Java程序辅导

A Counter servlet

The goal of this exercise is to create a servlet that maintains a counterof the number of times it has been invoked since it was last reloadedby the web server. It will use a class variable (i.e. static) tokeep count of the number of invocations. Each time the servlet isinvoked, it will print out the value of the counter.

There are 2 main ways to do this lab.

• Method 1 is to create a new Servlet (as per the Hello World Servlet lab) and name this CounterServlet, with the URL mapping of /Counter. You then add the required code to perform the desired task. This is fairly straight forward.
  
• Method 2 is to copy the existing Servlet and to add it into the deployment descriptor. This is a touch more complicated, so we will do this in this lab to show you how to copy an existing servlet class and insert it into the web project.

Copying an existing Servlet 

Begin by copying your Hello World servlet code, and putting it intoa file called CounterServlet.java. You may keep it inthe same project  (WAR file) as before, or put it in a separate WAR file (if youwould like more practice at the process of creating WAR files).

Make the following modifications to your Hello World servlet sourcecode:

1. You can cut and paste the HelloWorldServlet from the src folder. Rename the class to CounterServlet

2. Declare a class variable to hold the value of your counter.

     private static int numRequests = 0;

3. Declare a synchronized method to update the value of thenumRequests variable.

     private synchronized void incNumRequests() {    numRequests++;  }

4. Modify the code inside your existing doGet() methodto make it call the incNumRequests method you havejust created, and print out the current value of the counter.

       incNumRequests();  out.println("Number of requests since reload: "     + numRequests);  

Servlets and threading

The question you should be asking yourself right now is: why do I need to create an extra method just to increment the valueof numRequests? Why can't I just have my servlet increment the value directly?

The answer lies in the way servlets handle threading.By default, every servlet you create is multi-threaded.That means that at any given instant in time, multiple instancesof your doGet() method could be executing simultaneously.

Also recall the meaning of a class variable (i.e. declared asstatic) in Java. A static variable is one that is shared between all instances of a given class. So if you have10 instances of your CounterServlet executing, all 10 instances will be sharing a single copy of the numRequests variable.

What happens if two different instances of your servlet runningat the same time both try and update the value of numRequestsat the same time? In theory, this could lead to a semantic error inthe program because two threads are trying to update the same sharedresource simultaneously. [Although strictly speaking in this casethere is not much danger because all we are doing is incrementinga single value.]

The general principle that you should follow is that if you ever writea Java servlet where multiple threads will be accessing a single,non-shareable resource, you must programmatically ensure that atmost one thread can be accessing the resource at any one time.Java provides this facility through the use of itssynchronized keyword.

If you have studied operating systems, note that this is similarin concept to monitors and semaphores used to ensure at most oneprocess is executing a critical section at any given time.

Running your servlet

Follow the same steps you used when creating your Hello World servlet to edit and compile the code.

You now need to do some additional work..

Declare this servlet in your web.xml deployment descriptor. The following steps shows you how to do this via Eclipse/Workshop, but do note that it can be easier just to type the extra entries into the web.xml file directly! (see for the raw XML code to add)

1. Step 1: Right click on the Deployment Descriptor item. This should show you either the source or the design views. The main advantage of using the IDE is the guarantee of a correctly formatted xml file so let's select the design (also called Deployment Descriptor) tab.

2. Step 2: Right mouse click on the Servlets entry and select New Servlet

3. Step 3:
   You now create the servlet. You enter the servlet name (Counter) and the name of the servlet class (CounterServlet).
   Note that you can also use a JSP instead of a servlet class if you wish but we will wait till a later lab to try this out.

4. Step 4:
   You now enter the servlet URL mapping. We will use /Counter to point to our servlet.
   
   You can optionally add more mappings by using the Add button on the Servlet Properties page.
   
   One neat trick is to map sub-contexts (which act like virtual subdirectories on the URL) and file names to a servlet directly.
   
   For example, you could choose a mapping called /*.counter or index.html to map to CounterServlet!

5. 
   If you inspected the generated XML code in web.xml, you should have entries looking somewhat like:

     <servlet>    <servlet-name>Counter</servlet-name>    <servlet-class>CounterServlet</servlet-class>  </servlet>  

   and later in the file ...

     <servlet-mapping>    <servlet-name>Counter</servlet-name>    <url-pattern>/Counter</url-pattern>  </servlet-mapping>

   Note that the ORDER of these XML entities are important in web.xml. Misplacing them may result in an error or your web application to fail to deploy.

6. Eclipse automatically re-deploys the updated lab so you can now unit-test the application. Note that each time you reload the browser with the refresh button, the displayed counter should update.
   

7. You should now redeploy your WAR file by using the Export .. as WAR file menu and by copying the war file to your WebLogic autodeploy directory.

     cp HelloWorldApp.war ~/weblogic/autodeploy

   Test your new servlet, in your web browser, by viewing a URL likethe following (the filename on the end of the URL will dependupon the name you chose for the <url-pattern>