Command pattern can be used to implement Transactional behavior
(and Undo
).
But I could not find an example of these by googling. I could only find
You have to define undo(), redo() operations along with execute() in Command interface itself
.
example:
interface ChangeI {
enum State{ READY, DONE, UNDONE, STUCK } ;
State getState() ;
void execute() ;
void undo() ;
void redo() ;
}
Define a State in your ConcreteCommand
class. Depending on current State after execute
() method, you have to decide whether command should be added to Undo Stack
or Redo Stack
and take decision accordingly.
abstract class AbstractChange implements ChangeI {
State state = State.READY ;
public State getState() { return state ; }
public void execute() {
assert state == State.READY ;
try { doHook() ; state = State.DONE ; }
catch( Failure e ) { state = State.STUCK ; }
catch( Throwable e ) { assert false ; }
}
public void undo() {
assert state == State.DONE ; }
try { undoHook() ; state = State.UNDONE ; }
catch( Failure e ) { state = State.STUCK ; }
catch( Throwable e ) { assert false ; }
}
public void redo() {
assert state == State.UNDONE ;
try { redoHook() ; state = State.DONE ; }
catch( Failure e ) { state = State.STUCK ; }
catch( Throwable e ) { assert false ; }
}
protected abstract void doHook() throws Failure ;
protected abstract void undoHook() throws Failure ;
protected void redoHook() throws Failure { doHook() ;} ;
}
Have a look at this undo-redo command article for better understanding.
public final class Ping implements Callable<Boolean> {
private final InetAddress peer;
public Ping(final InetAddress peer) {
this.peer = peer;
}
public Boolean call() {
/* do the ping */
...
}
}
...
final Future<Boolean> result
= executorService.submit(new Ping(InetAddress.getByName("google.com")));
System.out.println("google.com is " + (result.get() ? "UP" : "DOWN"));
Command Patterns are used in a lot of places.
Here is a site which provides as example of command pattern used for callback. http://www.javaworld.com/article/2077569/core-java/java-tip-68--learn-how-to-implement-the-command-pattern-in-java.html?page=2
In one of our projects, we have the following requirement:
To perform this in a transactional manner, each operation is implemented as a command with undo operation. At the end of each step, the command is pushed onto a stack. If the operation fails at some step, then we pop the commands from the stack and call undo operation on each of the command popped out. The undo operation of each step is defined in that command implementation to reverse the earlier command.execute().
Hope this helps.