[java] What is the best Java library to use for HTTP POST, GET etc.?

imho: Apache HTTP Client

usage example:

import org.apache.commons.httpclient.*;
import org.apache.commons.httpclient.methods.*;
import org.apache.commons.httpclient.params.HttpMethodParams;

import java.io.*;

public class HttpClientTutorial {

  private static String url = "http://www.apache.org/";

  public static void main(String[] args) {
    // Create an instance of HttpClient.
    HttpClient client = new HttpClient();

    // Create a method instance.
    GetMethod method = new GetMethod(url);

    // Provide custom retry handler is necessary
    method.getParams().setParameter(HttpMethodParams.RETRY_HANDLER, 
            new DefaultHttpMethodRetryHandler(3, false));

    try {
      // Execute the method.
      int statusCode = client.executeMethod(method);

      if (statusCode != HttpStatus.SC_OK) {
        System.err.println("Method failed: " + method.getStatusLine());
      }

      // Read the response body.
      byte[] responseBody = method.getResponseBody();

      // Deal with the response.
      // Use caution: ensure correct character encoding and is not binary data
      System.out.println(new String(responseBody));

    } catch (HttpException e) {
      System.err.println("Fatal protocol violation: " + e.getMessage());
      e.printStackTrace();
    } catch (IOException e) {
      System.err.println("Fatal transport error: " + e.getMessage());
      e.printStackTrace();
    } finally {
      // Release the connection.
      method.releaseConnection();
    }  
  }
}

some highlight features:

• Standards based, pure Java, implementation of HTTP versions 1.0 and 1.1
  • Full implementation of all HTTP methods (GET, POST, PUT, DELETE, HEAD, OPTIONS, and TRACE) in an extensible OO framework.
  • Supports encryption with HTTPS (HTTP over SSL) protocol.
  • Granular non-standards configuration and tracking.
  • Transparent connections through HTTP proxies.
  • Tunneled HTTPS connections through HTTP proxies, via the CONNECT method.
  • Transparent connections through SOCKS proxies (version 4 & 5) using native Java socket support.
  • Authentication using Basic, Digest and the encrypting NTLM (NT Lan Manager) methods.
  • Plug-in mechanism for custom authentication methods.
  • Multi-Part form POST for uploading large files.
  • Pluggable secure sockets implementations, making it easier to use third party solutions
  • Connection management support for use in multi-threaded applications. Supports setting the maximum total connections as well as the maximum connections per host. Detects and closes stale connections.
  • Automatic Cookie handling for reading Set-Cookie: headers from the server and sending them back out in a Cookie: header when appropriate.
  • Plug-in mechanism for custom cookie policies.
  • Request output streams to avoid buffering any content body by streaming directly to the socket to the server.
  • Response input streams to efficiently read the response body by streaming directly from the socket to the server.
  • Persistent connections using KeepAlive in HTTP/1.0 and persistance in HTTP/1.1
  • Direct access to the response code and headers sent by the server.
  • The ability to set connection timeouts.
  • HttpMethods implement the Command Pattern to allow for parallel requests and efficient re-use of connections.
  • Source code is freely available under the Apache Software License.