JAVA EE: Servlets : How Servlets Works

Saturday, 22 March 2014

Servlets : How Servlets Works - Flow chart

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Introduction: Behind the Scenes of a Servlet

When a user clicks a button or navigates to a URL on a website, a request travels over the network to the web server. But once it arrives at the Servlet Container (such as Apache Tomcat), what actually happens under the hood?

Understanding the internal lifecycle and thread execution flow helps developers build high-performance web applications and troubleshoot concurrency or state issues with confidence.

The Complete Step-by-Step Servlet Workflow

Client Issues HTTP Request: The browser generates an HTTP request (GET, POST, etc.) and transmits it across the network to the server.
Web Server Routes to Servlet Container: The web server identifies that dynamic processing is required and hands over the raw request bytes to the servlet engine.
Request & Response Object Creation: The container instantiates two key Java objects:
- HttpServletRequest: Encapsulates incoming parameters, headers, cookies, and payload data.
- HttpServletResponse: Provides helper methods and output streams to construct the server's reply.
Class Loading & Initialization: If the servlet class has not been loaded previously, the container loads the bytecode, instantiates a single instance, and calls init(ServletConfig).
Thread Allocation & service() Invocation: The container allocates a thread from its thread pool and passes the request and response objects into the servlet's service() method.
Business Logic Execution: The service() method delegates to the appropriate handler (such as doGet() or doPost()), which executes custom business logic and writes data to the response stream.
Response Sent to Client: Once execution finishes, the container packages the HTTP response status code, headers, and body back to the client browser.
Garbage Collection & Destruction: The request and response objects are dereferenced for garbage collection. When the container unloads or shuts down, the servlet's destroy() method executes once.

Single Instance, Multi-Threaded Execution Model

A crucial architectural feature of Java Servlets is their single-instance model:

Only one instance of a given servlet class is maintained in memory by default.
Each incoming client request is handled by a separate lightweight thread executing the service() method concurrently on that single instance.
Best Practice: Avoid declaring mutable instance variables inside servlets to prevent race conditions and thread-safety bugs. Always use local variables within methods whenever possible.