Advanced
Java
Programming
Lecture 08
Trasmitting Java Objects
over the Network
dr inż. Wojciech Bieniecki
wbieniec@kis.p.lodz.pl
http://wbieniec.kis.p.lodz.pl
Sockets
… allow low-level network communication.
A socket is one of the ends of two-way connection between the client
and server.
Server is a program, which opens a sockets (well-known port) and waits until a client
connects.
A client is connectig using any free temporary port (ephemeral port).
After the client and the servers get connected, the server proposes to move the
„conversation” to another port.
2
Ports
Service name
TCP port
UDP port
RFC document
echo
7
7
862
discard
9
9
863
daytime
13
13
867
chargen
19
19
864
time
37
37
868
The most common Internet services and corresponding port numbers
3
Opening a socket
Example: Connect to the echo server and use this service to receive and
send a text (until the server send the text "Bye").
To create a socket, you must initialize a class Socket from java.net package:
1.
create a socket (create a connection)
s = new Socket("zly.kis.p.lodz.pl", 7);
2. read a text given from the user using a BufferedReader stream (here the text is
read from the standard input – a keyboard)
BufferedReader kr =
new BufferedReader(new InputStreamReader(System.in));
3. Build a stream that will send a text to the server and a stream reading from the
server
PrintWriter pw=new PrintWriter(s.getOutputStream(),true);
BufferedReader br=
new BufferedReader(new InputStreamReader(s.getInputStream()));
4
Opening a socket
4. Implement communication procedures (receive the text until we send „bye”)
while( !line.equals("bye") )
{
line=kr.readLine();
pw.println(line);
line= br.readLine();
System.out.println("Line received: " + line);
}
5. Close the streams.
br.close();
s.close();
5
Server sockets
1. Create an instance of the server socket
try
{
fServerSocket = new ServerSocket(kPortNumber);
System.out.println("TServer started...");
}
catch (IOException e)
{ … }
2. Wait for the connection request from a client
Socket theClientSocket;
while (true)
{
if (fServerSocket == null)
return;
try
{
theClientSocket = fServerSocket.accept();
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Server sockets
3. Accept the client request and send the message to the client
PrintWriter theWriter = new PrintWriter(
new OutputStreamWriter(theClientSocket.getOutputStream())) ;
theWriter.println(new java.util.Date().toString()) ;
theWriter.flush();
4. Break the connection
theWriter.close() ;
theClientSocket.close();
}
catch (IOException e)
{
System.err.println("Ugly exception: " + e.getMessage ());
System.exit(1);
}
}
7
Datagram socket
Provides a mechanism for sending and receiving datagrams.
The most common case are UDP packets.
We do not distinguish here a Server and a Client, the link is not established.
Packets are simply sent out to the world and there is no guarantee that they arrive
and will be prosecced in a correct order
Construction:
DatagramSocket()
Create a socket creation and bind it a free port on the local machine.
DatagramSocket(int port)
Create a socket creation and bind it a specific port on the local machine.
DatagramSocket(int port, InetAddress laddr)
Create a socket creation and bind it a specific port to the given machine.
DatagramSocket(SocketAddress bindaddr)
Create a socked and bind it to a given socket address
Datagram socket
Methods for handling the sockets:
void bind(SocketAddress addr)
bind the socket to a given address
void close()
close the socket
void connect(InetAddress address, int port),
void connect(SocketAddress addr)
bind the socket with a given remote address and the port (the
communication is limited only to this address).
void disconnect()
disconnecting a socket.
DatagramSocket class has two communication methods
send(DatagramPacket data)
Enables to send a single packet.
recive(DatagramPacket data).
holds the current thread until receive a message.
Datagram packet
A datagram packet (usually UDP) is represented by DatagramPacket
class.
It provides both input and output buffers.
It enables configuration of some UDP packet parameters and add
data field.
Most important UDP packed fields in DatagramPacket class are:
buf – a byte array for transmitted data,
length – count of data bytes;
offset – data shift from the beginning of the array;
address – an address of the source or destination socket
Datagram packet
Initialization of the DatagramPacket object
DatagramPacket(byte[] buf, int length)
DatagramPacket(byte[] buf, int length, InetAddress address, int port)
DatagramPacket(byte[] buf, int offset, int length)
DatagramPacket(byte[] buf, int offset, int length, InetAddress address, int port)
DatagramPacket(byte[] buf, int offset, int length, SocketAddress address)
DatagramPacket(byte[] buf, int length, SocketAddress address)
where:
buf – a byte array for transmitted data,
length – count of data bytes;
offset – data shift from the beginning of the array;
address – an address of the source or destination socket
Multicast socket
The MulticastSocket class is deigned to send and receive multicast datagram
packets.
The multicast connection cannot be routed, it resembles UDP.
In multicast packets single IP address (source or destination) is replaced by
multicast group address, which a host can join.
Group addresses have a D class IP number and a standard port number
D class is a range <224.0.0.0; 239.255.255.255 >
244.0.0.0 is reserved and should not be used in a standard communication
Multicast sockets behave as usual sockets, but they have some additional methods
joinGroup(InetAddress adr)
leaveGroup(InetAddress adr)
The methods are used to join or leave a multicast group.
After joining a group, each member can send and receive packets.
Multicast example
String msg = "Hello";
InetAddress group = InetAddress.getByName("228.5.6.7");
MulticastSocket s = new MulticastSocket(6789);
s.joinGroup(group);
DatagramPacket hi =
new DatagramPacket(msg.getBytes(),
msg.length(), group, 6789);
s.send(hi); // get their responses!
byte[] buf = new byte[1000];
DatagramPacket recv = new DatagramPacket(buf, buf.length);
s.receive(recv);
// OK, I'm done talking - leave the group...
s.leaveGroup(group);
Java RMI
Remote Method Invocation – Remote procedure call
RMI system allows an object running in one Java virtual machine to invoke methods on
an object running in another Java virtual machine.
Compared to object serialization through the socket it provides higher level of
abstraction
In fact – it is implemented on
sockets and serialization
Objective RPC for Java
The mechanism is simple
Th brand product
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The Idea of RPC (remote procedure call)
Local procedure
call
Remote procedure call paradigm: Birel and
Nelson (1984)
The client-server model allows manage a wide range of problems, but it has
some limits:
• he reference to input and output (receive / send)
• there is a problem of data representation (heterogeneous systems)
15
• the model is data oriented
The Idea of RPC
Possible problems:
Calling and called procedures run on different machines in different
address spaces
Problem in pointer usage.
The machines can have diferent systems of data representation.
There is a problem of emergency situations handling
The connectors between server and client are:
Client stub – represents a server at the client side
Server stub – represents a client at the server side. The procedure call is
replaced by the client stub call.
16
How does
remote procedure work
1. The application calls the client stub
2. The stub creates a message and
passes it to the OS kernel
3. The kernel sends the message to the
remote machine
4. The server stub deserializes the
parameters and calls the server
5. Server runs the procedure and returns
the result to the server stub
6. The stub packs the result and sends it
back in the same way
7. The client stub receives the message
and deserializes the result as Java Object
17
The properties of RPC
advantages:
•
•
•
Simplicity
Good efficiency
Commonness of the standard
drawbacks:
•
•
•
•
•
•
No multithreading support
Weak possibilities of the client authorization
Possibility of program number conflict
Function header may have only one argument
Difficult use of pointers in function arguments
C support only
18
Objective processing model
Properties of the objective processing model
•Encapsulation
•Module programming
•Inheritance
•Reusable code
•Polymorfism
•Development simplification
19
Objective processing model
Motivation:
Provide the client with object-oriented RPC mechanism.
Objective RPC + services = a platform for distributted processing
The client sees only an interface of the object, without its implementation
The examples:
– CORBA (Common Object Request Broker Architecture) – OMG
– DCOM (Distributed Component Object Model) – Microsoft
– SOAP (Simple Object Access Protocol ) – Microsoft
– .NET Remoting – Microsoft
– Ninf, NetSolve/GridSolve (grids)
– Java RMI (Remote Method Invocation) – Sun Microsystems
– Globe – Vrije Uniwersyty
20
RMI development
1
Define a remote
interface
2
Implement
the interface
(.java)
3
8
Client
implementation
(.java)
9
javac
uses
javac
4
Client stub
(.class)
(.class)
rmic
Server skel
(.class)
Server class
(.class)
(.class)
10
5
Run the client
6
7
Run RMI registry
Run server objects
Register remote objects
21
CLIENT
SERVER
RMI Example: Create an Interface
Interface declares the methods (and arguments to
them) which will be available in a class which
implements it.
public interface myRMIInterface extends java.rmi.Remote
{
public java.util.Date getDate() throws java.rmi.RemoteException;
}
Create class which implements
interfejs myRMIInterface
Class must implement the interface which has been created in last
step.
import java.rmi.*;
import java.rmi.server.UnicastRemoteObject;
public class myRMIImpl
extends UnicastRemoteObject
implements myRMIInterface
{
public java.util.Date getDate()
{
return new java.util.Date();
}
// …
A Remote Object
●
●
●
A remote object is an instance of a class that
implements a remote interface.
This class has to extend
java.rmi.UnicastRemoteObject.
It includes a constructor which exports the object
to the RMI system when it is created, making it the
object visible to the outside world.
Create class which implements
interfejs myRMIInterface (part 2)
//. . .
public myRMIImpl(String name)
throws RemoteException
{
super();
try
{
Naming.rebind(name, this);
}
catch(Exception e)
{
System.out.println("Exception occurred: " + e);
}
}
}
Create server
In this example server has two roles:
●
●
●
●
Installs new RMISecurityManager
Creates an instance of the myRMIImpl class, (with
name "myRMIImplInstance").
myRMIImpl object register object with the RMI
registry.
After running code, object will be available as
remote clients as "rmi://"
Create server (part 2)
import java.rmi.*;
import java.rmi.server.UnicastRemoteObject;
public class myRMIServer
{
public static void main(String[] argv)
{
System.setSecurityManager(new RMISecurityManager());
try
{
myRMIImpl implementation =
new myRMIImpl("myRMIImplInstance");
}
catch (Exception e)
{
System.out.println("Exception occurred: " + e);
}
}
}
Create the client
Client will connect to the server by using
naming.lookup()
On the begining, the Client installs a new RMI Security Manager,
then uses the static method
naming.lookup()
to get a reference to the remote object.
Create a client
import java.rmi.*;
import java.rmi.registry.*;
import java.rmi.server.*;
import java.util.Date;
public class myRMIClient{
public static void main(String[] argv){
System.setSecurityManager(new RMISecurityManager());
try{
myRMIInterface myServerObject = (myRMIInterface)
Naming.lookup("rmi://"+argv[0]+"/myRMIImplInstance");
//invoke method on server object
Date d = myServerObject.getDate();
System.out.println("Date on server is " + d);
}
catch(Exception e){
System.out.println("Exception occured: " + e);
System.exit(0);
}
System.out.println("RMI connection successful");
}
}
RMI and memory management
Main problem: distributed garbage collection
For each remote object there is a counter of its remote
references.
RMI protocol provides its update.
Remote objects may be destroyed only after all local and
remote references disappear.
30
RMI and memory management
For example: we would like to close the server when it is no longer
needed - the last remote object disappeares.
Solution: the server should implement Unreferenced interface.
The method unreferenced() is invoked when a list of references is
empty.
class Server … implements Unreferenced
…
void unreferened()
{
}
31
Advanced RMI capabilities
Tunelling over HTTP
RMI is able to provide connectivity even in the case of blocking such direct communication
through the firewall. In this case, you can use the HTTP protocol.
Stub download
RMI may automatically download servers stubs, which wants to use it.
Although, it requires a bit more labour (you need to create a security manager)
Running the services on demand
java.rmi.server.UnicastRemoteObject is not the only server class.
We can use java.rmi.activation.Activatable instead.
This class, along with a set of auxiliary classes, provide a fairly complicated way to run
services on demand.
In particular, this means that remote references may be valid after the destruction of the
object to which they pointed, even after restarting the virtual machine on which the object
is located. It is not possible for UnicastRemoteObject.
Use of CORBA
RMI uses its own protocol JRMP (Java Remote Method Protocol).
But CORBA is also available - RMI over IIOP (Internet Inter-ORB Protocol).
You may write an RMI interface, generate skeletons and stubs using CORBA and an IDL file.
32
Summary
RMI is slower than RPC and even CORBA
Java integrated
To remotely invoke a method it does not need to convert the interfaces to IDL.
In DCOM or CORBA this conversion is necessary.
Due to the strongly association with Java RMI is able to interact with inherited
classes. This is not possible in DCOM or CORBA, because those approaches
are typically static.
Parameters transmitted when calling methods between virtual machines can be
ordinary Java objects - Impossible in DCOM and CORBA.
Java-RMI supports the Distributed Garbage Collection, which works as local
Garbage Collectors in each of the virtual machines
Allows rapid and easy distributed application development
Built-in multithreading allows running other program functions
33
Serialization of Java Objects
Remote methods may use objects as parameters and return value.
public MyObject2 myMethod(MyObject1 theObject)
RMI employs the mechanism of object serialization.
Each object to be sent via RMI must implement the interface
java.io.Serializable
As well as in explicit object serialization, in case we need and a custom
serialization we have to oveeride additional methods for the object:
private void writeObject(java.io.ObjectOutputStream out)
throws IOException;
private void readObject(java.io.ObjectInputStream in)
throws IOException, ClassNotFoundException;
Registration of servers
Object factories
rmiregistry – starts the registry for remote services
The connection string for the service:
[rmi:]//<DNS name or IP>[:<port>]/<service name>
Methos used for serivce registration
public
public
public
public
public
static
static
static
static
static
void bind(String name, Remote obj)
void unbind(String name)
void rebind(String name, Remote obj)
String[] list(String name)
Remote lookup(String name)
The mechanism of the factory: we register one master object, which has methods
for creating slave objects on demand – those are particular remote services. The
methods return the references to these objects.
Another example – using factory
„Factory” will be the master remote object.
This object will start and manage the slave objects „Server” and
„Client”
36
Implementation – the factory
37
Implementation – the server
38
Implementation – the client
39
Implemetation – the client
40
Running the application
Compilation
javac *.java
rmic theFactory theClient theServer
Running the server
rmiregistry
java theFactory
Running the client:
java theClient
41