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This chapter is from the book

RCX Bean

Now we need to create a JSP to interact with the bean. Don't worry if you are not familiar with JSP technology. In a nutshell, a JSP contains code that is executed on the server machine. The JSP code determines what the HTML code that is eventually sent to the client's browser looks like. JSP code can access Java classes on the server, just like regular Java, as long as it can access them somewhere in the CLASSPATH. In our example, we will be using the RCXBean class for communications with the RCX.

The bean uses no constructor, so all properties are set using methods. This includes the COM port the bean will use. If this method is not called before the bean is used, then the bean throws an exception from the JSP code (and the users see an exception on their Web page).

  • public String getComPort()
    Returns a String representing the name of the COM port used by the bean.

  • public void setComPort(String value) throws IOException
    Change or set which COM port should be used for communication with the RCX.



    A String representing the COM port (e.g., COM1, /dev/ttyS0, or USB.

  • public void sendInt(int v) throws IOException
    Sends an int value as four bytes.



    The data.

  • public void send(byte b) throws IOException
    Sends a single byte value.



    The data.

  • public void send(byte[] b) throws IOException
    Sends an array of byte values.



    The data in the form of an array of bytes.

  • public byte receive() throws IOException
    Receives a single byte of data, will wait until one appears in the buffer.

  • public int receiveInt() throws IOException
    Receives an int value composed of four bytes.

  • public byte[] receive(int n) throws IOException
    Receive n bytes from the RCX.



    The number of bytes to receive, also determines the size of the array.

  • public synchronized void lock(Object o) throws IOException
    The lock() method creates a lock on the RCX bean. The RCX bean cannot be locked by another object before the free() method has been run. If lock() is called while it is already locked then the bean throws an IOException.



    The lock is bound to this object. It should therefore be unique for the thread.


    The lock() method does not prevent other threads from using the RCX bean!

  • public synchronized void free(Object o)
    Make the RCX bean available for other threads.



    The object that has the lock.

  • public void close()
    Close this RCX bean.


    For RCXBean to work with a JSP page, the CLASSPATH setting for leJOS must be accessible to the environment the Web server is running in. If you have set up CLASSPATH as a system variable, this is probably already working.

Because a Web page can be accessed simultaneously by many people on the Web, it is necessary to restrict access to the IR tower so only one person will access it at a time. The RCXBean uses two methods for restricting the use of the RCX bean: lock() and free(). The lock() method blocks other users from accessing the bean until free() is called. The rest of the JSP page is pretty standard, and is used for displaying an HTML page for the user to interact with (Figure 11–11). The following code can be entered in a simple text editor, or even in an IDE like JCreator. Keep in mind you should not attempt to compile it. Let's have a look at the code:

 1.  <%@ page contentType="text/html" language='java' import='pc.irtower.comm.*'%>
 2.  <jsp:useBean id='rcx' class='pc.irtower.comm.RCXBean' scope='application'>
 3.    <jsp:setProperty name='rcx' property='comPort' value='COM2'/>
 4.  </jsp:useBean>
 5.  <HTML>
 6.  <HEAD><TITLE>Russian Roulette</TITLE></HEAD>
 7.  <BODY>
 8.  Select the amount of time to spin the wheel.
 9.  Less than 800 means you are safe,
10.  greater than 800 means you have lost.
11.  <FORM>
12.    <SELECT name='SpinTime'>
13.      <OPTION value='1'>1 second
14.      <OPTION value='3'>3 seconds
15.      <OPTION value='5'>5 seconds
16.    </SELECT>
17.    <INPUT type='SUBMIT'>
18.  </FORM><BR>   
19.  <%   
20.    try {   
21.      String sensorIDString = request.getParameter("SpinTime");   
22.      if (sensorIDString != null) {
23.        int time = Integer.decode(sensorIDString).intValue();
24.        rcx.lock(request);   
25.        rcx.send((byte)time);
26.        int value = rcx.receiveInt();
27.        if(value < 800)   
28.        out.println("<b>"+value+"</b>"+"...click.");
29.        else   
30.          out.println("<b>"+value+"... BANG!</b>");
31.      }   
32.    } catch (NumberFormatException e) {
33.      out.println("The value of <code>time</code> is invalid.");   
34.    } finally {   
35.    rcx.free(request);   
36.    }   
37.  %>
38.  </body>
39.  </html>

Figure 11-11 The JSP page displayed in a browser.

Save this file with the name roulette.jsp. The file should be saved to the Resin doc directory, which is used to store the Web pages on the server. The very first line of code imports the packages used by the code—in this case pc.irtower.comm. The second and third lines define the JavaBean being used by the JSP, which is RCXBean. The bean is designated with a variable of rcx in the code. This is the variable name it is referred to by the JSP code. Because this is a bean, various parameters can be set, including the communications port (make sure this is set to the COM port your computer is using). The body of the HTML page contains a drop-down menu to select one of three options (Figure 11–11). Next comes the Java code, enclosed in the <% %> braces. As you can see, this is just regular Java code so it should be easy to understand what is happening in it. Now we just need one final component to run the project—a LEGO model.


The root HTML directory for Resin can be changed in the resin.conf file located in the conf directory. The default setting is <app-dir>doc</app-dir>.

Step 1 The motor must be mounted on its back. Attach a wire brick to the motor with the wire facing downward. Attach a 2 _ 4 brick to the bottom of the motor.

Step 2 Attach the light sensor to the 2 __4 brick and add a 1 __2 brick with a hole to shield the sensor.

Step 3 Secure the sensor in place by placing two 1 __2 plates and 1 __2 bricks on each side of the sensor, then cap it off with a gray 2 __4 plate.

Step 4 Attach the green 3 ´ 6 plate without corners to a green 2 ´ 8 plate as shown in the diagram. Attach the 2 ´ 8 plate to a 40-tooth gear by placing the plate studs into the holes on the gear. Attach the gear to the motor axle.

Step 5 Place a black friction pin in the center hole of a 1 _ 8 beam and attach it to another 1 _ 8 beam. Attach the double beam to the back of the motor.

Step 6 Attach two gray 2 __2 plates to the double beam and attach two 1 __2 bricks with holes to the plate as shown.

Step 7 Insert black friction pins into the bottom holes of the double beam. Attach two more black friction pins in the 1 ´ 2 brick holes. Now attach the two 1 ´ 12 beams by clipping them onto friction pins.

Step 8 Attach support legs to the base as shown.

We're all ready to test out the project now. First, upload the Roulette class to the RCX brick and press the Run button. The brick should be in front of the IR tower, with the IR port facing the tower. Next, make sure the Resin Server is running and go to the following Web address in your browser:


You should be presented with a Web page displaying an option to choose the delay time (Figure 11–11). Choose one and click Submit Query. At this point the green light should appear on the IR tower, and the arm will begin spinning. When the arm stops a sensor reading is sent back to the computer, and the Web page displays the sensor value as well as a win–lose message.


If the Web page seems to freeze when you submit the value and the green light does not go on, this is almost a sure sign the COM port is not properly set. Edit the third line in the roulette.jsp file so the value is equal to the correct port.


If the Web page returns an error message, it might indicate it was not able to access certain classes. Make sure your CLASSPATH environment variable is set to include jdk1.3.1\lib\comm.jar and lejos\lib\classes.jar.

Now that we know the Web page is working locally, why not try to get others to browse to it? First you'll need to connect to the Internet and find your IP address. Windows users can find this by going to a command prompt and typing


Linux users can use a similar program in the super-user directory:


These commands bring up data that includes the IP Address. Record this number and call a friend. Have your friend go to your IP address and make sure he or she types the prefix http:// in the address line; for example: 


If your computer is behind a firewall there may be problems contacting it. Also, some Internet connections use nonroutable or dynamic IPs that can cause connection problems.


Not all code in the JSP file Roulette.jsp is sent to the client's Web browser. Only the HTML code (including scripting languages like JavaScript) makes it to the client. The JSP-related code is executed by the server, and it produces an HTML document to return to the client. Any code enclosed in <% %> braces, or using the JSP tag <jsp: .. /jsp: ..> will never be seen by a client. To view this, try going to View Source in your browser.

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