- java.lang.Object
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- java.beans.EventHandler
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- All Implemented Interfaces:
InvocationHandler
public class EventHandler extends Object implements InvocationHandler
TheEventHandlerclass provides support for dynamically generating event listeners whose methods execute a simple statement involving an incoming event object and a target object.The
EventHandlerclass 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, theEventHandlerfor a connection from aJCheckBoxto 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
EventHandlerclass handles only a subset of what is possible using inner classes. However,EventHandlerworks better with the long-term persistence scheme than inner classes. Also, usingEventHandlerin 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
EventHandlerhave such a small footprint is that theProxyclass, on which theEventHandlerrelies, shares implementations of identical interfaces. For example, if you use theEventHandler createmethods to make all theActionListeners in an application, all the action listeners will be instances of a single class (one created by theProxyclass). In general, listeners based on theProxyclass 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
EventHandlerinstances. Instead, you use one of theEventHandlercreatemethods to create an object that implements a given listener interface. This listener object uses anEventHandlerobject 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 thecreatemethods.Examples of Using EventHandler
The simplest use ofEventHandleris to install a listener that calls a method on the target object with no arguments. In the following example we create anActionListenerthat invokes thetoFrontmethod on an instance ofjavax.swing.JFrame.
WhenmyButton.addActionListener( (ActionListener)EventHandler.create(ActionListener.class, frame, "toFront"));myButtonis pressed, the statementframe.toFront()will be executed. One could get the same effect, with some additional compile-time type safety, by defining a new implementation of theActionListenerinterface 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(); } });EventHandleris 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 anActionListenerthat sets thenextFocusableComponentproperty 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()); } }EventHandlerthat just passes the incoming event object to the target's action. If the fourthEventHandler.createargument 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); } }EventHandleris 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 anActionListenerthat 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
EventHandlerultimately relies on reflection to invoke a method we recommend against targeting an overloaded method. For example, if the target is an instance of the classMyTargetwhich is defined as:public class MyTarget { public void doIt(String); public void doIt(Object); }Then the methoddoItis 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
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Constructor Summary
Constructors Constructor Description EventHandler(Object target, String action, String eventPropertyName, String listenerMethodName)Creates a newEventHandlerobject; you generally use one of thecreatemethods instead of invoking this constructor directly.
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Method Summary
All Methods Static Methods Instance Methods Concrete Methods Modifier and Type Method Description static <T> Tcreate(Class<T> listenerInterface, Object target, String action)Creates an implementation oflistenerInterfacein which all of the methods in the listener interface apply the handler'sactionto thetarget.static <T> Tcreate(Class<T> listenerInterface, Object target, String action, String eventPropertyName)/** Creates an implementation oflistenerInterfacein 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 thetarget.static <T> Tcreate(Class<T> listenerInterface, Object target, String action, String eventPropertyName, String listenerMethodName)Creates an implementation oflistenerInterfacein which the method namedlistenerMethodNamepasses the value of the event expression,eventPropertyName, to the final method in the statement,action, which is applied to thetarget.StringgetAction()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.StringgetEventPropertyName()Returns the property of the event that should be used in the action applied to the target.StringgetListenerMethodName()Returns the name of the method that will trigger the action.ObjectgetTarget()Returns the object to which this event handler will send a message.Objectinvoke(Object proxy, Method method, Object[] arguments)Extract the appropriate property value from the event and pass it to the action associated with thisEventHandler.
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Constructor Detail
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EventHandler
@ConstructorProperties({"target","action","eventPropertyName","listenerMethodName"}) public EventHandler(Object target, String action, String eventPropertyName, String listenerMethodName)
Creates a newEventHandlerobject; you generally use one of thecreatemethods instead of invoking this constructor directly. Refer tothe general version of createfor a complete description of theeventPropertyNameandlistenerMethodNameparameter.- Parameters:
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 action- Throws:
NullPointerException- iftargetis nullNullPointerException- ifactionis null- See Also:
EventHandler,create(Class, Object, String, String, String),getTarget(),getAction(),getEventPropertyName(),getListenerMethodName()
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Method Detail
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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)
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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)
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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)
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getListenerMethodName
public String getListenerMethodName()
Returns the name of the method that will trigger the action. A return value ofnullsignifies 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)
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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 thisEventHandler.- Specified by:
invokein interfaceInvocationHandler- Parameters:
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, ornullif interface method takes no arguments. Arguments of primitive types are wrapped in instances of the appropriate primitive wrapper class, such asjava.lang.Integerorjava.lang.Boolean.- Returns:
- the result of applying the action to the target
- See Also:
EventHandler
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create
public static <T> T create(Class<T> listenerInterface, Object target, String action)
Creates an implementation oflistenerInterfacein which all of the methods in the listener interface apply the handler'sactionto thetarget. This method is implemented by calling the other, more general, implementation of thecreatemethod with both theeventPropertyNameand thelistenerMethodNametaking the valuenull. Refer tothe general version of createfor a complete description of theactionparameter.To create an
ActionListenerthat shows aJDialogwithdialog.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 fortarget- the object that will perform the actionaction- the name of a (possibly qualified) property or method on the target- Returns:
- an object that implements
listenerInterface - Throws:
NullPointerException- iflistenerInterfaceis nullNullPointerException- iftargetis nullNullPointerException- ifactionis nullIllegalArgumentException- if creating a Proxy forlistenerInterfacefails for any of the restrictions specified byProxy.newProxyInstance(java.lang.ClassLoader, java.lang.Class<?>[], java.lang.reflect.InvocationHandler)- See Also:
create(Class, Object, String, String),Proxy.newProxyInstance(java.lang.ClassLoader, java.lang.Class<?>[], java.lang.reflect.InvocationHandler)
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create
public static <T> T create(Class<T> listenerInterface, Object target, String action, String eventPropertyName)
/** Creates an implementation oflistenerInterfacein 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 thetarget. This method is implemented by calling the more general, implementation of thecreatemethod with thelistenerMethodNametaking the valuenull. Refer tothe general version of createfor a complete description of theactionandeventPropertyNameparameters.To create an
ActionListenerthat sets the the text of aJLabelto the text value of theJTextFieldsource 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()); } };- Type Parameters:
T- the type to create- Parameters:
listenerInterface- 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 event- Returns:
- an object that implements
listenerInterface - Throws:
NullPointerException- iflistenerInterfaceis nullNullPointerException- iftargetis nullNullPointerException- ifactionis nullIllegalArgumentException- if creating a Proxy forlistenerInterfacefails for any of the restrictions specified byProxy.newProxyInstance(java.lang.ClassLoader, java.lang.Class<?>[], java.lang.reflect.InvocationHandler)- See Also:
create(Class, Object, String, String, String),Proxy.newProxyInstance(java.lang.ClassLoader, java.lang.Class<?>[], java.lang.reflect.InvocationHandler)
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create
public static <T> T create(Class<T> listenerInterface, Object target, String action, String eventPropertyName, String listenerMethodName)
Creates an implementation oflistenerInterfacein which the method namedlistenerMethodNamepasses the value of the event expression,eventPropertyName, to the final method in the statement,action, which is applied to thetarget. All of the other listener methods do nothing.The
eventPropertyNamestring 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 theeventPropertyName. Alternatively if you want the incoming event object passed directly to the target method use the empty string. The format of theeventPropertyNamestring 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}*wherepropertyNamematches a method or property. For example, to extract thepointproperty from aMouseEvent, you could use either"point"or"getPoint"as theeventPropertyName. To extract the "text" property from aMouseEventwith aJLabelsource use any of the following aseventPropertyName:"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 aRuntimeExceptionwill be thrown at dispatch time. For example, if the incoming event object is null, andeventPropertyNameis non-null and not empty, aRuntimeExceptionwill be thrown.The
actionargument is of the same format as theeventPropertyNameargument where the last property name identifies either a method name or writable property.If the
listenerMethodNameisnullall methods in the interface trigger theactionto be executed on thetarget.For example, to create a
MouseListenerthat sets the target object'soriginproperty to the incomingMouseEvent's location (that's the value ofmouseEvent.getPoint()) each time a mouse button is pressed, one would write:
This is comparable to writing aEventHandler.create(MouseListener.class, target, "origin", "point", "mousePressed");
MouseListenerin which all of the methods exceptmousePressedare 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 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 action- Returns:
- an object that implements
listenerInterface - Throws:
NullPointerException- iflistenerInterfaceis nullNullPointerException- iftargetis nullNullPointerException- ifactionis nullIllegalArgumentException- if creating a Proxy forlistenerInterfacefails for any of the restrictions specified byProxy.newProxyInstance(java.lang.ClassLoader, java.lang.Class<?>[], java.lang.reflect.InvocationHandler)- See Also:
EventHandler,Proxy.newProxyInstance(java.lang.ClassLoader, java.lang.Class<?>[], java.lang.reflect.InvocationHandler)
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