Java Servlet in Details

Last Updated on October 17, 2023 by Ankit Kochar

In the world of web development, Java servlet architecture plays a pivotal role in powering dynamic and interactive web applications. Servlets are server-side Java components that extend the functionality of web servers, enabling developers to create robust and scalable web applications. Understanding the Servlet architecture in Java is essential for any Java web developer. In this article, we will explore the Java Servlet architecture, its key components, and how it facilitates the creation of dynamic web applications.

What is Java Servlet Architecture?

Java Servlet is a Java class that is used to extend the capabilities of a server. Java Servlets are designed to process and respond to HTTP requests from clients, such as browsers. They can generate dynamic web pages and handle request-response processing in a consistent and efficient manner.

Java Servlets are typically executed on a web server and are used to create dynamic web pages that interact with users. Java Servlets can receive input from the user, process it and then return a response. They can perform functions like processing form data, tracking user sessions, accessing databases, and sending e-mails.

A java Servlet can be thought of as a replacement for Common Gateway Interface (CGI) scripts, which have been used to create dynamic web pages for many years. The main difference between CGI scripts and Java Servlets is that CGI scripts are executed as separate processes on the server, while Java Servlets are executed within the memory of the web server, resulting in better performance and scalability.

To create a Java Servlet, you need to write a Java class that implements the javax.servlet.Servlet interface. This interface defines several methods that must be implemented, including the init() method, which is called when the Java Servlet is first loaded into memory, and the service() method, which is called for each request received by the Java Servlet.

Once a Java Servlet has been written, it must be compiled and deployed to a web server. The web server must also have the Servlet API installed, which provides the classes and interfaces needed to support Java Servlets.

Architecture of servlet architecture in Java

Java Servlet is a technology used for building web applications. The architecture of a Java Servlet is based on the Model-View-Controller (MVC) design pattern and follows a client-server architecture.

In a Java Servlet, the client sends a request to the server, and the server returns a response. The Java servlet acts as a controller, which receives requests from the client, processes the request, and generates a response. The response can be in the form of HTML, XML, or JSON, which is sent back to the client.

The Model component can be thought of as the data or business logic that the Java servlet manipulates, while the View component is responsible for generating the output or presentation of the data to the client.

Overall, the architecture of a Java Servlet provides a flexible and extensible framework for building dynamic, scalable, and secure web applications.
To use the java servlet the user has to implement the java servlet interface first by using the command given below:

import javax.servlet.*;

Java servlet architecture Life Cycle

The life cycle of a java servlet refers to the complete duration from the creation of the java servlet to its destruction. It consists of three stages we will discuss all three stages below.

1. init()
   Any Java Servlet starts out in its init() phase. The init() function is called when a URL peculiar to a certain java servlet is activated.
   When the servers are started, there is another situation in which the init() function is used. The relevant java servlets for each server start-up along with it, as does the init() procedure.
   One significant feature of the init() function is that it is only ever called once throughout the whole life of the Java Servlet and does not respond to any user instructions.
   If the java servlet instance does not already exist, the web container:

   • the java servlet class is loaded
   • creates a java servlet class object.
   • calls the init procedure to start it up.

   Before the java servlet can accept any requests, the init method must succeed. If the init method either throws a ServletException or does not return within the Web server-specified amount of time, the java servlet container cannot put the java servlet into service.

   The init() method is represented as shown below:

   public void init() throws ServletException {
   
   //init() method initializing  
   
   }

2. Service()
   Only once the servlet’s init() function has successfully finished is this method invoked.
   When a client makes a request, the Container invokes the service() function, which then understands the HTTP request type (GET, POST, PUT, DELETE, etc.) and invokes the appropriate doGet, doPost, doPut, doDelete, etc. method.
   The service() function is in charge of determining the type of request that was made by the client and responding appropriately by creating a new thread or collection of threads according to the specifications and carrying out the operation using the following methods.
   The various services provided to client are:

   • PUT
   • GET
   • DELETE

   • UPDATE

     public void service(ServletRequest request, ServletResponse response) 
     throws ServletException, IOException {
     }

3. Destroy()
   The destruct() function, like the init() method, is only used once for the whole life of a Java Servlet.

   The Java Servlet conducts cleaning tasks such as when the destroy() function is called.

   Putting an end to the active or background threads
   creates a disc recovery list for any relevant data, such as cookies.
   The Java Servlet is then badged and prepared for the garbage collector to clear it.

   public void destroy() {
       // 
   }

Advantages of servlet architecture in Java

Java Servlets offer several key features that make them a popular choice for building web applications:

• Platform Independent: Java Servlets are written in Java, which is a platform-independent language. This means that Java servlets can run on any operating system that has a Java Virtual Machine (JVM) installed.
• Dynamic Web Content: Java Servlets are capable of generating dynamic web content based on user requests. This allows for more interactive web applications that can change their content based on user input.
• Session Management: Java Servlets provide built-in support for session management, which allows developers to track user activity across multiple requests. This is useful for maintaining user authentication and authorization.
• Scalability: Java Servlets are designed to handle a large number of concurrent requests efficiently. They can be easily scaled to accommodate increased traffic by adding more resources to the server.
• Security: Java Servlets offer a secure platform for building web applications. Java has built-in security features, and Java servlets can be configured to use SSL (Secure Sockets Layer) for encrypted communication.
• Extensibility: Java Servlets can be easily extended by writing custom Java servlets or by using third-party libraries. This allows developers to add new functionality to their applications quickly and easily.
• MVC Architecture: Java Servlets follow the Model-View-Controller (MVC) architecture, which helps to separate the application logic from the presentation logic. This makes it easier to maintain and update the application.

Java Servlets offer a flexible, scalable, and secure platform for building dynamic web applications. Their platform-independent nature, dynamic content generation capabilities, and built-in support for session management, security, and extensibility make them a popular choice for web development.

Disadvantages of servlet architecture in Java

Below are the disadvantages of java servlet.

• Performance: Java Servlets can be slower compared to other server-side technologies, especially for applications with high traffic. The overhead of the JVM and the overhead of the Java servlet API can make Java servlets less suitable for high-performance applications.
• Complexity: Java Servlets can be complex to set up and use, especially for developers who are not familiar with Java. This can make it difficult for new developers to get started with Java servlets.
• Steep Learning Curve: Java Servlets have a steep learning curve, and require a solid understanding of Java, the Java servlet API, and web development in general. This can be a barrier for developers who are not familiar with these technologies.
• Limited Functionality: Java Servlets are primarily designed for building web applications, and do not provide built-in support for other types of applications. This can make it difficult to use Java servlets for other types of projects.
• No Built-in Template Engine: Java Servlets do not have a built-in template engine, which makes it difficult to separate presentation logic from application logic. Developers must use a third-party template engine or write their own templates.
• Old Technology: Java Servlets were first introduced in 1997, and have since been replaced by more modern technologies such as JSP, JavaServer Faces (JSF), and Spring MVC. This can make Java servlets seem outdated to some developers and organizations.

Java Servlet Request

Java Servlets can handle a variety of requests from a web client. The following are the different types of requests that a Java servlet can handle:

• HTTP GET Requests: The most common type of request is the HTTP GET request. This request is used to retrieve data from a web server. A GET request does not modify any data on the server and is considered to be safe.
• HTTP POST Requests: The HTTP POST request is used to submit data to the server. A POST request can modify data on the server and is not considered to be as safe as a GET request.
• HTTP PUT Requests: The HTTP PUT request is used to update an existing resource on the server. This request is similar to a POST request, but is used specifically for updating existing data.
• HTTP DELETE Requests: The HTTP DELETE request is used to delete a resource on the server. This request is typically used to remove a resource that is no longer needed.
• HTTP HEAD Requests: The HTTP HEAD request is used to retrieve the headers of a resource on the server. This request is similar to a GET request, but only retrieves the headers and not the actual content of the resource.
• HTTP OPTIONS Requests: The HTTP OPTIONS request is used to retrieve information about the communication options for a resource on the server. This request is used to determine the methods that are supported by the server for a particular resource.
• HTTP TRACE Requests: The HTTP TRACE request is used to retrieve a diagnostic trace of the request and response messages exchanged between the client and server. This request is used for debugging purposes.
• HTTP CONNECT Requests: The HTTP CONNECT request is used to establish a network connection to a resource. This request is typically used for secure communication over the internet.

When a Java servlet receives a request, it can process the request and generate a response. The response can be in the form of an HTML document, a JSON object, or any other format that the client is capable of understanding. The Java servlet API provides several classes and methods that make it easy for Java servlets to process requests and generate responses.

For example, the HttpServletRequest class provides methods for accessing the headers, parameters, and attributes of a request. The HttpServletResponse class provides methods for setting the headers, status, and body of a response. The Java Servlet API also provides classes for reading and writing data to and from the request and response objects.

Cookies in Java Servlet

Cookies are small text files that are stored on a client’s computer by a web server. In the context of Java Servlets, cookies are used to store information about a user’s session or preferences. This information can be retrieved later by the server to personalize the user’s experience or to maintain the state of a user’s session.

A cookie is created using the HttpServletResponse object’s addCookie method. The cookie can then be retrieved using the HttpServletRequest object’s getCookies method. Cookies have a name, a value, and various other attributes such as an expiration date and a domain. The name and value of the cookie are used to store the data, while the other attributes control how the cookie is stored and used.

For example, if a user logs into a web application, the server can store the user’s information in a cookie. This information can then be used to automatically log the user in when they return to the site at a later time. Similarly, if a user has selected some preferences, such as the font size or background color of the website, the server can store these preferences in a cookie and use them to personalize the user’s experience every time they visit the site.

In summary, cookies are small text files that are used to store information about a user’s session or preferences. They are created and retrieved using the HttpServletResponse and HttpServletRequest objects, respectively. Cookies are a useful tool for building web applications that need to remember information about a user’s session or preferences.

Session Tracking in Java Servlet

Session tracking in Java Servlet is the process of maintaining a user’s state across multiple requests. A session is created for each unique user that visits a web application. The session is identified by a unique session ID, and it is stored on the server side. When a user makes a request, the session ID is included in the request, allowing the server to associate the request with the correct session.

CGI

CGI (Common Gateway Interface) is a standard for executing scripts on a web server in order to produce dynamic web pages. In simple terms, it is a way for a web server to interact with scripts and programs to produce dynamic content. When a user requests a page from a web server, the server can pass the request to a CGI script, which will then generate the content for the page based on the information provided by the user. The output of the script is then returned to the web server and displayed on the user’s web browser.

CGI is a widely used technology for server-side programming, allowing developers to write scripts in various programming languages including Perl, Python, Ruby, and PHP. These scripts can interact with databases, perform calculations, or retrieve information from external sources. CGI scripts are usually executed on the server and are not processed by the client’s browser, making it a secure and efficient way to process and display dynamic content.

CGI has been around since the early days of the web and is still widely used, although it has largely been replaced by more modern technologies such as server-side JavaScript and serverless computing. However, CGI remains an important and relevant technology for many web developers and continues to be used for a variety of applications, from small personal websites to large e-commerce platforms.

Cgi vs Servlet

CGI (Common Gateway Interface) and Servlets are two technologies used for server-side programming in web development. Both are used to produce dynamic content in response to user requests, but they differ in their implementation and architecture.

Java Servlets	CGI
Sharing data is possible	Sharing data is not possible
Less expensive than CGI	More expensive than Java Servlets
Are efficient and portable	Not portable
Java Servlets can communicate directly with the web server	CGI cannot communicate directly with the web server.
It can handle the cookies	It cannot handle the cookies

Advantages of CGI

Below is the list of advantages of CGI.

• Portability: CGI is a standard that can be implemented on any platform that supports a web server, making it highly portable and accessible.
• Language Support: CGI scripts can be written in any programming language that can produce output in the required format, giving developers a wide range of options for implementation.
• Easy to learn: CGI is simple and easy to learn, making it accessible to many developers, even those with limited programming experience.

Disadvantages of CGI

Here we have shown various disadvantages of CGI.

• Performance: CGI scripts are executed each time a user requests a page, which can result in slower performance and increased server load.
• Security: CGI scripts can be vulnerable to security threats, such as code injection and buffer overflow attacks, making it important to take measures to secure them.
• Resource-Intensive: CGI scripts consume server resources, such as memory and CPU, each time they are executed, which can result in decreased performance and scalability issues.

Conclusion
Java servlet architecture forms the backbone of dynamic web applications by enabling server-side processing of HTTP requests and responses. Understanding the Servlet architecture in Java, including the role of servlet classes, servlet containers, deployment descriptors, and the Servlet API, is essential for Java web developers. Servlets have stood the test of time and remain a crucial technology for building scalable and high-performance web applications.

Frequently Asked Questions Related to Servlet Architecture in Java

Here are some FAQs related to Servlet Architecture in Java.

1. What are the advantages of using Java Servlets?
Java Servlets offer platform independence, scalability, and robustness. They are highly efficient for handling HTTP requests, making them suitable for developing web applications of various sizes.

2. How is a servlet different from a JSP (JavaServer Pages)?
Servlets are Java classes that handle HTTP requests and generate responses programmatically. JSPs, on the other hand, are templates that mix HTML with Java code and are compiled into servlets by the servlet container.

3. Can I use servlets for handling other types of requests, not just HTTP?
While servlets are primarily designed for handling HTTP requests, they can be extended to support other protocols or custom communication mechanisms.

4. Are there any alternatives to Java Servlets for building web applications in Java?
Yes, there are alternatives like JavaServer Faces (JSF), Spring MVC, and Play Framework. However, servlets remain a fundamental building block even within these frameworks.