Transcript Thursday February 6

MC365
Application Servers:
Servlets
Today We Will Cover:
• What a servlet is
• The HTTPServlet and some of its more important
methods
• How to configure the application server to use
your servlets
• Some example servlets
• Using properties files
• Some common errors and how to debug servlets
What is a Servlet?
• Sun describes a servlet in this way:
– “A servlet is a Java programming language class used
to extend the capabilities of servers that host
applications access via a request-response programming
model. Although servlets can respond to any type of
request, they are commonly used to extend the
applications hosted by Web servers. For such
applications, Java Servlet technology defines HTTPspecific servlet classes. “
• The javax.servlet and javax.servlet.http
packages provide interfaces and classes for writing
servlets. All servlets must implement the Servlet
interface, which defines life-cycle methods.
HttpServlet Class
• When implementing a generic service, you can use
or extend the GenericServlet class provided with
the Java Servlet API.
• The HttpServlet class, though, provides methods,
such as doGet and doPost, for handling HTTPspecific services.
• HttpServlets are best used to generate responses to
HTTP requests.
– This is the basis for web-based applications.
The LifeCycle of a Servlet?
• The life cycle of a servlet is controlled by the container in which the
servlet has been deployed. When a request is mapped to a servlet, the
container performs the following steps.
1. If an instance of the servlet does not exist, the Web container
a. Loads the servlet class.
b. Creates an instance of the servlet class.
c. Initializes the servlet instance by calling the init method.
2. Invokes a service method, passing a request and response object.
• If the container needs to remove the servlet, it finalizes the servlet by
calling the servlet's destroy method.
Important HTTPServlet Methods
• init
– After the Web container loads and instantiates the
servlet class and before it delivers requests from clients,
the Web container initializes the servlet.
– You can customize this process to allow the servlet to
read persistent configuration data, initialize resources,
and perform any other one-time activities by overriding
the init method of the Servlet interface.
– A servlet that cannot complete its initialization process
should throw UnavailableException.
Important HTTPServlet Methods
• doGet and doPost
– Called when browser makes an HTTP get request
– Most calls are gets
• doPost
– Called when browser makes an HTTP post request
• Usually you can handle both types of calls
similarly
Important HTTPServlet Methods
• Finalize
– When a servlet container determines that a servlet should be
removed from service (for example, when a container wants to
reclaim memory resources, or when it is being shut down), it calls
the destroy method of the Servlet interface.
• In this method, you release any resources the servlet is using and save
any persistent state.
– All of a servlet's service methods should be complete when a
servlet is removed. The server tries to ensure this completion by
calling the destroy method only after all service requests have
returned or after a server-specific grace period, whichever comes
first.
An Example of a Servlet
• To see the source code for a very simple
servlet, go to:
http://www2.bc.edu/~bernier/MC365/Lecture Notes/SimpleServlet.java
Configuring Tomcat to Use New
Servlets
• Set context path
– The context path is the first path name that comes after
the url of your server
•
•
•
•
In the url http://localhost:8080/test
http://localhost:8080 is the server name and port
/test is the context path
Set the context path for new applications in the C:\Program
Files\Apache Group\Tomcat 4.1\conf\server.xml.
– Search for “context path” to find out where to make this change
– You can see a sample server.xml file here.
• You must restart the app server when you update the
server.xml config file.
Configuring Tomcat to Use New
Servlets
• Set servlet mappings
– Servlet mappings refers to the mapping of servlets to
url patterns.
– In web.xml
• Invoker
– To use a servlet name you must have invoker in servlet mapping
in web.xml
– test with http://localhost:8080/test/servlet/SimpleServlet
• Mappings
– If you want to use a url other than the servlet name, use the
servlet mapping tags.
• See a sample web.xml configuration file here.
Configuring Tomcat to Use New
Servlets
• Autoloading of servlets:
– When you update servlets or web.xml you do
not need to restart Tomcat
– This is a nice feature of Tomcat.
Debugging Servlets
• Some common errors:
– HTTP Status 404:
• This means that the page/servlet is not found.
• Either the servlet class is not in the classes folder or you did
not configure the app server correctly.
– HTTP Status 500:
• This is the error code you get when a servlet abends (ends
abnormally).
• To correct this, fix the servlet.
– Use logging to debug servlet issues.
• If you do not define your own logger, app server messages go
to the stdout.log and stderr.log files in the logs directory.