The write methods ObjectOutputStream

While this discussion provides examples of implementation decisions that force two copies to occur, its important to note that, even without such examples, clients should be written as if the servers make independent copies. That is, clients are written to use interfaces. They should not, and cannot, make assumptions about server-side implementations of the interfaces.

10.2 Using Serialization

Serialization is a mechanism built into the core Java libraries for writing a graph of objects into a stream of data. This stream of data can then be programmatically manipulated, and a deep copy of the objects can be made by reversing the process. This reversal is often called deserialization. In particular, there are three main uses of serialization: As a persistence mechanism If the stream being used is FileOutputStream , then the data will automatically be written to a file. As a copy mechanism If the stream being used is ByteArrayOutputStream , then the data will be written to a byte array in memory. This byte array can then be used to create duplicates of the original objects. As a communication mechanism If the stream being used comes from a socket, then the data will automatically be sent over the wire to the receiving socket, at which point another program will decide what to do. The important thing to note is that the use of serialization is independent of the serialization algorithm itself. If we have a serializable class, we can save it to a file or make a copy of it simply by changing the way we use the output of the serialization mechanism. As you might expect, serialization is implemented using a pair of streams. Even though the code that underlies serialization is quite complex, the way you invoke it is designed to make serialization as transparent as possible to Java developers. To serialize an object, create an instance of ObjectOutputStream and call the writeObject method; to read in a serialized object, create an instance of ObjectInputStream and call the readObject object.

10.2.1 ObjectOutputStream

ObjectOutputStream , defined in the java.io package, is a stream that implements the writing-out part of the serialization algorithm. [ 2] The methods implemented by ObjectOutputStream can be grouped into three categories: methods that write information to the stream, methods used to control the streams behavior, and methods used to customize the serialization algorithm. [ 2] RMI actually uses a subclass of ObjectOutputStream to customize its behavior.

10.2.1.1 The write methods

The first, and most intuitive, category consists of the write methods: public void writebyte[] b; public void writebyte[] b, int off, int len; public void writeint data; public void writeBooleanboolean data; public void writeByteint data; public void writeBytesString data; public void writeCharint data; public void writeCharsString data; public void writeDoubledouble data; public void writeFields ; public void writeFloatfloat data; public void writeIntint data; public void writeLonglong data; public void writeObjectObject obj; public void writeShortint data; public void writeUTFString s; public void defaultWriteObject ; For the most part, these methods should seem familiar. writeFloat , for example, works exactly as you would expect after reading Chapt er 1 ™it takes a floating-point number and encodes the number as four bytes. There are, however, two new methods here: writeObject and defaultWriteObject . writeObject serializes an object. In fact, writeObject is often the instrument of the serialization mechanism itself. In the simplest and most common case, serializing an object involves doing two things: creating an ObjectOuptutStream and calling writeObject with a single top-level instance. The following code snippet shows the entire process, storing an object™and all the objects to which it refers™into a file: FileOutputStream underlyingStream = new FileOutputStreamC:\\temp\\test; ObjectOutputStream serializer = new ObjectOutputStreamunderlyingStream; serializer.writeObjectserializableObject; Of course, this works seamlessly with the other methods for writing data. That is, if you wanted to write two floats, a String, and an object to a file, you could do so with the following code snippet: FileOutputStream underlyingStream = new FileOutputStreamC:\\temp\\test; ObjectOutputStream serializer = new ObjectOutputStreamunderlyingStream; serializer.writeFloatfirstFloat; serializer.writeFloatsecongFloat; serializer.writeUTFaString; serializer.writeObjectserializableObject; ObjectOutputStream s constructor takes an OutputStream as an argument. This is analagous to many of the streams we looked at in Chapt er 1 . ObjectOutputStream and ObjectInputStream are simply encoding and transformation layers. This enables RMI to send objects over the wire by opening a socket connection, associating the OutputStream with the socket connection, creating an ObjectOutputStream on top of the sockets OutputStream , and then calling writeObject . The other new write method is defaultWriteObject . defaultWriteObject makes it much easier to customize how instances of a single class are serialized. However, defaultWriteObject has some strange restrictions placed on when it can be called. Heres what the documentation says about defaultWriteObject : Write the nonstatic and nontransient fields of the current class to this stream. This may only be called from the writeObject method of the class being serialized. It will throw the NotActiveException if it is called otherwise. That is, defaultWriteObject is a method that works only when it is called from another specific method at a particular time. Since defaultWriteObject is useful only when you are customizing the information stored for a particular class, this turns out to be a reasonable restriction. Well talk more about defaultWriteObject later in the chapter, when we discuss how to make a class serializable.

10.2.1.2 The stream manipulation methods