Distributed Programming in Java

Transcription

1 Distributed Programming in Java Networking (1) Motivating Scenario apps apps apps apps 2/28 apps nslookup A program for looking up IP addresses 3/28 1

2 InetAddress This class represents an IP address Each host on an IP network has a host name and a numeric address. 4/28 NsLookup Simple Java version of nslookup 5/28 Sockets The java.net package provides an objectoriented framework for using sockets Sockets offer a fast and flexible way for the communication between objects The Socket class represents an endpoint of a connection between two processes There are two flavors of sockets: datagram (UDP) and stream (TCP) sockets 6/28 2

3 Datagram Sockets Datagram sockets use the Unreliable Datagram Protocol (UDP) Low-level connectionless No guarantee about packet delivery Neither is their order guaranteed Packets transmitted by the sender either make it to the receiver or not Typically used in bandwidth-limited applications (e.g. real-time audio, webtv) 7/28 Stream Sockets Stream sockets use the Transmission Control Protocol (TCP) Reliable connection-oriented Packets are guaranteed to be delivered Packets are delivered in order Packets not received within a set time are assumed lost and requested again Typically used when lost packets cannot be tolerated (e.g. message passing) 8/28 Socket To create a client-side Socket to connect to a specific host and port number To create a server-side Socket to listen on a ServerSocket for a connection request 9/28 3

4 Client Socket Key Socket methods 10/28 ServerSocket Key ServerSocket methods 11/28 NameServer Simple name server which replies to a name with a hostname:port or unknown:0 The server creates a ServerSocket, then waits in a loop to accept() connections, and processes the name lookup request 12/28 4

5 NameServer 13/28 NameServer 14/28 NameServer 15/28 5

6 Notes Following good practice we make the PORT on which the server listens a constant The classes DataInputStream and DataOutStream are filter streams that wrap around an input and output stream These streams read and write primitive Java types: readint(), writeint(), etc. Strings are Unicode, which results in the methods: readutf(), and writeutf() 16/28 Name 17/28 Advanced Sockets: MulticastSocket Useful for sending/receiving IP multicast pkts Multicast socket is a UDP datagram socket Can join groups of multicast hosts Class is java.net.multicastsocket APIs: send(datagrampacket) joingroup(inetaddress) leavegroup(inetaddress) 18/28 6

7 MulticastSocket Multicast group specified by class D IP address and standard UDP port number Address: /28 MulticastSocket When one sends a message to a multicast group, all subscribing recipients to that host and port receive the message (within the time-to-live range of the packet). The socket needn't be a member of the multicast group to send messages to it 20/28 MulticastSocket When a socket subscribes to a multicast group/port, it receives datagrams sent by other hosts to the group/port, as do all other members of the group and port. A socket relinquishes membership in a group by the leavegroup(inetaddress addr) method. Multiple MulticastSocket's may subscribe to a multicast group and port concurrently, and they will all receive group datagrams 21/28 7

8 Multicast DNS Used for service discovery JMDNS Java implementation of service discovery protocol compatible with rendevous from Apple 22/28 Streams The java.io package allows us to exchange formatted data over connections Streams support reading data serially from a source, or writing to a destination 23/28 Byte Streams FilterInputStream BufferedInputStream InputStream FileInputStream DataInputStream FilterOutputStream BufferedOutputStream OutputStream FileOutputStream DataOutputStream 24/28 8

9 Character Streams 25/28 Serialization Object serialization is a process through which objects are flattened into a form that can be written to and read from a stream Flattened objects can be written to a file or even sent across a network connection Supported by the java.io package Key component of the implementation of distributed object frameworks in Java 26/28 Serializable Serialization is almost automatic in Java To use serialization, your objects need to implement the Serializable interface This is an example of a marking interface it does not have any methods... but indicates (to the JVM) this class is designed to be serialized However, the internal machinery required makes serialization complex and costly 27/28 9

10 Example Serializable Initial declaration of TestObject 28/28 Object Streams Objects can be... serialized with an ObjectOutputStream deserialized with an ObjectInputStream Similar to DataInput(Output)Stream Methods for writing and reading binary representations of primitive types But they add the ability to write and read non-primitive objects and array values 29/28 Object Streams 30/28 10

11 Not All Objects (and Data) Should be Serialized Inherently platform-specific objects e.g. FileDescriptors Redundant objects (eg that cache values) Only objects that implement Serializable can be serialized (must skip others) Can prevent fields from being serialized by declaring them as transient 31/28 Example Serializable Revised declaration of TestObject both name and panel are now transient assumes that name is derived from value in this case (for sake of the example) 32/28 References Online chapter on network streams Harold, Java I/O, Chapter 5 er/ch05.html More on using the serialization API Greanier, Flatten Your Objects jw /jw-0714-flatten.html 33/28 11