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UNIVERSITY OF
NIVERSITY OF M
MASSACHUSETTS
ASSACHUSETTS, A
, AMHERST
MHERST •
•Department of Computer Science
Department of Computer Science
14 – The Memento Pattern; Java Serialization
CMPSCI 220 (291A)
Programming Methodology
Spring 2009
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Memento Pattern and Java Serialization: Saving and Restoring State in Computer Science, Study notes of Computer Science
An overview of the memento pattern, its goals, benefits, considerations, and implementation using java serialization. The memento pattern is a behavioral design pattern that enables saving and restoring the important state of an object while maintaining its encapsulation. Java serialization is a popular approach for saving and restoring connected groups of objects. The process of making a memento in java, shallow versus deep copies, and creating game snapshot mementos. It also explains the concept of cloning objects and the differences between in-memory and saved copies.
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U
U
NIVERSITY OFNIVERSITY OF
M
M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science
14 – The Memento Pattern; Java Serialization
CMPSCI 220 (291A)
Programming Methodology
Spring 2009
U
U
NIVERSITY OFNIVERSITY OF
M
M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science
The Memento Pattern
Goals:
Save important state of a system’s key object …
While maintaining the key object’s
encapsulation
Benefits:
Separating saved state from the key object maintains cohesion
and the key object’s
encapsulation
Provides easy-to-implement recovery capability
Considerations:
Saving and restoring state can be time consuming
Java Serialization is a good implementation approach
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NIVERSITY OFNIVERSITY OF
M
M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science
Memento Pattern in the Project
BreakoutGame/BOEngine //when save is requestedGameSnapshot snapshot =
GameSnapshot.makeSnapshot(
(game, gameState); // **
//when restoring (a new game)… = new BreakoutGame(
gameEngine, snapshot); // ***
BreakoutGame/GameState GameState snapshotvoid saveGame() {
//gather state: code at **
} BreakoutGame(Game Engine e,
GameSnapshot s); {
//restore state: code at ***
} // do other Key Object stuff
GameSnapshot savedBallsavedBlocks…
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NIVERSITY OFNIVERSITY OF
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M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science
Making a Memento in Java
Often want to make a
copy
of an object
In Java, this is called
cloning
Class Object has a default clone method
Signature:
protected
Object
clone()
Creates a duplicate whose
fields
have the same values as
the original
This is a
shallow copy
(more in a moment)
Must implement the Cloneable marker interface inorder to clone an object
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NIVERSITY OFNIVERSITY OF
M
M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science
Shallow versus Deep Copies
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NIVERSITY OFNIVERSITY OF
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M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science
Shallow versus Deep Copies
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U
NIVERSITY OFNIVERSITY OF
M
M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science
Game snapshot Mementos
You need to clone the Sprites of the game
There is only one Ball and one Paddle (easy)
There can be many Blocks and PowerUps
Use SpriteGroup.getSprites() to obtain them
Add non-null active Sprites to a List that is part ofthe Memento object’s state
You will also need to clone the GameState
You need to mark these Cloneable
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NIVERSITY OFNIVERSITY OF
M
M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science
Another detail
In Object, clone() is marked
protected
You may need to declare a
public
method:
public
Object
clone
throws
CloneNotSupportedException
return
super.clone();
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NIVERSITY OFNIVERSITY OF
M
M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science
In-memory or saved?
Cloning objects just gives an in-memory copy
Fine for undoing or returning to a recent state
Fine if we don’t crash
With additional effort, we can save to a file
Called
serializing
the memento object
We now look at Java serialization in more detail
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NIVERSITY OFNIVERSITY OF
M
M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science
Java Serialization
Goal:
To save a connected group of objects to a
stream, so that we can restore them,
with their same
connections,
later.
Serialize = to turn into a series of bytes
Why is this tricky?
Shared
objects in the structure
Cycles
of pointers in the structure
Might be read back into a
different program
Could have different version of some of the classes
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NIVERSITY OFNIVERSITY OF
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M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science
What about sharing and cycles?
The objects written are
numbered:
The
first
time we write an object we proceed
as normal
If we see other references to object number k
, we write a special entry meaning “I refer to
previous object
k
This works for both sharing
and
cycles.
(There is also a special value for
null
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U
NIVERSITY OFNIVERSITY OF
M
M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science
A fine point
The
classes
mentioned are also numbered
The
first
time a class is mentioned, we write:
Its full name, such as java.util.ArrayList
Its
version number
, which allows for evolution
After the first mention, we just write a specialentry, “previous class number
n
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NIVERSITY OFNIVERSITY OF
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M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science
A small example
Write: com.xyz.product.package.A, version 1
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U
NIVERSITY OFNIVERSITY OF
M
M
ASSACHUSETTS ASSACHUSETTS
, A
, A
MHERST MHERST •
Department of Computer Science Department of Computer Science