What is the purpose of Serialization in Java?

I have read quite a number of articles on Serialization and how it is so nice and great but none of the arguments were convincing enough. I am wondering if someone can really tell me what is it that we can really achieve by serializing a class?

Let's define serialization first, then we can talk about why it's so useful.

Serialization is simply turning an existing object into a byte array. This byte array represents the class of the object, the version of the object, and the internal state of the object. This byte array can then be used between JVM's running the same code to transmit/read the object.

Why would we want to do this?

There are several reasons:

• Communication: If you have two machines that are running the same code, and they need to communicate, an easy way is for one machine to build an object with information that it would like to transmit, and then serialize that object to the other machine. It's not the best method for communication, but it gets the job done.

• Persistence: If you want to store the state of a particular operation in a database, it can be easily serialized to a byte array, and stored in the database for later retrieval.

• Deep Copy: If you need an exact replica of an Object, and don't want to go to the trouble of writing your own specialized clone() class, simply serializing the object to a byte array, and then de-serializing it to another object achieves this goal.

• Caching: Really just an application of the above, but sometimes an object takes 10 minutes to build, but would only take 10 seconds to de-serialize. So, rather than hold onto the giant object in memory, just cache it out to a file via serialization, and read it in later when it's needed.

• Cross JVM Synchronization: Serialization works across different JVMs that may be running on different architectures.

While you're running your application, all of its objects are stored in memory (RAM). When you exit, that memory gets reclaimed by the operating system, and your program essentially 'forgets' everything that happened while it was running. Serialization remedies this by letting your application save objects to disk so it can read them back the next time it starts. If your application is going to provide any way of saving/sharing a previous state, you'll need some form of serialization.

I can share my story and I hope it will give some ideas why serialization is necessary. However, the answers to your question are already remarkably detail.

I had several projects that need to load and read a bunch of text files. The files contained stop words, biomedical verbs, biomedical abbreviations, words semantically connected to each other, etc. The contents of these files are simple: words!

Now for each project, I needed to read the words from each of these files and put them into different arrays; as the contents of the file never changed, it became a common, however redundant, task after the first project.

So, what I did is that I created one object to read each of these files and to populate individual arrays (instance variables of the objects). Then I serialized the objects and then for the later projects, I simply deserialized them. I didn't have to read the files and populate the arrays again and again.

http://oreilly.com/catalog/javarmi/chapter/ch10.html">In essense:

> Serialization is the process of > converting a set of object instances > that contain references to each other > into a linear stream of bytes, which > can then be sent through a socket, > stored to a file, or simply > manipulated as a stream of data

See uses from http://en.wikipedia.org/wiki/Serialization">Wiki</a>;:

Serialization has a number of advantages. It provides:

> 1. a method of persisting objects which > is more convenient than writing > their properties to a text file on > disk, and re-assembling them by > reading this back in. > 2. a method of > issuing remote procedure calls, > e.g., as in SOAP > 3. a method for > distributing objects, especially in > software componentry such as COM, > CORBA, etc. > 4. a method for detecting > changes in time-varying data.

The most obvious is that you can transmit the serialized class over a network, and the recepient can construct a duplicate of the original instanstance. Likewise, you can save a serialized structure to a file system.

Also, note that serialization is recursive, so you can serialize an entire heterogenous data structure in one swell foop, if desired.

Serialized objects maintain state in space, they can be transferred over the network, file system, etc... and time, they can outlive the JVM that created them.

Sometimes this is useful.

I use serialized objects to standardize the arguments I pass to functions or class constructors. Passing one serialized bean is much cleaner than a long list of arguments. The result is code that is easier to read and debug.

For the simple purpose of learning (notice, I said learning, I did not say best, or even good, but just for the sake of understanding stuff), you could save your data to a text file on the computer, then have a program that reads that info, and based on the file, you could have your program respond differently. If you were more advanced, it wouldn't necessarily have to be a txt file, but something else.

Serializing on the other hand, puts things directly into computer language. It's like you're telling a Spanish computer something in Spanish, rather than telling it something in French, forcing it to learn French, then save things into its native Spanish by translating everything. Not the most tech-intensive answer, I'm just trying to create an understandable example in a common language format.

Serialization is also faster, because in Java, objects are handled on the heap, and take much longer than if they were represented as primitives on the stack. Speed, speed, speed. And less file processing from a programmer point of view.