Chapter 23 Creating Java Servlets
After you have created or obtained the Java class that implements your servlet's functionality, you must define a new servlet in Jaguar Manager, associate it with your class, then configure the properties that control how the servlet's class is loaded and executed.
In Jaguar Manager, servlets that are installed in a Jaguar server display in the Installed Servlets folder under the server's icon. All servlets that have been defined are displayed in the top-level Servlets folder. You must install a servlet in a server before that server's clients can execute the servlet.
When defining a new servlet, you can install it in a server at the same time, or you can define the servlet in the top-level Servlets folder, then install it in one or more servers later.
You must install servlets in a server before that server's clients can invoke the servlet. You can install a servlet into multiple servers. To install a servlet into a server:
Uninstalling a servlet from a server makes that servlet unavailable to clients of that server. The server definition persists in Jaguar Manager, under the top level Servlets folder. To uninstall a servlet:
Deleting a servlet from the top-level Servlets folder removes it entirely from Jaguar Manager. To delete a servlet definition:
The settings in the Servlet Properties dialog box specify the Java class for the servlet and control how the Jaguar server loads and runs instances of the class. The dialog contains the tabs described below.
Properties on the general tab define the basic information required to load and run the servlet.
Servlets may require initialization parameters that are specified outside of the source code. For example, you might specify the name of a Jaguar connection cache as an initialization parameter. You can use the Init-args properties to define optional initialization parameters for the server.
The Init-Params tab lists the initialization parameters that have been defined for the servlet. Click Add to define a new initialization parameter. Enter the parameter name and the text of the value. The servlet can retrieve the value as a Java String, as explained below. To change a parameter's value, highlight the parameter in the list, then click Modify. To remove a parameter, highlight it, then click Delete.
Your servlet's init method can retrieve the specified settings using the ServletConfig.getInitParameter(String) and ServletConfig.getInitParameterNames() methods. The following code fragment shows how:
void init(ServletConfig config) throws ServletException {
....
Enumeration paramNames = config.getInitParameterNames();
while (paramNames.hasMoreElements())
{
String name = (String) paramNames.nextElement();
String value = config.getParameter(name);
}
By default, Jaguar loads one instance of a servlet class and calls methods from multiple threads--to service multiple clients, multiple threads may call the service method simultaneously. If an instance of your servlet cannot safely execute in multiple threads, you must configure the Threading tab to specify that the servlet class is single threaded. You can also specify how Jaguar should serialize invocations of the service method for a single-threaded servlet.
Check the Single Threaded option if calls to your servlet's service method must be serialized. When this option is selected, you can specify the number of instances that Jaguar creates to serve client requests. Calls to the service method within a given instance are serialized. Jaguar creates multiple instances to minimize the time that clients have to wait for a blocked service request. Jaguar calls the service method in an instance that is not already busy serving a previous request. If all available instances are busy, the request is delayed until a service call returns.
If multiple instances are created, calls to the service method are not necessarily serialized; service calls may occur simultaneously in different instances. If your service method changes static variables, you must add code to synchronize these changes or configure the servlet properties so that only one instance is created.
Implementing the SingleThreadModel interface
Your servlet class can implement the SingleThreadModel interface
to indicate that calls to an instance's service method
must be serialized. Instances of these classes are always single-threaded
by Jaguar, regardless of whether the Single Threaded option is enabled.
The Java Servlet API provides classes to create a session between a given HTTP client and servlets running on a Jaguar server. You can use the session to record data related to the end-users session. If your servlet uses sessions, configure the following properties on the Sessions tab:
To support the Java Servlet Specification
version 2.2, the session timeout value for Web application
servlets is stored in minutes.
The All Properties tab allows you to edit package property
settings as they are stored in the Jaguar configuration repository.
You can only delete properties that you have added--you
cannot delete default properties, such as the com.sybase.jaguar.servlet.name
property.
You can set properties as follows:
You must set the following properties on the All Properties tab. There is no other means to configure these properties:
com.sybase.jaguar.servlet.java.classes
- Specifies
a list of additional Java classes that must be reloaded when you
refresh the servlet. By default, Jaguar reloads only the servlet
implementation class. These classes must be deployed under the Jaguar java/classes subdirectory.
com.sybase.jaguar.servlet.files
- Associates
any additional files that are with the servlet definition. If you
synchronize the server where the servlet is installed, and elect
to synchronize servlet files, these files will be transferred to
the target servers. See "Jaguar Clusters and Synchronization" in
the Jaguar CTS System Administration Guide for more
information on this feature.
Repository property names are listed in the documentation for the Jaguar::Repository IDL interface, in the description of the define method. To read this documentation, load your server's main HTML page into a Web browser, and follow the "Interface Repository" link.
When to use the All Properties tab
Though you can use the All Properties tab to set any property
prefixed with "com.sybase.jaguar.servlet," Sybase
recommends that you use this tab to set properties only as specified
by the Jaguar documentation or by Sybase Technical Support. Most
properties can be configured graphically elsewhere in the Jaguar
Manager user interface.
Though you can deploy servlet classes under any codebase that is specified in the CODEBASE environment variable, servlet classes should be deployed under the Jaguar java/classes subdirectory to simplify debugging. Only classes deployed under this codebase can be refreshed.
The refresh feature is useful for debugging, since it allows
you to load a changed version of the implementation class without
restarting the Jaguar server. If your implementation relies on other
classes that must also be reloaded when the implementation is refreshed,
specify them in the setting of the com.sybase.servlet.java.classes
property
on the All Properties tab in the Servlet Properties window. See "All Properties settings".
All servlets that are not installed in a Web application are considered to be part of the default Web application, and all servlets within the same Web application are refreshed at the same time.
To refresh all servlets in the Installed Servlets folder:
To refresh all the servlets in a Web application:
When you refresh a servlet, Jaguar calls the servlet's Servlet.destroy() method, reloads
the implementation class and any classes specified by the com.sybase.servlet.java.classes
property,
and then calls the Servlet.init() method in the
new instance.
At times you may wish to stop and restart the servlet without reloading the class. Also, starting a servlet causes Jaguar to load the implementation class if it has not already been loaded at startup or in response to a client request.
When you stop the servlet, Jaguar calls the Servlet.destroy() method. When you start the servlet, Jaguar calls the Servlet.init() method, unless it has already been called on the current instance of the implementation class.
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