To integrate Java code into a web application that uses HTML, CSS, and JavaScript, you typically use Java on the backend to handle business logic, while the frontend is handled by HTML/CSS/JS.

A common approach is to use a Java web framework such as **Spring Boot** or **Java EE**. These frameworks allow you to create REST APIs or serve web pages.

**Basic workflow:**

1. Use Java to create backend services (controllers, business logic, database access).

2. Serve static HTML, CSS, and JS files from the Java server or host them separately.

3. Use AJAX/fetch or form submissions from your frontend to communicate with Java backend APIs.

**Example:**

- With Spring Boot, place your static files (HTML, CSS, JS) in the `src/main/resources/static` folder.

- Create REST controllers in Java to handle requests at URLs.

- From JavaScript, call these APIs using `fetch` or `XMLHttpRequest`.

**Resources:**

- [Spring Boot Serving Static Content](https://spring.io/guides/gs/serving-web-content/)

- [Building RESTful Web Services with Spring](https://spring.io/guides/gs/rest-service/)

This separation keeps frontend and backend concerns clean and scalable.