Introduction

HttpServletRequest represents an HTTP request class used to receive HTTP requests from clients. Many attributes in HttpServletRequest correspond to request header attributes, so understanding request headers helps in learning this class.

An HTTP request consists of four parts:

1. First line = Method + URL + Version
2. Request headers: Composed of key-value pairs
3. Empty line
4. Body

Common HttpServletRequest Methods

Obtaining First Line Information and Header Data

@WebServlet("/requestInfo")
public class RequestInfoHandler extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        StringBuilder responseBuilder = new StringBuilder();

        // 1. Get first line information
        responseBuilder.append(req.getMethod());
        responseBuilder.append("<br>");
        responseBuilder.append(req.getRequestURI());
        responseBuilder.append("<br>");
        responseBuilder.append(req.getProtocol());
        responseBuilder.append("<br>");
        
        // 2. Get request header information
        Enumeration<String> headerNames = req.getHeaderNames();
        while (headerNames.hasMoreElements()) {
            String key = headerNames.nextElement();
            String value = req.getHeader(key);
            responseBuilder.append(key + ": " + value + "<br>");
        }

        resp.getWriter().write(responseBuilder.toString());
    }
}

Obtaining Query String from Request

The query string, often containing user information like login credentials, is frequently used in business logic. Its location in the request varies depending on the HTTP method:

• For GET requests: Query string is in the URL
• For POST requests: Query string is in the body

Note: The URL technically includes the query string, but Servlet methods like getRequestURL() return a URL without it.

GET Method

For GET requests, the query string is in the first line of the request. We can use getParameter() to retrieve values.

@WebServlet("/getParams")
public class GetParameterHandler extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // Retrieve query string parameters
        // Assuming format: username=value&password=value
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        System.out.println("Username: " + username);
        System.out.println("Password: " + password);
    }
}

When accessing "http://127.0.0.1:8080/yourapp/getParams?username=john&password=12345", the console displays the username and password.

POST Method

For POST requests, the query string is in the body. Different body formats require different parsing approaches:

1. Form data format
2. JSON format

Form Data Format

When the body is organized as form data with Content-Type: application/x-www-form-urlencoded, we use the same getParameter() method as with GET requests.

JSON Format

For JSON-formatted bodies, native Servlet doesn't provide parsing capabilities. We need the Jackson library, which is Spring's recommended JSON processing library.

First, add the Jackson dependency to your pom.xml:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.15.0</version>
</dependency>

Then, create a class to map JSON data:

class UserRequest {
    public String username;
    public String password;
}

Now, handle the JSON data in your Servlet:

@WebServlet("/Params")
public class JsonParameterHandler extends HttpServlet {
    private ObjectMapper objectMapper = new ObjectMapper();

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // Convert JSON string to Java object
        UserRequest userRequest = objectMapper.readValue(req.getInputStream(), UserRequest.class);

        // Process parameters
        String username = userRequest.username;
        String password = userRequest.password;
        String userInfo = "Username: " + username + "<br>Password: " + password;
        System.out.println(userInfo);

        // Convert Java object back to JSON for response
        String Response = objectMapper.writeValueAsString(userInfo);
        resp.getWriter().write(Response);
    }
}

@WebServlet("/setStatusCode")
public class StatusCodeHandler extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // Set status code directly
        // resp.setStatus(404);
        
        // More flexible approach for error messages
        resp.sendError(404, "The requested resource was not found");
    }
}

@WebServlet("/autoRefresh")
public class AutoRefreshHandler extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setHeader("refresh", "1"); // Refresh every second
        resp.getWriter().write("" + System.currentTimeMillis());
    }
}

@WebServlet("/redirectPage")
public class RedirectHandler extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // Manual approach
        // resp.setStatus(302);
        // resp.setHeader("Location", "https://example.com");
        
        // Simplified approach
        resp.sendRedirect("https://example.com");
    }
}

<html>
<head>
    <title>Redirection Example</title>
    <script>
        window.onload = function() {
            setTimeout(function() {
                window.location.href = "https://www.example.com";
            }, 3000); // 3 seconds
        };
    </script>
</head>
<body>
    <h1>This page will redirect to example.com in 3 seconds</h1>
</body>
</html>