Manual redes - network programming with j2 me wireless devices

Network Programming with J2ME Wireless Devices
by Yu Feng
Overview
The wireless devices such as cell phones and two-way pagers keep their owners
connected to the outside world at anytime from anywhere. They offer great
connectivity that other types of devices couldn’t offer. Application development for
these wireless devices is going to be in great demand for the next couple years.
Network programming plays an important role in wireless application development
to take advantage of the connectivity these devices have to offer.
Sun’s Java 2 Micro Edition (J2ME) offers a great development platform for
developing applications for the embedded electronics and mobile devices. In Java 2
Micro Edition (J2ME ), the Connected Limited Device Configuration (CLDC) defines a
generic "configuration" for a broad range of handheld devices. On top of CLDC, the
Mobile Information Device Profile (MIDP) is defined specifically for wireless devices
such as cell phones and two-way pagers. Wireless device manufacturers need to
implement MIDP in order to support Java applications on their devices. For
example, Motorola is going to release the MIDP implementation for its iDEN mobile
phone family in the Q1 of 2001. Research In Motion is also going to release the
MIDP implementation for its Blackberry pager family soon.
In the next section, we will take a look what’s new in J2ME network programming.
Later in this article, we will examine the HttpConnection class in J2ME MIDP. We will
conclude with a complete example of how to use the HttpConnection class.
The J2ME Generic Connection Framework
In Java 2 Standard Edition (J2SE), network programming is fairly straightforward.
Most of the network related classes are packaged in java.net. There are about 20
classes available in this package. These classes provide a rich set of functionality to
support the network communication in Java applications. However, the java.net
package is not suitable for the wireless applications that run on cell phones and
two-way pagers. The size of java.net is around 200KB. It is too big to fit in with the
wireless devices that only have a few hundred kilobytes of total memory and
storage budget. Besides the size issue, there is another challenge when dealing
with wireless devices: J2ME needs to support a variety of mobile devices that come
with different sizes and shapes, different networking capabilities, and different file
I/O requirements.
The networking in J2ME has to be very flexible to support a variety of devices and
has to be very device specific at the same time. To meet these challenges, the
Generic Connection framework is first introduced in the CLDC. The idea of the
Generic Connection framework is to define the abstractions of the networking and
file I/O as general as possible to support a broad range of handheld devices, and
leave the actual implementations of these abstractions to individual device
manufacturers. These abstractions are defined as Java interfaces. The device
manufacturers choose which one to implement in their MIDP based on the actual
device capabilities.
There is 1 class (Connector) and 7 connection interfaces (Connection,
ContentConnection, DatagramConnection, InputConnection, OutputConnection,
StreamConnection, and StreamConnectionNotifier) defined in the Generic

Connection framework. They can be found in the javax.microedition.io package that
comes with J2ME CLDC. Please note that there is no implementation of the
connection interfaces at the CLDC level. The actual implementation is left to MIDP.
The 7 connection interfaces define the abstractions of 6 basic types of
communications: basic serial input, basic serial output, datagrams communications,
sockets communications, notification mechanism in a client-server communication,
and basic HTTP communication with a Web server.
The relationships between these interfaces are illustrated in the following diagram:
As shown in the figure, Connection is the base interface and the root of the
connection interface hierarchy. All the other connection interfaces derive from
Connection. StreamConnection derives from InputConnection and
OutputConnection. It defines both the input and output capabilities for a stream
connection. ContentConnection derives from StreamConnection. It adds three
additional methods from MIME handling on top of the I/O methods in
StreamConnection.
The Connector class is the core of Generic Connection framework. The reason we
say that is because all connection objects we mentioned above are created by the
static method open defined in Connector class. Different types of communication
can be created by the same method with different parameters. The connection
could be file I/O, serial port communication, datagram connection, or an http
connection depending on the string parameter passed to the method. Such design
makes J2ME implementation very extensible and flexible in supporting new devices
and products.
Here is how the method is being used:

Connector.open(String connect);
The parameter connect_is a String variable. It has a URL-like format:
{protocol}:[{target}][{params}] and it consists of three parts: protocol, target,
and params.
protocol dictates what type of connection will be created by the method. There are
several possible values for protocol: file, socket, comm, datagram and http. file
indicates that the connection will be used for file I/O, comm indicates that the
connection will be used for serial port communication, and http indicates that the
connection is created for accessing web servers.
target can be a host name, a network port number, a file name, or a
communication port number.
params is optional, it specifies the additional information needed to complete the
connect string.
The following examples illustrate how to use the open method to create different
types of communication based on different protocols:
HTTP communication:
Connection hc = Connector.open("http://www.wirelessdevnet.com");
Stream-based Socket communication:
Connection sc = Connector.open("socket://localhost:9000");
Datagram-based socket communication:
Connection dc = Connector.open("datagram://:9000");
Serial port communication:
Connection cc = Connector.open("comm:0;baudrate=9000");
File I/O
Connection fc = Connector.open("file://foo.dat");
As we mentioned earlier, the support for these protocols varies from vendor to
vendor. Developers have to check the documentation from each MIDP device
manufacturers to see if they support the specific protocol. I tested the socket,
datagrams, and http connections with MotoSDK (a development kit from Motorola).

All three types of connections are supported by the Motorola’s MIDP. Sun’s MIDP
reference implementation supports only the http connections. We will talk more
about this later. If your program is trying to create a connection based on a
protocol that is not supported by your device manufacturer, a
ConnectionNotFoundException exception will be thrown.
The HttpConnection Class
The HttpConnection class is defined in J2ME MIDP to allow developer to handle http
connections in their wireless applications. The HttpConnection class is guaranteed
available on all MIDP devices.
Theoretically you may use either sockets, or datagrams for remote communication
in your J2ME applications if your MIDP device manufacturer supports them. But this
creates a portability issue for your applications, because you can’t always count on
that other device manufacturers will support them as well. This means that your
program may run on one MIDP device but fail on others. So you should consider
using the HttpConnection class in your application first, because HTTPConnection is
mandatory for all MIDP implementations.
In this section, we will take a hard look at the HttpConnection class. A complete
sample program using HttpConnection is shown in the next section.
The HttpConnection interface derives from the ContentConnection interface in
CLDC. It inherits all I/O stream methods from StreamConnection, all the MIME
handling methods from ContentConnection and adds several additional methods for
handling http protocol specific needs.
Here is a laundry list of all the methods available in HttpConnection:
I/O related:
DataInputStream openDataInputStream()
InputStream openInputStream()
DataOutputStream openDataOutputStream()
OutputStream openOutputStream()
An input stream can be used to read contents from a Web server while output
stream can be used to send request to a Web server. These streams can be
obtained from the HttpConnection object after the connection has been established
with the Web Server. In the following example, an http connection is established
with Web server www.wirelessdevnet.com at port 80 (the default http port). Then
an input stream is obtained for reading response from the Web:
HttpConnection hc = (HttpConnection);
Connector.open("http://www.wirelessdevnet.com");
InputStream is = new hc.openInputStream();
int ch;
// Check the Content-Length first
long len = hc.getLength();

if(len!=-1) {
for(int i = 0;i<len;i++)
if((ch = is.read())!= -1)
System.out.print((char) ch));
} else {
// if the content-length is not available
while ((ch = is.read()) != -1)
System.out.print((char) ch));
}
is.close();
hc.close();
MIME related:
String getEncoding()
long getLength()
String getType()
Once the http connection is established, these three methods can be used to obtain
the value of the following three fields in the HTTP header: Content-Length,
Content-Encoding and Content-Type. For more information on HTTP protocol,
please visit http://www.w3.org/Protocols/rfc2068/rfc2068.
Http Protocol related:
long getDate()
long getExpiration()
String getFile()
String getHeaderField(int index)
String getHeaderField(String name)
long getHeaderFieldDate(String name, long def)
int getHeaderFieldInt(String name, int def)
String getHeaderFieldKey(int n)
String getHost()
long getLastModified()
int getPort()
String getProtocol()
String getQuery()

String getRef()
String getRequestMethod()
String getRequestProperty(String key)
int getResponseCode()
String getResponseMessage()
String getURL()
void setRequestMethod(String method)
void setRequestProperty(String key, String value)
GetResponseCode and getResponseMessage can be used to check the response
status code and message from the Web server. In this typical response message
from a Web server: "HTTP/1.1 401 Unauthorized", you will get a response code
"401" and a response message "Unauthorized".
The following methods are used for obtaining additional header information:
getDate, getExpiration, getFile, getHeaderField, getHeaderField,
getHeaderFieldDate, getHeaderFieldInt, getHeaderFieldKey and getLastModified
The following methods are used for retrieving the individual components parsed out
of the URL string: GetHost, getPort, getProtocol, getQuery, getRef,
getRequestMethod, getRequestProperty, getResponseCode, getResponseMessage,
and getURL.
These three methods are used for dealing with the HEAD, GET, and POST requests:
setRequestMethod, setRequestProperty, and getRequestMethod.
A Complete Example
The following example is a MIDlet program that communicates with a Web server to
perform a string reversing operation.
Here is the program flow:
1. User types in a string on the phone (see Figure 2)

Figure 2: Entering The String
2. The string is then sent to a Java servlet on a Web server using the HTTP
POST method. The servlet is located at
http://www.webyu.com/servlets/webyu/wirelessdevnetsample1.
3. After the Java servlet receives the string, it reverses the string and sends it
back to the MIDlet application for display (see Figure 3).

Figure 3: Displaying the reversed string
This is a trivial string operation that can be dealt with locally on the phone.
However, the purpose of this program is to demonstrate how to use HttpConnection
in a MIDlet program.
Code Listing: Sample1.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;
import javax.microedition.io.*;
import java.io.*;
public class Sample1 extends MIDlet
implements CommandListener {
/*
* the default value for the URL string is
* http://www.webyu.com/servlets/webyu/wirelessdevnetsample1
*/
private static String defaultURL =
"http://www.webyu.com/servlets/webyu/wirelessdevnetsample1";

// GUI component for user to enter String
private Display myDisplay = null;
private Form mainScreen;
private TextField requestField;
// GUI component for displaying header information
private Form resultScreen;
private StringItem resultField;
// the "SEND" button used on the mainScreen
Command sendCommand = new Command("SEND", Command.OK, 1);
// the "BACK" button used on the resultScreen
Command backCommand = new Command("BACK", Command.OK, 1);
public Sample1(){
// initializing the GUI components for entering Web URL
myDisplay = Display.getDisplay(this);
mainScreen = new Form("Type in a string:");
requestField =
new TextField(null, "GREAT ARTICLE", 100,
TextField.ANY);
mainScreen.append(requestField);
mainScreen.addCommand(sendCommand);
mainScreen.setCommandListener(this);
}
public void startApp() {
myDisplay.setCurrent(mainScreen);
}
public void pauseApp() {
}
public void destroyApp(boolean unconditional) {
}
public void commandAction(Command c, Displayable s) {
if (c == sendCommand) {
// retrieving the String that user entered
String requeststring = requestField.getString();
// sending a POST request to Web server
String resultstring = sendPostRequest(requeststring);
// displaying the response back from Web server
resultScreen = new Form("Result String:");
resultField = new StringItem(null, resultstring);
resultScreen.append(resultField);
resultScreen.addCommand(backCommand);
resultScreen.setCommandListener(this);
myDisplay.setCurrent(resultScreen);
} else if (c == backCommand) {
// do it all over again
requestField.setString("SOMETHING GOOD");
myDisplay.setCurrent(mainScreen);
}
}
// send a POST request to Web server
public String sendPostRequest(String requeststring) {

HttpConnection hc = null;
DataInputStream dis = null;
DataOutputStream dos = null;
StringBuffer messagebuffer = new StringBuffer();
try {
// Open up a http connection with the Web server
// for both send and receive operations
hc = (HttpConnection)
Connector.open(defaultURL, Connector.READ_WRITE);
// Set the request method to POST
hc.setRequestMethod(HttpConnection.POST);
// Send the string entered by user byte by byte
dos = hc.openDataOutputStream();
byte[] request_body = requeststring.getBytes();
for (int i = 0; i < request_body.length; i++) {
dos.writeByte(request_body[i]);
}
dos.flush();
dos.close();
// Retrieve the response back from the servlet
dis = new DataInputStream(hc.openInputStream());
int ch;
// Check the Content-Length first
long len = hc.getLength();
if(len!=-1) {
for(int i = 0;i

Manual redes - network programming with j2 me wireless devices

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