Download Java Essential I

Java API Essentials
Web Languages Course 2009
University of Trento
Lab Objective

Refresh basic Java concepts on
• I/O Streams;
• Object Serialization;
• Simple Client-Server applications through
Sockets
2
I/O Streams in Java
Task objectives:
 Refresh the principles of
I/O usage in Java;
 Understand the difference between
Streams and Reader/Writer objects;
Introduction





A stream is a sequence of data
An I/O Stream represents an input source or an output
destination.
A program uses an input stream to read data from a
source, one item at a time
A program uses an output stream to write data to a
destination, one item at time
Three streams are created for us automatically:
• System.in -> standard input stream
• System.out -> standard output stream
• System.err -> standard error
4
Basic input and output classes


The java.io package has a large number of classes
Most of the classes consist of:
•
•


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Byte streams  subclasses of InputStream or
OutputStream
Character streams  subclasses of Reader and Writer
InputStream and OutputStream classes read and
write 8-bit bytes
Reader and Writer classes read and write 16-bit
Unicode characters
ObjectInputStream and ObjectOutputStream
classes read and write entire objects
5
What to use?

Depends…

Use InputStream and OutputStream classes if
you use binary data (e.g., integers, doubles)

Use Reader and Writer classes if you use text data

Use ObjectInputStream and
ObjectOutputStream classes if your data is an
object
6
Sources and sinks for data

What is the source of your data?

What will be consuming your output data?

Many ways to input or output your data: files, strings,
arrays of characters, sockets.

Any of these can be a source for an InputStream or
Reader or a sink for an OutputStream or Writer
7
Input stream classes
8
Output stream classes
9
I/O from the Command Line
1

the standard input stream System.in read bytes from
the console, while the standard ouput stream
System.out can print number and strings

Since data from keyboard are composed of characters,
the byte-to-character "bridge" streams
InputStreamReader is used to convert bytes in
characters:
InputStreamReader reader =
new InputStreamReader(System.in);
10
I/O from the Command Line
2

InputStreamReader reads one character at time

What if I want to read a whole string?

The class BufferedReader is used
BufferedReader console = new BufferedReader
(new InputStreamReader(System.in));

The method readLine()is used to read the string
String input = console.readLine();
11
Exercise 1:
ConsoleInputTest.java


Write a simple program that reads a
string of characters from the console and
print them into the std output
(System.out)
The program ends when “BYE” is digited
12
HINTs
Exercise 1:
ConsoleInputTest.java
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
BufferedReader console = new BufferedReader(new InputStreamReader(System.in));
…
String input = console.readLine();
…
System.out.println(input);
13
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
Exercise 1:
ConsoleInputTest.java
public class ConsoleInputTest {
public ConsoleInputTest() { }
/** @param args the command line arguments */
public static void main(String[] args) throws IOException {
BufferedReader console = new BufferedReader(new InputStreamReader(System.in));
System.out.println("This is a simple example on how to read data from the console");
System.out.println("Please, enter characters");
while(true) {
String input = console.readLine();
if (input.trim().equals("BYE")) System.exit(0);
System.out.println(input);
}
}
}
14
File I/O byte streams



1
Class FileInputStream: obtains input bytes from a
file in a file system
Constructors: FileInputStream(String
filename), FileInputStream(File file)
Methods:
• int
• int
read(): reads a byte of data from this input stream.
read(byte[] b): reads up to b.length bytes of data
from this input stream into an array of bytes.
• void close(): closes this file input stream and releases any
system resources associated with it.
15
File I/O byte streams
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2
Class FileOutputStream: output stream for writing
data to a File
Constructors: FileOutputStream(String
filename), FileOutputStream(File file)
Methods:
• int
write(int b): write the specified byte to this output
stream.
• int
write(byte[] b): writes b.length bytes from the
specified byte array to this file output stream.
• void close(): closes this file output stream and releases any
system resources associated with it.
16
Exercise 2:
Copy bytes from file1 to file2

Hints
•
•
Create a text file (e.g., input.txt) from where to read data
Write a Java program that uses FileInputStream and
FileOutputStream classes and their methods
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
...
try {
in = new FileInputStream("input.txt");
out = new FileOutputStream("output.txt");
int c;
...
} catch (IOException e) {
System.out.println(e.getMessage());
}
….
17
Exercise 2:
Copy bytes from file1 to file2

Solution
package it.unitn.web.streams;
while ((c = in.read()) != -1) {
out.write(c);
}
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
in.close();
out.close();
public class CopyBytes {
System.out.println(“done!");
public static void main(String[] args) {
} catch (IOException e) {
System.out.println(e.getMessage());
}
}
FileInputStream in = null;
FileOutputStream out = null;
try {
}
in = new FileInputStream("input.txt");
out = new FileOutputStream("output.txt");
int c;
18
Exercise 3:
Copy characters from file1 to
file2

Hints
• Use the same text file (e.g., input.txt) as before
• Write a Java program that uses FileReader and
FileWriter classes and their methods
• http://java.sun.com/j2se/1.5.0/docs/api/index.html
19
Exercise 3:
Copy characters from file1 to
file2

Solution
package it.unitn.web.streams;
while ((c = in.read()) != -1) {
out.write(c);
}
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
in.close();
out.close();
public class CopyBytes {
System.out.println(“done!");
public static void main(String[] args) {
} catch (IOException e) {
System.out.println(e.getMessage());
}
}
FileReader in = null;
FileWriter out = null;
try {
}
in = new FileReader("input.txt");
out = new FileWriter("output.txt");
int c;
20
Exercise 4:
Copy lines from file1 to file2

Hints
•
•
Create a text file (e.g., input.txt) from where to read data
Write a Java program that uses BufferedReader and
PrintWriter classes and their methods
…
import java.io.BufferedReader;
...
try {
in = new BufferedReader( new FileReader("input.txt"));
out = new PrintWriter( new FileWriter("output.txt"));
String line;
...
} catch (IOException e) {
System.out.println(e.getMessage());
}
….
21
Exercise 4:
Copy lines from file1 to file2

Solution
package it.unitn.web.streams;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.PrintWriter;
public class CopyLines {
out = new PrintWriter( new
FileWriter("output.txt"));
String line;
while ( (line = in.readLine()) != null) {
out.println();
}
in.close();
out.close();
public static void main(String[] args) {
System.out.println(“done!");
BufferedReader in = null;
PrintWriter out = null;
try {
in = new BufferedReader( new
FileReader("input.txt"));
} catch (IOException e) {
System.out.println(e.getMessage());
}
}
}
22
Buffered Streams

CopyBytes.java and CopyCharacters.java used
unbuffered I/O. This means each read or write request
is handled directly by the underlying OS.

ConsoleInputTest.java and CopyLines.java
used buffered I/O streams.

Buffered input/output streams read/write data from/to a
memory area known as a buffer; the native input API is
called only when the buffer is empty/full
23
Exercise 2:
GZIP compression
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Create a text file
Compress the file by using the GZIP format;
Decompress the GZIP file and put it into another file
Check if the two files are the same
Hint: Use GZIPOutputStream and GZIPInputStream
24
Object Serialization
Example of writing on file
The serialization process
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
1
The ability for an object to exist beyond the
execution of the program which has created it, is
called persistence
The Java Serialization API provides a standard
mechanism for developers to implement object
persistence.
•
•
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.
Such mechanism allows, for example, to save the object’s
state in a file, move it through the network and use it in
distributed applications.
26
The serialization process
2
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An object’s state is composed by its class, its
primitive fields and its referenced objects
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Serializing an object means to transfer the entire
web of object references into a stream
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In Java few lines of code allow to save/restore the
state of an object into/from a stream
•
Classes ObjectOutputStream and ObjectInputStream
are used for serialization and deserialization
27
The serialization process
3
Rule #1: The object to be persisted must implement the
Serializable interface or inherit that implementation
from its object hierarchy.
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Not all the classes implement the interface Serializable (e.g,
Object, Thread, Sockets)
•
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Try the command “serialver –show” to check if a class is serializable
What if we have a class that contains an instance of Thread?
Rule #2: The object to be persisted must mark all nonserializable
fields transient
28
Example:
Serialization on file

Steps:
1. Create a persistent object (e.g., current time and date)
2. Save object into a file
3. Restore this info from the file, print it on the screen
together with time and date occurring at the time of
printing

Expected output (after step 3) :
Flattened time: Thu Sep 15 14:21:09 CEST 2005
Current time: Thu Sep 15 14:22:11 CEST 2005
29
Step 1:
Create a persistent object
package flatten;
import java.io.Serializable;
import java.util.Date;
import java.util.Calendar;
public class PersistentTime implements Serializable {
private Date time;
PersistentTime() {
time = Calendar.getInstance().getTime();
}
//Return the date
public Date getTime() {
return time;
}
}
30
Step 2:
Save object into a file
import java.io.ObjectOutputStream;
import java.io.FileOutputStream;
...
public class FlattenTime {
public static void main(String [] args) {
...
PersistentTime time = new PersistentTime();
...
fos = new FileOutputStream(filename);
out = new ObjectOutputStream(fos);
out.writeObject(time);
out.close();
...
}
}
31
import java.io.ObjectOutputStream;
import java.io.FileOutputStream;
Import java.io.IOExcception
public class FlattenTime {
public static void main(String [] args) {
String filename = "time.ser";
if(args.length > 0)
filename = args[0];
PersistentTime time = new PersistentTime();
FileOutputStream fos = null;
ObjectOutputStream out = null;
try {
fos = new FileOutputStream(filename);
out = new ObjectOutputStream(fos);
out.writeObject(time);
out.close();
}
catch(IOException ex) {
ex.printStackTrace();
}
}
}
32
Exercise (30 min.)
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Execute serialization on file (step1, step2)
Implement step3: write a Java program that, given a
serialized file
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Restore its content (e.g., time and date)
Print it on the screen
For checking, display also time and date occurring at the time
of printing
Hints: Use classes, FileInputStream,
ObjectInputStream and method readObject
Expected output:
Flattened time: Thu Sep 15 14:21:09 CEST 2005
Current time: Thu Sep 15 14:22:11 CEST 2005
33
public class InflateTime {
public static void main(String [] args) {
String filename = "time.ser";
if(args.length > 0)
filename = args[0];
Solution
PersistentTime time = null;
FileInputStream fis = null;
Explicit Cast
ObjectInputStream in = null;
try {
fis = new FileInputStream(filename);
in = new ObjectInputStream(fis);
time = (PersistentTime)in.readObject();
in.close();
}
catch(IOException ex) { ex.printStackTrace(); }
catch(ClassNotFoundException ex) {
ex.printStackTrace(); }
System.out.println("Flattened time: " +
time.getTime());
System.out.println();
System.out.println("Current time: " +
Calendar.getInstance().getTime());
}
}
34
Exercise: object serialization
(20 min.)
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Create an ArrayList object and fill it with
3-5 String object;
Save the ArrayList object on disk
Restore the object from disk and copy it
in another ArrayList object
Check if the two objects are the same
35
Client-Server Applications
through Sockets
The package java.net.*
Outline
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Client Server Architecture
Simple Client Server Application
The java.net package
Example Using Object Serialization
37
Client Server Architecture
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The commonly used model for
Distributed Applications

The client is a process accessing the
remote resource and the server provides
the access to the resource
38
A simple client server application

Client
 Connects to a server
 Sends a message to the server
 Waits for response
 Prints the response to the standard output

Server
 Waits for a client to connect
 Receives a message from the client
 Send the message back to the client
39
The java.net package

The package java.net provides support for
sockets programming

The package java.net contains: Socket, URL,
InetAddress, ServerSocket, etc.

Sockets + Object Serialization: combine java.net
and java.io.Serializable to transport Java
objects through the network
40
The class InetAddress
1
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The java.net.InetAddress class represents Internet
Protocol (IP) address

It converts numeric addresses to host names and
host names to numeric addresses.

It is used by other network classes like Socket and
ServerSocket to identify hosts

There are no public InetAddress() constructors.
Arbitrary addresses may not be created.
41
The class InetAddress
2

Host name-to-IP address resolution is accomplished
through the use of a combination of local machine
configuration information and network naming
services such as the Domain Name System (DNS)
and Network Information Service(NIS)

The particular naming services(s) being used is by
default the local machine configured one. For any
host name, its corresponding IP address is returned.
42
The class InetAddress
3
Static methods:

getLocalHost()  Returns the local host.

getByName(String host)  Returns the IP address of a
host, given the host's name

getAllByName(String host)  Returns an array of its IP
addresses, based on the
configured name service
on the system.
43
Exercise: InetAddressTest.java
import java.net.*;
/**
* Testing class InetAddress:
*/
class InetAddressTest {
/** Creates a new instance of InetAddressTest */
public InetAddressTest() { }
/** @param args the command line arguments */
public static void main(String[] args) throws UnknownHostException {
InetAddress address = InetAddress.getLocalHost();
System.out.println(address);
System.out.println(address.getHostName());
}
}
44
Example:
NSLookupApp.java
…
try {
if(args.length!=1){
System.out.println("Usage: java NSLookupApp hostName");
return;
}
InetAddress host = InetAddress.getByName(args[0]);
String hostName = host.getHostName();
System.out.println("Host name: "+hostName);
System.out.println("IP address: "+host.getHostAddress());
}catch(UnknownHostException ex) {
System.out.println("Unknown host");
return;
45
Sockets

Endpoint of a two-way communication link between two
programs running on the network.

An endpoint is a combination of an IP address and a
port number.

Every TCP connection can be uniquely identified by its
two endpoints.
46
TCP Sockets

Connection-oriented sockets, almost always based on
TCP
java.net.ServerSocket

java.net.Socket

• Implements the server side of the connection
• Used to listen for connection requests from clients;
• Should be bound to a known port to listen on,
• Its accept method blocks until a client requests a connection
• Implements the client side of the connection
• Used to connect to a specific port in a server machine
47
Socket TCP
Constructor
Create
Socket(String host, int
port)
a stream socket and connects it to the
specified port number on the named
host.
Socket(InetAddress address,
int port)
a stream socket and connects it to the
specified port number at the
specified IP address
ServerSocket(int port)
a server socket, bound to the specified
port .
ServerSocket(int port, int
backlog )
a server socket and binds it to the
specified local port number, with the
specified backlog (maximum queue
length for incoming connection)
48
Example: “ the echo application”


EchoClient .java
•
•
•
•
Creates a socket thereby getting a connection to the Echo server.
Reads input from the user on the standard input stream
Forwards that text to the Echo server by writing it to the socket
Reads and displays the data passed back to it from the server
EchoServer.java
•
•
•
•
Creates a server socket to accept connections from clients.
Establishes connection to the client
Reads from the socket (receives data from the client)
Writes to the socket (sends data to the client)
49
…
EchoClient
try {
//Create connection to the server "localhost" on port 8189
s = new Socket("localhost",8189);
//Set streams to read from and write to a socket
s_in = new BufferedReader(new
InputStreamReader(s.getInputStream()));
s_out = new PrintWriter(s.getOutputStream(),true);
System.out.println(s_in.readLine());
//Set input stream for reading from console
console = new BufferedReader(new InputStreamReader(System.in));
String user_input; String received;
while(true){
user_input = console.readLine();
s_out.println(user_input);
received = s_in.readLine();
System.out.println(received);
}
…
50
EchoServer
try {
ServerSocket s = new ServerSocket (8189);
//Wait for the client
Socket incoming = s.accept ();
System.out.println(“Connection requested from host: " +
incoming.getInetAddress().getHostName() +
"\nto port: " + incoming.getLocalPort() +
"\nfrom port: " + incoming.getPort() +
"\nwith IP: " +
incoming.getInetAddress().getHostAddress());
BufferedReader in = new BufferedReader (new InputStreamReader
(incoming.getInputStream ()));
PrintWriter out = new PrintWriter (incoming.getOutputStream
(), true);
out.println ("Hello! Enter BYE to exit.");
51
EchoServer
boolean go = true;
String line;
while (go){
line = in.readLine ();
out.println ("Echo: " + line);
if (line.trim().equals("BYE")){
go = false;
}
}
incoming.close ()
} catch (IOException e) {
System.out.println ("Error. " + e);
}
52
Socket Timeout

If data are not available (e.g., the host is not reachable),
the reading methods remain blocked

To solve the problem: use the setSoTimeout()
method
Socket s = new Socket(…);
s.setSoTimeout(1000); //time out in millisecond

When a timeout is set, all the following operations throw
the exception InterruptedIOException
53
References

Bruce Eckel, Thinking in Java, 3rd Edition

IBM developerWorks: New to Java technology

http://java.sun.com/developer/technicalArticles/Progr
amming/serialization/

http://java.sun.com/developer/technicalArticles/ALT/s
ockets/
54