Java Custom Serialization

I have an object that contains a few unserializable fields that I want to serialize. They are from a separate API that I cannot change, so making them Serializable is not an option. The main problem is the Location class. It contains four things that can be serialized that I'd need, all ints. How can I use read/writeObject to create a custom serialization method that can do something like this:

// writeObject:
List<Integer> loc = new ArrayList<Integer>();
loc.add(location.x);
loc.add(location.y);
loc.add(location.z);
loc.add(location.uid);
// ... serialization code

// readObject:
List<Integer> loc = deserialize(); // Replace with real deserialization
location = new Location(loc.get(0), loc.get(1), loc.get(2), loc.get(3));
// ... more code

How can I do this?

Java supports Custom Serialization. Read the section Customize the Default Protocol.

To quote:

There is, however, a strange yet crafty solution. By using a built-in feature of the serialization mechanism, developers can enhance the normal process by providing two methods inside their class files. Those methods are:

private void writeObject(ObjectOutputStream out) throws IOException;

private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException;

In this method, what you could do is serialize it into other forms if you need to such as the ArrayList for Location that you illustrated or JSON or other data format/method and reconstruct it back on readObject()

With your example, you add the following code:



private void writeObject(ObjectOutputStream oos)
throws IOException {
    // default serialization 
    oos.defaultWriteObject();
    // write the object
    List loc = new ArrayList();
    loc.add(location.x);
    loc.add(location.y);
    loc.add(location.z);
    loc.add(location.uid);
    oos.writeObject(loc);
}

private void readObject(ObjectInputStream ois)
throws ClassNotFoundException, IOException {
    // default deserialization
    ois.defaultReadObject();
    List loc = (List)ois.readObject(); // Replace with real deserialization
    location = new Location(loc.get(0), loc.get(1), loc.get(2), loc.get(3));
    // ... more code

}

Similar to @momo's answer but without using a List and auto-boxed int values which will make it much more compact.

private void writeObject(ObjectOutputStream oos) throws IOException {
    // default serialization 
    oos.defaultWriteObject();
    // write the object
    oos.writeInt(location.x);
    oos.writeInt(location.y);
    oos.writeInt(location.z);
    oos.writeInt(location.uid);
}

private void readObject(ObjectInputStream ois) throws ClassNotFoundException, IOException {
    // default deserialization
    ois.defaultReadObject();
    location = new Location(ois.readInt(), ois.readInt(), ois.readInt(), ois.readInt());
    // ... more code

}

If it must be Java serialization, the only way I know of is to redefine the readObject() and writeObject() in all classes having a reference to an instance of Location as shown in Momo's answer. Note that this will not allow you to serialize a Location[], and require you to subclass all Collection<Location> appearing in your code. Moreover, it requires the fields of type Location to be marked transient, which will exclude their definitions from being written to the serialization stream, possible foiling the detection of incompatible class changes.

A better way would be to simply override ObjectOutputStream.writeObject. Alas, that method is final. You could override ObjectOutputStream.writeObjectOverride() instead, but that method can not delegate the default implementation, ObjectOutputStream.writeObject0() because that method is private. Of course, you could invoke the private method using reflection, but ...

Therefore, I recommend verifying your constraints. Does it have to be Java serialization? Can you really not change the definition of class Location?

If you have the source code to class Location, it's quite trivial to add implements Serializable and add it to your classpath. Yes, you'll have to do this again whenever you upgrade the library, but it might be better than the alternative ...

Java Custom Serialization

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