What Is a Java Servlet: A Comprehensive Guide
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What Is a Java Servlet? A Comprehensive Guide

Ene Ayegba
Ene Ayegba
Hosting Expert 12 min read

What is a Java Servlet? A Java servlet is an integral part of Java web development. It provides a means of processing and responding to client requests on the server side of a web application. Servlets also offer several advantages over traditional common gateway interface (CGI) scripts, including improved performance and scalability.

This article will explore the fundamentals of Java servlets, including key features, perks, downsides, and how to write and deploy servlets.

Let’s get into it.

Key Highlights

  • Web developers use Java servlets to create dynamic web pages and process and store user data
  • Platform independence, scalability, reusability, and security are key features of Java servlets
  • Java servlets are efficient, flexible, widely acceptable, and easy to deploy
  • Complexity and overhead are some of the downsides of Java servlets. They are also challenging to scale and time-consuming

What Are Java Servlets?

Over 12,035 websites use Java Servlets

Source: Web Tech Survey

Java Servlets are server-side components for creating dynamic web applications in Java. A Servlet is a Java class that processes HTTP requests and generates HTTP responses, allowing web servers to generate content dynamically. Servlets are a key component of Java Enterprise Edition (Java EE) technology, providing a powerful and flexible means of building server-side applications.

Web developers use Java Servlet for various purposes. For example, they use them to create dynamic web pages, process, store user data, and integrate with other web technologies. Since they can handle a high volume of requests and provide a consistent interface for web developers, Java Servlets are widely used in enterprise web development.

Servlets operate inside a web container– a Java runtime environment offering services for servlets lifecycle management, consisting of loading, initialization, and invocation. In addition, Java Servlet API offers categories of classes and interfaces that highlight the contract between servlets and web containers.

Features of Java Servlets

Java Servlets offer unique features for the effective development of web applications. Here are major Java Servlets characteristics:

1. Platform Independent

Java Servlets are platform-independent because they are written in the Java programming language and executed on a Java Virtual Machine (JVM). The JVM is an abstract machine providing a runtime environment for Java code execution.

When you compile a Java Servlet, it is compiled into platform-independent bytecode, which can be executed on any system with a JVM installed. Consequently, a Java Servlet can work on any platform that supports Java, whether it’s Windows, Linux, macOS, and others, without requiring any adjustments to the Servlet code.

Additionally, the Java platform provides a set of APIs that allow Servlets to interact with the underlying operating system, network, and other resources in a consistent and platform-independent way. As a result, Java Servlets are highly portable and are used to build web applications that run on various hardware and software platforms.

2. Scalability

Java Servlets offer high scalability. They are designed to handle several requests at the same time. They also adopt connection pooling, caching, and asynchronous processing. Lastly, they can be load balanced across many servers.

  • Multithreading: Java Servlets are built to manage multiple requests at the same time. Every request is managed by a different thread. Therefore, every server can handle several requests concurrently without blocking.
  • Pooling: Java Servlet containers adopt connection pooling to handle database connections. Therefore, it mitigates the overhead of establishing a new connection for each request, enhancing performance.
  • Caching: Servlet containers harness caching to keep frequently used data in memory. This reduces the need to query the database multiple times, increasing performance.
  • Asynchronous processing: Servlets can also use asynchronous processing to handle requests. It enables the server to free up resources while waiting for long-running operations to finish, enhancing scalability.
  • Load balancing: You can configure servlet containers to distribute requests across several servers. This helps balance the load and enhance scalability.

3. Reusability

Java Servlets are reusable components used in many web applications. They are structured to manage HTTP requests and responses, the standard means of communication between cloud servers and clients. A Java Servlet can manage several requests and responses. You can also use them to create web applications that are highly modular and flexible.

As a web developer, you can reuse Java servlets in many ways. For instance, you can deploy them in a Java web container like Tomcat or Jetty. You can also access them through several web applications running in that container. In addition, it is possible to reduce maintenance costs and time, and duplication by converting servlets to Java archive files for several projects.

4. Security

Java Servlets enable developers to build web applications securely. For instance, they can authenticate users, authorize resource access, and safeguard against attacks and SQL injection. They also help with fixing cross site scripting (XSS). Below is how Java Servlets offer security for web applications.

  • Authentication: Servlets adopt various authentication schemes to verify users’ identity before granting them access to safeguarded resources. You can also do this with standard mechanisms like basic authentication, form-based authentication, and custom authentication schemes.
  • Authorization: Servlets implement access control policies to restrict resources users can access based on their roles after they’ve been authenticated. Authorization can be achieved through several methods like role-based access control (RBAC) or attribute-based access control (ABAC).
  • Encryption: Servlets use different encryption strategies to protect sensitive data transmitted over the network. These mechanisms can vary from SSL/TLS to encrypt HTTP traffic, encrypting data stored in databases or files, or using protected session management methods to safeguard session data.

Generally, Java servlets provide a secure framework for building web applications that protect against a wide range of security threats, making them a popular choice for developing enterprise-grade applications.

How Do Java Servlets Work?

Java servlets work by receiving HTTP requests from web clients (such as web browsers) and generating HTTP responses to the clients.

When a client sends an HTTP request to a web server, the web server passes the request to a servlet container responsible for managing servlets. The servlet container then locates the appropriate servlet based on the URL requested and passes the request to the servlet.

Java servlets work in various steps. Let’s look at them.

1. Web Container Initialization

This step allows Java servlets to be deployed and executed on the web server. This process leads to the creation of unique and scalable web applications. When a web server receives a request for a servlet or JSP page, the servlet container loads the appropriate servlet or JSP page and ensures it generates the required response.

In addition, during container initialization, the servlet container sets up the runtime environment for Java servlets, including creating and managing a pool of Servlet instances, managing requests and responses, and handling servlet lifecycle events.

Lastly, the Servlet container offers a set of standards for Java Servlet APIs developers can adopt to create Servlets and JSP pages. This enables them to develop dynamic web content and engage with web users.

2. Servlet Initialization

After the web container recognizes the Servlet that can handle requests, it loads the Servlet class and initializes it by categorizing it as the init () method. The init() method performs any necessary setup tasks like initializing variables, opening database connections, or loading configuration files.

3. Request Handling

When the Servlet has been initialized to process the request, the web container adopts the service () method of the Servlet. Afterward, it receives a request object and a response object. The request object is the data about the user’s request. They include URLs, headers, and query parameters. However, the response object is used to generate a response to the client.

4. Processing the Request

The service method examines the databases and files and creates a response to handle requests. The response formats differ based on users’ requests. For instance, the response could be in HTML, XML, or JSON.

5. Response Generation

After the response is produced, the web container directs it to the client and has an HTTP response. The response includes response codes, response body, and headers. The client showcases the response to the header once it is received.

6. Servlet Destruction

Servlet destruction occurs when the service () method is complete. Similar to how a servlet that maintains a database connection might use the destroy () method to close the database collection, a servlet that uses resources like files or network connections uses the destroy () method to release these resources. The destroy () method is primarily called only once during a Servlet’s life cycle. It allows the server to clean up resources that it has been holding onto, including open files or database connections.

Java Servlets HTTP Requests

Java Server HTTP Requests refers to the process by which a Java server-side application handles and responds to HTTP requests from client-side applications, such as web browsers or mobile apps. This section discusses the various aspects of a Java Servlet request and how to access and process them through a Servlet.

1. HTTP Request Header

An HTTP request header is a set of metadata information that a client sends to a server when making an HTTP request. The header contains data about the request, such as the type of request the user makes, and other extra details the server requires to process a request. The Servlet obtains the request header using the HttpServletRequest interface’s getHeader () and getHeader () methods.

An example of a code snippet showing how to retrieve the User-Agent header from a request include:

String userAgent = request.getHeader("User-Agent");

2. HTTP Request Parameters

HTTP Request Parameters are information that are sent from a client to a server as part of an HTTP request. They offer extra details about the requested information. They are also used to adjust the behavior of the server. In HTTP, parameters are set as key-value pairs. This is where the key is a string that highlights the parameter, and the value is the precise information that is delivered.

The Servlet reviews the request parameters using the getParameter () and getParameterValues () methods of the HttpServletRequest interface. For instance, the code snippet below shows how to restore a parameter named “username” from a request:

String username = request.getParameter("username")

3. HTTP Request InputStream

HTTP Request InputStream is a stream of data consisting of the data sent by a user to a server with the HTTP protocol. The Servlet obtains the InputStream with the getInputStream() method of the HttpServletRequest interface.

Here is a code snippet showing how to read the request body as a string:

InputStream inputStream = request.getInputStream();
byte[] buffer = new byte[1024];
int bytes read = -1;
StringBuilder stringBuilder = new StringBuilder();
while ((bytes read = inputStream.read(buffer)) != -1) {
String chunk = new String(buffer, 0, bytesRead);
stringBuilder.append(chunk);
}
String requestBody = stringBuilder.toString();

4. HTTP Request Context

The HTTP Request Context contains details related to the request context, including the Servlet context, server name, and server port: The Servlet accesses the request context with the getServletContext () and getRequestURL () methods of the HttpServletRequest interface.

Here is an example of a snippet that shows how to restore the context path and the URL of a request.

String contextPath = request.getContextPath();
String requestURL = request.getRequestURL().toString();

5. HTTP Request Session

The HTTP Request Session is a server-side object that saves information related to a client’s session. The Servlet accesses the session with the getSession () procedure of the HttpServletRequest interface.

An example of a code snippet showing how to retrieve an attribute called “cart” from the session is:

HttpSession session = request.getSession();
ShoppingCart cart = (ShoppingCart) session.getAttribute("cart");

Java Servlet HTTP Response

The response is transferred to the client after processing a request in a Java Servlet. Here are the various aspects of a Java Servlet response and how they are used to produce responses.

1. HTTP Response Header

An HTTP Response Header is a set of metadata that is sent by a web server in response to an HTTP request made by a client. Typically, the HTTP Response Header contains data about the server, the content being returned, and the instructions for how the client should handle the response, including the content type, encoding, cookies, and cache control.

Moreover, it’s possible for the Servlet to set the response header using the HttpServletResponse interface’s setHeader () and addHeader () methods.

Here is an example of a code snippet displaying how to set the content type to “text/html.”

response.setHeader("Content-Type", "text/html");

2. HTTP Response Content-Type

The HTTP response content header field that tells the user the format of the information the server returns. The HttpServletResponse interface’s setContentType () method sets the content type. For instance, this code snippet reveals how to see the content type to the “application/Jason.

response.setContentType("application/json");

3. HTTP Response Content-Length

The HTTP Response Content-Length shows the payload size or body in the HTTP response text. It shows users how much data to expect when a response is made. This allows them to read and process the response correctly.

You can set the content length with the setContentLength () method of the HttpServletResponse interface. For example, the code snippet below shows how to set the content length to 1024 bytes.

response.setContentLength(1024);

2. HTTP Response Write HTML

The HTTP Response Write HTML method creates HTML content for the message body of an HTTP response. As a result, the server can generate HTML content as per the client’s request dynamically.

The Servlet uses the getWriter () method of the HttpServletResponse interface to access a PrintWriter object. You can also use it to write HTML content for the response. For instance, this code snippet below shows how to write a “Hello World” message to the response body:

PrintWriter out = response.getWriter();
out.println("<html>");
out.println("<head><title>Hello World</title></head>");
out.println("<body>");
out.println("<h1>Hello World</h1>");
out.println("</body>");
out.println("</html>");

5. HTTP Response Redirection

This involves responding to a client’s request with a status code indicating that the requested resource has been removed or is no longer accessible at the requested URL.

Next, the server obtains the response from the user using a different URL. The Servlet utilizes the sendRedirect () method with the HttpServletResponse interface to redirect the user. For example, this code snippet reveals how to direct the client to another URL:

response.sendRedirect("http://example.com");

Advantages of Java Servlet

Java Servlets provide several benefits that enhance web applications. Let’s discuss some of these benefits below:

1. Fast and Efficient

Java servlets can produce unique web pages in record time with the information available on the server. Consequently, they reduce the time needed to transfer data over a network and improve web applications’ performance.

Moreover, Java Servlet utilizes multithreading to manage multiple requests at the same time without hassle. Java servlets are then combined into bytecode and written in Java. The result is that they are more effective than interpreted languages.

2. Easy to Create

Java Servlets are relatively easy to use for developers familiar with the Java programming language because they are written in Java. Moreover, Java Servlets provide a simple and consistent interface for building web applications, providing seamless development processes.

3. Great Compatibility

Not only are Java servlets built on the extensively-used Java platform, but they are also supported by various web servers, including Apache Tomcat, Jetty, and IBM software.

Various gaming servers, web containers, and application servers also support Java Servlets. As a result, Servlets can be deployed on various platforms and connected to various technologies like the JavaServer Pages (JSP) and JavaServer Faces (JSF).

4. Flexible

Java Servlets can run on any platform supporting Java, including e-commerce sites, social networks, and content management platforms. Additionally, developers can customize request processing methods, including doGet, doPost, doPut, doDelete, etc.

Java Servlets flexibility enables dynamic content generation, enabling developers to create custom web pages, dynamic data-driven applications, etc. Finally, Servlet provides additional functionality by enabling integration with other Java technologies like JavaBeans and Enterprise JavaBeans (EJB).

Disadvantage Of Using Java Servlet

Although Java Servlets have numerous advantages, they have their downsides too. Here are some of the disadvantages to keep in mind as a developer.

1. Complexity

Java Servlets are not necessarily complex themselves, but they can be complicated for beginners and also based on the requirements of the web application being developed.

Unlike other web development structures offering higher-level abstractions, Servlets require developers to handle many low-level details, including request and response processing, thread safety in Java, and session management.

Additionally, as the number of servlets grows, the complexity also increases. So, while servlets may not be inherently complex, the complexity of the web application they are a part of can make them seem so.

2. Overhead

Each Servlet runs in a separate thread; it can be resource-intensive for the server to handle multiple requests simultaneously. In addition, Java servlets can be more difficult to learn and use compared to other web development frameworks, making them less accessible to developers who are beginners or who prefer streamlined development tools.

Finally, Java Servlets require a huge amount of memory and processing power. This hampers the application’s performance, especially if the server has to handle many requests.

3. State Management

Managing application state in Java can be complex for many reasons. First, Java applications use multiple threads to perform repetitive tasks. In addition, as the size and complexity of a Java application grow, managing its state becomes more challenging.

Lastly, complex data model application comes with tasking state management processes. This is because developers have to run state management manually to prevent errors, bugs, and Java security vulnerabilities.

4. Scalability

Scaling Java servlets can be challenging depending on the specific requirements and complexity of the application being scaled. For example, high availability and fault tolerance is required for large-scale applications. To efficiently scale Java servlets, you must also take into account the application architecture, load balancing, and data management procedures.

5. Testing

The complexity of the application, the number of servlets involved, and the testing methodology can all affect how long it takes to test Java servlets. It is difficult to test servlets because they frequently interact with databases, third-party services, network resources, and other application components.

Conclusion

If you are looking for an effective way to carry out HTTP requests and create responses for web applications, then Java servlets can prove useful. Java servlets also cover many servers, can be added to many applications, and are customizable.

On the flip side, it might take some time for newbies to understand the state management system because of the complex models and combinations. This article discusses Java servlets’ key features, benefits, and disadvantages, including prominent Java servlets terminologies and how to write code snippets.

Next Steps: What Now?

Further Reading – Useful Resources

Frequently Asked Questions

What is the lifecycle of a Java Servlet?

The Servlet is loaded and instantiated, and the innit () method is called during the initialization process. The Servlet controls the clients’ requests by invoking its service () method during request processing. Finally, when the Servlet is no longer needed, the container calls its destroy () method once during the life cycle of a servlet to clean up the tasks the Servlet needs to perform.

What are some popular Java Servlet Containers?

GlassFish, JBoss, and Apache Tomcat are some of the most popular Java servlet containers. They provide a channel to deploy and manage Java servlets and other web applications.

What are the advantages of using Java Servlets for web development?

Java servlets are scalable, flexible, and efficient. They also offer platform independence and extensibility. Finally, they work effectively for applications requiring high performance, security, and reliability.

What are the disadvantages of using Java Servlets for web development?

Java servlets have a steep learning curve. Additionally, they don’t have built-in security features, so developers have to implement built-in security features themselves. Other disadvantages of Java servlets include complex state management, testing, and scalability.

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