Last updated: 01 May 2017
FXML is a scriptable, XML-based markup language for constructing Java object graphs. It provides a convenient alternative to constructing such graphs in procedural code, and is ideally suited to defining the user interface of a JavaFX application, since the hierarchical structure of an XML document closely parallels the structure of the JavaFX scene graph.
This document introduces the FXML markup language and explains how it can be used to simplify development of JavaFX applications.
In FXML, an XML element represents one of the following:
Class instances, instance properties, static properties, and define blocks are discussed in this section below. Scripting is discussed in a later section.
Class instances can be constructed in FXML in several ways. The most common is via instance declaration elements, which simply create a new instance of a class by name. Other ways of creating class instances include referencing existing values, copying existing values, and including external FXML files. Each is discussed in more detail below.
If an element's tag is considered an instance declaration if the tag begins with uppercase letter (and the class is imported) or, as in Java, it denotes a fully-qualified (including the package name) name of a class. When the FXML loader (also introduced later) encounters such an element, it creates an instance of that class.
Importing a class is done using the "import" processing instruction (PI). For example, the following PI imports the javafx.scene.control.Label class into the current FXML document’s namespace:
<?import javafx.scene.control.Label?>
This PI imports all classes from the javafx.scene.control package into the current namespace:
<?import javafx.scene.control.*?>
Any class that adheres to JavaBean constructor and property naming conventions can be readily instantiated and configured using FXML. The following is a simple but complete example that creates an instance of javafx.scene.control.Label and sets its "text" property to "Hello, World!":
<?import javafx.scene.control.Label?> <Label text="Hello, World!"/>
Note that the Label’s "text" property in this example is set using an XML attribute. Properties can also be set using nested property elements. Property elements are discussed in more detail later in this section. Property attributes are discussed in a later section.
Classes that don't conform to Bean conventions can also be constructed in FXML, using an object called a "builder". Builders are discussed in more detail later.
Internally, the FXML loader uses an instance of com.sun.javafx.fxml.BeanAdapter to wrap an instantiated object and invoke its setter methods. This (currently) private class implements the java.util.Map interface and allows a caller to get and set Bean property values as key/value pairs.
If an element represents a type that already implements Map (such as java.util.HashMap), it is not wrapped and its get() and put() methods are invoked directly. For example, the following FXML creates an instance of HashMap and sets its "foo" and "bar" values to "123" and "456", respectively:
<HashMap foo="123" bar="456"/>
The fx:value attribute can be used to initialize an instance of a type that does not have a default constructor but provides a static valueOf(String) method. For example, java.lang.String as well as each of the primitive wrapper types define a valueOf() method and can be constructed in FXML as follows:
<String fx:value="Hello, World!"/> <Double fx:value="1.0"/> <Boolean fx:value="false"/>
Custom classes that define a static valueOf(String) method can also be constructed this way.
The fx:factory attribute is another means of creating objects whose classes do not have a default constructor. The value of the attribute is the name of a static, no-arg factory method for producing class instances. For example, the following markup creates an instance of an observable array list, populated with three string values:
<FXCollections fx:factory="observableArrayList">
<String fx:value="A"/>
<String fx:value="B"/>
<String fx:value="C"/>
</FXCollections>
A third means of creating instances of classes that do not conform to Bean conventions (such as those representing immutable values) is a "builder". The builder design pattern delegates object construction to a mutable helper class (called a "builder") that is responsible for manufacturing instances of the immutable type.
Builder support in FXML is provided by two interfaces. The javafx.util.Builder interface defines a single method named build() which is responsible for constructing the actual object:
public interface Builder<T> {
public T build();
}
A javafx.util.BuilderFactory is responsible for producing builders that are capable of instantiating a given type:
public interface BuilderFactory {
public Builder<?> getBuilder(Class<?> type);
}
A default builder factory, JavaFXBuilderFactory, is provided in the javafx.fxml package. This factory is capable of creating and configuring most immutable JavaFX types. For example, the following markup uses the default builder to create an instance of the immutable javafx.scene.paint.Color class:
<Color red="1.0" green="0.0" blue="0.0"/>
Note that, unlike Bean types, which are constructed when the element's start tag is processed, objects constructed by a builder are not instantiated until the element's closing tag is reached. This is because all of the required arguments may not be available until the element has been fully processed. For example, the Color object in the preceding example could also be written as:
<Color>
<red>1.0</red>
<green>0.0</green>
<blue>0.0</blue>
</Color>
The Color instance cannot be fully constructed until all three of the color components are known.
When processing markup for an object that will be constructed by a builder, the Builder instances are treated like value objects - if a Builder implements the Map interface, the put() method is used to set the builder's attribute values. Otherwise, the builder is wrapped in a BeanAdapter and its properties are assumed to be exposed via standard Bean setters.
The <fx:include> tag creates an object from FXML markup defined in another file. It is used as follows:
<fx:include source="filename"/>
where filename is the name of the FXML file to include. Values that begin with a leading slash character are treated as relative to the classpath. Values with no leading slash are considered relative to the path of the current document.
For example, given the following markup:
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<VBox xmlns:fx="http://javafx.com/fxml">
<children>
<fx:include source="my_button.fxml"/>
</children>
</VBox>
If my_button.fxml contains the following:
<?import javafx.scene.control.*?> <Button text="My Button"/>
the resulting scene graph would contain a VBox as a root object with a single Button as a child node.
Note the use of the "fx" namespace prefix. This is a reserved prefix that defines a number of elements and attributes that are used for internal processing of an FXML source file. It is generally declared on the root element of a FXML document. Other features provided by the "fx" namespace are described in the following sections.
<fx:include> also supports attributes for specifying the name of the resource bundle that should be used to localize the included content, as well as the character set used to encode the source file. Resource resolution is discussed in a later section.
<fx:include source="filename" resources="resource_file" charset="utf-8"/>
The <fx:constant> element creates a reference to a class constant. For example, the following markup sets the value of the "minWidth" property of aButton instance to the value of the NEGATIVE_INFINITY constant defined by the java.lang.Double class:
<Button>
<minHeight><Double fx:constant="NEGATIVE_INFINITY"/></minHeight>
</Button>
The <fx:reference> element creates a new reference to an existing element. Wherever this tag appears, it will effectively be replaced by the value of the named element. It is used in conjunction with either the fx:id attribute or with a script variables, both of which are discussed in more detail in later sections. The "source" attribute of the <fx:reference> element specifies the name of the object to which the new element will refer.
For example, the following markup assigns a previously-defined Image instance named "myImage" to the "image" property of an ImageView control:
<ImageView>
<image>
<fx:reference source="myImage"/>
</image>
</ImageView>
Note that, since it is also possible to dereference a variable using the attribute variable resolution operator (discussed later in the Attributes section), fx:reference is generally only used when a reference value must be specified as an element, such as when adding the reference to a collection:
<ArrayList>
<fx:reference source="element1"/>
<fx:reference source="element2"/>
<fx:reference source="element3"/>
</ArrayList>
For most other cases, using an attribute is simpler and more concise.
The <fx:copy> element creates a copy of an existing element. Like <fx:reference>, it is used with the fx:id attribute or a script variable. The element's "source" attribute specifies the name of the object that will be copied. The source type must define a copy constructor that will be used to construct the copy from the source value.
At the moment, no JavaFX platform classes provide such a copy constructor, so this element is provided primarily for use by application developers. This may change in a future release.
The <fx:root> element creates a reference to a previously defined root element. It is only valid as the root node of an FXML document. <fx:root> is used primarily when creating custom controls that are backed by FXML markup. This is discussed in more detail in the FXMLLoader section.
Elements whose tag names begin with a lowercase letter represent object properties. A property element may represent one of the following:
If an element represents a property setter, the contents of the element (which must be either a text node or a nested class instance element) are passed as the value to the setter for the property.
For example, the following FXML creates an instance of the Label class and sets the value of the label's "text" property to "Hello, World!":
<?import javafx.scene.control.Label?>
<Label>
<text>Hello, World!</text>
</Label>
This produces the same result as the earlier example which used an attribute to set the "text" property:
<?import javafx.scene.control.Label?> <Label text="Hello, World!"/>
Property elements are generally used when the property value is a complex type that can't be represented using a simple string-based attribute value, or when the character length of the value is so long that specifying it as an attribute would have a negative impact on readability.
FXML uses "type coercion" to convert property values to the appropriate type as needed. Type coercion is required because the only data types supported by XML are elements, text, and attributes (whose values are also text). However, Java supports a number of different data types including built-in primitive value types as well as extensible reference types.
The FXML loader uses the coerce() method of BeanAdapter to perform any required type conversions. This method is capable of performing basic primitive type conversions such as String to boolean or int to double, and will also convert String to Class or String to Enum. Additional conversions can be implemented by defining a static valueOf() method on the target type.
A read-only list property is a Bean property whose getter returns an instance of java.util.List and has no corresponding setter method. The contents of a read-only list element are automatically added to the list as they are processed.
For example, the "children" property of javafx.scene.Group is a read-only list property representing the group's child nodes:
<?import javafx.scene.*?>
<?import javafx.scene.shape.*?>
<Group xmlns:fx="http://javafx.com/fxml">
<children>
<Rectangle fx:id="rectangle" x="10" y="10" width="320" height="240"
fill="#ff0000"/>
...
</children>
</Group>
As each sub-element of the <children> element is read, it is added to the list returned by Group#getChildren().
A read-only map property is a bean property whose getter returns an instance of java.util.Map and has no corresponding setter method. The attributes of a read-only map element are applied to the map when the closing tag is processed.
The "properties" property of javafx.scene.Node is an example of a read-only map property. The following markup sets the "foo" and "bar" properties of a Label instance to "123" and "456", respectively:
<?import javafx.scene.control.*?>
<Button>
<properties foo="123" bar="456"/>
</Button>
Note that a read-only property whose type is neither a List nor a Map will be treated as if it were a read-only map. The return value of the getter method will be wrapped in a BeanAdapter and can be used in the same way as any other read-only map.
A class may define a "default property" using the @DefaultProperty annotation defined in the javafx.beans package. If present, the sub-element representing the default property can be omitted from the markup.
For example, since javafx.scene.layout.Pane (the superclass of javafx.scene.layout.VBox) defines a default property of "children", a <children> element is not required; the loader will automatically add the sub-elements of the VBox to the container's "children" collection:
<?import javafx.scene.*?>
<?import javafx.scene.shape.*?>
<VBox xmlns:fx="http://javafx.com/fxml">
<Button text="Click Me!"/>
...
</VBox>
Note that default properties are not limited to collections. If an element's default property refers to a scalar value, any sub-element of that element will be set as the value of the property.
For example, since javafx.scene.control.ScrollPane defines a default property of "content", a scroll pane containing a TextArea as its content can be specified as follows:
<ScrollPane>
<TextArea text="Once upon a time..."/>
</ScrollPane>
Taking advantage of default properties can significantly reduce the verbosity of FXML markup.
An element may also represent a "static" property (sometimes called an "attached property"). Static properties are properties that only make sense in a particular context. They are not intrinsic to the class to which they are applied, but are defined by another class (often, the parent container of a control).
Static properties are prefixed with the name of class that defines them. For example, The following FXML invokes the static setter for GridPane's "rowIndex" and "columnIndex" properties:
<GridPane>
<children>
<Label text="My Label">
<GridPane.rowIndex>0</GridPane.rowIndex>
<GridPane.columnIndex>0</GridPane.columnIndex>
</Label>
</children>
</TabPane>
This translates roughly to the following in Java:
GridPane gridPane = new GridPane();
Label label = new Label();
label.setText("My Label");
GridPane.setRowIndex(label, 0);
GridPane.setColumnIndex(label, 0);
gridPane.getChildren().add(label);
The calls to GridPane#setRowIndex() and GridPane#setColumnIndex() "attach" the index data to the Label instance. GridPane then uses these during layout to arrange its children appropriately. Other containers, including AnchorPane, BorderPane, and StackPane, define similar properties.
As with instance properties, static property elements are generally used when the property value cannot be efficiently represented by an attribute value. Otherwise, static property attributes (discussed in a later section) will generally produce more concise and readable markup.
The <fx:define> element is used to create objects that exist outside of the object hierarchy but may need to be referred to elsewhere.
For example, when working with radio buttons, it is common to define a ToggleGroup that will manage the buttons' selection state. This group is not part of the scene graph itself, so should not be added to the buttons' parent. A define block can be used to create the button group without interfering with the overall structure of the document:
<VBox>
<fx:define>
<ToggleGroup fx:id="myToggleGroup"/>
</fx:define>
<children>
<RadioButton text="A" toggleGroup="$myToggleGroup"/>
<RadioButton text="B" toggleGroup="$myToggleGroup"/>
<RadioButton text="C" toggleGroup="$myToggleGroup"/>
</children>
</VBox>
Elements in define blocks are usually assigned an ID that can be used to refer to the element's value later. IDs are discussed in more detail in later sections.
An attribute in FXML may represent one of the following:
Each are discussed in more detail in the following sections.
Like property elements, attributes can also be used to configure the properties of a class instance. For example, the following markup creates a Button whose text reads "Click Me!":
<?import javafx.scene.control.*?> <Button text="Click Me!"/>
As with property elements, property attributes support type coercion. When the following markup is processed, the "x", "y", "width", and "height" values will be converted to doubles, and the "fill" value will be converted to a Color:
<Rectangle fx:id="rectangle" x="10" y="10" width="320" height="240"
fill="#ff0000"/>
Unlike property elements, which are applied as they are processed, property attributes are not applied until the closing tag of their respective element is reached. This is done primarily to facilitate the case where an attribute value depends on some information that won't be available until after the element's content has been completely processed (for example, the selected index of a TabPane control, which can't be set until all of the tabs have been added).
Another key difference between property attributes and property elements in FXML is that attributes support a number of "resolution operators" that extend their functionality. The following operators are supported and are discussed in more detail below:
As strings, XML attributes cannot natively represent typed location information such as a URL. However, it is often necessary to specify such locations in markup; for example, the source of an image resource. The location resolution operator (represented by an "@" prefix to the attribute value) is used to specify that an attribute value should be treated as a location relative to the current file rather than a simple string.
For example, the following markup creates an ImageView and populates it with image data from my_image.png, which is assumed to be located at a path relative to the current FXML file:
<ImageView>
<image>
<Image url="@my_image.png"/>
</image>
</ImageView>
Since Image is an immutable object, a builder is required to construct it. Alternatively, if Image were to define a valueOf(URL) factory method, the image view could be populated as follows:
<ImageView image="@my_image.png"/>
The value of the "image" attribute would be converted to a URL by the FXML loader, then coerced to an Image using the valueOf() method.
Note that whitespace values in the URL must be encoded; for example, to refer to a file named "My Image.png", the FXML document should contain the following:
<Image url="@My%20Image.png"/>
rather than:
<Image url="@My Image.png"/>
In FXML, resource substitution can be performed at load time for localization purposes. When provided with an instance of java.util.ResourceBundle, the FXML loader will replace instances of resource names with their locale-specific values. Resource names are identified by a "%" prefix, as shown below:
<Label text="%myText"/>
If the loader is given a resource bundle defined as follows:
myText = This is the text!
the output of the FXML loader would be a Label instance containing the text "This is the text!".
An FXML document defines a variable namespace in which named elements and script variables may be uniquely identified. The variable resolution operator allows a caller to replace an attribute value with an instance of a named object before the corresponding setter method is invoked. Variable references are identified by a "$" prefix, as shown below:
<fx:define>
<ToggleGroup fx:id="myToggleGroup"/>
</fx:define>
...
<RadioButton text="A" toggleGroup="$myToggleGroup"/>
<RadioButton text="B" toggleGroup="$myToggleGroup"/>
<RadioButton text="C" toggleGroup="$myToggleGroup"/>
Assigning an fx:id value to an element creates a variable in the document's namespace that can later be referred to by variable dereference attributes, such as the "toggleGroup" attribute shown above, or in script code, discussed in a later section. Additionally, if the object's type defines an "id" property, this value will also be passed to the objects setId() method.
If the value of an attribute begins with one of the resource resolution prefixes, the character can be escaped by prepending it with a leading backslash ("\") character. For example, the following markup creates a Label instance whose text reads "$10.00":
<Label text="\$10.00"/>
Attribute variables as shown above are resolved once at load time. Later updates to the variables value are not automatically reflected in any properties to which the value was assigned. In many cases, this is sufficient; however, it is often convenient to "bind" a property value to a variable or expression such that changes to the variable are automatically propagated to the target property. Expression bindings can be used for this purpose.
An expression binding also begins with the variable resolution operator, but is followed by a set of curly braces which wrap the expression value. For example, the following markup binds the value of a text input's "text" property to the "text" property of a Label instance:
<TextField fx:id="textField"/>
<Label text="${textField.text}"/>
As the user types in the text input, the label's text content will be automatically updated.
More complex expression are also supported. A list of supported constants and operators follows:
| Constant / Operator | Description |
|---|---|
| "string" 'string' | A string constant |
| true false | A boolean constant |
| null | A constant representing the null value |
| 50.0 3e5 42 | A numerical constant |
| - (unary operator) | Unary minus operator, applied on a number |
| ! (unary operator) | Unary negation of a boolean |
| + - * / % | Numerical binary operators |
| && || | Boolean binary operators |
| > >= < <= == != |
Binary operators of comparison. Both arguments must be of type Comparable |
Attributes representing static properties are handled similarly to static property elements and use a similar syntax. For example, the earlier GridPane markup shown earlier to demonstrate static property elements could be rewritten as follows:
<GridPane>
<children>
<Label text="My Label" GridPane.rowIndex="0" GridPane.columnIndex="0"/>
</children>
</TabPane>
In addition to being more concise, static property attributes, like instance property attributes, support location, resource, and variable resolution operators, the only limitation being that it is not possible to create an expression binding to a static property.
Event handler attributes are a convenient means of attaching behaviors to document elements. Any class that defines a setOnEvent() method can be assigned an event handler in markup.
FXML supports three types of event handler attributes: script event handlers, controller method event handlers and expressions. Each are discussed below.
A script event handler is an event handler that executes script code when the event is fired, similar to event handlers in HTML. For example, the following script-based handler for the button's "onAction" event uses JavaScript to write the text "You clicked me!" to the console when the user presses the button:
<?language javascript?>
...
<VBox>
<children>
<Button text="Click Me!"
onAction="java.lang.System.out.println('You clicked me!');"/>
</children>
</VBox>
Note the use of the language processing instruction at the beginning of the code snippet. This PI tells the FXML loader which scripting language should be used to execute the event handler. A page language must be specified whenever inline script is used in an FXML document, and can only be specified once per document. However, this does not apply to external scripts, which may be implemented using any number of supported scripting languages. Scripting is discussed in more detail in the next section.
Note: to turn off automatic compilation of script code place the processing instruction <?compile false?> before the element that contains the script. To turn on compilation of script code again use the processing instruction <?compile true?> (or short: <?compile?>). The compile processing instruction can be used repeatedly to turn compilation of script code off and on.
Note: The JavaScript script engine is disabled by default. If the JDK has a JavaScript script engine, it can be enabled using a system property javafx.allowjs=true.
A controller method event handler is a method defined by a document's "controller". A controller is an object that is associated with the deserialized contents of an FXML document and is responsible for coordinating the behaviors of the objects (often user interface elements) defined by the document.
A controller method event handler is specified by a leading hash symbol followed by the name of the handler method. For example:
<VBox fx:controller="com.foo.MyController"
xmlns:fx="http://javafx.com/fxml">
<children>
<Button text="Click Me!" onAction="#handleButtonAction"/>
</children>
</VBox>
Note the use of the fx:controller attribute on the root element. This attribute is used to associate a controller class with the document. If MyController is defined as follows:
package com.foo;
public class MyController {
public void handleButtonAction(ActionEvent event) {
System.out.println("You clicked me!");
}
}
the handleButtonAction() will be called when the user presses the button, and the text "You clicked me!" will be written to the console.
In general, a handler method should conform to the signature of a standard event handler; that is, it should take a single argument of a type that extends javafx.event.Event and should return void (similar to an event delegate in C#). The event argument often carries important and useful information about the nature of the event; however, it is optional and may be omitted if desired. So this is also a valid handler:
package com.foo;
public class MyController {
public void handleButtonAction() {
System.out.println("You clicked me!");
}
}
Controllers are discussed in more detail in a later section.
Any expression that point to a variable of javafx.event.EventHandler type can be used as an expression handler.
Previous example using an expression handler:
<VBox fx:controller="com.foo.MyController"
xmlns:fx="http://javafx.com/fxml">
<children>
<Button text="Click Me!" onAction="$controller.onActionHandler"/>
</children>
</VBox>
With the controller that contains a field like this
public class MyController {
@FXML
public EventHandler<ActionEvent> onActionHandler = new EventHandler<>() { ... }
...
}
Note that other kinds of expressions, like binding expressions are not supported in this context.
Collections and object properties cannot be listen to using setOnEvent() methods. For these reason, special handler methods need to be used. ObservableList, ObservableMap or ObservableSet uses a special onChange attribute that points to a handler method with a ListChangeListener.Change, MapChangeListener.Change or SetChangeListener.Change parameter, respectively.
<VBox fx:controller="com.foo.MyController"
xmlns:fx="http://javafx.com/fxml">
<children onChange="#handleChildrenChange"/>
</VBox>
where the handler method looks like this:
package com.foo;
import javafx.collections.ListChangeListener.Change;
public class MyController {
public void handleChildrenChange(ListChangeListener.Change c) {
System.out.println("Children changed!");
}
}
Similarly, the property handlers are methods that have the same parameters as changed method of ChangeListener :
changed(ObservableValue<? extends T> observable, T oldValue, T newValue)
A handler for parent property would look like this
public class MyController {
public void handleParentChange(ObservableValue value, Parent oldValue, Parent newValue) {
System.out.println("Parent changed!");
}
}
For convenience, the first parameter can be a subclass of ObservableValue, e.g. Property
For registering to a property, a special on<propertyName>Change attribute must be used.
<VBox fx:controller="com.foo.MyController"
xmlns:fx="http://javafx.com/fxml" onParentChange="#handleParentChange"/>
Note that collections and properties do not currently support scripting handlers.
The <fx:script> tag allows a caller to import scripting code into or embed script within a FXML file. Any JVM scripting language can be used, including JavaScript, Groovy, and Clojure, among others. Script code is often used to define event handlers directly in markup or in an associated source file, since event handlers can often be written more concisely in more loosely-typed scripting languages than they can in a statically-typed language such as Java.
Note: The JavaScript script engine is disabled by default. If the JDK has a JavaScript script engine, it can be enabled using a system property javafx.allowjs=true.
Scripts are compiled by default, when they are first loaded, if the ScriptEngine implements the javax.script.Compilable interface. If compilation fails, the FXMLLoader will fall back to interpreted mode.
Note: to turn off automatic compilation of script code place the processing instruction <?compile false?> before the script element. To turn on compilation of script code again use the processing instruction <?compile true?> (or short: <?compile?>). The compile processing instruction can be used repeatedly to turn compilation of script code off and on.
The following example markup defines a function called handleButtonAction() that is called by the action handler attached to the Button element:
<?language javascript?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<VBox xmlns:fx="http://javafx.com/fxml">
<fx:script>
function handleButtonAction(event) {
java.lang.System.out.println('You clicked me!');
}
</fx:script>
<children>
<Button text="Click Me!" onAction="handleButtonAction(event);"/>
</children>
</VBox>
Clicking the button triggers the event handler, which invokes the function, producing output identical to the previous examples.
Script code may also be defined in external files. The previous example could be split into an FXML file and a JavaScript source file with no difference in functionality:
<?language javascript?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<VBox xmlns:fx="http://javafx.com/fxml">
<fx:script source="example.js" charset="cp1252"/>
<children>
<Button text="Click Me!" onAction="handleButtonAction(event);"/>
</children>
</VBox>
function handleButtonAction(event) {
java.lang.System.out.println('You clicked me!');
}
It is often preferable to separate code from markup in this way, since many text editors support syntax highlighting for the various scripting languages supported by the JVM. It can also help improve readability of the source code and markup.
Note that script blocks are not limited to defining event handler functions. Script code is executed as it is processed, so it can also be used to dynamically configure the structure of the resulting output. As a simple example, the following FXML includes a script block that defines a variable named "labelText". The value of this variable is used to populate the text property of a Label instance:
<fx:script> var myText = "This is the text of my label."; </fx:script> ... <Label text="$myText"/>
Warning: As of JavaFX 8, importClass() javascript function is no longer supported. You have to use fully qualified names as in the example above or load a nashorn compatibility script.
load("nashorn:mozilla_compat.js");
importClass(java.lang.System);
function handleButtonAction(event) {
System.out.println('You clicked me!');
}
While it can be convenient to write simple event handlers in script, either inline or defined in external files, it is often preferable to define more complex application logic in a compiled, strongly-typed language such as Java. As discussed earlier, the fx:controller attribute allows a caller to associate a "controller" class with an FXML document. A controller is a compiled class that implements the "code behind" the object hierarchy defined by the document.
As shown earlier, controllers are often used to implement event handlers for user interface elements defined in markup:
<VBox fx:controller="com.foo.MyController"
xmlns:fx="http://javafx.com/fxml">
<children>
<Button text="Click Me!" onAction="#handleButtonAction"/>
</children>
</VBox>
package com.foo;
public class MyController {
public void handleButtonAction(ActionEvent event) {
System.out.println("You clicked me!");
}
}
In many cases, it is sufficient to simply declare event handlers in this manner. However, when more control over the behavior of the controller and the elements it manages is required, the controller can define an initialize() method, which will be called once on an implementing controller when the contents of its associated document have been completely loaded:
public void initialize();
This allows the implementing class to perform any necessary post-processing on the content. It also provides the controller with access to the resources that were used to load the document and the location that was used to resolve relative paths within the document (commonly equivalent to the location of the document itself).
For example, the following code defines an initialize() method that attaches an action handler to a button in code rather than via an event handler attribute, as was done in the previous example. The button instance variable is injected by the loader as the document is read. The resulting application behavior is identical:
<VBox fx:controller="com.foo.MyController"
xmlns:fx="http://javafx.com/fxml">
<children>
<Button fx:id="button" text="Click Me!"/>
</children>
</VBox>
package com.foo;
public class MyController implements Initializable {
public Button button;
@Override
public void initialize(URL location, Resources resources)
button.setOnAction(new EventHandler<ActionEvent>() {
@Override
public void handle(ActionEvent event) {
System.out.println("You clicked me!");
}
});
}
}
Note that, in the previous examples, the controller member fields and event handler methods were declared as public so they can be set or invoked by the loader. In practice, this is not often an issue, since a controller is generally only visible to the FXML loader that creates it. However, for developers who prefer more restricted visibility for controller fields or handler methods, the javafx.fxml.FXML annotation can be used. This annotation marks a protected or private class member as accessible to FXML. If the class being annotated is in a named module, the module containing that class must open the containing package to at least the javafx.fxml module.
For example, the controllers from the previous examples could be rewritten as follows:
package com.foo;
public class MyController {
@FXML
private void handleButtonAction(ActionEvent event) {
System.out.println("You clicked me!");
}
}
package com.foo;
public class MyController implements Initializable {
@FXML private Button button;
@FXML
protected void initialize()
button.setOnAction(new EventHandler<ActionEvent>() {
@Override
public void handle(ActionEvent event) {
System.out.println("You clicked me!");
}
});
}
}
In the first version, the handleButtonAction() is tagged with @FXML to allow markup defined in the controller's document to invoke it. In the second example, the button field is annotated to allow the loader to set its value. The initialize() method is similarly annotated.
Controller instances for nested FXML documents loaded via the <fx:include> element are mapped directly to member fields of the including controller. This allows a developer to easily access functionality defined by an include (such as a dialog window presented by an application's main window controller). For example, given the following code:
<VBox fx:controller="com.foo.MainController">
<fx:define>
<fx:include fx:id="dialog" source="dialog.fxml"/>
</fx:define>
...
</VBox>
public class MainController extends Controller {
@FXML private Window dialog;
@FXML private DialogController dialogController;
...
}
when the controller's initialize() method is called, the dialog field will contain the root element loaded from the "dialog.fxml" include, and the dialogController field will contain the include's controller. The main controller can then invoke methods on the included controller, to populate and show the dialog, for example. Note that as the content of the file referenced by fx:include otherwise would become part of the scene graph spanned from main_window_content.fxml, it is necessary to wrap fx:include by fx:define to separate the scene graphs of both windows.
The FXMLLoader class is responsible for actually loading an FXML source file and returning the resulting object graph. For example, the following code loads an FXML file from a location on the classpath relative to the loading class and localizes it with a resource bundle named "com.foo.example". The type of the root element is assumed to be a subclass of javafx.scene.layout.Pane, and the document is assumed to define a controller of type MyController:
URL location = getClass().getResource("example.fxml");
ResourceBundle resources = ResourceBundle.getBundle("com.foo.example");
FXMLLoader fxmlLoader = new FXMLLoader(location, resources);
Pane root = (Pane)fxmlLoader.load();
MyController controller = (MyController)fxmlLoader.getController();
Note that the output of an FXMLLoader#load() operation is an instance hierarchy that reflects the actual named classes in the document, not org.w3c.dom nodes representing those classes. Internally, FXMLLoader uses the javax.xml.stream API (also known as the Streaming API for XML, or StAX) to load an FXML document. StAX is an extremely efficient event-based XML parsing API that is conceptually similar to its W3C predecessor, SAX. It allows an FXML document to be processed in a single pass, rather than loaded into an intermediate DOM structure and then post-processed.
The setRoot() and setController() methods of FXMLLoader allow a caller to inject document root and controller values, respectively, into the document namespace, rather than delegating creation of these values to FXMLLoader itself. This allows a developer to easily create reusable controls that are internally implemented using markup, but (from an API perspective) appear identically to controls implemented programmatically.
For example, the following markup defines the structure of a simple custom control containing a TextField and a Button instance. The root container is defined as an instance of javafx.scene.layout.VBox:
<?import javafx.scene.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<fx:root type="javafx.scene.layout.VBox" xmlns:fx="http://javafx.com/fxml">
<TextField fx:id="textField"/>
<Button text="Click Me" onAction="#doSomething"/>
</fx:root>
As mentioned earlier, the <fx:root> tag creates a reference to a previously defined root element. The value of this element is obtained by calling the getRoot() method of FXMLLoader. Prior to calling load(), the caller must specify this value via a call to setRoot(). The caller may similarly provide a value for the document's controller by calling setController(), which sets the value that will be used as the document's controller when the document is read. These two methods are commonly used together when creating custom FXML-based components.
In the following example, the CustomControl class extends VBox (the type declared by the <fx:root> element), and sets itself as both the root and controller of the FXML document in its constructor. When the document is loaded, the contents of CustomControl will be populated with the contents of the previous FXML document:
package fxml;
import java.io.IOException;
import javafx.beans.property.StringProperty;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.control.TextField;
import javafx.scene.layout.VBox;
public class CustomControl extends VBox {
@FXML private TextField textField;
public CustomControl() {
FXMLLoader fxmlLoader = new FXMLLoader(getClass().getResource("custom_control.fxml"));
fxmlLoader.setRoot(this);
fxmlLoader.setController(this);
try {
fxmlLoader.load();
} catch (IOException exception) {
throw new RuntimeException(exception);
}
}
public String getText() {
return textProperty().get();
}
public void setText(String value) {
textProperty().set(value);
}
public StringProperty textProperty() {
return textField.textProperty();
}
@FXML
protected void doSomething() {
System.out.println("The button was clicked!");
}
}
Now, callers can use instances of this control in code or in markup, just like any other control; e.g.:
HBox hbox = new HBox();
CustomControl customControl = new CustomControl();
customControl.setText("Hello World!");
hbox.getChildren().add(customControl);
<HBox>
<CustomControl text="Hello World!"/>
</HBox>
If FXMLLoader is used to load types in a named module, the application must ensure that all types that are referenced in the FXML files, including the controller class and any custom Node classes, are reflectively accessible to the javafx.fxml module. A type is reflectively accessible if the module opens the containing package to at least the javafx.fxml module.
For example, if com.foo.MyController is in the foo.app module, the module-info.java might look like this:
module foo.app {
opens com.foo to javafx.fxml;
}
Alternatively, a type is reflectively accessible if the module exports the containing package unconditionally.
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