How to Handle Trailing Slashes in Spring Boot

Introduction

Two types of slashes exist, the backward slash and the forward slash. A good way to differentiate the two is that a backward slash leans backward(), while a forward slash leans forward(/)

The slash we want to handle in Spring Boot is the forward slash. We want to handle trailing forward slash at the end of an API endpoint.

The problem

When you send a request to this API endpoint api/users/add it will return a state code of 200, but when you send a request to this API endpoint api/users/add/ it will return a status code of 404.
Now we want a situation where api/users/add/ will redirect or point to the same resource that api/users/add points to.
To fix this problem, In Spring Boot 2.7.x we have to add the following configuration.

@Configuration
class WebConfiguration implements WebFluxConfigurer {

    @Override
    public void configurePathMatching(PathMatchConfigurer configurer) {
        configurer.setUseTrailingSlashMatch(true);
    }
}

Sadly, in Spring Boot v3.x.x setUseTrailingSlashMatch(true); was deprecated and if you check this GitHub issue   you see a lot of people who complained of wasting a lot of time trying to resolve the issue of a trailing slash returning a status code of 404 and also you see the Spring Team defending why they deprecated this method.

The Solution

Kindly note that this is not a hack or a workaround.
Now what this solution does is to remove trailing slashes from API endpoints or URLs, to do this issue we have to create a custom filter.

The code above is a Spring component that implements a servlet filter, this is designed to remove trailing slashes from URLs. Here’s a step-by-step explanation of how this filter works:

1. The TrailingSlashFilter class is annotated with @Component, which means it’s a Spring component and will automatically get detected during classpath scanning. This filter is designed to intercept HTTP requests and perform some processing before they reach the controller.

2. The doFilter method is overridden from the Filter interface. This method is called by the servlet container each time a request/response pair is passed through the chain due to a client request for a resource at the end of the chain. The doFilter method takes three parameters: ServletRequest, ServletResponse, and FilterChain. The ServletRequest and ServletResponse are the request and response objects, and the FilterChain is used to pass the request and response objects to the next filter in the chain.

3. Inside the doFilter method, the ServletRequest is cast to an HttpServletRequest, which allows access to HTTP-specific methods. The request URI is then retrieved and checked if it ends with a trailing slash.

4. If the URI ends with a trailing slash, a new URI without the trailing slash is created. A new HttpRequestWrapper object is created with the original request and the new URI. This wrapper overrides the getRequestURI and getRequestURL methods to return the new URI and URL.

5. The doFilter method of the FilterChain is then called with the new request and response objects. If the URI does not end with a trailing slash, the doFilter method of the FilterChain is called with the original request and response objects.

6. The HttpRequestWrapper is a private static class that extends HttpServletRequestWrapper. It overrides the getRequestURI and getRequestURL methods to return the new URI and URL. This is necessary because once a request has been wrapped, the original URI cannot be changed.

In essence, the filter above removes trailing slashes from URLs. It carries this function by creating a new request with the modified URI and passing this new request to the next filter in the chain. If the URI does not end with a trailing slash, the original request is passed to the next filter. This filter does not affect the performance of your API

Don’t Try This

When I was trying to implement this solution, I removed the else statement, because I thought it was redundant.

This code modification caused my application to break at runtime, you can find it in this GitHub Issue  

I had to return the else statement

Frequently Asked Questions

How do I fix a trailing slash in a URL?
To fix a trailing slash with an .htaccess file, you can utilize mod_rewrite to modify the address. Keep in mind that the RewriteRule command triggers a redirection from the original address to the modified one. If you wish to skip this redirection, you can omit the R=301 flag from the RewriteRule command.

What is the trailing slash rule?
A trailing slash is the / at the end of a web address. It helps differentiate between a folder (with the slash) and a file (without). While it’s a helpful practice, it’s not a strict rule, just a good suggestion for clarity.

What is the problem with trailing slash?
Having a trailing slash at the end of your website’s URL might lead to duplicate content problems. In simple terms, Google doesn’t appreciate identical content spread across various pages.

What is the best practice for trailing slash URL?
it’s best not to have multiple versions of the same webpage—one with a trailing slash and one without. If you choose to use a trailing slash, make sure that when someone accesses the URL without it, they are automatically redirected to the correct page instead of seeing an error or a new page. This helps maintain a consistent and user-friendly experience on your website.