Chapter 20 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 EAServer 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 EAServer 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.
| Control Name | Specifies | Comment |
|---|---|---|
| Description | An optional comment describing the servlet. | All servlets |
| Servlet's fully qualified class name | The name of the Java class that implements the
servlet functionality, in Java dot notation. For example, com.sybase.jaguar.DemoServlet
In a Web application, you can map a servlet name to either a servlet class or a JSP file. |
All servlets |
| Load during startup | Choose Yes if the servlet must be loaded
and initialized when the server starts. If you choose No, the class
is loaded when the first client requests to run the servlet. Classes
that perform lengthy processing in the init method can
be loaded at start-up so that the first client to invoke the servlet
does not experience increased response time.
If you choose Yes, servlets are reloaded when you refresh the Web application. |
All servlets |
| Startup load sequence position | EAServer loads servlets serially. If you choose Yes, to load the servlet during startup, define the order, relative to other servlets in the application. To load the servlet first, enter 1. | All servlets |
| Web component type | Choose Servlet or JSP | Web application servlets only |
Servlets may require initialization parameters that are specified outside of the source code. For example, you might specify the name of an EAServer 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.getInitParameter(name);
}
By default, EAServer 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 EAServer 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 EAServer creates to serve client requests. Calls to the service method within a given instance are serialized. EAServer creates multiple instances to minimize the time that clients have to wait for a blocked service request. EAServer 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 EAServer, 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 EAServer. 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:
On the Java Classes tab, specify a list of additional Java classes that must be reloaded when you refresh the servlet. By default, EAServer reloads only the servlet implementation class. You can configure the classes to be custom loaded at the servlet, Web application, J2EE application, or server level. For more information, see Chapter 28, "Configuring Custom Java Class Lists"
The Additional Files tab enables you to associate additional files 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 Chapter 6, "Clusters and Synchronization" in the EAServer System Administration Guide for more information on this feature.
You can specify the name of a Java class or package to be added to the archive, using the Java dot notation. For example, "com.sybase.CORBA" adds all files in the com.sybase.CORBA package.
Any other files must be separated by commas and specified relative to EAServer's Repository subdirectory or with a full path. Full paths require that any server to which you synchronize share the same directory structure.
When you include additional files, you can either enter the file names individually, or you can use the Additional Files wizard to add multiple files, packages, classes, and directories.
To enter file names individually:
To add multiple items:
Configures an alternate identity used for authentication of component invocations from the servlet or JSP. By default, component invocations use the Web client's identity. The settings are:
specified
to
configure an alternate identity. The default, client
,
specifies that the Web client identity is used.
To enable use of the run-as identity for EJB component calls
to remote servers, you must specify corbaname
URLs
in the EJB Reference properties for the Web application where the
servlet is installed. For more information, see "Interoperable naming URLs" and "EJB references".
The All Properties tab allows you to edit property settings
as they are stored in the EAServer 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. Repository
property names are documented in the chapter, "Repository
Properties Reference," in the EAServer System
Administration Guide.
You can set properties as follows:
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 EAServer 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 EAServer's 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 server. If your implementation relies on other classes that must also be reloaded when the implementation is refreshed, specify them on the Java Classes tab in the Servlet Properties window. See "Java Classes 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, EAServer calls the servlet's Servlet.destroy() method, reloads the implementation class and any classes specified on the Java Classes tab, 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 EAServer 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, EAServer calls the Servlet.destroy() method. When you start the servlet, EAServer calls the Servlet.init() method, unless it has already been called on the current instance of the implementation class.
To start a servlet:
To stop a servlet:
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