CSC/ECE 517 Fall 2009/wiki2 18 cc
Introduction
Immutable classes are used to ensure the object state never gets changed. On the other hand the concept of serialization is that applications can send and receive objects across the network; this is a step further than just keeping the object alive (persistence) which is the ability to call objects remotely, however private instances may be exposed by performing serialization.
Now the problem arises when serializing an immutable class and the immutability must be preserved between applications.
The Serialization Proxy Pattern is capable to handle this kind of situations, henceforth the main objectives are to explain:
1. It's idiomatic usage.
2. How it is designed and give details of its implementation.
3. How the issue described above is handled.
When a Class is Immutable?
If a class state cannot be modified after being created then it is called immutable, and viceverza a mutable class can be modified after being created. There are also partially immutable classes, in which some of their attributes can be defined as immutable.
The following is an example to illustrate immutable property:
String course_id= "ECE517" ; course_id.toLowerCase();
In this case the course_id didn’t change to lower case, it remains as "ECE517" , although the output is “ece517" , instead the output is a new string that can be assigned to another variable, therefore and for the moment the values are still immutable, however the following code can change the value.
course_id = course_id.toLowerCase();
How to Make a Class Immutable?
Java Example
To make a class immutable we need to keep the class state safe by any means, in other words, assignments should never occur, the following code is an example of immutable class in JAVA
final class Immutable_ece517Class
{
// To make the variables private the term “final” is to restrict writing private
final string nombre;
// Constructor where we can provide the constant value
public Immutable_ece517Class (string paramNombre)
{
nombre = paramNombre;
}
// Define the methods to return variable value without writing it
public string getNombre
{
return nombre;
}
}
Ruby Example
To implement immutable class in Ruby, the programmer must use the Struct.immutable instead of using Struct.new, the following is the code example implementation.
def Struct.immutable(*args) # Factory method for immutable classes
Struct.new(*args).class_eval do # Define struct, then modify it
undef []= # Undefine general mutator
args.each do |sym| # For each field of the struct
mutator = :"#{sym.to_s}=" # Symbol for setter method
remove_method mutator # Undefine that method
end
self # Return the class
end
end
What is Serialization?
It is known as the process from a source application to convert an object into a sequence of bits and to store or sent them through the network, in this way the same sequence can be retrieved by another end application that reconstructs or clones the original object in order to use it, this capability to store entire objects instead of simple parameters make possible to have high level distributed applications, without serialization the object would persist only while it is being executed (transient) and the application interaction would not be possible, e.g. remote procedure calls. Figure 1 Shows the Serialization/Deserialization process.
There is an option when an object is serialized, the state can also recorded in the byte stream, when it is retrieved its last state is also recovered, this feature allow the possibility to save the object state in memory and retrieve for its use later by the same application, although there is also an application named as persistent that does the same, serialization can be also used as a persistent procedure to keep alive classes in local memory storage after being executed.
It is also possible to serialize complex data structures like applets and complete GUI, but the scope of this wiki will be on class serialization.
How to Implement Serialization of Classes?
Java Serialization
The next step would be in how to implement serialization of classes. In this case Java.io package offers the “writeObject()” to serialize and for un-serialize it uses “readObject( )” both are methods from ObjectOuputStream and ObjectOuputStream respectively, for an object to be serialized must have implemented the java.io.Serializable interface, the reason behind this is security, some private classes should not be exposed when de-serialize them, this is why a class must explicity declare as serializable.
Below is the serialization code implementation in JAVA: Part taken from wiki/Object_serialization
import java.io.*;
/* The object to serialize. */
class ObjectToSerialize implements Serializable {
static private final long serialVersionUID = 42L;
private String firstAttribute;
private int secondAttribute;
public ObjectToSerialize(String firstAttribute, int secondAttribute) {
this.firstAttribute = firstAttribute;
this.secondAttribute = secondAttribute;
}
@Override
public String toString() {
return firstAttribute + ", " + secondAttribute;
}
}
public static void main(String[] args) {
ObjectToSerialize original = new ObjectToSerialize("some text", 123);
System.out.println(original);
try {
saveObject(original, "object.ser");
ObjectToSerialize loaded = (ObjectToSerialize) loadObject("object.ser");
System.out.println(loaded);
} catch (Exception e) {
e.printStackTrace();
}
}
}
Ruby Serialization
Ruby uses the methods of dump and load from the Marshal module to perform serialization, however not all objects can be serialized, the exeptions are Bindings, Procedures and Singleton objects,
Below is the code implementation in Ruby.
class Klass
def initialize(str)
@str = str
end
def sayHello
@str
end
end
o = Klass.new("hello\n")
data = Marshal.dump(o)
obj = Marshal.load(data)
obj.sayHello » "hello\n"
The Main Issue when De-serialize an Immutable Class
The weakness in serialization is that the details of the hidden private instances may be exposed in the other end application which is not good, however in order to make remote call the end application must understand the de-serialized objects, this is why serialization formats have been developed to keep proprietary software, one clear example is that a password and login should never be serialized, it is too risky if this information is exposed in the form of a serialized variable class.
The main issue is that hidden private instances (immutable classes) can be exposed whe de-serialize in the other end application and the idea is that they must remain safe and unchanged across applications. The solution proposed is the use of the Proxy Serialization Pattern.
What is a Proxy Pattern?
The Proxy Pattern, which is one of the most used design patterns, defines a proxy object that interatcs between source and sink applications within a network, one common use of it is the ability to call a remote tasks and to share information trough a network.
There are two types of proxies:
The first one is virtual proxy which creates the proxy obtect that interacts with the same application or other application within the same computer (stored in local memory), its porpouse is to keep an object state alive after its execution so that it can be used later on.
The second one is remote proxy which creates the proxy object to be used in a different address space, commonly in a remote object on a distant host. It creates the ilusion of interacting with the object or task directly, however there is a proxy between them that makes the interaction possible.
So, What is Proxy Serialization Pattern?
In general and summarized, the proxy serialization means that a class is serialized along with the method “writeReplace()” (Java), the result of this procedure is the so called “proxy serialization”, on the other end side the class is de-serialized along with the method “readResolve()” (Java) to reconstruct the class state (private properties).
The process to make a Proxy Serialization in more detail is listed below:
1. Before class serialization takes place, Java checks if the class contains the method “Object writeReplace()”.
2. If it does then the return value of “Object writeReplace()” is serialized together with the class (called Proxy Serialization), other wise only perform normal serialization on the class.
3. The proxy serialized value is used to keep the state of the original object to write (e.g. immutability), so that in the other end application the state can be recovered.
4. When de-serializing, Java checks if the class has the implementation of the method “Object readResolve()” if it does then it completes the de-serialization process and then the return value of the “readResolve()” is added to the de-serialization, which contains the state of the original object to write (e.g. immutability)
Figure 2 shows the process of Proxy Serialization Pattern
The key advantage of Proxy Serialization process is that proxy is implemented as a private static nested class, in figure 2 shows that its state is serialized (control of the read/write process) as well as the class itself, on the other end side the state is de-serialized (control of the read/write process) along with the class, in this way the class state is kept in the cloned class, this would keep the properties for example of an immutable implementation class.
Proxy Serialize Implementation in JAVA
The following code shows an example in Java; the first one is a typical immutable class, the second one is the serialization proxy implemented.
Immutable Class
public class Tot_price {
final int mPrice;
final int mTax;
public int price() { return mPrice; }
public int tax() { return mTax; }
public int total() {
return price() + tax();
}
Immutable Class Proxy Serialized
public class Tot_price implements Serializable {
private final mPrice;
private final mTax;
public Tot_price(double price, double tax) { mPrice = price; mTax=tax; }
...
private Object writeReplace() {
return new SerializationProxy(this);
}
private static class SerializationProxy
implements Serializable {
int mPrice; int mTax;
public SerializationProxy() { }
public SerializationProxy(Tot_price cost) {
mPrice = cost.price(); mTax = cost.tax();
}
Object readResolve() {
return new Tot_price(mPrice,mTax);
}
}
}
Proxy Serilization Identification
A random serial ID number is added whenever a class is Proxy Serialized by default. However, it is highly recommended to add it within the code in order to preserve forward compatibility, otherwise if the class changes the serialize process would be aborted, the following code shows how to add the ID number:
public class Tot_price implements Serializable {
....
static final long serialVersionUID = 200120012001L;
....
}
Every time Java serializes an object, the ID number is computed through reflection, this causes to slow down the serializationàde-serialization process, for this reason it is recommended to define in the code the ID number.
How is the Main Issue Solved?
Since the Proxy Pattern is a private class then its inheritance is also private, if there is an immutable class, then its immutable implementation is also private and its details remains hidden after de-serializating the proxy pattern.
In this way the immutable proporties are kept even in the other end side application.
References
http://stackoverflow.com/questions/702357/what-is-the-serialization-proxy-pattern
http://java.sun.com/javase/6/docs/technotes/guides/serialization/
http://my.safaribooksonline.com/9781598635655/ch10lev1sec4
http://my.safaribooksonline.com/0596006209/jenut3-CHP-10
http://my.safaribooksonline.com/0130844667/ch01lev1sec4