what is HttpClient and why do we use it(Http Client)

The Hyper-Text Transfer Protocol (HTTP) is perhaps the most significant protocol used on the Internet today. Web services, network-enabled appliances and the growth of network computing continue to expand the role of the HTTP protocol beyond user-driven web browsers, while increasing the number of applications that require HTTP support.

Although the java.net package provides basic functionality for accessing resources via HTTP, it doesn't provide the full flexibility or functionality needed by many applications. The Jakarta Commons HttpClient component seeks to fill this void by providing an efficient, up-to-date, and feature-rich package implementing the client side of the most recent HTTP standards and recommendations. See the Features page for more details on standards compliance and capabilities.

Designed for extension while providing robust support for the base HTTP protocol, the HttpClient component may be of interest to anyone building HTTP-aware client applications such as web browsers, web service clients, or systems that leverage or extend the HTTP protocol for distributed communication.

There are many projects that use HttpClient to provide the core HTTP functionality. Some of these are open source with project pages you can find on the web while others are closed source that you would never see or hear about. The Apache Source License provides maximum flexibility for source and binary reuse. Please see theApplications page for projects using HttpClient....

there are some below example

Http Get

The following is an example an HTTP Get request via HttpClient.

HttpClient client = new DefaultHttpClient();
HttpGet request = new HttpGet("http://www.vogella.com");
HttpResponse response = client.execute(request);

// Get the response
BufferedReader rd = new BufferedReader
  (new InputStreamReader(response.getEntity().getContent()));
    
String line = "";
while ((line = rd.readLine()) != null) {
  textView.append(line);
} 

Http Post


The following will send a post request to the URL "http://vogellac2dm.appspot.com/register" and include a parameter "registrationid" which an random value.


package de.vogella.web.httpclient;

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.NameValuePair;
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 SimpleHttpPut { 
  public static void main(String[] args) {
    HttpClient client = new DefaultHttpClient();
    HttpPost post = new HttpPost("http://vogellac2dm.appspot.com/register");
    try {
      List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(1);
      nameValuePairs.add(new BasicNameValuePair("registrationid",
          "123456789"));
      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);
      }

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



The following request adds several parameters to the post request.


package de.vogella.web.httpclient;

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.NameValuePair;
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 TestHttpClient {
  public static void main(String[] args) {
    HttpClient client = new DefaultHttpClient();
    HttpPost post = new HttpPost("https://www.google.com/accounts/ClientLogin");

    try {

      List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(1);
      nameValuePairs.add(new BasicNameValuePair("Email", "youremail"));
      nameValuePairs
          .add(new BasicNameValuePair("Passwd", "yourpassword"));
      nameValuePairs.add(new BasicNameValuePair("accountType", "GOOGLE"));
      nameValuePairs.add(new BasicNameValuePair("source",
          "Google-cURL-Example"));
      nameValuePairs.add(new BasicNameValuePair("service", "ac2dm"));

      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);
        if (line.startsWith("Auth=")) {
          String key = line.substring(5);
          // do something with the key
        }

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







References:-

http://www.vogella.com/tutorials/ApacheHttpClient/article.html

http://hc.apache.org/httpclient-3.x/tutorial.html