Transcript J2EE Servlets Internet i jego zastosowania 1

Internet i jego zastosowania
J2EE Servlets
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Internet i jego zastosowania
Agenda
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Overview
Servlet Interface
Servlet Context
Request
Response
Sample Servlet
Sessions
Dispatching Request
Web Applications
Deployment Descriptor
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Internet i jego zastosowania
What is a Sevlet?
• Web component managed by servlet container
• Generates dynamic content, usually HTML
• Small, platform-independent classes compiled to
bytecode
• Loaded dynamically and run by a container in
conjunction with a web server
• Interacts with a web client via request - response
model based on the the HTTP protocol
• Can access databases and other storage systems
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What is a Servlet Container?
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Execution context of the servlet
Provides network services over which requests and
responses are exchanged
Contains and manages servlets through their lifecycle
Can be either built-in into a web server or web enabled
application server or installed as an add-on component
Must support the HTTP Protocol (at minimum HTTP/1.0)
May place security restrictions on the environment that a
servlet executes in (e.g. Number of simultenously running
threads)
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Benefits of Servlets
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Performance
Scalability
Distributed environment (distributable web applications)
Multithreaded environment
Portability
Platform independence
Ease of development
Part of J2EE (Servlet API is a required API of J2EE)
Standard supported by many vendors and web servers
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Typical Scenario
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Request is submitted from a web browser
Request is converted to a name-value list by the servlet
container
The service() method of a servlet is called with the namevalue list together with the response as arguments
The servlet processes the request by invoking EJBs,
performing other business logic, connecting to databases
or application server extensions
The servlet prepares HTML code to return either directly
printing to the response stream
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The Servlet Interface
void init(ServletConfig config);
void service(ServletRequest req, ServletResponse res);
void destroy();
ServletConfig getServletConfig();
java.lang.String getServletInfo();
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Central abstraction of the Servlet API
Interface implemented either directly or by
extending
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Servlet Life Cycle
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Loading and instantiating
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At start time
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Delayed until the service method of the servlet is requested
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The servlet container load the servlet class (file system, remote file system,
other network services)
The Servlet Container instantiates an object instance
There can be more than one instance (SingleThreadModel) of a given servlet
(several servlet definitions which point to the same class) or only one instance
(MultipleThreadModel)
Initialization
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Point where servlet can read any configuration data, persistent data, make a
connection to a database and other costly resources, perform one-time activities
The Servlet Container calls init method of the servlet and passes ServletConfig access to name-value initialization parameters
On error throw ServletException or UnavailableException
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Servlet Life Cycle
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End of service
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A servlet instance may be kept active in a container for any amount
of time
The container allow any threads executing service method to
complete
The container executes destroy method - point where a servlet may
release any resources and save its state to any persistance storage
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No requests can be routed to this instance of the servlet
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The instance is released so that it can be garbage collected
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Servlet Life Cycle
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Request handling
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Each request represented by ServletRequest object
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Servlet can create the response by using object of type
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ServletRequest and ServletResponse are provided as parameters to
ServletResponse
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HTTP - HttpServletRequest, HttpServletResponse
Errors during request handling
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ServletException
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UnavailableException
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Permanent - servlet should be removed from the service (destroy is
called)
Temporary - SERVICE_UNAVAILABLE (503) is returned with RetryAfter header
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Multithreading Requests
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MultipleThreadModel (default)
One instance of a servlet class per servlet definition
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SingleThreadModel (marker interface)
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Multiple instances of the servlet. Ensures that servlets handle only one
request at a time.
No two threads will execute concurrently in the servlet's service
method. The servlet container can make this guarantee by
synchronizing access to a single instance of the servlet, or by
maintaining a pool of servlet instances and dispatching each new
request to a free servlet.
The servlet is thread safe. SingleThreadedModel does not prevent
synchronization problems that result from servlets accessing shared
resources (static class variables, classes outside the scope of the
servlet).
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The Servlet Context
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A servlet's view of the web application within which it is running
One instance per web application (per VM for distributed
applications)
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The container implements the ServletContext interface
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ServletContext
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allows a servlet to set and store attributes, log events, obtain URL
references to resources
provides a set of methods that a servlet uses to communicate with
its servlet container
Context attributes - available to any other servlet of the web
application (exits only locally in the VM)
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Resources - direct access to static documents (HTML, GIF,
JPEG)
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The Request
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Encapsulates information from the client
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HttpServletRequest - HTTP headers and message body
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Parameters
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getParameter, getParameterNames, getParameterValues
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String parameters stored as name-value pairs
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Possible multiple parameter values
Attributes
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getAttribute, getAttributeNames, getAttributeValues
Objects associated with a request set by the container or a
servlet to communicate with another servlet
Single values
Cookies
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getCookies
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Path Elements & Translation
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Request path is composed of
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Context Path
getContextPath
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Servlet Path
getServletPath
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Following is always true
RequestURI = contextPath + servletPath
Pattern:
/garden
Servlet:
GardenServlet
/catalog/garden/implements ->
ContextPath:
/catalog
ServletPath:
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The Response
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Encapsulates information to be returned to the client
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HttpServletResponse - HTTP headers and message
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Cookies
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Helper methods
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body
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sendRedirect
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sendError
Typical use
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Either getOutputStream (binary data) or getWriter (text)
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Sample Servlet
public class SampleServlet extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException
{
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<html><body>");
out.println("Hello World!");
out.println("</body></html>");
}
}
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Sessions
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HTTP - stateless protocol
Different requests from the same client should be associated
with each other
Strategies for session tracking
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Hidden variables
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URL rewriting
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Cookies
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Session tracking implemented by servlet container
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HttpSession provided as an convenient interface
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A session persists for a specified period of time
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HttpSession
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View and manipulate information about a session, such as the session
identifier, creation time, last accessed time
Bind objects to sessions, allowing user information to persist across
multiple user connections
Session information is scoped only to the current web application
(ServletContext)
Binding attributes into a session
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getAttribute, setAttribute, removeAttribute
HttpSessionBindingListener - causes an object to be notified
when it is bound to or unbound from a session
Session timeouts because there is no explicit client termination
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Session Semantics
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Multiple servlets may have access to a single session
object
Synchronization is the responsibility of a developer
For applications marked as distributable session objects
must implement java.io.Serializable interface
Scalability - in distributed environment session objects can
be moved from any active node to some other node of the
system
All windows of a client are participating in the same
session
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Dispatching Requests
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forward - pass processing to another servlet
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Including output of another servlet in the response
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RequestDispatcher; can be obtained from ServletContext
include
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Can be called at any time
Included servlet cannot set headers or call any method that affects
the headers of the response
forward
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Can only be called if no output has been commited to a client
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Web Applications
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Collection of servlets, JSPs, HTML pages and other resources bundled
and run on possibly multiple containers
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Rooted at a specific path eg. http://www.company.com/catalog
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By default one instance must only be run on one VM
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Distributable web applications - components distributed across multiple
containers
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1:1 mapping between web application and ServletContext
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Representation
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Structured hierarchy of directories
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Archive file
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Open Directory Structure
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Root directory eg. /catalog
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Special WEB-INF directory - not a part of public document tree
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Sample directory structure:
/index.html
/howto.jsp
/feedback.jsp
/images/logo.gif
/WEB-INF/web.xml
/WEB-INF/lib/jspbeans.jar
/WEB-INF/classes/com/dsrg/servlets/Test.class
/WEB-INF/classes/com/dsrg/util/Helper.class
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Deployment Descriptor
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Elements and configuration information of a web application
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Contents of a deployment descriptor
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ServletContext init parameters
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Session configuration
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Servlet/JSP definitions
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Servlet/JSP mappings
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Mime Type mappings
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Welcome file list
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Error pages
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Security info
XML document
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