Loading Resources

How resources are stored in a project, and how to access them in an application.

Resource Cheat Sheet
Importing JavaScript and CSS
Storing Resources

Applications can use various types of resources, such as JavaScript code, CSS, and images. This article describes how they are stored in a project and then accessed in an application.

Resource Cheat Sheet

The following tables show how to load various resources and where the resource files should be placed in a project.

Table 1. In non-Spring projects
File type	Import	File location
CSS files	`@CssImport("./my-styles/styles.css")`[1]	`/frontend/my-styles/styles.css`
JavaScript, TypeScript and Lit templates	`@JsModule("./src/my-script.js")`[1]	`/frontend/src/my-script.js`
Static files, such as images and icons (such as the favicon)	`new Image("img/flower.jpg", "A flower")`	`/src/main/webapp/img/flower.jpg` + `/src/main/webapp/favicon.icon`

Table 2. In Spring projects
File type	Import	File location
CSS files	`@CssImport("./my-styles/styles.css")`[1]	`/frontend/my-styles/styles.css`
JavaScript, TypeScript and Lit templates	`@JsModule("./src/my-script.js")`[1]	`/frontend/src/my-script.js`
Static files, such as images and icons (for example, the favicon)	`new Image("img/flower.jpg", "A flower")`	`/src/main/resources/META-INF/resources/img/flower.jpg` `/src/main/webapp/favicon.icon`

Table 3. In add-ons
File type	Import	File location
CSS files	`@CssImport("./my-styles/styles.css")`[1]	`/src/main/resources/META-INF/resources/frontend/my-styles/styles.css`
JavaScript, TypeScript and Lit templates	`@JsModule("./src/my-script.js")`[1]	`/src/main/resources/META-INF/resources/frontend/src/my-script.js`
Static files, such as images and icons (for example, the favicon)	`new Image("img/flower.jpg", "A flower")`	`/src/main/resources/META-INF/resources/img/flower.jpg` `/src/main/webapp/favicon.icon`

Footnotes

The @JsModule and @CssImport annotations can also be used to import from an npm package. In this case, the path is defined as @JsModule("@polymer/paper-input") or @CssImport("some-package/style.css"). Paths referring to the local front-end directory should be prefixed with ./.

Importing JavaScript and CSS

You can add JavaScript files in two ways: either by using annotations or through the Page object.

Sometimes the way client-side resources are loaded by the browser affects the functionality of the application. Vaadin Flow provides advanced methods to configure the loading of these resources.

Using Annotations

The following example shows how to import JavaScript files into CustomComponent:

@Tag("div")
@JavaScript("/js/script.js")
@JsModule("./src/my-module.js")
@StyleSheet(value = "/css/big_style_file.css")
static class CustomComponent extends Component
        implements HasText {
  // implementation omitted
}

The following annotations are available:

@JsModule: Defines a JavaScript module dependency. These dependencies are loaded first.

Lit templates should be imported using @JsModule (see Creating a Simple Component Using the Template API for more information).
@JavaScript: Defines a JavaScript file dependency. The file is loaded according to the load mode, as described in Load Mode.
@CssImport: Imports a local style sheet, which can be bundled in the front-end bundle.

See Importing Bundled Style Sheets.
@StyleSheet: Imports an external or linked style sheet.

See Importing External Style Sheets.

All the resource annotations are repeatable. Add one annotation for each file that you need to add.

Loading JavaScript with the Page Object

You can use the addJavaScript(String url) method in Page to load JavaScript. The Page object also has an addJsModule(String url) method, which you can use to load an external JavaScript module.

The following example uses addJavaScript(String url) to import JavaScript files:

UI.getCurrent().getPage().addJavaScript("/js/script.js");
// external JavaScript module
UI.getCurrent().getPage()
        .addJsModule("https://unpkg.com/lodash@4.17.15");

Dependency Loading Order

Imported dependency files of the same type load in the order they are defined in the class. For example, CSS files load in the @CssImport annotation definition order, JavaScript files in the @JsModule and @JavaScript annotation definition order.

The loading order of imported dependencies is only guaranteed for one file type, in one class. Specifically, loading order isn’t guaranteed between classes; annotations on class A could be imported before or after annotations on class B.

Front-end resources bundled by Vite also have a type group ordering; JavaScript files loaded by the @JsModule annotation always come before JavaScript files loaded by @JavaScript, and both of those come before CSS files loaded by @CssImport. The exception to this rule is @JsModule annotations of files annotated with @Theme. All JavaScript modules found on such classes are imported before other file types. This covers both the Lumo and Material themes, as well as custom themes created by the developer.

For example, you could have multiple imported dependencies of different file types in a single class, as follows:

@JavaScript("1.js")
@JsModule("a.js")
@CssImport("1.css")
@JavaScript("2.js")
@JsModule("b.js")
@CssImport("2.css")
static class OrderedDependencies extends Div {
}

The loading order of the files would be: a.js, b.js, 1.js, 2.js, 1.css, and 2.css.

Imports on other classes could be before or after the imports present here (within each file group).

You can control the load order of dependencies of different file types by adding imports within a JavaScript import.

In the following example, using JavaScript imports ensures that custom-css.js runs before javascript-file.js. The custom-css.js code uses the technique for wrapping CSS into JavaScript presented in Importing Style Sheets.

import '../styles/custom-css.js';
import './javascript-file.js';

Load Mode

Resources referenced with annotations or loaded with the methods in the Page object can be imported with different levels of "eagerness". This is controlled with the load mode.

The load mode doesn’t affect files that are bundled by Vite. These files are included into the front-end resource bundle and are available after the bundle has been loaded.

Three load modes are available:

LoadMode.EAGER (default)

This is the default load mode for all dependencies. The "eager" mode ensures that the dependency is loaded as soon as possible, and before the initial page load.

The "eager" mode is usually suitable. Use it if you are unsure which mode to use.

LoadMode.INLINE

The dependencies are included inline in the body of the page. This mode eliminates round trips when fetching dependencies. If the contents can’t be fetched, an exception is thrown and loading stops.

Note	Pay attention to URLs used for inline dependencies; the URLs may change and could be incorrect after loading.

LoadMode.LAZY

The dependencies are loaded in the background, after all eager and inline dependencies have loaded. "Lazy" dependency loading is independent of page initialization.

"Lazy" mode is suitable when you need to load the dependency, but it isn’t important when it’s loaded.

You can give the load mode as a parameter for annotations that load the resources.

The following example shows how to use annotations to add resource files:

@Tag("div")
@StyleSheet(value = "/css/big_style_file.css",
        loadMode = LoadMode.INLINE)
@JavaScript(value = "/js/animation.js",
        loadMode = LoadMode.LAZY)
public class MainLayout extends Component {
    // implementation omitted
}

When loading resources with the Page object, you can use the following methods:

addStyleSheet(String url, LoadMode loadMode)
addJavaScript(String url, LoadMode loadMode)

For example:

  public MainLayout() {
      UI.getCurrent().getPage().addStyleSheet(
            "/css/big_style_file.css", LoadMode.INLINE);
      UI.getCurrent().getPage().addJavaScript(
            "/js/animation.js", LoadMode.LAZY);
  }
}

Load-Order Guarantees

All "eager" and inline dependencies are guaranteed to load before "lazy" dependencies.

For example, a component could use JavaScript animation, say /js/animation.js. It’s optional and not required to display when the page is loaded. You can postpone its loading, giving priority to other resources.

Dependencies with the same load mode are guaranteed to load in the order defined in the component. This is true for all load modes.

Storing Resources

Resources can be loaded as individual files or bundled into the front-end bundle that also includes all Vaadin web components and other resources.

Bundled Front-End Resources

Vaadin bundles all the web components used in an application into a single front-end bundle file, which can be loaded efficiently when the application page is loaded. You can include your own files into the bundle as well.

Static resources that are bundled using Vite and referenced with the @JavaScript, @JsModule, and @CssImport annotations should be placed under {project directory}/frontend. This includes normal JavaScript and TypeScript files, Lit template files, and CSS files. When importing files using these annotations, prefix the path with ./, which signifies the frontend/ directory. For example, a CSS file my-custom.css under {project directory}/frontend/styles/my-custom.css would be referenced @CssImport("./styles/my-custom.css").

If the ./ prefix is missing from an @JsModule annotation, the import is treated as a reference to an npm module under the node_modules/ folder.

Static Resources

This section covers static resource locations for resources that shouldn’t be bundled by Vite.

You can place your resource files (CSS style sheets and JavaScript files, and other static resources) in any folder in your Web Archive (WAR) file, except /VAADIN, which is reserved for internal framework use.

VaadinServlet handles static resource requests, if you have mapped it to /*. If not, the servlet container takes care of static resource requests.

If you use relative URLs, it’s irrelevant whether your application is deployed in the root context (for example http://mysite.com/) or in a sub-context (for example http://mysite.com/myapp/). Relative URLs are resolved using the page base URI, which is always set to match the servlet URL.

Using a Servlet Path

If you use a servlet path for the servlet, for example http://mysite.com/myapp/myservlet/, you need to take the servlet path into account when including resources. This is because the base URI is http://mysite.com/myapp/myservlet/, but static resources are deployed at http://mysite.com/myapp/.

You can use the context:// protocol, with the Page.addStyleSheet() method, for example. This ensures that the URL is relative to the context path. This protocol is only supported when including resources.

When you configure an element, for example setting the src attribute for an <img>, you can’t use the context:// protocol.

Your options are:

Take the servlet path into account with your relative URL, for example ../images/logo.png.
Use an absolute URL, for example /myapp/images/logo.png.
Deploy your static resources in a directory that matches your servlet path, for example /myservlet/.

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