1. Object Serialization in Java Or: The Persistence of Memory…

2. 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

3. 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 { … } }

4. 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

5. 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 }

6. Saving, cont’d public 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; }

7. 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); }

8. 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.

9. 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();

10. 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

11. The object graph and uniqueness MondoHashTable Entry “tyromancy” “zygopleural” Vector

12. 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?

13. 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

14. writeObject() in action public 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(); }

15. 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 }

16. 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);

17. Gotchas: #0.5 -- non-Ser. superclasses Suppose class Foo extends Bar implements Serializable But Bar itself isn’t serializable What happens?

18. 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

19. 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.

20. 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?

21. 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

22. 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; }

23. 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

24. 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;