How do you serialize an object to a file?

Answer 1

Imagine you want to save the state of one or more objects. If Java didn't have serialization, you'd have to use one of the I/O classes to write out the state of the instance variables of all the objects you want to save. The worst part would be trying to reconstruct new objects that were virtually identical to the objects you were trying to save. You'd need your own protocol for the way in which you wrote and restored the state of each object, or you could end up setting variables with the wrong values. For example, imagine you stored an object that has instance variables for height and weight. At the time you save the state of the object, you could write out the height and weight as two ints in a file, but the order in which you write them is crucial. It would be all too easy to re-create the object but mix up the height and weight values-using the saved height as the value for the new object's weight and vice versa.

The purpose of Serialization is to help us achieve whatever complicated scenario we just witnessed in an easier manner.

Serializing Objects

What does it really mean to save an object? If the instance variables are all primitive types, it's pretty straightforward. But what if the instance variables are themselves references to objects? What gets saved? Clearly in Java it wouldn't make any sense to save the actual value of a reference variable, because the value of a Java reference has meaning only within the context of a single instance of a JVM. In other words, if you tried to restore the object in another instance of the JVM, even running on the same computer on which the object was originally serialized, the reference would be useless.

But what about the object that the reference refers to? Look at this class:

class Car {

private Engine theEngine;

private int CarSize;

public Car(Engine Engine, int size) {

theEngine = Engine;

CarSize = size;

}

public Engine getEngine() { return theEngine; }

}

class Engine {

private int EngineSize;

public Engine(int size) { EngineSize = size; }

public int getEngineSize() { return EngineSize; }

}

Now make a Car... First, you make a Engine for the Car:

Engine c = new Engine(3);

Then make a new Car, passing it the Engine:

Car d = new Car(c, 8);

Now what happens if you save the Car? If the goal is to save and then restore a Car, and the restored Car is an exact duplicate of the Car that was saved, then the Car needs a Engine that is an exact duplicate of the Car's Engine at the time the Car was saved. That means both the Car and the Engine should be saved.

And what if the Engine itself had references to other objects-like perhaps a Color object? This gets quite complicated very quickly. If it were up to the programmer to know the internal structure of each object the Car referred to, so that the programmer could be sure to save all the state of all those objects. That would be a nightmare with even the simplest of objects.

Fortunately, the Java serialization mechanism takes care of all of this. When you serialize an object, Java serialization takes care of saving that object's entire "object graph." That means a deep copy of everything the saved object needs to be restored. For example, if you serialize a Car object, the Engine will be serialized automatically. And if the Engine class contained a reference to another object, THAT object would also be serialized, and so on. And the only object you have to worry about saving and restoring is the Car. The other objects required to fully reconstruct that Car are saved (and restored) automatically through serialization.

Remember, you do have to make a conscious choice to create objects that are serializable, by implementing the Serializable interface. If we want to save Car objects, for example, we'll have to modify the Car class as follows:

class Car implements Serializable {

// the rest of the code as before

// Serializable has no methods to implement

}

And now we can save the Car with the following code:

import java.io.*;

public class SerializeCar {

public static void main(String[] args) {

Engine c = new Engine(3);

Car d = new Car(c, 8);

try {

FileOutputStream fs = new FileOutputStream("testSer.ser");

ObjectOutputStream os = new ObjectOutputStream(fs);

os.writeObject(d);

os.close();

} Catch (Exception e) { e.printStackTrace(); }

}

}

But when we run this code we get a runtime exception something like this

java.io.NotSerializableException: Engine

What did we forget? The Engine class must ALSO be Serializable. If we modify the Engine class and make it serializable, then there's no problem:

class Engine implements Serializable {

// same

}

Here's the complete code:

import java.io.*;

public class SerializeCar {

public static void main(String[] args) {

Engine c = new Engine(3);

Car d = new Car(c, 5);

System.out.println("before: Engine size is "

+ d.getEngine().getEngineSize());

try {

FileOutputStream fs = new FileOutputStream("testSer.ser");

ObjectOutputStream os = new ObjectOutputStream(fs);

os.writeObject(d);

os.close();

} Catch (Exception e) { e.printStackTrace(); }

try {

FileInputStream fis = new FileInputStream("testSer.ser");

ObjectInputStream ois = new ObjectInputStream(fis);

d = (Car) ois.readObject();

ois.close();

} Catch (Exception e) { e.printStackTrace(); }

System.out.println("after: Engine size is "

+ d.getEngine().getEngineSize());

}

}

class Car implements Serializable {

private Engine theEngine;

private int CarSize;

public Car(Engine Engine, int size) {

theEngine = Engine;

CarSize = size;

}

public Engine getEngine() { return theEngine; }

}

class Engine implements Serializable {

private int EngineSize;

public Engine(int size) { EngineSize = size; }

public int getEngineSize() { return EngineSize; }

}

This produces the output:

before: Engine size is 3

after: Engine size is 3