CheerpJ Library Mode Test

--- title: "Run a Java Library" source: "https://cheerpj.com/docs/getting-started/Java-library" --- # Run a Java Library Use Java classes in JavaScript CheerpJ loads an existing Java library in the browser, so you can call its classes and methods from JavaScript without changing the original code. This page shows how to load a library’s JAR file in a web page and call it. You need the library’s compiled JAR file, not its source code. **To get started you will need:** - Your library’s JAR file. - An HTML page to load it from. - An HTTP server that supports Range requests, to test the page locally. ## 1. Create a project directory Create a folder for the project and copy your JAR file into it. The HTML file goes in the same folder. ```shell mkdir my-project ``` ## 2. Create a basic HTML file Create an `index.html` file in the project directory. Add CheerpJ’s loader script to the ``, then call [`cheerpjInit`](https://cheerpj.com/docs/reference/cheerpjInit.md) to initialise the CheerpJ runtime environment: `index.html` ```html CheerpJ Library Mode Test ``` The script is a module (`type="module"`) so that it can use `await` at the top level. In a classic ` ``` Loading a class, constructing an object (`await new`) and calling a method are asynchronous, so `await` each one. A method call resolves to the method’s return value, converted as described in [`CJ3Library`](https://cheerpj.com/docs/reference/CJ3Library.md). Public fields are read and written directly, without `await`. To use only the Java standard library, pass an empty string: `await cheerpjRunLibrary("")`. ## 4. Host your page Serve the project directory with an HTTP server, such as http-server: ```shell cd my-project npx http-server -p 8080 ``` Then open `http://localhost:8080` in the browser. > **Info** > > Opening the page directly from disk (a file:// URL) does not work. The server must support HTTP Range requests: npx http-server and npx serve do, Python’s http.server does not. See basic server setup, and use HTTPS in production. ## The result CheerpJ starts in the browser and loads your library. Once it has loaded, your JavaScript calls the library’s methods. Output the library writes to `System.out` appears in the browser console. Depending on the size of the library and the optimisations applied, this can take a few seconds before your code starts running. If nothing happens, open the browser console. A JavaScript error or a 404 for the JAR in the Network tab usually means the path to the JAR is wrong or is missing the `/app/` prefix. ## Further reading - [Learn more about library mode](https://cheerpj.com/docs/guides/library-mode.md) - [Interoperability tutorial](https://cheerpj.com/docs/tutorials/interoperability-tutorial.md) - [`cheerpjRunLibrary` reference](https://cheerpj.com/docs/reference/cheerpjRunLibrary.md)