Servlet Execution in Java

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Servlet Execution
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Now that we've seen how to develop a servlet, and we can imagine how it's invoked, we can discuss some of the details of what goes on behind the scenes For performance and scalability reasons, it's of particular interest to know how the servlet container works
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Servlet Containers
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First, let's consider the term container versus an older term used to describe this entity: engine When you think of a container, you think of a host a mechanism for storing a resource such as a servlet In contrast, an engine connotes a mechanism for processing, which is not at all what happens Container is a better, more accurate term to be sure, since the real execution has to do with the code written by the servlet programmer Interestingly, this shift in terminology fits well into a discussion we'll have later about the role of servlets in the overall application architecture, but for now let's return to understanding what containers are all about At the minimum, a servlet container is an intermediary/facilitator between the Web server and servlet It's responsible for
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Creating a servlet instance Calling the servlet init() method Calling the servlet service() method whenever a request is made Calling the servlet destroy() method before the servlet is terminated Destroying the servlet instance
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Like a Web server, a servlet container is a continually running process It takes care of creating a servlet instance when needed and using threads to execute the code in it Specifically, when a request comes in for a servlet, the container does the following:
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Maps the request to identify the servlet class being requested Marshals input and output data by constructing Java objects corresponding to the request (HttpRequest) and reply (HttpResponse) descriptors; it's through these descriptors that servlet developers can interrogate things like request parameters and produce dynamic output Sets up the environment associated with the request; the Java classes HttpSession and ServletContext represent request-level state, and application-level state, respectively Creates an instance of the servlet class if one doesn't already exist Creates a thread and executes the corresponding servlet method(s)
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If multiple requests arrive for a servlet at the same time, threads are created for each one of them; however, the same servlet object instance is used Using threads to execute the servlet instance code is efficient because it eliminates the need to continually recreate and destroy the servlet instance (which may have an expensive initialization section) The obvious minor drawback is that it requires programmers to write thread-safe code When a servlet instance hasn't been requested for a while, it may be destroyed by the servlet container However, the lifetime of servlet instance is a servlet container configuration parameter and can be adjusted as needed Furthermore, the servlet container can be designated to create instances of various servlets upon startup before the servlets are requested by clients
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Servlets and Multithreading
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By default, a servlet container can service concurrent requests by associating each one with a thread and executing the servlet service() method for it This is generally considered a feature because it increases parallelism during application processing Since Web applications are primarily I/O bound (waiting for requests from the client or for output from an external resource such as a database), this concurrency is often realized during execution, resulting in more efficient applications Also, using threads as opposed to processes for each request is generally more scalable As we've discussed a few times already, memory and CPU resources can be shared more efficiently by deploying multithreaded servers instead of multiple processes Although parallel request processing is efficient, there are certain cases where a developer needs to serialize request execution The most common instances involve third-party data structures that are not thread-safe Incoming requests that use these data structures should do so one at a time For such cases, there are two options for serialization:
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Explicit synchronization of methods or use of the synchronized keyword to designate serialized execution within an instance Implementation of the servlet under the SingleThreadModel to ensure that only one thread at a time is executing the service() call (and its descendants, such as doGet()) of a particular instance
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The first option is well understood by most Java programmers By synchronizing the doPost() or doGet() method, for example, you ensure that only one thread is executing these methods for that instance at a given time Consider the very simple example in Listing 7-3, which echoes the ID of the user who invokes it Listing 7-3 Example of Unsafe Membership Processing (Requires Serialization)
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/** * Process new member - not thread safe * Adds new member to the system **/ public class ProcessMembershipUnsafe extends HttpServlet { private int m_nextUserId; /** * Initialize instance-specific data structures **/ public void init() { m_nextUserId = 0; } /** * Insert new member in the database **/ public UserLogin addMember(String a_first, String a_last, String a_street, String a_city, String a_state, String a_zip) { m_nextUserId++; /** * Simulate inserting a member into database * To grossly demonstrate a synchronization issue, we sleep * for 5 seconds when inserting the first user Though contrived * here, unpredictable orderings of request completions are * common, especially when reliant on external sources (like a * database) **/ if (m_nextUserId == 1) { try { Threadsleep(5000);
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} catch (Exception e) { } } /** * At this point, all processing using m_nextUserId when * the first member is created is particularly unsafe because * addMember may have been called during the sleep time above **/ /* Generate user login information */ UserLogin userLogin = new UserLogin(); userLoginusername = m_nextUserId; userLoginpassword = userLoginusername+"123"; return userLogin; }
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To underline the concurrency problem, the code sleeps for 5 seconds when processing the first user ID Thus, if the first and second users are submitted at roughly the same time, the second user will finish before the first and more alarmingly any processing done on her behalf that involves m_nextUserId (such as creation of UserLogin) will be incorrect The implication is that if requests don't take a deterministic time (nearly always the case when relying on an external resource such as a database), their order of completion won't be predictable Thus, state maintained in the context of a call isn't guaranteed to be consistent To solve this synchronization problem, you can use the synchronized keyword on a code block or method For example, we can change addMember() to
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public synchronized UserLogin addMember(String a_first, String a_last, String a_street, String a_city, String a_state, String a_zip)
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Many developers believe that the same effect can be achieved by having the servlet implement the SingleThreadModel class So, instead of synchronizing the addMember() method, it would seem that we should do the following:
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