The Java serialization algorithm revealed 

Serialization is the process of saving an object's state to a sequence of bytes; deserialization is the process of rebuilding those bytes into a live object. The Java Serialization API provides a standard mechanism for developers to handle object serialization. In this tip, you will see how to serialize an object, and why serialization is sometimes necessary. You'll learn about the serialization algorithm used in Java, and see an example that illustrates the serialized format of an object. By the time you're done, you should have a solid knowledge of how the serialization algorithm works and what entities are serialized as part of the object at a low level.

Why is serialization required?

In today's world, a typical enterprise application will have multiple components and will be distributed across various systems and networks. In Java, everything is represented as objects; if two Java components want to communicate with each other, there needs be a mechanism to exchange data. One way to achieve this is to define your own protocol and transfer an object. This means that the receiving end must know the protocol used by the sender to re-create the object, which would make it very difficult to talk to third-party components. Hence, there needs to be a generic and efficient protocol to transfer the object between components. Serialization is defined for this purpose, and Java components use this protocol to transfer objects.

Figure 1 shows a high-level view of client/server communication, where an object is transferred from the client to the server through serialization.

Figure 1. A high-level view of serialization in action (click to enlarge)

How to serialize an object

In order to serialize an object, you need to ensure that the class of the object implements the

java.io.Serializable

interface, as shown in Listing 1.

Listing 1. Implementing Serializable

[code=plain]import java.io.Serializable;

class TestSerial implements Serializable {
public byte version = 100;
public byte count = 0;
}

[/code]
In Listing 1, the only thing you had to do differently from creating a normal class is implement the

java.io.Serializable

interface. The

Serializable

interface is a marker interface; it declares no methods at all. It tells the serialization mechanism that the class can be serialized.

Now that you have made the class eligible for serialization, the next step is to actually serialize the object. That is done by calling the

writeObject()

method of the

java.io.ObjectOutputStream

class, as shown in Listing 2.

Listing 2. Calling writeObject()

[code=plain]public static void main(String args[]) throws IOException {
FileOutputStream fos = new FileOutputStream("temp.out");
ObjectOutputStream oos = new ObjectOutputStream(fos);
TestSerial ts = new TestSerial();
oos.writeObject(ts);
oos.flush();
oos.close();
}

[/code]
Listing 2 stores the state of the

TestSerial

object in a file called

temp.out

.

oos.writeObject(ts);

actually kicks off the serialization algorithm, which in turn writes the object to

temp.out

.

To re-create the object from the persistent file, you would employ the code in Listing 3.

Listing 3. Recreating a serialized object

[code=plain]public static void main(String args[]) throws IOException {
FileInputStream fis = new FileInputStream("temp.out");
ObjectInputStream oin = new ObjectInputStream(fis);
TestSerial ts = (TestSerial) oin.readObject();
System.out.println("version="+ts.version);
}

[/code]
In Listing 3, the object's restoration occurs with the

oin.readObject()

method call. This method call reads in the raw bytes that we previously persisted and creates a live object that is an exact replica of the original object graph. Because

readObject()

can read any serializable object, a cast to the correct type is required.

Executing this code will print

version=100

on the standard output.

The serialized format of an object

What does the serialized version of the object look like? Remember, the sample code in the previous section saved the serialized version of the

TestSerial

object into the file

temp.out

. Listing 4 shows the contents of

temp.out

, displayed in hexadecimal. (You need a hexadecimal editor to see the output in hexadecimal format.)

Listing 4. Hexadecimal form of TestSerial

[code=plain]AC ED 00 05 73 72 00 0A 53 65 72 69 61 6C 54 65
73 74 A0 0C 34 00 FE B1 DD F9 02 00 02 42 00 05
63 6F 75 6E 74 42 00 07 76 65 72 73 69 6F 6E 78
70 00 64

[/code]
If you look again at the actual

TestSerial

object, you'll see that it has only two byte members, as shown in Listing 5.

Listing 5. TestSerial's byte members

[code=plain]	public byte version = 100;
public byte count = 0;

[/code]
The size of a byte variable is one byte, and hence the total size of the object (without the header) is two bytes. But if you look at the size of the serialized object in Listing 4, you'll see 51 bytes. Surprise! Where did the extra bytes come from, and what is their significance? They are introduced by the serialization algorithm, and are required in order to to re-create the object. In the next section, you'll explore this algorithm in detail.

Java's serialization algorithm

By now, you should have a pretty good knowledge of how to serialize an object. But how does the process work under the hood? In general the serialization algorithm does the following:

It writes out the metadata of the class associated with an instance.

It recursively writes out the description of the superclass until it finds

java.lang.object

.

Once it finishes writing the metadata information, it then starts with the actual data associated with the instance. But this time, it starts from the topmost superclass.

It recursively writes the data associated with the instance, starting from the least superclass to the most-derived class.

I've written a different example object for this section that will cover all possible cases. The new sample object to be serialized is shown in Listing 6.

Listing 6. Sample serialized object

[code=plain]class parent implements Serializable {
int parentVersion = 10;
}

class contain implements Serializable{
int containVersion = 11;
}
public class SerialTest extends parent implements Serializable {
int version = 66;
contain con = new contain();

public int getVersion() {
return version;
}
public static void main(String args[]) throws IOException {
FileOutputStream fos = new FileOutputStream("temp.out");
ObjectOutputStream oos = new ObjectOutputStream(fos);
SerialTest st = new SerialTest();
oos.writeObject(st);
oos.flush();
oos.close();
}
}

[/code]
This example is a straightforward one. It serializes an object of type

SerialTest

, which is derived from

parent

and has a container object,

contain

. The serialized format of this object is shown in Listing 7.

Listing 7. Serialized form of sample object

[code=plain]AC ED 00 05 73 72 00 0A 53 65 72 69 61 6C 54 65
73 74 05 52 81 5A AC 66 02 F6 02 00 02 49 00 07
76 65 72 73 69 6F 6E 4C 00 03 63 6F 6E 74 00 09
4C 63 6F 6E 74 61 69 6E 3B 78 72 00 06 70 61 72
65 6E 74 0E DB D2 BD 85 EE 63 7A 02 00 01 49 00
0D 70 61 72 65 6E 74 56 65 72 73 69 6F 6E 78 70
00 00 00 0A 00 00 00 42 73 72 00 07 63 6F 6E 74
61 69 6E FC BB E6 0E FB CB 60 C7 02 00 01 49 00
0E 63 6F 6E 74 61 69 6E 56 65 72 73 69 6F 6E 78
70 00 00 00 0B

[/code]
Figure 2 offers a high-level look at the serialization algorithm for this scenario.

Figure 2. An outline of the serialization algorithm

Let's go through the serialized format of the object in detail and see what each byte represents. Begin with the serialization protocol information:

AC ED

:

STREAM_MAGIC

. Specifies that this is a serialization protocol.

00 05

:

STREAM_VERSION

. The serialization version.

0x73

:

TC_OBJECT

. Specifies that this is a new

Object

.

The first step of the serialization algorithm is to write the description of the class associated with an instance. The example serializes an object of type

SerialTest

, so the algorithm starts by writing the description of the

SerialTest

class.

0x72

:

TC_CLASSDESC

. Specifies that this is a new class.

00 0A

: Length of the class name.

53 65 72 69 61 6c 54 65 73 74

:

SerialTest

, the name of the class.

05 52 81 5A AC 66 02 F6

:

SerialVersionUID

, the serial version identifier of this class.

0x02

: Various flags. This particular flag says that the object supports serialization.

00 02

: Number of fields in this class.

Next, the algorithm writes the field

int version = 66;

.

0x49

: Field type code. 49 represents "I", which stands for

Int

.

00 07

: Length of the field name.

76 65 72 73 69 6F 6E

:

version

, the name of the field.

And then the algorithm writes the next field,

contain con = new contain();

. This is an object, so it will write the canonical JVM signature of this field.

0x74

:

TC_STRING

. Represents a new string.

00 09

: Length of the string.

4C 63 6F 6E 74 61 69 6E 3B

:

Lcontain;

, the canonical JVM signature.

0x78

:

TC_ENDBLOCKDATA

, the end of the optional block data for an object.

The next step of the algorithm is to write the description of the

parent

class, which is the immediate superclass of

SerialTest

.

0x72

:

TC_CLASSDESC

. Specifies that this is a new class.

00 06

: Length of the class name.

70 61 72 65 6E 74

:

SerialTest

, the name of the class

0E DB D2 BD 85 EE 63 7A

:

SerialVersionUID

, the serial version identifier of this class.

0x02

: Various flags. This flag notes that the object supports serialization.

00 01

: Number of fields in this class.

Now the algorithm will write the field description for the

parent

class.

parent

has one field,

int parentVersion = 100;

.

0x49

: Field type code. 49 represents "I", which stands for

Int

.

00 0D

: Length of the field name.

70 61 72 65 6E 74 56 65 72 73 69 6F 6E

:

parentVersion

, the name of the field.

0x78

:

TC_ENDBLOCKDATA

, the end of block data for this object.

0x70

:

TC_NULL

, which represents the fact that there are no more superclasses because we have reached the top of the class hierarchy.

So far, the serialization algorithm has written the description of the class associated with the instance and all its superclasses. Next, it will write the actual data associated with the instance. It writes the parent class members first:

00 00 00 0A

: 10, the value of

parentVersion

.

Then it moves on to

SerialTest

.

00 00 00 42

: 66, the value of

version

.

The next few bytes are interesting. The algorithm needs to write the information about the

contain

object, shown in Listing 8.

Listing 8. The contain object

[code=plain]contain con = new contain();

[/code]
Remember, the serialization algorithm hasn't written the class description for the

contain

class yet. This is the opportunity to write this description.

0x73

:

TC_OBJECT

, designating a new object.

0x72

:

TC_CLASSDESC

.

00 07

: Length of the class name.

63 6F 6E 74 61 69 6E

:

contain

, the name of the class.

FC BB E6 0E FB CB 60 C7

:

SerialVersionUID

, the serial version identifier of this class.

0x02

: Various flags. This flag indicates that this class supports serialization.

00 01

: Number of fields in this class.

Next, the algorithm must write the description for

contain

's only field,

int containVersion = 11;

.

0x49

: Field type code. 49 represents "I", which stands for

Int

.

00 0E

: Length of the field name.

63 6F 6E 74 61 69 6E 56 65 72 73 69 6F 6E

:

containVersion

, the name of the field.

0x78

:

TC_ENDBLOCKDATA

.

Next, the serialization algorithm checks to see if

contain

has any parent classes. If it did, the algorithm would start writing that class; but in this case there is no superclass for

contain

, so the algorithm writes

TC_NULL

.

0x70

:

TC_NULL

.

Finally, the algorithm writes the actual data associated with

contain

.

00 00 00 0B

: 11, the value of

containVersion

.

Conclusion

In this tip, you have seen how to serialize an object, and learned how the serialization algorithm works in detail. I hope this article gives you more detail on what happens when you actually serialize an object.

About the author

Sathiskumar Palaniappan has more than four years of experience in the IT industry, and has been working with Java-related technologies for more than three years. Currently, he is working as a system software engineer at the Java Technology Center, IBM Labs. He also has experience in the telecom industry.

Resources

Read the Java object serialization specification. (Spec is a PDF.)

"Flatten your objects: Discover the secrets of the Java Serialization API" (Todd M. Greanier, JavaWorld, July 2000) offers a look into the nuts and bolts of the serialization process.

Chapter 10 of Java RMI (William Grosso, O'Reilly, October 2001) is also a useful reference.