org.prevayler.util.memento
Class MementoTransaction

java.lang.Object
  org.prevayler.util.memento.MementoTransaction

All Implemented Interfaces:

Serializable

Direct Known Subclasses:

AccountCreation, AccountDeletion, Deposit, Transfer, Withdrawal

public abstract class MementoTransaction
extends Object
implements Serializable

An atomic transaction to be executed on a PrevalentSystem. Any operation which changes the observable state of a PrevalentSystem must be encapsulated as a Command. This command adds the capability to create mementos of objects before they get modified. In the event of a failure, these mementos are restored and the system is left in the state it was in before the command got executed. The execution is split in four parts:

• Find the objects: find all the objects needed for checking the precondition, creating the mementos and execution, and store them in transient fields;
• Check the precondition: verify that it is safe to execute;
• Create the mementos: create a memento for each object that is possibly going to be modified;
• Execute: modify the objects.

Version:

2.0

Author:

Johan Stuyts

See Also:

Serialized Form

Constructor Summary

MementoTransaction()

Method Summary

protected abstract void	checkPrecondition() Check the precondition.
protected abstract void	createMementos(MementoCollector collector) Create mementos for all objects which (possibly) get modified.
protected abstract Account	execute(MementoCollector collector) Execute the actual command.
Account	execute(MementoCollector collector, Bank prevalentSystem) Executes this command on the received system.
protected abstract void	findObjects(Bank prevalentSystem) Find the objects this command modifies.

Methods inherited from class java.lang.Object

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

Constructor Detail

MementoTransaction

public MementoTransaction()

Method Detail

execute

public Account execute(MementoCollector collector,
                       Bank prevalentSystem)
                throws Exception

Executes this command on the received system. See prevayler.demos for examples. The returned object has to be Serializable in preparation for future versions of Prevayler that will provide fault-tolerance through system replicas.

Parameters:

collector - The memento collector to which to add the mementos. A memento collector instead of a Prevayler instance is passed, so the command will not easily invoke subcommands through the prevayler (which is not allowed).

prevalentSystem - The system on which to execute the command.

Returns:

The object returned by the execution of this command. Most commands simply return null.

Throws:

Exception

findObjects

protected abstract void findObjects(Bank prevalentSystem)
                             throws Exception

Find the objects this command modifies.

Parameters:

prevalentSystem - The prevalent system in which to find the objects.

Throws:

Exception

checkPrecondition

protected abstract void checkPrecondition()
                                   throws Exception

Check the precondition.

Throws:

Exception

createMementos

protected abstract void createMementos(MementoCollector collector)

Create mementos for all objects which (possibly) get modified.

Parameters:

collector - The memento collector to which to add the mementos.

execute

protected abstract Account execute(MementoCollector collector)
                            throws Exception

Execute the actual command.

Parameters:

collector - The memento collector which can be used to execute subcommands.

Returns:

The object returned by the execution of this command. Most commands simply return null.

Throws:

Exception