Practical Session #09 Exceptions & Networking. Tom Mahler

Transcription

1 Practical Session #09 Exceptions & Networking Tom Mahler 1

2 In This Recitation We ll Cover Exceptions Java Sockets Datagram vs. Stream The Client-Server Model 2

3 Exceptions 3

4 Exceptions Java uses exceptions to handle errors and other exceptional events Examples: Reading a file that does not exist or is inaccessible Dividing a number by zero Network is down, URL does not exist Accessing an array element at a certain index that is not within the bounds of the array 4

5 Examples See code (See ExceptionExample) 5

6 What Is An Exception? An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions When an error occurs within a method, the method creates an object and hands it off to the runtime system The object, called an exception object, contains information about the error, including its type and the state of the program when the error occurred 6

7 What Is An Exception? Creating an exception object and handing it to the runtime system is called throwing an exception After a method throws an exception, the runtime system attempts to find something to handle it 7

8 What Is An Exception? The set of possible "somethings" to handle the exception is the ordered list of methods that had been called to get to the method where the error occurred The list of methods is known as the call stack (see the next figure) 8

9 What Is An Exception? The runtime system searches the call stack for a method that contains a block of code that can handle the exception This block of code is called an exception handler 9

10 What Is An Exception? The search begins with the method in which the error occurred and proceeds through the call stack in the reverse order in which the methods were called 10

11 What Is An Exception? If no handler is found, the exception is thrown to the JVM, which terminates the program and prints the exception together with its call stack to the console 11

12 Exception Types: Unchecked Exceptions Errors: exceptional conditions that are external to the application, and that the application usually cannot anticipate or recover from 12

13 Exception Types: Unchecked Exceptions They are indicated by Error and its subclasses Example: OutOfMemoryError signals that the application ran out of memory 13

14 Exception Types: Unchecked Exceptions Runtime Exceptions: exceptional conditions that are internal to the application, and that the application usually cannot anticipate or recover from 14

15 Exception Types: Unchecked Exceptions These usually indicate programming bugs, such as logic errors or improper use of an API Runtime exceptions are those indicated by RuntimeException and its subclasses 15

16 Exception Types: Unchecked Exceptions Examples: NullPointerException - signals that the application is trying to access a null pointer (an uninitialized object) ArrayIndexOutOfBoundsEx ception - signals that the application is trying to access an array outside of its boundaries 16

17 Exception Types: Checked Exceptions Exceptional conditions that a well-written application should anticipate and recover from They are inherited from the core Java class Exception 17

18 Exception Types: Checked Exceptions Checked exceptions must be handled in your code (catch it), or passed to the calling code for handling (declare that your method might throw it using the throws keyword at the method signature) 18

19 Exception Types: Checked Exceptions Example: FileNotFoundExceptio n - signals that the application is trying to access a file that does not exist 19

20 Throwing An Exception If a method wants to signal that something went wrong during its execution, it throws an exception Exceptions may be caught and handled by another part of the program 20

21 Throwing An Exception Throwing an exception involves: Creating an exception object that encloses information about the problem that occurred Using the throw statement to notify about the exception 21

22 Examples See code (See Clock) 22

23 Creating New Exceptions You can create a new exception by extending Exception, or one of its subclasses 23

24 Examples See code (See Clock) 24

25 Catching Exceptions A method catches an exception using a combination of the try and catch keywords A try/catch block is placed around the code that might generate an exception Each catch block is an exception handler and handles the type of exception indicated by its argument, which must inherits from the Throwable class 25

26 Examples See code (See FirstLine) 26

27 Catching Exceptions If an exception extends another exception and you would like to handle each separately, then the catch block that handles the sub exception should be before the catch block that handles the base exception 27

28 Catching Exceptions Exceptions don't have to be handled in the method in which they occurred You can let a method further up the call stack handle the exception This is done by adding a throws clause to the method declaration 28

29 Handling Exceptions When handling exceptions you should restore stable state and do one of the following: Change conditions and retry For example, when using a fail-fast iterator Clean up and fail (re-throw exception) For example, when trying to open a file that does not exist 29

30 Examples See code (See FailFastExample) 30

31 The Finally Block The finally block comes right after all catch blocks in a try-catch clause The code found in the finally block is always executed before exiting the try/catch block, even if an exception occurred, or a return/continue/break statement were encountered It is usually used for cleanup code, since it allows the programmer to avoid having cleanup code accidentally bypassed by a return, continue, or break 31

32 Examples See code (See FirstLine2) 32

33 Try-With-Resource While the code in the previous section seems to handle all possible exceptions - it does overlook the possibility that the close() function of the BufferedReader will produce an IOException! Should such an exception occur within the finally block - it would not be caught by the previous try/catch block, but rather thrown upwards 33

34 Try-With-Resource This could obviously be solved by a second set of try/catch statements, but fortunately, Java SE 7 provides a more elegant solution in the form of try-with-resource statements The code in the previous section can be replaced with the following equivalent: 34

35 Examples See code (See FirstLine2) 35

36 Try-With-Resource In the above code, a BufferedReader is declared as part of the "try" statement If an exception occurs, either during the creation of the BufferedReader, or its use within the "try" statement - the corresponding.close() function will be called automatically and no further exceptions will be thrown 36

37 Java Sockets 37

38 Internet Protocol: IP Addresses The Internet is a global system of interconnected computer networks, it is a network of networks that consists of millions of private and public, academic, business, and government networks of local to global scope that are linked by a broad array of electronic and optical networking technologies 38

39 Internet Protocol: IP Addresses Every machine on the Internet has a unique identifying number, called an IP Address The IP stands for Internet Protocol, which is the language that computers use to communicate over the Interne The original designers of TCP/IP defined an IP address as a 32-bit number and this system, known as Internet Protocol Version 4 or IPv4, is still in use today 39

40 Internet Protocol: IP Addresses However, due to the enormous growth of the Internet and the resulting depletion of available addresses, a new addressing system (IPv6), using 128 bits for the address, was developed in 1995 IPv4 addresses are usually represented in dot-decimal notation (four numbers, each ranging from 0 to 255) Each part represents 8 bits of the address, and is therefore called an octet 40

41 Internet Protocol: IP Addresses Protocol rules of conversation A typical IP address (decimal format) A typical IP address (binary format)

42 TCP/IP Model The TCP/IP model is a description framework for computer network protocols TCP/IP provides end-to-end connectivity specifying how data should be formatted, addressed, transmitted, routed and received at the destination 42

43 TCP/IP Model TCP/IP is generally described as having four abstraction layers: Link Layer Internet Layer Transport Layer Application Layer 43

44 Restricted IP Addresses is reserved for the default network is used for broadcasts (Messages that are intended for all computers on a network) (Loopback) - is used as the loopback address This means that it is used by the host computer to send a message back to itself Also aliased as localhost 44

45 Domain Name System (DNS) Remembering IP addresses is hard for humans The DNS service maps text names to IP addresses automatically For example: > xxx.xxx.xxx.xxx 45

46 Domain Name System (DNS) An often used analogy to explain the Domain Name System is that it serves as the "phone book" for the Internet by translating human-friendly computer hostnames into IP addresses 46

47 Internet Servers And Clients All of the machines on the Internet are either servers or clients (or both) The machines that provide services to other machines are servers And the machines that are used to connect to those services are clients 47

48 Internet Servers And Clients A server has a static IP address that does not change very often A home machine that is dialing up through a modem, typically has a dynamic IP address - A session unique IP address assigned by the ISP (usually assigned by a server using DHCP) every time you dial in (could be different the next time you dial in) 48

49 Ports Any server machine makes its services available using numbered ports - one for each service that is available on the server Think of the IP:Port combo as this: The IP is the building address, where the Port denotes which apartment in that building In our world, IP is the computer machine, where the Port is the application that listens to it and can send and receive messages through it 49

50 Network Console Tools ipconfig (Microsoft Windows console application) displays all current TCP/IP network configuration values. ifconfig (Unix-like console application) - configure, control, and query TCP/IP network interface parameters. 50

51 Network Console Tools ping (Unix-like & Windows) - utility used to test whether a particular host is reachable across an Internet Protocol (IP) network and to measure the round-trip time for packets sent from the local host to a destination computer curl is a computer software project providing a library and command-line tool for transferring data using various protocols 51

52 Network Console Tools Telnet client (Unix-like & Windows) Telnet is a network protocol used to provide a bidirectional interactive communications facility Typically, telnet provides access to a command-line interface on a remote host via a virtual terminal connection Putty can be used on recent Windows-es which don't have telnet 52

53 Sockets Sometimes your applications require low-level communication For example connecting to a database or implementing instant messaging Networking allows processes running on different hosts to exchange messages 53

54 Sockets Message destinations are specified as socket addresses Each socket address is a communication identifier that consists of a port number and an Internet address 54

55 Sockets When messages are sent, they are queued at the sending socket until the underlying network protocol has transmitted them When they arrive, the messages are queued at the receiving socket until the receiving process makes the necessary calls to receive them 55

56 Datagram vs. Stream 56

57 Datagram vs. Stream Transport Layer is a group of methods and protocols within a layered architecture of network components There are two communication protocols that one can use for socket programming: Datagram communication (UDP) Stream communication (TCP) 57

58 Datagram vs. Stream In this practical session we focus on sessionoriented, reliable connections implemented by the TCP protocol and used through the socket interface 58

59 Datagram Communication The datagram communication protocol, known as UDP (user datagram protocol), is a connectionless protocol It means that a datagram can be sent at any moment without prior connection preparation as in TCP You just send the datagram and hope the receiver is able to handle it 59

60 Datagram Communication There is absolutely no guarantee that the datagram will be delivered to the destination host In reality, the failure rate is very low on the Internet and nearly null on a LAN unless the bandwidth is full 60

61 Datagram Communication Not only the datagram can be undelivered, but it can be delivered in an incorrect order It means you can receive a packet before another one, even if the second has been sent before the first you just received You can also receive the same packet twice 61

62 Datagram Communication The main advantages for UDP are that you can broadcast, and it is fast The main disadvantage is unreliability and therefore complicated to program at the application level 62

63 Stream Communication The stream communication protocol is known as TCP (transfer control protocol) Unlike UDP, TCP is a connection-oriented protocol In order to do communication over the TCP protocol, a connection must first be established between the pair of sockets 63

64 Stream Communication While one of the sockets listens for a connection request (server), the other asks for a connection (client) Once two sockets have been connected, they can be used to transmit data in both (or either one of the) directions 64

65 Stream Communication Being stream oriented means that the data is a plain bytes sequence The receiver has no means of knowing how the data was actually transmitted The sender can send many small data chunks and the receiver receive only one big chunk, or the sender can send a big chunk, the receiver receiving it in a number of smaller chunks 65

66 Stream Communication The only thing that is guaranteed is that all data sent will be received without any error and in the correct order Should any error occur, it will automatically be corrected (retransmitted as needed) or the error will be notified if it can t be corrected 66

67 Stream Communication: The following diagram presents the way we will handle text streams in java 67

68 Stream Communication: This illustrates the duplex communication model 68

69 Stream Communication Running netstat -p when both the server and the client are on the same computer should show Active Internet connections (w/o servers) Proto Recv-Q Send-Q Local Address Foreign Address S3htate PID/Program name tcp6 0 0 localhost:4500 localhost:42705 ESTABLISHED 6830/java tcp6 0 0 localhost:42705 localhost:4500 ESTABLISHED 6857/java 69

70 Stream Communication The main advantages for TCP are that it guarantees three things: Correctness Order No duplication The main disadvantage is: It cannot be used for broadcast or multicast transmission It might have a bad throughput on high latency link 70

71 TCP/UDP Overhead: The following image illustrates the header size for TCP and UDP packets 71

72 TCP/UDP Overhead: The following image illustrates the header size for TCP and UDP packets 72

73 TCP/UDP Overhead It is quite evident that the TCP header (sent along with each data packet!) is significantly larger than the UDP header This creates additional overhead, reducing throughput of TCP communication 73

74 The Client-Server Model 74

75 The Client-Server Model The client-server is a very common model in many networking applications The server provides some service, such as processing database queries or sending out current stock prices The client uses the service provided by the server, either displaying database query results to the user or making stock purchase recommendations to an investor 75

76 The Client-Server Model A socket connection based on top of a TCPconnection is symmetric between the two ends - except for the connection establishment stage 76

77 The Client-Server Model To establish a connection, the model determines that: 1. The server waits for connection requests from clients 2. The server only reacts to the initiative of the client 77

78 The Client-Server Model To establish a connection, the model determines that: 3. To remain available for further connections, the server does not open the connection directly with the client on the incoming socket Instead, it creates a private socket which the client can keep connected for the whole period of the session 78

79 The Client-Server Model Servers must be built to maximize availability that is, they must handle requests for connections as fast as possible and then return to the mode where they can receive and handle more requests from other clients 79

80 Examples There are three examples In each folder you will find code for client and for server Compile both files and open two console window Place them so you can see them both First run the server and supply the server with a port to listen at For example: > java EchoServer

81 Examples Then run the client, provide it with the host name and the port of the server For example, if you use the same machine for both, then > java EchoClient localhost 4444 If you run on different machine replace "localhost" with the server computer name 81

82 1. Simple Line Printer A very simple client-server application The client sends the server a line of text read from the user console and the server prints it 82

83 Examples See code (See LPServer, LPClient) 83

84 2. Echo Server with UTF-8 Different computers, run time environment languages and compilers can use different symbols representation If we want to communicate we need uniform coding for the symbols Since we are transferring objects larger than a single byte (integers, chars) over streams of bytes, we need to encode them 84

85 2. Echo Server with UTF-8 Should we run on one side java -Dfile.encoding=US-ASCII LPServer 4500 And on another java LPClient localhost 4500 (assuming default UTF-8) 85

86 2. Echo Server with UTF-8 Any text which has characters not included in the similar 127 first characters in the encodings is not transferred correct E.g. Hebrew We can notice other problems using UTF-16 and UTF- 32 For example line endings no longer recognized as message ends since they have different numerical values 86

87 Examples We can also see that Unicode strings have different lengths when represented in different encodings See code (See TestEncodings) 87

88 2. Echo Server with UTF-8 In the next example the client and the server are communicate using the "UTF-8" format You will learn more about Encoding in the lectures This time, the server sends the client back the message it received using UTF-8, we create the readers and writers with the encoding specified (EchoServer.java, EchoClient.java). 88

89 Examples See code (See EchoServer, EchoClient) 89

90 3. Protocol Interface We don t want to hold the implementation of a protocol inside the server s code It is complicated enough that it has to handle the communication, and not testable while a protocol usually follows a specification 90

91 3. Protocol Interface We would like to have the protocol implemented somewhere else For that purpose we define an interface ServerProtocol which has two methods for protocol processing: process - for processing the received message and construct a response message isend - identifies the end of a protocol 91

92 3. Protocol Interface Note that since we have an independent protocol processor we can hold the protocol s state Once again we have an Echo Server, but this time, in addition to sending back the message it is also numbered 92

93 Examples See code (See ProtocolServer) 93

94 4. Http Client Hypertext Transfer Protocol is an application protocol for distributed, collaborative, hypermedia information systems We can see a few attributes to many communication protocol designs An HTTP request is defined as a header, followed by an empty line and an optional body 94

95 4. Http Client The request includes the protocol version An HTTP response is defined as a header, followed by an empty line and an optional body 95

96 4. Http Client The response may end after the body, or in HTTP/1.1 each response may contain: Content-Length header field indicating the number of bytes for the response A response with Transfer-Encoding: chunked with each chunk containing the chunk size 96

97 4. Http Client If we try to run java Http We should receive a proper HTTP response 97