What Is OncePerRequestFilter in spring boot?

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OncePerRequestFilter is a subclass of the Filter class that ensures a particular filter is only executed once per request. It is used to provide certain functionality that should only be applied once per request, rather than on every request-response pair that passes through the filter chain.

For example, you might use a OncePerRequestFilter to ensure that a user is only authenticated once per request, even if the request passes through multiple filters that check for authentication. To do this, the OncePerRequestFilter would store a flag in the request indicating that the user has already been authenticated, and other filters could check for this flag before attempting to re-authenticate the user.

In general, OncePerRequestFilter is a useful utility for implementing filters that should only be applied once per request, without the need to manually keep track of whether the filter has already been applied. It is often used in web frameworks to provide various types of request-scoped functionality, such as authentication, request logging, and so on.

How to use OncePerRequestFilter for Synchronous Requests

To use OncePerRequestFilter for synchronous requests in a Java web application, you will need to do the following:

  1. Create a subclass of OncePerRequestFilter. This subclass should override the doFilterInternal method, which is where you will implement the filter’s logic.
  2. In the doFilterInternal method, check for a flag in the request indicating whether the filter has already been applied. You can use a request attribute or request parameter for this purpose. If the flag is present, return immediately without executing the filter’s logic.
  3. If the flag is not present, set the flag in the request to indicate that the filter has been applied. You can then proceed to execute the filter’s logic.
  4. After the filter’s logic has been executed, remove the flag from the request. This will ensure that the filter can be applied again on subsequent requests.

Here is an example of a OncePerRequestFilter that checks for an “already_filtered” request attribute:

public class MyFilter extends OncePerRequestFilter {

    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {
        if (request.getAttribute("already_filtered") != null) {
            // Filter has already been applied, so return immediately
            return;
        }
        // Set the "already_filtered" attribute to indicate that the filter has been applied
        request.setAttribute("already_filtered", true);

        // Execute the filter's logic here

        // Remove the "already_filtered" attribute
        request.removeAttribute("already_filtered");
    }
}

To use this filter in your web application, you will need to register it in your application’s web.xml file or configure it using some other means (e.g., using Java-based configuration if you are using a framework like Spring).

How to use OncePerRequestFilter for asynchronous Requests

To use OncePerRequestFilter for asynchronous requests in a Java web application, you will need to do the following:

  1. Create a subclass of OncePerRequestFilter as described above.
  2. In the doFilterInternal method, wrap the HttpServletRequest and HttpServletResponse objects in HttpServletRequestWrapper and HttpServletResponseWrapper classes, respectively. These wrapper classes will allow you to store request-specific information in the request and response objects, even after the original request and response have been dispatched to an async thread.
  3. In the wrapper classes, define a method to check for the presence of the “already_filtered” flag, as well as methods to set and remove the flag.
  4. In the doFilterInternal method, check for the presence of the “already_filtered” flag using the method defined in the wrapper class. If the flag is present, return immediately without executing the filter’s logic.
  5. If the flag is not present, set the flag using the method defined in the wrapper class. You can then proceed to execute the filter’s logic.
  6. After the filter’s logic has been executed, remove the flag using the method defined in the wrapper class. This will ensure that the filter can be applied again on subsequent requests.

Here is an example of how you might implement the wrapper classes:

public class MyHttpServletRequestWrapper extends HttpServletRequestWrapper {

    private boolean alreadyFiltered = false;

    public MyHttpServletRequestWrapper(HttpServletRequest request) {
        super(request);
    }

    public boolean isAlreadyFiltered() {
        return alreadyFiltered;
    }

    public void setAlreadyFiltered(boolean alreadyFiltered) {
        this.alreadyFiltered = alreadyFiltered;
    }
}

public class MyHttpServletResponseWrapper extends HttpServletResponseWrapper {

    private boolean alreadyFiltered = false;

    public MyHttpServletResponseWrapper(HttpServletResponse response) {
        super(response);
    }

    public boolean isAlreadyFiltered() {
        return alreadyFiltered;
    }

    public void setAlreadyFiltered(boolean alreadyFiltered) {
        this.alreadyFiltered = alreadyFiltered;
    }
}

And here is an example of how you might use the wrapper classes in your filter:

public class MyFilter extends OncePerRequestFilter {

    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {
        MyHttpServletRequestWrapper wrappedRequest = new MyHttpServletRequestWrapper(request);
        MyHttpServletResponseWrapper wrappedResponse = new MyHttpServletResponseWrapper(response);
        if (wrappedRequest.isAlreadyFiltered() || wrappedResponse.isAlreadyFiltered()) {
            // Filter has already been applied, so return immediately
            return;
        }
        wrappedRequest.setAlreadyFiltered(true);
        wrappedResponse.setAlreadyFiltered(true);

        // Execute the filter's logic here

        wrappedRequest.setAlreadyFiltered(false);
        wrappedResponse.setAlreadyFiltered(false);
    }
}

OncePerRequestFilter is a useful utility for implementing filters that should only be applied once per request in a Java web application. It can be used for both synchronous and asynchronous requests, although the implementation may be slightly different in each case. To use OncePerRequestFilter, you will need to create a subclass of the filter and override the doFilterInternal method, where you will implement the filter’s logic and check for the presence of a flag indicating that the filter has already been applied. If the flag is present, the filter should return immediately without executing its logic. If the flag is not present, the filter should set the flag, execute its logic, and then remove the flag when it is finished. This will ensure that the filter is only applied once per request.

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