dec00_ericg.txt

TechTips j2me的常用技巧. 网络功能

To make an HTTP request, use the Connector.open method
and a conventional URL, casting the result to an HttpConnection:

    import javax.microedition.io.*;
    
    HttpConnection conn = Connector.open( 
                                "http://www.java.sun.com/jdc" );
    
The HttpConnection interface defines all the usual methods you 
would expect to see for making HTTP requests and processing the 
replies. This includes setRequestMethod, getHeaderField and
openInputStream. The HttpConnection interface makes it simple 
to interact with any web site on the Internet.

One thing you will soon discover is that you won't always get 
the response you expect when you connect to a web site. In 
particular, many web sites redirect you to another URL. The 
Java Developer Connection web site does this, for example, when 
you attempt to open the page at http://www.java.sun.com/jdc -- it
redirects you to http://developer.java.sun.com/developer/. 
Sometimes a site redirects you because the resource you are 
asking for has moved. Or else the site wants you to login first. 
If you are accessing a web site that's not under your control, 
you have to be prepared to handle redirections. You need to 
follow the redirections to obtain the data you really want.

Here's a simple class called HttpConnectionHelper that
uses the HttpConnection interface. You can use the class to
automatically follow web site redirections:

    import java.io.*;
    import javax.microedition.io.*;
    
    public class HttpConnectionHelper {
  
        public interface Callback {
            void prepareRequest( String originalURL, 
                                          HttpConnection conn ) 
                 throws IOException;
        }
    
        public static HttpConnection connect( String url ) 
                                           throws IOException {
            return connect( url, null );
        }
    
        public static HttpConnection connect(
                           String url, Callback callback ) 
                                            throws IOException {
            HttpConnection conn = null;
            String         originalURL = url;
    
            while( url != null ){
                HttpConnection conn = (HttpConnection) 
                                           Connector.open( url );
    
                if( callback != null ){
                    callback.prepareRequest( originalURL, conn );
                }
    
                int rc = conn.getResponseCode();
   
                switch( rc ){
                    case HttpConnection.HTTP_MOVED_PERM:
                    case HttpConnection.HTTP_MOVED_TEMP:
                    case HttpConnection.HTTP_SEE_OTHER:
                    case HttpConnection.HTTP_TEMP_REDIRECT:
                        url = conn.getHeaderField( "Location" );
                        if( url != null && url.startsWith( 
                                                        "/" ) ){
                            StringBuffer b = new StringBuffer();
                            b.append( "http://" );
                            b.append( conn.getHost() );
                            b.append( ':' );
                            b.append( conn.getPort() );
                            b.append( url );
                            url = b.toString();
                        }
                        conn.close();
                        break;
                    default:
                        url = null;
                        break;
                }
            }
    
            return conn;
       }
    }

To use this class, call the connect method, passing it the 
original URL:

    HttpConnection conn = HttpConnectionHelper.connect( 
                                    "http://java.sun.com/jdc" );
    int            rc = conn.getResponseCode();
    
    if( rc == HttpConnection.HTTP_OK ){
        InputStream in = conn.openInputStream();
        // do something with the response....
    } else {
        // unexpected response code
    }
  
When invoked, the connect method connects to the original URL 
and gets the HTTP response code. If the response code is the 
redirection code HTTP_TEMP_REDIRECT, the connect method gets the 
new URL from the Location response header and connects to that 
URL. The method loops until a non-redirection response code is 
returned. At that point, the method returns an HttpConnection to 
the final destination.

For special cases, such as when you want to make a Post request 
instead of a Get request, you can use the two-parameter form of 
the connect method. Here you pass in the URL as the first
parameter, and an object implementing the 
HttpConnectionHelper.Callback interface as the second parameter:

    class MyCallback implements HttpConnectionHelper.Callback {
        public void prepareRequest( String originalURL, 
                                          HttpConnection conn ) 
                                           throws IOException {
            conn.setRequestMethod( HttpConnection.POST );
            conn.setRequestProperty( "User-Agent",
                   "Profile/MIDP-1.0 Configuration/CLDC-1.0" );
            conn.setRequestProperty( 
                                 "Content-Language", "en-US" );
        }
    }
    
    HttpConnection conn = HttpConnection.connect( 
                 "http://java.sun.com/jdc", new MyCallback() );
    
If you define a callback, it's called each time a connection is
made, that is, before the response code is obtained. This lets you 
modify the HTTP request before it's actually made.

For further information about the Mobile Information Device 
Profile (MIDP), see http://java.sun.com/products/midp.

For more information about the HTTP 1.1 protocol, refer to
ftp://ftp.isi.edu/in-notes/rfc2616.txt.

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J2ME Tech Tips 
December 18, 2000


* As used in this document, the terms "Java virtual machine" 
  or "JVM" mean a virtual machine for the Java platform.