6 write a program in java to demonstrate the complete life cycle of a servlet
The web container maintains the life cycle of a servlet instance. Let's see the life cycle of the servlet:. As displayed in the above diagram, there are three states of a servlet: new, ready and end. The servlet is in new state if servlet instance is created. After invoking the init method, Servlet comes in the ready state. In the ready state, servlet performs all the tasks.
The Lifecycle of a Servlet
The init method is called when the servlet is first created and is 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 can be created when a user first invokes a URL corresponding to the servlet or when the server is first started, depending on how you have registered the servlet with the Web server. It will be created for the first user request if it is not explicitly registered but is instead just placed in one of the standard server directories.
There are two versions of init : one that takes no arguments and one that takes a ServletConfig object as an argument. The first version is used when the servlet does not need to read any settings that vary from server to server.
The method definition looks like this:. The second version of init is used when the servlet needs to read server-specific settings before it can complete the initialization. For example, the servlet might need to know about database settings, password files, server-specific performance parameters, hit count files, or serialized cookie data from previous requests. The second version of init looks like this:. Notice two things about this code. First, the init method takes a Servlet- Config as an argument.
ServletConfig has a getInitParameter method with which you can look up initialization parameters associated with the servlet. Just as with the getParameter method used in the init method of applets, both the input the parameter name and the output the parameter value are strings. Note that although you look up parameters in a portable manner, you set them in a server-specific way. For example, with Tomcat, you embed servlet properties in a file called web.
This call is critical! The ServletConfig object is used elsewhere in the servlet, and the init method of the superclass registers it where the servlet can find it later. So, you can cause yourself huge headaches later if you omit the super.
Each time the server receives a request for a servlet, the server spawns a new thread and calls service. Now, if you have a servlet that needs to handle both POST and GET requests identically, you may be tempted to override service directly as below, rather than implementing both doGet and doPost.
HttpServletResponse response. This is not a good idea. Instead, just have doPost call doGet or vice versa , as below. Although this approach takes a couple of extra lines of code, it has five advantages over directly overriding service :. You can add support for other services later by adding doPut , doTrace , etc. Overriding service directly precludes this possibility.
You can add support for modification dates by adding a get- LastModified method. If you use doGet , the standard service method uses the getLastModified method to set Last-Modified headers and to respond properly to conditional GET requests those containing an If-Modified-Since header. You get automatic support for HEAD requests. The system just returns whatever headers and status codes doGet sets, but omits the page body.
These methods contain the real meat of your servlet. Note that there is no doHead method. Normally, the system makes a single instance of your servlet and then creates a new thread for each user request, with multiple simultaneous threads running if a new request comes in while a previous request is still executing.
This means that your doGet and doPost methods must be careful to synchronize access to fields and other shared data, since multiple threads may be trying to access the data simultaneously. If you want to prevent this multithreaded access, you can have your servlet implement the SingleThreadModel interface, as below.
If you implement this interface, the system guarantees that there is never more than one request thread accessing a single instance of your servlet. It does so either by queuing up all the requests and passing them one at a time to a single servlet instance, or by creating a pool of multiple instances, each of which handles one request at a time.
You do , however, still have to synchronize access to class variables static fields or shared data stored outside the servlet. Synchronous access to your servlets can significantly hurt performance latency if your servlet is accessed extremely frequently.
So think twice before using the SingleThreadModel approach. The server may decide to remove a previously loaded servlet instance, perhaps because it is explicitly asked to do so by the server administrator, or perhaps because the servlet is idle for a long time. 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.
Be aware, however, that it is possible for the Web server to crash. After all, not all Web servers are written in reliable programming languages like Java; some are written in languages such as ones named after letters of the alphabet where it is easy to read or write off the ends of arrays, make illegal typecasts, or have dangling pointers due to memory reclamation errors. Activities like hit counting or accumulating lists of cookie values that indicate special access should also proactively write their state to disk periodically.
Listing 1 shows a servlet that reads the message and repeats initializationparameters when initialized. Remember that, although servlets read init parameters in a standard way, developers set init parameters in a server-specific manner. Please refer to your server documentation for authoritative details. Because the process of setting init parameters is server-specific, it is a good idea to minimize the number of separate initialization entries that have to be specified.
This will limit the work you need to do when moving servlets that use init parameters from one server to another. If you need to read a large amount of data, I recommend that the init parameter itself merely give the location of a parameter file, and that the real data go in that file. Listing 1 - ShowMessage. Here, the message.
Either way, do nothing in catch,. Listing 2 shows the setup file used to supply initialization parameters to servlets used with Tomcat 3. The idea is that you first associate a name with the servlet class file, then associate initialization parameters with that name not with the actual class file.
Listing 3 shows the properties file used to supply initialization parameters to servlets in the JSWDK. As with Tomcat, you first associate a name with the servlet class, then associate the initialization parameters with the name. Listing 2 - web. Listing 3 - servlets. Register servlet via servletName. Set init params via.
Standard setting. Set this to keep servlet source code built from JSP. Even with servers that support persistent keep-alive HTTP connections and keep a socket open for multiple client requests that occur close together in time, there is no built-in support for maintaining contextual information. This lack of context causes a number of difficulties.
Similarly, when clients decide to proceed to checkout, how can the server determine which previously created shopping carts are theirs? There are three typical solutions to this problem: cookies, URL-rewriting, and hidden form fields.
You can use HTTP cookies to store information about a shopping session, and each subsequent connection can look up the current session and then extract information about that session from some location on the server machine.
For example, a servlet could do something like the following:. This is an excellent solution and is the most widely used approach for session handling. Still, it would be nice to have a higher-level API that handles some of these details. Besides, due to real and perceived privacy concerns over cookies, some users disable them. So, it would be nice to have alternative implementation approaches in addition to a higher-level protocol. With this approach, the client appends some extra data on the end of each URL that identifies the session, and the server associates that identifier with data it has stored about that session.
However, it has most of the same problems as cookies, namely, that the server-side program has a lot of straightforward but tedious processing to do. In addition, you have to be very careful that every URL that references your site and is returned to the user even by indirect means like Location fields in server redirects has the extra information appended.
And, if the user leaves the session and comes back via a bookmark or link, the session information can be lost. HTML forms can have an entry that looks like the following:. For details, see Section This hidden field can be used to store information about the session but it has the major disadvantage that it only works if every page is dynamically generated.
This high-level interface is built on top of cookies or URL-rewriting. In fact, most servers use cookies if the browser supports them, but automatically revert to URL-rewriting when cookies are unsupported or explicitly disabled. Using sessions in servlets is straightforward and involves looking up the session object associated with the current request, creating a new session object when necessary, looking up information associated with a session, storing information in a session, and discarding completed or abandoned sessions.
Behind the scenes, the system extracts a user ID from a cookie or attached URL data, then uses that as a key into a table of previously created HttpSession objects. But this is all done transparently to the programmer: you just call getSession.
If getSession returns null , this means that the user is not already participating in a session, so you can create a new session. Just pass true to getSession. Thus, your first step usually looks like this:. If you care whether the session existed previously or is newly created, you can use isNew to check.
These session objects have a built-in data structure that lets you store any number of keys and associated values. In version 2. The return type is Object , so you have to do a typecast to whatever more specific type of data was associated with that attribute name in the session. The return value is null if there is no such attribute, so you need to check for null before calling methods on objects associated with sessions.
Nevertheless, since not all commercial servlet engines yet support version 2. In most cases, you have a specific attribute name in mind and want to find the value if any already associated with that name. However, you can also discover all the attribute names in a given session by calling getValueNames , which returns an array of strings.
This method is your only option for finding attribute names in version 2.
The lifecycle of a servlet is controlled by the container in which the servlet has been deployed. Service methods are discussed in Writing Service Methods. to get events for various operations on the particular web application context. In this example we are going to examine what is the servlet lifecycle and how it the whole process of creating and initializing the servlet, using it and destroying it push the button to make the request, only then the servlet will be initialized. 5. Java Interview Questions. 6. Spring Interview Questions. 7.
The entire life cycle of a Servlet is managed by the Servlet container which uses the javax. Servlet interface to understand the Servlet object and manage it. Initializing a Servlet : After the Servlet is instantiated successfully, the Servlet container initializes the instantiated Servlet object. The container initializes the Servlet object by invoking the Servlet.
This is the first step of JSP life cycle. In translation phase, container validates the syntactic correctness of JSP page and tag files.
The init method is called when the servlet is first created and is not called again for each user request. So, it is used for one-time initializations, just as with the init method of applets.
Life Cycle of a Servlet
L et us see one example on servlet life cycle. Do you have any doubts related to the concept? Thanks a lot Sivateja. Excellent website that i have ever seen. Suppose there are two servlet deployed on server.
Servlets - Life Cycle
Comment 0. A servlet is a server component where 'serv' means server and 'let' means component. A servlet is a java file which takes and process the request, and as a response provides an HTML page to that client. It is called only once after the instance is created. We can perform any initialization task, create a database connection, or open a file which will be needed for the servlet. See the below code. When a user calls the method, a new thread is created for each new request, no matter if you are the same user or a new user. We can perform any clean-up code before the servlet is killed, such as closing the database connection, freeing threads and cookies, closing a file, etc. To perform any clean-up task, we can override this method, as shown below. That's it.
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Introduction to Java Servlets
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 init method is called only once. It is called only when the servlet is created, and not called for any user requests afterwards. 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. 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 service method is the main method to perform the actual task. The servlet container i. Each time the server receives a request for a servlet, the server spawns a new thread and calls service. The service method is called by the container and service method invokes doGet, doPost, doPut, doDelete, etc. 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.
JSP Life Cycle: Introduction, Phases, Methods
A Java servlet is a Java software component that extends the capabilities of a server. Although servlets can respond to many types of requests, they most commonly implement web containers for hosting web applications on web servers and thus qualify as a server-side servlet web API. Servlets could in principle communicate over any client—server protocol, but they are most often used with HTTP. Thus "servlet" is often used as shorthand for "HTTP servlet". The Java servlet API has, to some extent, been superseded by two standard Java technologies for web services:. To deploy and run a servlet, a web container must be used. A web container also known as a servlet container is essentially the component of a web server that interacts with the servlets. The web container is responsible for managing the lifecycle of servlets, mapping a URL to a particular servlet and ensuring that the URL requester has the correct access rights. A Servlet is an object that receives a request and generates a response based on that request. The basic Servlet package defines Java objects to represent servlet requests and responses, as well as objects to reflect the servlet's configuration parameters and execution environment.
