Object Serialization in Java

Or: The Persistence of Memory…

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So you want to save your data…

Common problem: You’ve built a large, complex object

Spam/Normal statistics tables Game state Database of student records Etc…

Want to store on disk and retrieve later Or: want to send over network to another

Java process In general: want your objects to be

persistent -- outlive the current Java process

Answer I: Homebrew file formats

You’ve got file I/O nailed, so… Write a set of methods for saving/loading

each class that you care about

public class MyClass { public void saveYourself(Writer o) throws IOException { … } public static MyClass loadYourself(Reader r) throws IOException { … }}

Coolnesses of Approach 1:

Can produce arbitrary file formats Know exactly what you want to store and

get back/don’t store extraneous stuff Can build file formats to interface w/ other

codes/programs XML Tab-delimited/spreadsheet Etc.

If your classes are nicely hierarchical, makes saving/loading simple

Saving/Loading Recursive Data Structs

public interface Saveable { public void saveYourself(Writer w) throws IOException; // should also have this // public static Object loadYourself(Reader r) // throws IOException; // but you can’t put a static method in an // interface in Java}

Saving, cont’dpublic class MyClassA implements Saveable { public MyClassA(int arg) { // initialize private data members of A } public void saveYourself(Writer w) throws IOException { // write MyClassA identifier and private data on // stream w } public static MyClassA loadYourself(Reader r) throws IOException { // parse MyClassA from the data stream r MyClassA tmp=new MyClassA(data); return tmp; }}

Saving, cont’d

public class MyClassB implements Saveable { public void MyClassB(int arg) { … } private MyClassA _stuff; public void saveYourself(Writer w) { // write ID for MyClassB _stuff.saveYourself(w); // write other private data for MyClassB w.flush(); } public static MyClassB loadYourself(Reader r) { // parse MyClassB ID from r MyClassA tmp=MyClassA.loadYourself(r); // parse other private data for MyClassB return new MyClassB(tmp); }}

Painfulnesses of Approach 1:

This is called recursive descent parsing (and formatting)

We’ll use it in project 2, and there are plenty of places in the Real World (TM) where it’s terribly useful.

But... It’s also a pain in the a** If all you want to do is store/retrieve data,

do you really need to go to all of that effort? Fortunately, no. Java provides a shortcut

that takes a lot of the work out.

Approach 2: Enter Serialization...

Java provides the serialization mechanism for object persistence

It essentially automates the grunt work for you Short form:

public class MyClassA implements Serializable { ... }

// in some other code elsewhere...

MyClassA tmp=new MyClassA(arg);

FileOutputStream fos=new FileOutputStream(“some.obj”);

ObjectOutputStream out=new ObjectOutputStream(fos);

out.writeObject(tmp);

out.flush();

out.close();

In a bit more detail...

To (de-)serialize an object, it must implements Serializable All of its data members must also be

marked serializable And so on, recursively... Primitive types (int, char, etc.) are all

serizable automatically So are Strings, most classes in java.util,

etc. This saves/retrieves the entire object

graph, including ensuring uniqueness of objects

The object graph and uniqueness

MondoHashTable

Entry

Entry

“tyromancy”

“zygopleural”

Vector

Now for some subtleties...

static fields are not automatically serialized Not possible to automatically serialize them

b/c they’re owned by an entire class, not an object

Options: final static fields are automatically

initialized (once) the first time a class is loaded

static fields initialized in the static {} block will be initialized the first time a class is loaded

But what about other static fields?

When default serialization isn’t enough

Java allows writeObject() and readObject() methods to customize output

If a class provides these methods, the serialization/deserialization mechanism calls them instead of doing the default thing

writeObject() in actionpublic class DemoClass implements Serializable { private int _dat=3; private static int _sdat=2;

private void writeObject(ObjectOutputStream o) throws IOException { o.writeInt(_dat); o.writeInt(_sdat); } private void readObject(ObjectInputStream i) throws IOException, ClassNotFoundException { _dat=i.readInt(); _sdat=i.readInt(); }}

Things that you don’t want to save

Sometimes, you want to explicitly not store some non-static data Computed vals that are cached simply

for convenience/speed Passwords or other “secret” data that

shouldn’t be written to disk Java provides the “transient” keyword. transient foo==don’t save foo

public class MyClass implements Serializable {

private int _primaryVal=3; // is serialized

private transient int _cachedVal=_primaryVal*2;

// _cachedVal is not serialized

}

Gotchas: #0 -- non Serializable fields

What happens if class Foo has a field of type Bar, but Bar isn’t serializable?

If you just do this:

You get a NotSerializableException (bummer)

Answer: use read/writeObject to explicitly serialize parts that can’t be handled otherwise

Need some way to get/set necessary state

Foo tmp=new Foo();ObjectOutputStream out=new ObjectOutputStream;out.writeObject(tmp);

Gotchas: #0.5 -- non-Ser. superclasses

Suppose class Foo extends Bar implements Serializable

But Bar itself isn’t serializable What happens?

Non-Serializable superclasses, cont’d

Bar must provide a no-arg constructor Foo must use readObject/writeObject to

take care of Bar’s private data Java helps a bit with defaultReadObject

and defaultWriteObject Order of operations (for deserialization)

Java creates a new Foo object Java calls Bar’s no-arg constructor Java calls Foo’s readObject

Foo’s readObject explicitly reads Bar’s state data Foo reads its own data Foo reads its children’s data

Gotchas: #1 -- Efficiency

For your MondoHashTable, you can just serialize/deserialize it with the default methods

But that’s not necessarily efficient, and may even be wrong

By default, Java will store the entire internal _table, including all of its null entries!

Now you’re wasting space/time to load/save all those empty cells

Plus, the hashCode()s of the keys may not be the same after deserialziation -- should explicitly rehash them to check.

Gotchas: #2 -- Backward compatibility

Suppose that you have two versions of class Foo: Foo v. 1.0 and Foo v. 1.1

The public and protected members of 1.0 and 1.1 are the same; the semantics of both are the same

So Foo 1.0 and 1.1 should behave the same and be interchangable

BUT... The private fields and implementation of 1.0 and 1.1 are different

What happens if you serialize with a 1.0 object and deserialize with a 1.1? Or vice versa?

Backward compat, cont’d.

Issue is that in code, only changes to the public or protected interfaces matter

With serialization, all of a sudden, the private data memebers (and methods) count too

Have to be very careful to not muck up internals in a way that’s inconsistent with previous versions

E.g., changing the meaning, but not name of some data field

Backward compat, cont’d

Example:

// version 1.0

public class MyClass {

MyClass(int arg) { _dat=arg*2; }

private int _dat;

}

// version 1.1

public class MyClass {

MyClass(int arg) { _dat=arg*3; } // NO-NO!

private int _dat;

}

Backward compat, cont’d:

Java helps as much as it can Java tracks a “version number” of a class

that changes when the class changes “substantially” Fields changed to/from static or transient Field or method names changed Data types change Class moves up or down in the class

hierarchy Trying to deserialize a class of a different

version than the one currently in memory throws InvalidClassException

Yet more on backward compat

Java version number comes from names of all data and method members of a class

If they don’t change, the version number won’t change

If you want Java to detect that something about your class has changed, change a name

But, if all you’ve done is changed names (or refactored functionality), you want to be able to tell Java that nothing has changed

Can lie to Java about version number:static final long serialVersionUID = 3530053329164698194L;