What is the reason for Java configuration not functioning when web.xml is not present?

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This is because web.xml file is a crucial part of Java web application. The web.xml file contains configuration information for the Java EE application, including the mapping of servlet and JSP classes to URLs, security constraints, and resource references. Without this configuration information, the Java web application won't be able to function properly. Therefore, if the web.xml file is not present, the Java configuration settings won't be picked up by the web application.