by Todd Greanier; Reprinted from JavaWorld
July 2000
We all know the JavaTM platform allows us to create reusable objects in memory. However, all of those objects exist only as long as the JavaTM virtual machine1 remains running. It would be nice if the objects we create could exist beyond the lifetime of the virtual machine, wouldn't it? Well, with object serialization, you can flatten your objects and reuse them in powerful ways.
Object serialization is the process of saving an object's state to a sequence of bytes, as well as the process of rebuilding those bytes into a live object at some future time. The Java Serialization API provides a standard mechanism for developers to handle object serialization. The API is small and easy to use, provided the classes and methods are understood.
Throughout this article, we'll examine how to persist your Java objects, starting with the basics and proceeding to the more advanced concepts. We'll learn three different ways to perform serialization -- using the default protocol, customizing the default protocol, and creating our own protocol -- and we'll investigate concerns that arise with any persistence scheme such as object caching, version control, and performance issues.
By the conclusion of this article, you should have a solid comprehension of that powerful yet sometimes poorly understood Java API.
Let's start with the basics. To persist an object in Java, we must have a
persistent object. An object is marked serializable by implementing the java.io.Serializable
interface, which signifies to the underlying API that the object can be
flattened into bytes and subsequently inflated in the future.
Let's look at a persistent class we'll use to demonstrate the serialization mechanism:
10 import java.io.Serializable;
20 import java.util.Date;
30 import java.util.Calendar;
40 public class PersistentTime implements Serializable
50 {
60 private Date time;
70
80 public PersistentTime()
90 {
100 time = Calendar.getInstance().getTime();
110 }
120
130 public Date getTime()
140 {
150 return time;
160 }
170 }
As you can see, the only thing we had to do differently from creating a
normal class is implement the java.io.Serializable interface on
line 40. The completely empty Serializable is only a marker
interface -- it simply allows the serialization mechanism to verify that the
class is able to be persisted. Thus, we turn to the first rule of serialization:
Rule #1: The object to be persisted must implement the Serializable
interface or inherit that implementation from its object hierarchy.
The next step is to actually persist the object. That is done with the java.io.ObjectOutputStream
class. That class is a filter stream--it is wrapped around a
lower-level byte stream (called a node stream) to handle the
serialization protocol for us. Node streams can be used to write to file systems
or even across sockets. That means we could easily transfer a flattened object
across a network wire and have it be rebuilt on the other side!
Take a look at the code used to save the PersistentTime object:
10 import java.io.ObjectOutputStream;
20 import java.io.FileOutputStream;
30 import java.io.IOException;
40 public class FlattenTime
50 {
60 public static void main(String [] args)
70 {
80 String filename = "time.ser";
90 if(args.length > 0)
100 {
110 filename = args[0];
120 }
130 PersistentTime time = new PersistentTime();
140 FileOutputStream fos = null;
150 ObjectOutputStream out = null;
160 try
170 {
180 fos = new FileOutputStream(filename);
190 out = new ObjectOutputStream(fos);
200 out.writeObject(time);
210 out.close();
220 }
230 catch(IOException ex)
240 {
250 ex.printStackTrace();
260 }
270 }
280 }
The real work happens on line 200 when we call the ObjectOutputStream.writeObject()
method, which kicks off the serialization mechanism and the object is flattened
(in that case to a file).
To restore the file, we can employ the following code:
10 import java.io.ObjectInputStream;
20 import java.io.FileInputStream;
30 import java.io.IOException;
40 import java.util.Calendar;
50 public class InflateTime
60 {
70 public static void main(String [] args)
80 {
90 String filename = "time.ser";
100 if(args.length > 0)
110 {
120 filename = args[0];
130 }
140 PersistentTime time = null;
150 FileInputStream fis = null;
160 ObjectInputStream in = null;
170 try
180 {
190 fis = new FileInputStream(filename);
200 in = new ObjectInputStream(fis);
210 time = (PersistentTime)in.readObject();
220 in.close();
230 }
240 catch(IOException ex)
250 {
260 ex.printStackTrace();
270 }
280 catch(ClassNotFoundException ex)
290 {
300 ex.printStackTrace();
310 }
320 // print out restored time
330 System.out.println("Flattened time: " + time.getTime());
340 System.out.println();
350 // print out the current time
360 System.out.println("Current time: " +
Calendar.getInstance().getTime());
370 }
380}
In the code above, the object's restoration occurs on line 210 with the ObjectInputStream.readObject()
method call. The method call reads in the raw bytes that we previously persisted
and creates a live object that is an exact replica of the original. Because readObject()
can read any serializable object, a cast to the correct type is required. With
that in mind, the class file must be accessible from the system in which the
restoration occurs. In other words, the object's class file and methods are not
saved; only the object's state is saved.
Later, on line 360, we simply call the getTime() method to
retrieve the time that the original object flattened. The flatten time is
compared to the current time to demonstrate that the mechanism indeed worked as
expected.
The basic mechanism of Java serialization is simple to use, but there are
some more things to know. As mentioned before, only objects marked Serializable
can be persisted. The java.lang.Object class does not implement
that interface. Therefore, not all the objects in Java can be persisted
automatically. The good news is that most of them -- like AWT and Swing GUI
components, strings, and arrays -- are serializable.
On the other hand, certain system-level classes such as Thread, OutputStream
and its subclasses, and Socket are not serializable. Indeed, it
would not make any sense if they were. For example, thread running in my JVM
would be using my system's memory. Persisting it and trying to run it in your
JVM would make no sense at all. Another important point about java.lang.Object
not implementing the Serializable interface is that any class you
create that extends only Object (and no other serializable classes)
is not serializable unless you implement the interface yourself (as done with
the previous example).
That situation presents a problem: what if we have a class that contains an
instance of Thread? In that case, can we ever persist objects of
that type? The answer is yes, as long as we tell the serialization mechanism our
intentions by marking our class's Thread object as transient.
Let's assume we want to create a class that performs an animation. I will not actually provide the animation code here, but here is the class we'll use:
10 import java.io.Serializable;
20 public class PersistentAnimation implements Serializable, Runnable
30 {
40 transient private Thread animator;
50 private int animationSpeed;
60 public PersistentAnimation(int animationSpeed)
70 {
80 this.animationSpeed = animationSpeed;
90 animator = new Thread(this);
100 animator.start();
110 }
120 public void run()
130 {
140 while(true)
150 {
160 // do animation here
170 }
180 }
190 }
When we create an instance of the PersistentAnimation class, the
thread animator will be created and started as we expect. We've
marked the thread on line 40 transient to tell the serialization
mechanism that the field should not be saved along with the rest of that
object's state (in that case, the field speed). The bottom line:
you must mark transient any field that either cannot be serialized
or any field you do not want serialized. Serialization does not care about
access modifiers such as private -- all nontransient fields are
considered part of an object's persistent state and are eligible for
persistence.
Therefore, we have another rule to add. Here are both rules concerning persistent objects:
Serializable
interface or inherit that implementation from its object hierarchy
transient
Let's move on to the second way to perform serialization: customize the
default protocol. Though the animation code above demonstrates how a thread
could be included as part of an object while still making that object be
serializable, there is a major problem with it if we recall how Java creates
objects. To wit, when we create an object with the new keyword, the
object's constructor is called only when a new instance of a class is created.
Keeping that basic fact in mind, let's revisit our animation code. First, we
instantiate an object of type PersistentAnimation, which begins the
animation thread sequence. Next, we serialize the object with that code:
PersistentAnimation animation = new PersistentAnimation(10);
FileOutputStream fos = ...
ObjectOutputStream out = new ObjectOutputStream(fos);
out.writeObject(animation);
All seems fine until we read the object back in with a call to the readObject()
method. Remember, a constructor is called only when a new instance is created.
We are not creating a new instance here, we are restoring a persisted object.
The end result is the animation object will work only once, when it is first
instantiated. Kind of makes it useless to persist it, huh?
Well, there is good news. We can make our object work the way we want it to;
we can make the animation restart upon restoration of the object. To accomplish
that, we could, for example, create a startAnimation() helper
method that does what the constructor currently does. We could then call that
method from the constructor, after which we read the object back in. Not bad,
but it introduces more complexity. Now, anyone who wants to use that animation
object will have to know that method has to be called following the normal
deserialization process. That does not make for a seamless mechanism, something
the Java Serialization API promises developers.
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;
Notice that both methods are (and must be) declared private,
proving that neither method is inherited and overridden or overloaded. The trick
here is that the virtual machine will automatically check to see if either
method is declared during the corresponding method call. The virtual machine can
call private methods of your class whenever it wants but no other objects can.
Thus, the integrity of the class is maintained and the serialization protocol
can continue to work as normal. The serialization protocol is always used the
same way, by calling either ObjectOutputStream.writeObject() or ObjectInputStream.readObject().
So, even though those specialized private methods are provided, the object
serialization works the same way as far as any calling object is concerned.
Considering all that, let's look at a revised version of PersistentAnimation
that includes those private methods to allow us to have control over the
deserialization process, giving us a pseudo-constructor:
10 import java.io.Serializable;
20 public class PersistentAnimation implements Serializable, Runnable
30 {
40 transient private Thread animator;
50 private int animationSpeed;
60 public PersistentAnimation(int animationSpeed)
70 {
80 this.animationSpeed = animationSpeed;
90 startAnimation();
100 }
110 public void run()
120 {
130 while(true)
140 {
150 // do animation here
160 }
170 }
180 private void writeObject(ObjectOutputStream out) throws
IOException
190 {
200 out.defaultWriteObject();
220 }
230 private void readObject(ObjectInputStream in) throws IOException,
ClassNotFoundException
240 {
250 // our "pseudo-constructor"
260 in.defaultReadObject();
270 // now we are a "live" object again, so
let's run rebuild and start
280 startAnimation();
290
300 }
310 private void startAnimation()
320 {
330 animator = new Thread(this);
340 animator.start();
350 }
360 }
Notice the first line of each of the new private methods. Those calls do what
they sound like -- they perform the default writing and reading of the flattened
object, which is important because we are not replacing the normal process, we
are only adding to it. Those methods work because the call to ObjectOutputStream.writeObject()
kicks off the serialization protocol. First, the object is checked to ensure it
implements Serializable and then it is checked to see whether
either of those private methods are provided. If they are provided, the stream
class is passed as the parameter, giving the code control over its usage.
Those private methods can be used for any customization you need to make to the serialization process. Encryption could be added to the output and decryption to the input (note that the bytes are written and read in cleartext with no obfuscation at all). They could be used to add extra data to the stream, perhaps a company versioning code. The possibilities are truly limitless.
OK, we have seen quite a bit about the serialization process, now let's see
some more. What if you create a class whose superclass is serializable but you
do not want that new class to be serializable? You cannot unimplement an
interface, so if your superclass does implement Serializable, your
new class implements it, too (assuming both rules listed above are met). To stop
the automatic serialization, you can once again use the private methods to just
throw the NotSerializableException. Here is how that would be done:
10 private void writeObject(ObjectOutputStream out) throws IOException
20 {
30 throw new NotSerializableException("Not today!");
40 }
50 private void readObject(ObjectInputStream in) throws IOException
60 {
70 throw new NotSerializableException("Not today!");
80 }
Any attempt to write or read that object will now always result in the
exception being thrown. Remember, since those methods are declared private,
nobody could modify your code without the source code available to them -- no
overriding of those methods would be allowed by Java.
Our discussion would be incomplete not to mention the third option for
serialization: create your own protocol with the Externalizable
interface. Instead of implementing the Serializable interface, you
can implement Externalizable, which contains two methods:
public void writeExternal(ObjectOutput out) throws IOException;
public void readExternal(ObjectInput in) throws IOException,
ClassNotFoundException;
Just override those methods to provide your own protocol. Unlike the previous
two serialization variations, nothing is provided for free here, though. That
is, the protocol is entirely in your hands. Although it's the more difficult
scenario, it's also the most controllable. An example situation for that
alternate type of serialization: read and write PDF files with a Java
application. If you know how to write and read PDF (the sequence of bytes
required), you could provide the PDF-specific protocol in the writeExternal
and readExternal methods.
Just as before, though, there is no difference in how a class that implements
Externalizable is used. Just call writeObject() or readObject
and, voila, those externalizable methods will be called automatically.
There are a few things about the serialization protocol that can seem very strange to developers who are not aware. Of course, that is the purpose of the article -- to get you aware! So let's discuss a few of those gotchas and see if we can understand why they exist and how to handle them.
First, consider the situation in which an object is written to a stream and
then written again later. By default, an ObjectOutputStream will
maintain a reference to an object written to it. That means that if the state of
the written object is written and then written again, the new state will not be
saved! Here is a code snippet that shows that problem in action:
10 ObjectOutputStream out = new ObjectOutputStream(...);
20 MyObject obj = new MyObject(); // must be Serializable
30 obj.setState(100);
40 out.writeObject(obj); // saves object with state = 100
50 obj.setState(200);
60 out.writeObject(obj); // does not save new object state
There are two ways to control that situation. First, you could make sure to
always close the stream after a write call, ensuring the new object is written
out each time. Second, you could call the ObjectOutputStream.reset()
method, which would tell the stream to release the cache of references it is
holding so all new write calls will actually be written. Be careful, though --
the reset flushes the entire object cache, so all objects that have
been written could be rewritten.
With our second gotcha, imagine you create a class, instantiate it, and write it out to an object stream. That flattened object sits in the file system for some time. Meanwhile, you update the class file, perhaps adding a new field. What happens when you try to read in the flattened object?
Well, the bad news is that an exception will be thrown -- specifically, the java.io.InvalidClassException
-- because all persistent-capable classes are automatically given a unique
identifier. If the identifier of the class does not equal the identifier of the
flattened object, the exception will be thrown. However, if you really think
about it, why should it be thrown just because I added a field? Couldn't the
field just be set to its default value and then written out next time?
Yes, but it takes a little code manipulation. The identifier that is part of
all classes is maintained in a field called serialVersionUID. If
you wish to control versioning, you simply have to provide the serialVersionUID
field manually and ensure it is always the same, no matter what changes you make
to the classfile. You can use a utility that comes with the JDK distribution
called serialver to see what that code would be by default (it is
just the hash code of the object by default).
Here is an example of using serialver with a class called Baz:
> serialver Baz
> Baz: static final long serialVersionUID = 10275539472837495L;
Simply copy the returned line with the version ID and paste it into your
code. (On a Windows box, you can run that utility with the - show
option to simplify the copy and paste procedure.) Now, if you make any changes
to the Baz class file, just ensure that same version ID is
specified and all will be well.
The version control works great as long as the changes are compatible.
Compatible changes include adding or removing a method or a field. Incompatible
changes include changing an object's hierarchy or removing the implementation of
the Serializable interface. A complete list of compatible and
incompatible changes is given in the Java Serialization Specification.
Our third gotcha: the default mechanism, although simple to use, is not the
best performer. I wrote out a Date object to a file 1,000 times,
repeating that procedure 100 times. The average time to write out the Date
object was 115 milliseconds. I then manually wrote out the Date
object, using standard I/O the same number of iterations; the average time was
52 milliseconds. Almost half the time! There is often a trade-off between
convenience and performance, and serialization proves no different. If speed is
the primary consideration for your application, you may want to consider
building a custom protocol.
Another consideration concerns the aforementioned fact that object references are cached in the output stream. Due to that, the system may not garbage collect the objects written to a stream if the stream is not closed. The best move, as always with I/O, is to close the streams as soon as possible, following the write operations.
Serialization in Java is simple to instigate and almost as simple to implement. Understanding the three different ways of implementing serialization should aid in bending the API to your will. We have seen a lot of the serialization mechanism in that article, and I hope it made things clearer and not worse. The bottom line, as with all coding, is to maintain common sense within the bounds of API familiarity. That article has laid out a strong basis of understanding the Java Serialization API, but I recommend perusing the specification to discover more fine-grained details.
Reprinted with permission from the June 2000 edition of JavaWorld magazine. Copyright ITworld.com, Inc., an IDG Communications company. Register for editorial e-mailalerts
Todd Greanier,director of technology for ComTech Training, has been teaching and developing Java since it was introduced publicly. An expert in distributed Java technologies, he teaches classes in a wide range of topics, including JDBCTM, RMI, CORBA, UML, Swing, servlets/JSPTM, security, JavaBeansTM, Enterprise Java BeansTM, and multithreading. He also creates custom seminars for corporations, slanted to their specific needs. Todd lives in upstate New York with his wife, Stacey, and his cat, Bean.