Input/Output and Serialization

Java Programming: Advanced Topics1

Input/Output and Serialization


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


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


Byte-Oriented Stream Classes


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


The print and println Methods


Input Methods

• Input methods of the java.io.InputStream class:– 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)


Output Methods

• Output methods of the java.io.OutputStream class:– void close()– void flush()– void write( int b)– void write(byte[] buffer)– void write( byte[] buffer, int offset, int length)


Byte-Oriented Filter Streams


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


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


Console I/O Sample Program


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


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


File Navigation Methods


An Example of File Input and Output


An Example of File Input and Output (Cont.)


An Example of File Input and Output (Cont.)


Random-Access File I/O

• The class RandomAccessFile supports byte-oriented 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


Character Streams


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


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.


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


The New I/O (NIO) Programming Interface

• The java.nio package provides new features:– Support for buffer management– A new primitive I/O abstraction called a channel– File locking at the process level

– Memory mapping


Buffers• 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


Buffer Classes in java.nio


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


Channel Classes in java.nio.channels


An Example of File Channel I/O


An Example of File Channel I/O (Cont.)


An Example of File Channel I/O (Cont.)


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


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


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


The readObject and writeObject Methods


The validateObject Method


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


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


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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Input/Output and Serialization