Java如何侦听受约束的属性更改？
当bean的值即将更改时，将触发受约束的属性更改。当VetoableChangeListener否决值更改时，bean的值将回滚到上一个值。在这个例子中，我们有一个叫做interest的约束属性。
package org.nhooo.example.bean;
import java.beans.VetoableChangeSupport;
import java.beans.PropertyVetoException;
public class VetoBean {
private double interest;
private VetoableChangeSupport vcs = new VetoableChangeSupport(this);
public VetoBean() {
vcs.addVetoableChangeListener(new VetoChangeListener());
}
public double getInterest() {
return interest;
}
public void setInterest(double interest) {
try {
vcs.fireVetoableChange("interest", new Double(this.interest), new Double(interest));
this.interest = interest;
} catch (PropertyVetoException e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
VetoBean bean = new VetoBean();
bean.setInterest(10.99);
bean.setInterest(15.99);
// PropertyVetoException将被抛出，因为利息值
// 不得超过20.00。
bean.setInterest(20.99);
}
}package org.nhooo.example.bean;
import java.beans.VetoableChangeListener;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyVetoException;
public class VetoChangeListener implements VetoableChangeListener {
/**
* This method gets called when a constrained property is changed.
*
* @param evt a `PropertyChangeEvent` object describing the
* event source and the property that has changed.
* @throws java.beans.PropertyVetoException
* if the recipient wishes the property
* change to be rolled back.
*/
public void vetoableChange(PropertyChangeEvent evt) throws PropertyVetoException {
String eventName = evt.getPropertyName();
if (eventName.equalsIgnoreCase("interest")) {
double interest = ((Double) evt.getNewValue()).doubleValue();
if (interest > 20.00) {
throw new PropertyVetoException("Interest must be below 20.00", evt);
}
System.out.println("Interest applied = " + interest);
}
}
}