You can install this servlet in Java Web Server by opening the Web server in the administration tool, clicking the "Servlets" button, and adding HeaderServlet as "Header".
On the "Advanced" tab in "Basic Setup" make sure you have turned servlet chaining on. Whenever a file other than another alias is requested by a browser, it will be pumped through this servlet first. Just as developers have started getting a handle on Servlets 1. Servlets 1. Remember that these functions will initially only be supported by JWS 1. The Servlets 1. Here's a sneak peak at the new features:. Session Tracking The largest addition is certainly the HttpSession class.
HttpSession adds lightweight session tracking to servlet applications, using cookies or rewritten URLs. You can use HttpSession's putValue and getValue methods to store user identification information, shopping cart data, even database connections.
You can even use them carefully! Standardized Cookies The original servlets specification only supported cookies with a Java Web Server specific class. The actual interface is essentially the same as the Sun specific cookies discussed in Part 2 of this tutorial. The Single Thread Model Servlets 1. This is one of the few times that your HttpServlet subclass should override the service method. The new method should call super.
To ensure a clean shutdown, your destroy method should not release any shared resources until all the service requests have completed. One part of doing this is to check the service counter. Another part is to notify the long-running methods that it is time to shut down.
For this notification, another field is required. The field should have the usual access methods:. Here is an example of the destroy method using these fields to provide a clean shutdown:. The final step in providing a clean shutdown is to make any long-running methods behave politely. Methods that might run for a long time should check the value of the field that notifies them of shutdowns and should interrupt their work, if necessary:. The example shows how to develop a simple application by using the WebServlet , WebFilter , and WebListener annotations to create a servlet, a listener, and a filter.
The mood example application is comprised of three components: mood. MoodServlet , mood. TimeOfDayFilter , and mood. The log entries appear in the server log. Chapter 10 Java Servlet Technology Shortly after the Web began to be used for delivering services, service providers recognized the need for dynamic content.
The following topics are addressed here: What Is a Servlet? Servlet Lifecycle The lifecycle of a servlet is controlled by the container in which the servlet has been deployed. If an instance of the servlet does not exist, the web container Loads the servlet class. Creates an instance of the servlet class.
Defining the Listener Class You define a listener class as an implementation of a listener interface. ServletRequestAttributeListener and ServletRequestAttributeEvent Use the WebListener annotation to define a listener to get events for various operations on the particular web application context.
Classes annotated with WebListener must implement one of the following interfaces: javax. ServletContextListener javax. ServletContextAttributeListener javax. ServletRequestListener javax.
ServletRequestAttributeListener javax. HttpSessionListener javax. HttpSessionAttributeListener For example, the following code snippet defines a listener that implements two of these interfaces: import javax. ServletContextAttributeListener; import javax. ServletContextListener; import javax. Handling Servlet Errors Any number of exceptions can occur when a servlet executes.
When an exception occurs, the web container generates a default page containing the following message: A Servlet Exception Has Occurred But you can also specify that the container should return a specific error page for a given exception.
Sharing Information Web components, like most objects, usually work with other objects to accomplish their tasks. Web components can do so by Using private helper objects for example, JavaBeans components. Sharing objects that are attributes of a public scope. Using a database. Using Scope Objects Collaborating web components share information by means of objects that are maintained as attributes of four scope objects. ServletContext Web components within a web context. Session javax.
HttpSession Web components handling a request that belongs to the session. Request Subtype of javax. ServletRequest Web components handling the request. Page javax. Controlling Concurrent Access to Shared Resources In a multithreaded server, shared resources can be accessed concurrently. Concurrent access can arise in several situations: Multiple web components accessing objects stored in the web context. Multiple web components accessing objects stored in a session.
Creating and Initializing a Servlet Use the WebServlet annotation to define a servlet component in a web application. WebServlet; import javax. Writing Service Methods The service provided by a servlet is implemented in the service method of a GenericServlet , in the do Method methods where Method can take the value Get , Delete , Options , Post , Put , or Trace of an HttpServlet object, or in any other protocol-specific methods defined by a class that implements the Servlet interface.
For HTTP servlets, the correct procedure for populating the response is to do the following: Retrieve an output stream from the response. Fill in the response headers. Write any body content to the output stream. Getting Information from Requests A request contains data passed between a client and the servlet.
This interface defines methods for accessing the following information: Parameters, which are typically used to convey information between clients and servlets Object-valued attributes, which are typically used to pass information between the servlet container and a servlet or between collaborating servlets Information about the protocol used to communicate the request and about the client and server involved in the request Information relevant to localization You can also retrieve an input stream from the request and manually parse the data.
A query string can explicitly appear in a web page. Constructing Responses A response contains data passed between a server and the client. This interface defines methods that allow you to Retrieve an output stream to use to send data to the client. Set localization information, such as locale and character encoding.
HttpServletResponse , have fields representing HTTP headers, such as the following: Status codes, which are used to indicate the reason a request is not satisfied or that a request has been redirected. Filtering Requests and Responses A filter is an object that can transform the header and content or both of a request or response. The main tasks that a filter can perform are as follows: Query the request and act accordingly.
Block the request-and-response pair from passing any further. Interact with external resources. The following code snippet defines a filter, specifying an initialization parameter: import javax.
Filter; import javax. WebFilter; import javax. This method can perform the following actions: Examine the request headers. Customize the request object if the filter wishes to modify request headers or data. Customize the response object if the filter wishes to modify response headers or data.
Examine response headers after invoking the next filter in the chain. Throw an exception to indicate an error in processing. Programming Customized Requests and Responses There are many ways for a filter to modify a request or a response.
Specifying Filter Mappings A web container uses filter mappings to decide how to apply filters to web resources. Figure 10—1 Filter-to-Servlet Mapping Recall that a filter chain is one of the objects passed to the doFilter method of a filter. Double-click web. Click Filters at the top of the editor pane. Expand the Servlet Filters node in the editor pane.
Click Browse to locate the servlet class to which the filter applies. You can include wildcard characters so that you can apply the filter to more than one servlet. Click OK. To constrain how the filter is applied to requests, follow these steps. Expand the Filter Mappings node. Select the filter from the list of filters. Click Add. Invoking Other Web Resources Web components can invoke other web resources both indirectly and directly. Including Other Resources in the Response It is often useful to include another web resource, such as banner content or copyright information in the response returned from a web component.
To include another resource, invoke the include method of a RequestDispatcher object: include request, response ; If the resource is static, the include method enables programmatic server-side includes. It can write to the body of the response and commit a response. Transferring Control to Another Web Component In some applications, you might want to have one web component do preliminary processing of a request and have another component generate the response. Accessing the Web Context The context in which web components execute is an object that implements the ServletContext interface.
Maintaining Client State Many applications require that a series of requests from a client be associated with one another. Accessing a Session Sessions are represented by an HttpSession object. Associating Objects with a Session You can associate object-valued attributes with a session by name. You can also notify objects of certain events related to their association with a session such as the following: When the object is added to or removed from a session.
Session Management Because an HTTP client has no way to signal that it no longer needs a session, each session has an associated timeout so that its resources can be reclaimed. Click General at the top of the editor. In the Session Timeout field, type an integer value. Session Tracking To associate a session with a user, a web container can use several methods, all of which involve passing an identifier between the client and the server.
Finalizing a Servlet A servlet container may determine 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.
The remainder of this section explains how to do the following: Keep track of how many threads are currently running the service method. Tracking Service Requests To track service requests, include in your servlet class a field that counts the number of service methods that are running. A chain can be explicitly specified for certain requests, or it can be created on the fly when one servlet returns a content type that another servlet is registered to handle.
We'll use both techniques to run Deblink. The file servlet is a core Java Web Server servlet used to retrieve files. Normally it is the only servlet invoked to return an HTML file.
Here you will see which servlets are invoked for different kinds of URLs, as shown in Figure These mappings provide some insight into how the Java Web Server uses its core servlets.
You can change the default aliases or add new aliases. Right now, we're interested in adding another alias. After making this change, any file ending in. Try it yourself. Create a blinky. If everything's set up right, all evidence of the blink tags is removed. This technique has one large loophole: not all HTML comes from files with the. For example, HTML can come from a file with the. We can work around multiple file extensions with more aliases.
This, however, still doesn't catch dynamic content. We need our second technique for creating a servlet chain to plug that hole. The JavaServer Administration Tool does not yet include a graphical way to do this. Instead, we can make the change with a simple edit of a properties file. It contains directives like this:. Why is this necessary? Without it, the ssinclude servlet would handle only static files with the. In other words, every HTML page is deblinked forever or until the client stops the request, whichever comes first.
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