EventHandler (Java SE 25 & JDK 25)
Description: declaration: module: java.desktop, package: java.beans, class: EventHandler
Keywords: java.beans.EventHandler class

# Class EventHandler

java.lang.Object
java.beans.EventHandler

All Implemented Interfaces:
  `InvocationHandler`

public class EventHandler extends Object implements InvocationHandler
The `EventHandler` class provides support for dynamically generating event listeners whose methods execute a simple statement involving an incoming event object and a target object.

The `EventHandler` class is intended to be used by interactive tools, such as application builders, that allow developers to make connections between beans. Typically connections are made from a user interface bean (the event source) to an application logic bean (the target). The most effective connections of this kind isolate the application logic from the user interface. For example, the `EventHandler` for a connection from a `JCheckBox` to a method that accepts a boolean value can deal with extracting the state of the check box and passing it directly to the method so that the method is isolated from the user interface layer.

Inner classes are another, more general way to handle events from user interfaces. The `EventHandler` class handles only a subset of what is possible using inner classes. However, `EventHandler` works better with the long-term persistence scheme than inner classes. Also, using `EventHandler` in large applications in which the same interface is implemented many times can reduce the disk and memory footprint of the application.

The reason that listeners created with `EventHandler` have such a small footprint is that the `Proxy` class, on which the `EventHandler` relies, shares implementations of identical interfaces. For example, if you use the `EventHandler create` methods to make all the `ActionListener`s in an application, all the action listeners will be instances of a single class (one created by the `Proxy` class). In general, listeners based on the `Proxy` class require one listener class to be created per listener type (interface), whereas the inner class approach requires one class to be created per listener (object that implements the interface).

You don't generally deal directly with `EventHandler` instances. Instead, you use one of the `EventHandler` `create` methods to create an object that implements a given listener interface. This listener object uses an `EventHandler` object behind the scenes to encapsulate information about the event, the object to be sent a message when the event occurs, the message (method) to be sent, and any argument to the method. The following section gives examples of how to create listener objects using the `create` methods.

## Examples of Using EventHandler

The simplest use of `EventHandler` is to install a listener that calls a method on the target object with no arguments. In the following example we create an `ActionListener` that invokes the `toFront` method on an instance of `javax.swing.JFrame`.

>     myButton.addActionListener(
>         (ActionListener)EventHandler.create(ActionListener.class, frame, "toFront"));

When `myButton` is pressed, the statement `frame.toFront()` will be executed. One could get the same effect, with some additional compile-time type safety, by defining a new implementation of the `ActionListener` interface and adding an instance of it to the button:

>     //Equivalent code using an inner class instead of EventHandler.
>     myButton.addActionListener(new ActionListener() {
>         public void actionPerformed(ActionEvent e) {
>             frame.toFront();
>         }
>     });

The next simplest use of `EventHandler` is to extract a property value from the first argument of the method in the listener interface (typically an event object) and use it to set the value of a property in the target object. In the following example we create an `ActionListener` that sets the `nextFocusableComponent` property of the target (myButton) object to the value of the "source" property of the event.

>     EventHandler.create(ActionListener.class, myButton, "nextFocusableComponent", "source")

This would correspond to the following inner class implementation:

>     //Equivalent code using an inner class instead of EventHandler.
>     new ActionListener() {
>         public void actionPerformed(ActionEvent e) {
>             myButton.setNextFocusableComponent((Component)e.getSource());
>         }
>     }

It's also possible to create an `EventHandler` that just passes the incoming event object to the target's action. If the fourth `EventHandler.create` argument is an empty string, then the event is just passed along:

>     EventHandler.create(ActionListener.class, target, "doActionEvent", "")

This would correspond to the following inner class implementation:

>     //Equivalent code using an inner class instead of EventHandler.
>     new ActionListener() {
>         public void actionPerformed(ActionEvent e) {
>             target.doActionEvent(e);
>         }
>     }

Probably the most common use of `EventHandler` is to extract a property value from the source of the event object and set this value as the value of a property of the target object. In the following example we create an `ActionListener` that sets the "label" property of the target object to the value of the "text" property of the source (the value of the "source" property) of the event.

>     EventHandler.create(ActionListener.class, myButton, "label", "source.text")

This would correspond to the following inner class implementation:

>     //Equivalent code using an inner class instead of EventHandler.
>     new ActionListener {
>         public void actionPerformed(ActionEvent e) {
>             myButton.setLabel(((JTextField)e.getSource()).getText());
>         }
>     }

The event property may be "qualified" with an arbitrary number of property prefixes delimited with the "." character. The "qualifying" names that appear before the "." characters are taken as the names of properties that should be applied, left-most first, to the event object.

For example, the following action listener

>     EventHandler.create(ActionListener.class, target, "a", "b.c.d")

might be written as the following inner class (assuming all the properties had canonical getter methods and returned the appropriate types):

>     //Equivalent code using an inner class instead of EventHandler.
>     new ActionListener {
>         public void actionPerformed(ActionEvent e) {
>             target.setA(e.getB().getC().isD());
>         }
>     }

The target property may also be "qualified" with an arbitrary number of property prefixs delimited with the "." character. For example, the following action listener:

       EventHandler.create(ActionListener.class, target, "a.b", "c.d")

might be written as the following inner class (assuming all the properties had canonical getter methods and returned the appropriate types):

       //Equivalent code using an inner class instead of EventHandler.
       new ActionListener {
         public void actionPerformed(ActionEvent e) {
             target.getA().setB(e.getC().isD());
        }
    }

As `EventHandler` ultimately relies on reflection to invoke a method we recommend against targeting an overloaded method. For example, if the target is an instance of the class `MyTarget` which is defined as:

       public class MyTarget {
         public void doIt(String);
         public void doIt(Object);
       }

Then the method `doIt` is overloaded. EventHandler will invoke the method that is appropriate based on the source. If the source is null, then either method is appropriate and the one that is invoked is undefined. For that reason we recommend against targeting overloaded methods.

Since:
  1.4
See Also:
  - `Proxy`
  - `EventObject`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `EventHandler(Object target, String action, String eventPropertyName, String listenerMethodName)`
  Creates a new `EventHandler` object; you generally use one of the `create` methods instead of invoking this constructor directly.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `static <T> T`
  `create(Class<T> listenerInterface, Object target, String action)`
  Creates an implementation of `listenerInterface` in which all of the methods in the listener interface apply the handler's `action` to the `target`.
  `static <T> T`
  `create(Class<T> listenerInterface, Object target, String action, String eventPropertyName)`
  /** Creates an implementation of `listenerInterface` in which all of the methods pass the value of the event expression, `eventPropertyName`, to the final method in the statement, `action`, which is applied to the `target`.
  `static <T> T`
  `create(Class<T> listenerInterface, Object target, String action, String eventPropertyName, String listenerMethodName)`
  Creates an implementation of `listenerInterface` in which the method named `listenerMethodName` passes the value of the event expression, `eventPropertyName`, to the final method in the statement, `action`, which is applied to the `target`.
  `String`
  `getAction()`
  Returns the name of the target's writable property that this event handler will set, or the name of the method that this event handler will invoke on the target.
  `String`
  `getEventPropertyName()`
  Returns the property of the event that should be used in the action applied to the target.
  `String`
  `getListenerMethodName()`
  Returns the name of the method that will trigger the action.
  `Object`
  `getTarget()`
  Returns the object to which this event handler will send a message.
  `Object`
  `invoke(Object proxy, Method method, Object[] arguments)`
  Extract the appropriate property value from the event and pass it to the action associated with this `EventHandler`.

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Constructor Details

  - ### EventHandler

    @ConstructorProperties({"target","action","eventPropertyName","listenerMethodName"}) public EventHandler(Object target, String action, String eventPropertyName, String listenerMethodName)
    Creates a new `EventHandler` object; you generally use one of the `create` methods instead of invoking this constructor directly. Refer to `the general version of create` for a complete description of the `eventPropertyName` and `listenerMethodName` parameter.
    Parameters:
      `target` - the object that will perform the action
      `action` - the name of a (possibly qualified) property or method on the target
      `eventPropertyName` - the (possibly qualified) name of a readable property of the incoming event
      `listenerMethodName` - the name of the method in the listener interface that should trigger the action
    Throws:
      `NullPointerException` - if `target` is null
      `NullPointerException` - if `action` is null
    See Also:
      - `EventHandler`
      - `create(Class, Object, String, String, String)`
      - `getTarget()`
      - `getAction()`
      - `getEventPropertyName()`
      - `getListenerMethodName()`

- ## Method Details

  - ### getTarget

    public Object getTarget()
    Returns the object to which this event handler will send a message.
    Returns:
      the target of this event handler
    See Also:
      - `EventHandler(Object, String, String, String)`

  - ### getAction

    public String getAction()
    Returns the name of the target's writable property that this event handler will set, or the name of the method that this event handler will invoke on the target.
    Returns:
      the action of this event handler
    See Also:
      - `EventHandler(Object, String, String, String)`

  - ### getEventPropertyName

    public String getEventPropertyName()
    Returns the property of the event that should be used in the action applied to the target.
    Returns:
      the property of the event
    See Also:
      - `EventHandler(Object, String, String, String)`

  - ### getListenerMethodName

    public String getListenerMethodName()
    Returns the name of the method that will trigger the action. A return value of `null` signifies that all methods in the listener interface trigger the action.
    Returns:
      the name of the method that will trigger the action
    See Also:
      - `EventHandler(Object, String, String, String)`

  - ### invoke

    public Object invoke(Object proxy, Method method, Object[] arguments)
    Extract the appropriate property value from the event and pass it to the action associated with this `EventHandler`.
    Specified by:
      `invoke` in interface `InvocationHandler`
    Parameters:
      `proxy` - the proxy object
      `method` - the method in the listener interface
      `arguments` - an array of objects containing the values of the arguments passed in the method invocation on the proxy instance, or `null` if interface method takes no arguments. Arguments of primitive types are wrapped in instances of the appropriate primitive wrapper class, such as `java.lang.Integer` or `java.lang.Boolean`.
    Returns:
      the result of applying the action to the target
    See Also:
      - `EventHandler`

  - ### create

    public static <T> T create(Class<T> listenerInterface, Object target, String action)
    Creates an implementation of `listenerInterface` in which all of the methods in the listener interface apply the handler's `action` to the `target`. This method is implemented by calling the other, more general, implementation of the `create` method with both the `eventPropertyName` and the `listenerMethodName` taking the value `null`. Refer to `the general version of create` for a complete description of the `action` parameter.

    To create an `ActionListener` that shows a `JDialog` with `dialog.show()`, one can write:

    >     EventHandler.create(ActionListener.class, dialog, "show")

    Type Parameters:
      `T` - the type to create
    Parameters:
      `listenerInterface` - the listener interface to create a proxy for
      `target` - the object that will perform the action
      `action` - the name of a (possibly qualified) property or method on the target
    Returns:
      an object that implements `listenerInterface`
    Throws:
      `NullPointerException` - if `listenerInterface` is null
      `NullPointerException` - if `target` is null
      `NullPointerException` - if `action` is null
      `IllegalArgumentException` - if creating a Proxy for `listenerInterface` fails for any of the restrictions specified by `Proxy.newProxyInstance(ClassLoader, Class[], InvocationHandler)`
    See Also:
      - `create(Class, Object, String, String)`
      - `Proxy.newProxyInstance(ClassLoader, Class[], InvocationHandler)`

  - ### create

    public static <T> T create(Class<T> listenerInterface, Object target, String action, String eventPropertyName)
    /** Creates an implementation of `listenerInterface` in which all of the methods pass the value of the event expression, `eventPropertyName`, to the final method in the statement, `action`, which is applied to the `target`. This method is implemented by calling the more general, implementation of the `create` method with the `listenerMethodName` taking the value `null`. Refer to `the general version of create` for a complete description of the `action` and `eventPropertyName` parameters.

    To create an `ActionListener` that sets the the text of a `JLabel` to the text value of the `JTextField` source of the incoming event, you can use the following code:

    >     EventHandler.create(ActionListener.class, label, "text", "source.text");

    This is equivalent to the following code:

    >     //Equivalent code using an inner class instead of EventHandler.
    >     new ActionListener() {
    >         public void actionPerformed(ActionEvent event) {
    >             label.setText(((JTextField)(event.getSource())).getText());
    >          }
    >     };

    Type Parameters:
      `T` - the type to create
    Parameters:
      `listenerInterface` - the listener interface to create a proxy for
      `target` - the object that will perform the action
      `action` - the name of a (possibly qualified) property or method on the target
      `eventPropertyName` - the (possibly qualified) name of a readable property of the incoming event
    Returns:
      an object that implements `listenerInterface`
    Throws:
      `NullPointerException` - if `listenerInterface` is null
      `NullPointerException` - if `target` is null
      `NullPointerException` - if `action` is null
      `IllegalArgumentException` - if creating a Proxy for `listenerInterface` fails for any of the restrictions specified by `Proxy.newProxyInstance(ClassLoader, Class[], InvocationHandler)`
    See Also:
      - `create(Class, Object, String, String, String)`
      - `Proxy.newProxyInstance(ClassLoader, Class[], InvocationHandler)`

  - ### create

    public static <T> T create(Class<T> listenerInterface, Object target, String action, String eventPropertyName, String listenerMethodName)
    Creates an implementation of `listenerInterface` in which the method named `listenerMethodName` passes the value of the event expression, `eventPropertyName`, to the final method in the statement, `action`, which is applied to the `target`. All of the other listener methods do nothing.

    The `eventPropertyName` string is used to extract a value from the incoming event object that is passed to the target method. The common case is the target method takes no arguments, in which case a value of null should be used for the `eventPropertyName`. Alternatively if you want the incoming event object passed directly to the target method use the empty string. The format of the `eventPropertyName` string is a sequence of methods or properties where each method or property is applied to the value returned by the preceding method starting from the incoming event object. The syntax is: `propertyName{.propertyName}*` where `propertyName` matches a method or property. For example, to extract the `point` property from a `MouseEvent`, you could use either `"point"` or `"getPoint"` as the `eventPropertyName`. To extract the "text" property from a `MouseEvent` with a `JLabel` source use any of the following as `eventPropertyName`: `"source.text"`, `"getSource.text" "getSource.getText"` or `"source.getText"`. If a method can not be found, or an exception is generated as part of invoking a method a `RuntimeException` will be thrown at dispatch time. For example, if the incoming event object is null, and `eventPropertyName` is non-null and not empty, a `RuntimeException` will be thrown.

    The `action` argument is of the same format as the `eventPropertyName` argument where the last property name identifies either a method name or writable property.

    If the `listenerMethodName` is `null` all methods in the interface trigger the `action` to be executed on the `target`.

    For example, to create a `MouseListener` that sets the target object's `origin` property to the incoming `MouseEvent`'s location (that's the value of `mouseEvent.getPoint()`) each time a mouse button is pressed, one would write:

    >     EventHandler.create(MouseListener.class, target, "origin", "point", "mousePressed");

    This is comparable to writing a `MouseListener` in which all of the methods except `mousePressed` are no-ops:

    >     //Equivalent code using an inner class instead of EventHandler.
    >     new MouseAdapter() {
    >         public void mousePressed(MouseEvent e) {
    >             target.setOrigin(e.getPoint());
    >         }
    >     };

    Type Parameters:
      `T` - the type to create
    Parameters:
      `listenerInterface` - the listener interface to create a proxy for
      `target` - the object that will perform the action
      `action` - the name of a (possibly qualified) property or method on the target
      `eventPropertyName` - the (possibly qualified) name of a readable property of the incoming event
      `listenerMethodName` - the name of the method in the listener interface that should trigger the action
    Returns:
      an object that implements `listenerInterface`
    Throws:
      `NullPointerException` - if `listenerInterface` is null
      `NullPointerException` - if `target` is null
      `NullPointerException` - if `action` is null
      `IllegalArgumentException` - if creating a Proxy for `listenerInterface` fails for any of the restrictions specified by `Proxy.newProxyInstance(ClassLoader, Class[], InvocationHandler)`
    See Also:
      - `EventHandler`
      - `Proxy.newProxyInstance(ClassLoader, Class[], InvocationHandler)`