FileChannel Objects in Java, ByteBuffer Type Los Angeles CA

provided by: 
Originally published at Internet.com

Java Programming Notes # 1788 * Preface * Discussion and Sample Code * Run the Program * Summary * What's Next? * Complete Program Listing -----------------------------------

Preface

The recently released JavaTM 2 SDK, Standard Edition Version 1.4 contains a large number of new features.

Among the features which are completely new to version 1.4 is the concept of an IO channel. The previous lesson, entitled FileChannel Objects in Java, Background Information, introduced you to the concept of channels from a read/write IO viewpoint.

Data of type byte

This lesson will walk you through a sample program that uses FileChannel objects in conjunction with ByteBuffer objects to perform read/write file IO for data of the primitive type byte.

Mixed primitive types

In the next lesson, I will show you how to use the new I/O classes to transfer data of type double and data of type short between the computer's memory and a physical file. You will learn how to extend the concept to any primitive data type other than boolean.

In a future lesson, I will show you how to use the new I/O classes to create records consisting of sequences of data values of different primitive types, and how to transfer those records between the computer's memory and a physical file.

Memory-mapped IO

Future lessons will teach you how to do memory-mapped IO using channels.

A channel is not a stream

Note that a channel is not the same as a stream, but a channel may be based on a stream. Streams have been supported by Java since the earliest days of Java. Channels were not introduced until version 1.4.

The java.nio.channels package

Much of the support for channels is provided in the package named java.nio.channels. This package defines more than a dozen new classes, and about seven new interfaces, not counting the new exception classes. (In addition, there are four more new packages that begin with the name java.nio.)

Viewing tip

You may find it useful to open another copy of this lesson in a separate browser window. That will make it easier for you to scroll back and forth among the different listings and figures while you are reading about them.

Supplementary material

I recommend that you also study the other lessons in my extensive collection of online Java tutorials. You will find those lessons published at Gamelan.com. However, as of the date of this writing, Gamelan doesn't maintain a consolidated index of my Java tutorial lessons, and sometimes they are difficult to locate there. You will find a consolidated index at www.DickBaldwin.com.

Discussion and Sample Code

Before getting into the details of FileChannel objects, I need to tell you that much of what youu will see in this lesson will involve objects of the ByteBuffer class. The ByteBuffer class extends the Buffer class, and as such inherits a number of methods from the Buffer class.

It will be much easier for you to understand this discussion of FileChannel objects if you are already familiar with the features of Buffer and ByteBuffer. I have previously published lessons on these topics entitled Understanding the Buffer class in Java and The ByteBuffer Class in Java. You may find it useful to review those two lessons in preparation for your study of this lesson.

What is a FileChannel?

Sun describes an object of the FileChannel class simply as "A channel for reading, writing, mapping, and manipulating a file." In this lesson, I will show you how to use FileChannel objects for reading files and writing files. In a future lesson, I will show you how to use FileChannel objects for mapping files into memory.

Enough talk, let's see some code!

I will illustrate the FileChannel class using the sample program named Channel01. You will find a complete listing of the program in Listing 16 near the end of the lesson. As is my normal approach, I will discuss this program in fragments.

This program, which was tested using Java SDK version 1.4.0 under Win2000, first writes, and then reads a file using FileChannel objects. The data written to and read from the file is of the primitive type byte.

Import directives

The first fragment, shown in Listing 1, shows the beginning of the class named Channel01. The primary purpose of showing this listing at this point is to emphasize the use of the new java.nio packages, as illustrated by the import directives. import java.io.*; import java.nio.channels.*; import java.nio.*; class Channel01 { Listing 1

The showBufferData method

Listing 2 shows the signature of a static convenience method of my own design named showBufferData. As you might guess from the name and the incoming parameter type, the purpose of this method is to display the contents of a ByteBuffer object. This method will be used later in the program, so you need to understand how it behaves.   static void showBufferData (           ByteBuffer buf, String name){ Listing 2

A ByteBuffer parameter

As you can see, this method receives a reference to a ByteBuffer object as an incoming parameter. The method displays the byte data encapsulated by that object.

A String parameter

The method also receives a reference to a String object as an incoming parameter. This string is included in the display to make it easier to associate the output with the program code.

Saving the position property

When the ByteBuffer object is passed to the method, its position property may or may not be zero. This method needs to be careful to avoid modifying the value of the