Download Objectives

Input/Output and
Serialization
Chapter 3
Java Programming: Advanced Topics
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Objectives
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Learn how the Java platform supports I/O
Understand file I/O basics
Understand character streams
Use the new I/O (NIO) programming interface
Understand object serialization
Write appropriate code to read, write, and
update files
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Objectives (Cont.)
• Describe the permanent effects on the file
system of constructing and using
FileInputStream, FileOutputStream, and
RandomAccessFile objects
• Describe the connection between object
serialization and I/O streams
• Distinguish between classes that can be
serialized and those that cannot
• Write code to define a class that can be
serialized
• Learn about the compatibility of serialization
formats
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How the Java Platform
Supports I/O
• The package java.io supports console I/O and
file I/O
• Console I/O is character keyboard input and
output without the mouse graphics of a graphical
user interface
• File I/O involves reading and writing data to and
from a mass storage device, typically the
computer’s hard drive
• The new I/O (NIO) package java.nio
supplements the original I/O package
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Programming I/O
• In stream I/O, characters or bytes are read or
written sequentially
• The Java platform includes two dual hierarchies
of classes that support streams:
– byte-oriented input and output
– character-oriented input and output
• The class RandomAccessFile is used to read
and write arbitrary locations within a file without
first having to read or write all the bytes or
characters that precede that location
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Byte-Oriented Stream Classes
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Predefined Stream Objects
• All Java programs can use three stream objects
that are defined in the System class of the
java.lang package:
– System.in
• The field System.in is a java.io.BufferedInputStream object
– System.out
• The field System.out is a java.io.PrintStream object
– System.err
• The field System.err is a java.io.PrintStream object
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The print and println Methods
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Input Methods
• Input methods of the java.io.InputStream
class:
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int available()
void close()
void mark( int readlimit)
boolean markSupported()
int read()
int read(byte[] buffer)
int read( byte[] buffer, int offset, int length)
void reset()
long skip(long bytecount)
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Output Methods
• Output methods of the java.io.OutputStream
class:
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void close()
void flush()
void write( int b)
void write(byte[] buffer)
void write( byte[] buffer, int offset, int length)
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Byte-Oriented Filter Streams
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Other Byte I/O Classes
• Extensions of InputStream include the following
classes:
• The class ByteArrayInputStream lets you read an array
of bytes as though it were an InputStream object
• The class SequenceInputStream provides a
mechanism for concatenating the data from two or
more InputStream objects into a single, seamless
stream
• The class PipedInputStream implements half of a pipe
to connect the input stream of one thread or process to
the output stream of another thread process
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Other Byte I/O Classes
• Extensions of OutputStream include the
following classes:
– The class ByteArrayOutputStream sends its output
into an object of type byte
– The class PipedOutputStream is the
complementary class to PipedInputStream
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Console I/O Sample Program
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Parsing an Input Stream
• The next step after reading a line of input is to
parse it, or break it into separate tokens
• A token usually consists of a sequence of
characters that does not include a space,
newline, tab, or other nonprinting character
• To tokenize a string, you can use the
StringTokenizer class provided by the java.util
package
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File I/O Basics
• To use a file associate the file with a
FileInputStream or FileOutputStream object
• To access the data in a file in random-access
order, open it as a RandomAccessFile object, not
as a FileInputStream object
• In a network environment, applets can perform
I/O only on files that reside on the server from
which the applet originated
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File Navigation Methods
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An Example of File Input and
Output
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An Example of File Input and
Output (Cont.)
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An Example of File Input and
Output (Cont.)
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Random-Access File I/O
• The class RandomAccessFile supports byteoriented I/O to and from random-access files
• The class RandomAccessFile extends Object
• RandomAccessFile combines input and output
operations in one class
• The seek method selects the position within the
file where the next I/O operation will begin
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Character Streams
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Connecting Byte and Character
I/O Classes
• The Java platform includes adapter classes
that bridge between character I/O classes
and byte I/O classes:
– InputStreamReader
– OutputStreamWriter
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Using Other Character I/O
Classes
• Extensions of the Reader object include the
following:
– The class CharArrayReader lets you read an array
of characters as though it were a Reader object
– The class StringReader lets you read a String
object as though it were a Reader object
– The class PipedReader implements half of a pipe
and is especially useful for communication
between threads.
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Using Other Character
I/O Classes
• Extensions of the Writer object include the
following:
– The class CharArrayWriter sends its output into an
object of type char[]
– The class StringWriter lets you write to a
StringBuffer object as though it were a Writer
object
– The class PipedWriter is the complementary class
to PipedReader
– The class PrintWriter is the character I/O
equivalent of the PrintStream class
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The New I/O (NIO) Programming
Interface
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The java.nio package provides new features:
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Support for buffer management
A new primitive I/O abstraction called a channel
File locking at the process level
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Memory mapping
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Buffers
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Capacity: the maximum number of data
elements that the buffer can contain
Limit: a reflection of the amount of data that
the buffer currently contains and is defined as
the index of the first element in the buffer that
should not be read or written
Mark: the index to which the position value will
be set if the buffer is reset
Position: the index of the next element to be
read or written and can never exceed the limit
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Buffer Classes in java.nio
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Channels
• Channel: a flexible concept that includes
any open connection to a program entity
that is capable of I/O operations such as
read or write
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Channel Classes in
java.nio.channels
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An Example of File Channel I/O
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An Example of File Channel I/O
(Cont.)
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An Example of File Channel I/O
(Cont.)
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An Example of Mapped Buffers
and File Locking
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An Example of Mapped Buffers
and File Locking (Cont.)
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An Example of Mapped Buffers
and File Locking (Cont.)
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Object Serialization
• Object serialization: a general solution that
lets you write objects to I/O streams and then
read them, without defining any additional
methods
• Enables transmission of objects over a
network or save objects to files between runs
of your program
• The object serialization feature is an
important component of the Java Remote
Method Invocation (RMI) enterprise API
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Using Object Streams
• Two stream classes support object
serialization:
– ObjectOutputStream - a subclass of
OutputStream
– ObjectInputStream - a subclass of InputStream
• To create an ObjectOutputStream object,
provide an existing OutputStream object as
the argument of the constructor
• To create an ObjectInputStream object,
provide an existing InputStream object as
the argument of the constructor
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Suppressing Serialization
of Fields
• Include the qualifier transient when you
declare fields to indicate that they should not
be serialized with instances of the class
• Fields that have the transient qualifier are
not output when the object is serialized
• When the object is deserialized later,
transient fields are given the default value
normally used for fields of their type
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The readObject and writeObject
Methods
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The validateObject Method
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Forcing Changed Objects to
Be Serialized
• Subsequent serialization operations for the
same object copy only the object reference
into the stream, even if the object has
changed
• A simple solution to this problem:
– invoke the reset method for the
ObjectOutputStream object, which causes the
next serialization of an object to be treated as if
it were the first
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Specifying the Version Number
• A version number is calculated using a
formula that takes the name of the class and
its interfaces, fields, and methods
• Determine the value of the version number
with the serialver tool supplied with the SDK
• Define a serialVersionUID field and let the
JVM generate one for you
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Compatibility of Serialization
Formats
• To identify the serialization stream format
used, Java 2 defines two new constants:
PROTOCOL_VERSION_1 and
PROTOCOL_VERSION_2, in the
java.io.ObjectStreamConstants interface
• The useProtocolVersion method takes a
protocol version constant as input and
updates the ObjectOutputStream object to
use the corresponding serialization stream
version
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Summary
• Support for I/O is provided by the core classes
in the package java.io and is supplemented by
java.nio
• The Java platform supports byte-oriented
streams and the character-oriented streams
• The predefined stream objects are System.in,
System.out and System.err
• FileInputStream and FileOutputStream are the
classes used for reading and writing files
• Filter-stream classes are designed to wrap an
InputStream or an OutputStream class
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Summary (Cont.)
• The class RandomAccessFile is used to read
and write information at arbitrary locations within
a file
• To read and write objects as a whole, Java
provides object serialization
• Objects are written to and read from
ObjectOutputStream and ObjectInputStream
objects
• Only classes that implement the marker interface
Serializable can be serialized
• Transient fields are not serialized
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