Understanding Servlet Dispatcher (Beginner Guide)

🚦Understanding Servlet Dispatcher (Beginner Guide)

In web applications using Java Servlets, sometimes we want one servlet to pass a request to another servlet. This is where Servlet Dispatcher comes in.

🌐 What is Servlet Dispatcher?

A Servlet Dispatcher helps in passing the request from one servlet to another without going back to the network. This is fast and efficient.

There are two types of dispatchers:

Include Dispatcher
Forward Dispatcher

Another way to move from one servlet to another is by using sendRedirect(). But sendRedirect() goes back to the network, which is slower. That’s why RequestDispatcher is better inside a server.

🔄 Real-Life Example: Why Dispatcher?

Imagine three servlets: Servlet1, Servlet2, and Servlet3.

Without Dispatcher

If a user in the USA calls Servlet1 on a server in India:

First, the request goes to India (Servlet1).
Next, if the user wants Servlet2, the request must go back to the server again.
This back-and-forth is slow and increases network traffic.

With Dispatcher

If we use a dispatcher:

Servlet1 passes the request directly to Servlet2 inside the server.
No need to go back to the network.
It saves time and is called Servlet Chaining.

Also, the request data can travel from one servlet to another. This is not possible with sendRedirect().

💡Why Dispatcher is Useful

Faster: Works inside the server.
Data Sharing: Request data moves from servlet to servlet.
Efficient: Great when you need to split work among servlets.

But remember: if there’s a lot of data, or you don’t need to send data, sendRedirect() might be better.

🧩 Types of Request Dispatcher

1. 🧷 Include Dispatcher

The Include Dispatcher adds the response of another servlet into the current servlet’s response — like calling a helper method.

🔧 Example

IncludeServlet includes the response from IncludedServlet.
The browser sees both outputs combined.

Steps:

Create a Java Web Application.
Add IncludeServlet.java and IncludedServlet.java.

IncludeServlet Code:

RequestDispatcher dis = getServletContext().getRequestDispatcher("/IncludedServlet");
dis.include(request, response);

IncludedServlet Code:

out.println("<h1>" + request.getAttribute("command") + " Included Servlet response</h1>");

📝 Note: We removed <html> tags from IncludedServlet so that HTML tags don’t repeat.

✅ Output:

The browser shows:

Servlet IncludeServlet at /YourApp
As commanded by IncludeServlet Included Servlet response
This message will be included.
After Include

2. 📨 Forward Dispatcher

The Forward Dispatcher sends the request from one servlet to another. But this time, only the second servlet sends the response to the browser.

🔧 Example

ForwardServlet forwards the request to ForwardedServlet.
The browser only sees output from ForwardedServlet.

ForwardServlet Code:

RequestDispatcher dis = getServletContext().getRequestDispatcher("/ForwardedServlet");
dis.forward(request, response);

📝 Note: Any code after dis.forward() will not run.

ForwardedServlet Code:

out.println("<h1>" + request.getAttribute("forward") + " Forwarded Servlet response</h1>");

✅ Output:

The browser shows:

Servlet ForwardedServlet at /YourApp
As commanded by ForwardServlet Forwarded Servlet response
After Forwarded

🏁 Conclusion

Use RequestDispatcher when you want to pass data and control between servlets.
Choose Include when you want to combine responses.
Choose Forward when you want one servlet to hand over everything to another.
Use sendRedirect() only when the client needs to make a brand-new request.

Let me know if you'd like a diagram or flowchart to make this even easier to visualize!

Understanding Servlet Dispatcher (Beginner Guide)