Servlet Exception Handling

try-catch in doGet / doPost

The simplest way to handle exceptions in a servlet is with a standard try-catch block inside doGet() or doPost(). This gives you full control over the error response.

try-catch and sendError() in Servlet

package com.example;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.WebServlet;
import java.io.*;
import java.util.logging.*;

@WebServlet("/user")
public class UserServlet extends HttpServlet {

    private static final Logger logger = Logger.getLogger(UserServlet.class.getName());

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        response.setContentType("text/html;charset=UTF-8");

        try {
            String idParam = request.getParameter("id");

            // Validate parameter
            if (idParam == null || idParam.isEmpty()) {
                // Send HTTP 400 Bad Request
                response.sendError(HttpServletResponse.SC_BAD_REQUEST,
                                   "Parameter 'id' is required.");
                return;
            }

            int userId = Integer.parseInt(idParam); // may throw NumberFormatException

            // Simulate user lookup
            if (userId <= 0) {
                // Send HTTP 404 Not Found
                response.sendError(HttpServletResponse.SC_NOT_FOUND,
                                   "User with id=" + userId + " not found.");
                return;
            }

            // Normal response
            PrintWriter out = response.getWriter();
            out.println("<h2>User ID: " + userId + "</h2>");

        } catch (NumberFormatException e) {
            logger.warning("Invalid id parameter: " + e.getMessage());
            response.sendError(HttpServletResponse.SC_BAD_REQUEST,
                               "Invalid id format. Must be an integer.");
        } catch (Exception e) {
            logger.log(Level.SEVERE, "Unexpected error in UserServlet", e);
            // Forward to container's error handling
            throw new ServletException("Internal error processing user request", e);
        }
    }
}

web.xml Error Pages

Define global error pages in web.xml to handle HTTP error codes and uncaught exceptions across the entire application. The container sets special request attributes on the error page.

web.xml Error Pages and Error Attributes

<web-app>

    <!-- Map HTTP status codes to error handler servlet -->
    <error-page>
        <error-code>404</error-code>
        <location>/error</location>
    </error-page>

    <error-page>
        <error-code>500</error-code>
        <location>/error</location>
    </error-page>

    <!-- Map specific exception types -->
    <error-page>
        <exception-type>java.lang.NumberFormatException</exception-type>
        <location>/error</location>
    </error-page>

    <!-- Catch-all for any Throwable -->
    <error-page>
        <exception-type>java.lang.Throwable</exception-type>
        <location>/error</location>
    </error-page>

</web-app>

package com.example;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.WebServlet;
import java.io.*;

@WebServlet("/error")
public class ErrorServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        handleError(request, response);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        handleError(request, response);
    }

    private void handleError(HttpServletRequest request, HttpServletResponse response)
            throws IOException {

        // Read error attributes set by the container
        Integer statusCode = (Integer) request.getAttribute(
                "javax.servlet.error.status_code");
        String  message    = (String)  request.getAttribute(
                "javax.servlet.error.message");
        String  requestUri = (String)  request.getAttribute(
                "javax.servlet.error.request_uri");
        Throwable exception = (Throwable) request.getAttribute(
                "javax.servlet.error.exception");

        response.setContentType("text/html;charset=UTF-8");
        response.setStatus(statusCode != null ? statusCode : 500);

        PrintWriter out = response.getWriter();
        out.println("<!DOCTYPE html><html><head><title>Error</title></head><body>");
        out.println("<h1>Error " + statusCode + "</h1>");
        out.println("<p>" + (message != null ? message : "An error occurred.") + "</p>");
        if (requestUri != null) out.println("<p>URL: " + requestUri + "</p>");
        if (exception != null) out.println("<p>Cause: " + exception.getMessage() + "</p>");
        out.println("</body></html>");
    }
}

Logging Exceptions

Always log exceptions in servlets. Use java.util.logging (built-in) or a logging framework like SLF4J/Logback. The log() method on GenericServlet writes to the container's log.

Logging in Servlets

package com.example;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.WebServlet;
import java.io.*;
import java.util.logging.*;

@WebServlet("/process")
public class LoggingServlet extends HttpServlet {

    // java.util.logging — no extra dependency needed
    private static final Logger logger =
            Logger.getLogger(LoggingServlet.class.getName());

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        String data = request.getParameter("data");
        logger.info("Processing request with data: " + data);

        try {
            // Simulate processing
            if (data == null) throw new IllegalArgumentException("data is null");
            processData(data);
            logger.info("Processing completed successfully.");
            response.sendRedirect("success.jsp");

        } catch (IllegalArgumentException e) {
            // Log at WARNING level — expected business error
            logger.warning("Validation failed: " + e.getMessage());
            request.setAttribute("error", e.getMessage());
            request.getRequestDispatcher("form.jsp").forward(request, response);

        } catch (Exception e) {
            // Log at SEVERE level with full stack trace — unexpected error
            logger.log(Level.SEVERE, "Unexpected error during processing", e);
            // Also use GenericServlet.log() to write to container log
            log("Critical error in LoggingServlet", e);
            throw new ServletException("Processing failed", e);
        }
    }

    private void processData(String data) {
        // Business logic here
    }
}

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Servlet Exception Handling

try-catch in doGet / doPost

web.xml Error Pages

Logging Exceptions

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