSON you can also have XML
input.setContentType('application/json');

For JSON you can use JSONObject to create string representation of JSON.

JSONObject json = new JSONObject();
json.put('name1', 'value1');
json.put('name2', 'value2');
StringEntity se = new StringEntity( json.toString());

And for sending multiple parameter in post request:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;
public class Test {
 public static void main(String[] args) throws ClientProtocolException, IOException {
 HttpClient client = new DefaultHttpClient();
 HttpPost post = new HttpPost('http://restUrl');
 List nameValuePairs = new ArrayList(1);
 nameValuePairs.add(new BasicNameValuePair('name', 'value')); //you can as many name value pair as you want in the list.
 post.setEntity(new UrlEncodedFormEntity(nameValuePairs));
 HttpResponse response = client.execute(post);
 BufferedReader rd = new BufferedReader(new InputStreamReader(response.getEntity().getContent()));
 String line = '';
 while ((line = rd.readLine()) != null) {
 System.out.println(line);
 }
 }
}

2. Using Jersey

Jersey is the reference implementation forJSR-311 specification, the specification of REST support in Java. Jersey contains basically a REST server and a REST client. it provides a library to communicate with the server producing REST services. For http get method:

import java.io.IOException;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.UriBuilder;
import org.apache.http.client.ClientProtocolException;
import com.sun.jersey.api.client.Client;
import com.sun.jersey.api.client.WebResource;
import com.sun.jersey.api.client.config.ClientConfig;
import com.sun.jersey.api.client.config.DefaultClientConfig;
public class Test {
 public static void main(String[] args) throws ClientProtocolException, IOException {
 ClientConfig config = new DefaultClientConfig();
 Client client = Client.create(config);
 WebResource service = client.resource(UriBuilder.fromUri('http://restUrl').build());
 // getting XML data
 System.out.println(service. path('restPath').path('resourcePath').accept(MediaType.APPLICATION_JSON).get(String.class));
 // getting JSON data
 System.out.println(service. path('restPath').path('resourcePath').accept(MediaType.APPLICATION_XML).get(String.class));
 }
}

There are also other media formats in which you can get the response like PLAIN or HTML.
And for HTTP POST method:

import java.io.IOException;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.MultivaluedMap;
import javax.ws.rs.core.UriBuilder;
import org.apache.http.client.ClientProtocolException;
import com.sun.jersey.api.client.Client;
import com.sun.jersey.api.client.ClientResponse;
import com.sun.jersey.api.client.WebResource;
import com.sun.jersey.api.client.config.ClientConfig;
import com.sun.jersey.api.client.config.DefaultClientConfig;
import com.sun.jersey.core.util.MultivaluedMapImpl;
public class Test {
 public static void main(String[] args) throws ClientProtocolException, IOException {
 ClientConfig config = new DefaultClientConfig();
 Client client = Client.create(config);
 WebResource webResource = client.resource(UriBuilder.fromUri('http://restUrl').build());
 MultivaluedMap formData = new MultivaluedMapImpl();
 formData.add('name1', 'val1');
 formData.add('name2', 'val2');
 ClientResponse response = webResource.type(MediaType.APPLICATION_FORM_URLENCODED_TYPE).post(ClientResponse.class, formData);
 System.out.println('Response ' + response.getEntity(String.class));
 }
}

If you are using your POJO in the POST then you can do something like following:

ClientResponse response = webResource.path('restPath').path('resourcePath').
type(MediaType.APPLICATION_JSON).accept(MediaType.APPLICATION_JSON).post(ClientResponse.class, myPojo);
System.out.println('Response ' + response.getEntity(String.class));

Here myPojo is an instance of custom POJO class.
You can also use Form class from Jersey to submit multiple parameters in POST request:

import java.io.IOException;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.UriBuilder;
import org.apache.http.client.ClientProtocolException;
import com.sun.jersey.api.client.Client;
import com.sun.jersey.api.client.ClientResponse;
import com.sun.jersey.api.client.WebResource;
import com.sun.jersey.api.client.config.ClientConfig;
import com.sun.jersey.api.client.config.DefaultClientConfig;
import com.sun.jersey.api.representation.Form;
public class Test {
 public static void main(String[] args) throws ClientProtocolException, IOException {
 ClientConfig config = new DefaultClientConfig();
 Client client = Client.create(config);
 WebResource service = client.resource(UriBuilder.fromUri('http://restUrl').build());
 Form form = new Form();
 form.add('name1', 'value1');
 form.add('name2', 'value1');
 ClientResponse response = service.path('restPath').path('resourcePath').
 type(MediaType.APPLICATION_FORM_URLENCODED).post(ClientResponse.class, form);
 System.out.println('Response ' + response.getEntity(String.class));
 }
}

Happy coding and don’t forget to share!

Reference: Simple REST client in java from our JCG partner Harsh Raval at the harryjoy blog.