net.sf.basedb.util
Class Diff3

java.lang.Object
  net.sf.basedb.util.Diff3

public class Diff3
extends Object

This class is helpful when a client application needs to implement long-running transactions and still be able to update items with as few clashes as possible. Using this class it is possible to avoid ItemModifiedException:s even when two or more users are modifying the same item at the same time. The only requirement is that they don't change the same property to different values. Here is how to use this class:

• The oldItem is loaded and the user is presented with a screen to modify the properties of the item. The client application must keep the item in memory while the user is doing the modifications.
• When the user submits the changes the newItem is loaded. Both objects are passed to the Diff3(BasicItem, BasicItem) constructor. The construcor checks if both objects are of the same version or not.
• The client application uses the getValue(Object, Object, Object) method to set the properties on the newItem object.

edit.jsp
Label label = Label.getById(dbControl, labelId);
sessionControl.setSessionSetting("LABEL", label);

submit.jsp
Label oldLabel = (Label)sessionControl.getSessionSetting("LABEL");
Label newLabel = Label.getById(dbControl, labelId);
Diff3 d3 = new Diff3(oldLabel, newLabel);
newLabel.setName(d3.getValue(oldLabel.getName(), newLabel.getName(), 
   request.getParameter("name")));
// ... etc ...
dbControl.commit();

Additional code to check for session timeouts and creating new items is also required.

Version:

2.0

Author:

Nicklas

Last modified

$Date: 2009-04-06 14:52:39 +0200 (Mon, 06 Apr 2009) $

Field Summary

private boolean	sameVersion
private String	what

Constructor Summary

Diff3(BasicItem oldItem, BasicItem newItem)
Create a new Diff3 object to compare properties of the oldItem and newItem.

Method Summary

<T> T	getValue(T oldValue, T newValue, T requestValue) Get the correct value to set on the newItem object.
static boolean	isEqualOrNull(Object o1, Object o2) Check if two objects are both null or equal as defined by the Object.equals(Object) method.

Methods inherited from class java.lang.Object

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

Field Detail

sameVersion

private final boolean sameVersion

what

private final String what

Constructor Detail

Diff3

public Diff3(BasicItem oldItem,
             BasicItem newItem)
      throws InvalidDataException

Create a new Diff3 object to compare properties of the oldItem and newItem. Both objects mustn't be null and they must represent the same item in the database. For new objects, not yet in the database, both parameters must be the same instance.

Parameters:

oldItem - The "old" representation of the item

newItem - The "new" representation of the item

Throws:

InvalidDataException - If one of the parameters is null or they don't represent the same database item

Method Detail

getValue

public <T> T getValue(T oldValue,
                      T newValue,
                      T requestValue)
           throws ItemModifiedException

Get the correct value to set on the newItem object.

• If the old and new item have the same version or if the oldValue is the same as the newValue the requestValue is returned. This represents the case that no other user has modified the item, or it was modified but not this particular property.
• If the old and request value are equal the newValue is returned. This represents the case that the other user has modified the property, but the current user hasn't.
• If the new and request value are equal the newValue is returned. This represents the case that both users has modified the property and, fortunately, set it to the same value.
• Otherwise, it all three values are different, a ItemModifiedException is thrown.

Parameters:

oldValue - The value of the property on the old item

newValue - The value of the property on the new item

requestValue - The vale of the property the current user has set on the edit screen

Returns:

The most appropriate value as described above

Throws:

ItemModifiedException - If all values are different

isEqualOrNull

public static boolean isEqualOrNull(Object o1,
                                    Object o2)

Check if two objects are both null or equal as defined by the Object.equals(Object) method.

Parameters:

o1 - Object to compare

o2 - Object to compare

Returns:

TRUE if equal or null, FALSE otherwise.