Java.Servlet.How to handle exceptions thrown by another servlet in an application?

Handling exceptions thrown by other servlets is crucial for building robust, user-friendly web applications.

There are multiple ways to handle exceptions in a Java servlet-based application, and it all depends on how centralized and graceful you want the handling to be.

✅ 1. Handle Exceptions with web.xml – Centralized Error Pages

The classic and cleanest way is to configure exception handling in web.xml.

📄 Example:

<error-page>
    <exception-type>java.lang.NullPointerException</exception-type>
    <location>/error.jsp</location>
</error-page>

<error-page>
    <error-code>404</error-code>
    <location>/notfound.jsp</location>
</error-page>

🔧 What This Does:

• If any servlet in your app throws a NullPointerException, the container forwards the user to /error.jsp.
• If a 404 error (resource not found) occurs, user is redirected to /notfound.jsp.

You can use these placeholders in JSP pages:

<%= exception %>  <!-- To show the actual exception -->

✅ 2. Handle Exceptions in the Servlet Itself

You can always use try-catch in individual servlets:

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException {
    try {
        // risky code
    } catch (Exception e) {
        res.setStatus(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
        res.getWriter().write("Oops! Something went wrong.");
    }
}

✔️ Good for specific or critical operations
✖️ But clutters the servlet and doesn’t centralize error handling

✅ 3. Use a Filter for Global Exception Handling

If you want to catch exceptions from any servlet globally, create a Filter:

@WebFilter("/*")
public class ExceptionHandlerFilter implements Filter {
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        try {
            chain.doFilter(request, response);
        } catch (Throwable e) {
            HttpServletResponse res = (HttpServletResponse) response;
            res.setStatus(500);
            res.getWriter().write("Global Error Handler: " + e.getMessage());
        }
    }
}

✔️ Great for catching unexpected exceptions across the whole app
✔️ Centralized error logging
✖️ Not ideal for handling specific exception types differently

✅ 4. Custom Error Servlet

You can map exceptions to a dedicated servlet instead of a JSP:

<error-page>
    <exception-type>java.lang.ArithmeticException</exception-type>
    <location>/ErrorHandler</location>
</error-page>

🧠 In your ErrorHandler servlet:

public class ErrorHandler extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException {
        Throwable throwable = (Throwable) req.getAttribute("javax.servlet.error.exception");
        res.getWriter().write("Exception: " + throwable);
    }
}

🧠 Summary Table

Method	Scope	Use When…
web.xml <error-page>	Application-wide	You want clean and declarative error pages
try-catch in servlet	Servlet-specific	You want tight control over a risky block
Global Filter with try-catch	Entire app	You want global error logging or fallback
Error handling servlet	App-wide, customizable	You want structured error recovery