Programming Tutorials

Using HttpConnection in J2ME (Retrieve web content from a website to a phone)

By: David Hemphill  

This J2ME example will cover reading data using the MIDP HttpConnection. Note that this connection interface is not part of the CLDC or CDC, but is defined rather in the MIDP and Personal Profiles themselves. The behavior of HttpConnection is one that combines an InputStream and an OutputStream into a single connection. A single HttpConnection may open and use exactly one OutputStream and exactly one InputStream. The order in which the streams are used is important as well. The OutputStream, if used, must be used before the InputStream. Once the streams have been used the connection should be closed and a new HttpConnection should be opened to continue communications if necessary. This follows the HTTP request-response paradigm.

The HttpConnection is a bit more tricky to use than the socket or datagram connections because there is a lot that happens behind the scenes. There are three states to an HttpConnection:

• Setup
• Connected
• Closed

The setup state is the first state encountered after a connection is opened. While in this state, connection parameters can be set such as the request method (GET, POST or HEAD) using the setRequestMethod() method or any header properties using the setRequestProperty() method.

The transition from setup to connected is triggered by any methods that cause data to be sent to the server. The following is a list of methods that cause this transition.

• openInputStream
• openDataInputStream
• getLength
• getType
• getEncoding
• getHeaderField
• getResponseCode
• getResponseMessage
• getHeaderFieldInt
• getHeaderFieldDate
• getExpiration
• getDate
• getLastModified
• getHeaderField
• getHeaderFieldKey

Once the connection transitions to the connected state, any calls to setRequestMethod() and setRequestProperty() will throw an IOException. The state transition from setup to connected reflects the underlying handshake of the HttpConnection as headers are sent to the server and the connection prepares to send data. The following example demonstrates reading Web content from an HttpConnection.

HttpConnection c = null;
InputStream is = null;
StringBuffer sb = new StringBuffer();
try {
  c = (HttpConnection)Connector.open(
     "http://www.java-samples.com”, 
     Connector.READ_WRITE, true);
  c.setRequestMethod(HttpConnection.GET); //default
  is = c.openInputStream(); // transition to connected!
  int ch = 0;
  for(int ccnt=0; ccnt < 150; ccnt++) { // get the title.
    ch = is.read();
    if (ch == -1){
      break;
    }
    sb.append((char)ch);
  }
}
catch (IOException x){
	x.printStackTrace();
}
finally{
     try     {
       is.close();
          c.close();
     } catch (IOException x){
          x.printStackTrace();
     }
}
System.out.println(sb.toString());

In this example, the server at www.java-samples.com is contacted. Because this is an HttpConnection and no port is specified, port 80 is used by default. The request method is set to GET (note GET is the default and is explicitly set here only for the example).

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