Servlets
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This document provides information on Java servlets including: what servlets are and their advantages; the servlet architecture and lifecycle including initialization, processing requests via doGet and doPost methods, and destruction; how to deploy servlets in an application server; and how servlets handle form data submission. Servlets act as a middle layer between web requests and applications/databases, and allow dynamically generating web pages.
Servlets
- 2. List of Topics What are Servlet Servlet Architecture Life cycle Example FORM Servlet deployment Filter Packaging Debugging
- 3. What are Servlets ? Java Servlets are programs that run on a Web or Application server and act as a middle layer between a request coming from a Web browser or other HTTP client and databases or applications on the HTTP server. Using Servlets, you can collect input from users through web page forms, present records from a database or another source, and create web pages dynamically. Servlets advantages Performance is significantly better. Servlets execute within the address space of a Web server. It is not necessary to create a separate process to handle each client request. Servlets are platform-independent because they are written in Java. Java security manager on the server enforces a set of restrictions to protect the resources on a server machine. So servlets are trusted. The full functionality of the Java class libraries is available to a servlet. It can communicate with applets, databases, or other software via the sockets and RMI mechanisms that you have seen already.
- 5. Servlet Task Read the explicit data sent by the clients (browsers). This includes an HTML form on a Web page or it could also come from an applet or a custom HTTP client program. Read the implicit HTTP request data sent by the clients (browsers). This includes cookies, media types and compression schemes the browser understands, and so forth. Process the data and generate the results. This process may require talking to a database, executing an RMI or CORBA call, invoking a Web service, or computing the response directly. Send the explicit data (i.e., the document) to the clients (browsers). This document can be sent in a variety of formats, including text (HTML or XML), binary (GIF images), Excel, etc. Send the implicit HTTP response to the clients (browsers). This includes telling the browsers or other clients what type of document is being returned (e.g., HTML), setting cookies and caching parameters, and other such tasks.
- 6. Servlets Package Java Servlets are Java classes run by a web server that has an interpreter that supports the Java Servlet specification. Servlets can be created using the javax.servlet and javax.servlet.http packages, which are a standard part of the Java's enterprise edition, an expanded version of the Java class library that supports large-scale development projects. Java servlets have been created and compiled just like any other Java class. After you install the servlet packages and add them to your computer's Class path, you can compile servlets with the JDK's Java compiler or any other current compiler servlet-api.jar present in <Tomcat>/lib dir
- 7. Servlet – Life cycle A servlet life cycle can be defined as the entire process from its creation till the destruction. The following are the paths followed by a servlet The servlet is initialized by calling the init () method. The servlet calls service() method to process a client's request. The servlet is terminated by calling the destroy() method. Finally, servlet is garbage collected by the garbage collector of the JVM. The init() method : The init method is designed to be called only once. It is called when the servlet is first created, and not called again for each user request. So, it is used for one- time initializations, just as with the init method of applets. The servlet is normally created when a user first invokes a URL corresponding to the servlet, but you can also specify that the servlet be loaded when the server is first started.
- 10. Life cycle cont… When a user invokes a servlet, a single instance of each servlet gets created, with each user request resulting in a new thread that is handed off to doGet or doPost as appropriate. The init() method simply creates or loads some data that will be used throughout the life of the servlet. The init method definition looks like this: public void init() throws ServletException { // Initialization code... } The service() method : The service() method is the main method to perform the actual task. The servlet container (i.e. web server) calls the service() method to handle requests coming from the client( browsers) and to write the formatted response back to the client. Each time the server receives a request for a servlet, the server spawns a new thread and calls service. The service() method checks the HTTP request type (GET, POST, PUT, DELETE, etc.) and calls doGet, doPost, doPut, doDelete, etc. methods as appropriate.
- 11. Life cycle cont… Here is the signature of this method: public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException{ } The service () method is called by the container and service method invokes doGet, doPost, doPut, doDelete, etc. methods as appropriate. So you have nothing to do with service() method but you override either doGet() or doPost() depending on what type of request you receive from the client. The doGet() and doPost() are most frequently used methods with in each service request. Here is the signature of these two methods.
- 12. Life cycle cont… The doGet() Method A GET request results from a normal request for a URL or from an HTML form that has no METHOD specified and it should be handled by doGet() method. public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Servlet code } The doPost() Method A POST request results from an HTML form that specifically lists POST as the METHOD and it should be handled by doPost() method. public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Servlet code }
- 13. Life cycle cont… The destroy() method : The destroy() method is called only once at the end of the life cycle of a servlet. This method gives your servlet a chance to close database connections, halt background threads, write cookie lists or hit counts to disk, and perform other such cleanup activities. After the destroy() method is called, the servlet object is marked for garbage collection. The destroy method definition looks like this: public void destroy() { // Finalization code... }
- 14. Servlet types Servlet is an object that extends either generic servlet class or http servlet Generic servlet class define methods for building protocol independent servlet Http servlet class provide methods specific to HTTP protocol Servlets are protocol independent. Thus, servlet can support FTP commands, SMTP(Simple Mail Transfer Protocol), POP3 (Post Office Protocol), telnet, HTTP and other protocols.
- 15. Architecture diagram The following figure depicts a typical servlet life-cycle scenario. First the HTTP requests coming to the server are delegated to the servlet container. The servlet container loads the servlet before invoking the service() method. Then the servlet container handles multiple requests by spawning multiple threads, each thread executing the service() method of a single instance of the servlet.
- 16. Servlet FORM data Consider a situation when you need to pass some information from your browser to web server and ultimately to your backend program. The browser uses two methods to pass this information to web server. These methods are GET Method and POST Method GET method: The GET method sends the encoded user information appended to the page request. The page and the encoded information are separated by the ? character as follows: http://www.test.com/hello?key1=value1&key2=value2 The GET method is the default method to pass information from browser to web server and it produces a long string that appears in your browser's Location. Never use the GET method if you have password or other sensitive information to pass to the server. The GET method has size limitation : only 1024 characters can be in a request string.
- 17. FORM cont… POST method: A generally more reliable method of passing information to a backend program is the POST method. This packages the information in exactly the same way as GET methods, but instead of sending it as a text string after a ? in the URL it sends it as a separate message. This message comes to the backend program in the form of the standard input which you can parse and use for your processing. Servlet handles this type of requests using doPost() method. Parameters are encrypted.
- 18. Deploying Servlet in App server Steps Under the $CATALINA_HOME/webapps folder ( henceforth, just webapps ), create a new folder for your webapp; I'll be calling it servletTestApp. Under servletTestApp create another folder called WEB-INF. Under WEB-INF, create a folder called classes. Also, under WEB-INF, create a file called web.xml ( just create a text file and rename the extension ). Next, create the servlet, testServlet. I've put the servlet in a package, testServletPackage, to demonstrate. So, create a folder under classes called testServletPackage. Compile the servlet. Edit the web.xml to add the following servlet mapping: