Tomcat Filter: A Deep Dive into the Source Code

Web Technologies
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Tomcat Filter: A Deep Dive into the Source Code

In the realm of web development, the Servlet API provides a powerful tool for developers to create custom filters that can intercept and manipulate incoming requests and responses. In this article, we will delve into the source code of the Tomcat Filter, exploring its inner workings and the design patterns that make it tick.

Filter Overview

The Filter interface is a fundamental component of the Servlet API, allowing developers to create custom filters that can be applied to specific resources or URLs within a web application. By implementing the Filter interface in a Java class, developers can intercept incoming requests and responses, performing various operations such as authentication, logging, and caching.

StandardWrapperValve.invoke() Method

The StandardWrapperValve.invoke() method is a crucial component of the Tomcat Filter chain, responsible for creating a filter chain and storing servlet objects to be executed. This method is called when a filter is configured, and it determines the order of filters based on the web.xml configuration.

ApplicationFilterFactory.createFilterChain() Method

The ApplicationFilterFactory.createFilterChain() method is responsible for creating a filter chain, which is a sequence of filters that will be executed in a specific order. This method takes into account the filter-mapping rules configured in the web.xml file, adding filters to the chain in the order specified.

StandardWrapperValve.invoke() Method (Again)

When the servlet filter chain is not empty, the StandardWrapperValve.invoke() method is called again, starting the filter chain execution. Each filter in the chain performs its doFilter() method, which is the heart of the chain of responsibility design pattern.

ApplicationFilterChain.doFilter() → internalDoFilter()

The ApplicationFilterChain.doFilter() method is responsible for executing the filter chain, calling the doFilter() method of each filter in sequence. This process continues until all filters have been executed, at which point the servlet.service() method is called to execute the servlet.

The Chain of Responsibility Design Pattern

The Filter chain employs the chain of responsibility design pattern, where each filter is responsible for determining whether to invoke the next filter in the chain. This process continues until all filters have been executed, at which point the servlet is executed.

Code Snippets

Here are some key code snippets from the Tomcat Filter source code:

// StandardWrapperValve.invoke() method
public void invoke(Request request, Response response) {
    // Create a filter chain and store servlet objects to be executed
    FilterChain filterChain = createFilterChain(request, response);
    // Execute the filter chain
    filterChain.doFilter(request, response);
}

// ApplicationFilterFactory.createFilterChain() method
public FilterChain createFilterChain(Request request, Response response) {
    // Determine the order of filters based on web.xml configuration
    List<Filter> filters = new ArrayList<>();
    // Add filters to the chain in the order specified
    filters.add(filter1);
    filters.add(filter2);
    filters.add(filter3);
    // Return the filter chain
    return new ApplicationFilterChain(filters);
}

// ApplicationFilterChain.doFilter() method
public void doFilter(Request request, Response response) {
    // Execute each filter in the chain
    for (Filter filter : filters) {
        filter.doFilter(request, response, this);
    }
}

Conclusion

In this article, we have delved into the source code of the Tomcat Filter, exploring its inner workings and the design patterns that make it tick. By understanding how the Filter chain works, developers can create custom filters that can intercept and manipulate incoming requests and responses, enhancing the functionality of their web applications.