InvocationHandler
public class EventHandler extends Object implements InvocationHandler
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.
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
. WhenmyButton.addActionListener( (ActionListener)EventHandler.create(ActionListener.class, frame, "toFront"));
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: The next simplest use of//Equivalent code using an inner class instead of EventHandler. myButton.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e) { frame.toFront(); } });
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. This would correspond to the following inner class implementation:EventHandler.create(ActionListener.class, myButton, "nextFocusableComponent", "source")
It's also possible to create an//Equivalent code using an inner class instead of EventHandler. new ActionListener() { public void actionPerformed(ActionEvent e) { myButton.setNextFocusableComponent((Component)e.getSource()); } }
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: This would correspond to the following inner class implementation:EventHandler.create(ActionListener.class, target, "doActionEvent", "")
Probably the most common use of//Equivalent code using an inner class instead of EventHandler. new ActionListener() { public void actionPerformed(ActionEvent e) { target.doActionEvent(e); } }
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. This would correspond to the following inner class implementation:EventHandler.create(ActionListener.class, myButton, "label", "source.text")
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.//Equivalent code using an inner class instead of EventHandler. new ActionListener { public void actionPerformed(ActionEvent e) { myButton.setLabel(((JTextField)e.getSource()).getText()); } }
For example, the following action listener
might be written as the following inner class (assuming all the properties had canonical getter methods and returned the appropriate types):EventHandler.create(ActionListener.class, target, "a", "b.c.d")
The target property may also be "qualified" with an arbitrary number of property prefixs delimited with the "." character. For example, the following action listener://Equivalent code using an inner class instead of EventHandler. new ActionListener { public void actionPerformed(ActionEvent e) { target.setA(e.getB().getC().isD()); } }
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.Constructor | Description |
---|---|
EventHandler |
Creates a new EventHandler object; you generally use one of the create methods instead of invoking this constructor directly. |
Modifier and Type | Method | Description |
---|---|---|
static <T> T |
create |
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 |
/** 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 |
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 |
Extract the appropriate property value from the event and pass it to the action associated with this EventHandler . |
@ConstructorProperties({"target","action","eventPropertyName","listenerMethodName"}) public EventHandler(Object target, String action, String eventPropertyName, String listenerMethodName)
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.target
- the object that will perform the actionaction
- the name of a (possibly qualified) property or method on the targeteventPropertyName
- the (possibly qualified) name of a readable property of the incoming eventlistenerMethodName
- the name of the method in the listener interface that should trigger the actionNullPointerException
- if target
is nullNullPointerException
- if action
is nullpublic Object getTarget()
public String getAction()
public String getEventPropertyName()
public String getListenerMethodName()
null
signifies that all methods in the listener interface trigger the action.public Object invoke(Object proxy, Method method, Object[] arguments)
EventHandler
.invoke
in interface InvocationHandler
proxy
- the proxy objectmethod
- the method in the listener interfacearguments
- 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
.public static <T> T create(Class<T> listenerInterface, Object target, String action)
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")
T
- the type to createlistenerInterface
- the listener interface to create a proxy fortarget
- the object that will perform the actionaction
- the name of a (possibly qualified) property or method on the targetlistenerInterface
NullPointerException
- if listenerInterface
is nullNullPointerException
- if target
is nullNullPointerException
- if action
is nullIllegalArgumentException
- if creating a Proxy for listenerInterface
fails for any of the restrictions specified by Proxy.newProxyInstance(java.lang.ClassLoader, java.lang.Class<?>[], java.lang.reflect.InvocationHandler)
public static <T> T create(Class<T> listenerInterface, Object target, String action, String eventPropertyName)
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:
This is equivalent to the following code:EventHandler.create(ActionListener.class, label, "text", "source.text");
//Equivalent code using an inner class instead of EventHandler. new ActionListener() { public void actionPerformed(ActionEvent event) { label.setText(((JTextField)(event.getSource())).getText()); } };
T
- the type to createlistenerInterface
- the listener interface to create a proxy fortarget
- the object that will perform the actionaction
- the name of a (possibly qualified) property or method on the targeteventPropertyName
- the (possibly qualified) name of a readable property of the incoming eventlistenerInterface
NullPointerException
- if listenerInterface
is nullNullPointerException
- if target
is nullNullPointerException
- if action
is nullIllegalArgumentException
- if creating a Proxy for listenerInterface
fails for any of the restrictions specified by Proxy.newProxyInstance(java.lang.ClassLoader, java.lang.Class<?>[], java.lang.reflect.InvocationHandler)
public static <T> T create(Class<T> listenerInterface, Object target, String action, String eventPropertyName, String listenerMethodName)
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:
This is comparable to writing aEventHandler.create(MouseListener.class, target, "origin", "point", "mousePressed");
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()); } };
T
- the type to createlistenerInterface
- the listener interface to create a proxy fortarget
- the object that will perform the actionaction
- the name of a (possibly qualified) property or method on the targeteventPropertyName
- the (possibly qualified) name of a readable property of the incoming eventlistenerMethodName
- the name of the method in the listener interface that should trigger the actionlistenerInterface
NullPointerException
- if listenerInterface
is nullNullPointerException
- if target
is nullNullPointerException
- if action
is nullIllegalArgumentException
- if creating a Proxy for listenerInterface
fails for any of the restrictions specified by Proxy.newProxyInstance(java.lang.ClassLoader, java.lang.Class<?>[], java.lang.reflect.InvocationHandler)
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