Chng 6
Lp trnh Socket cho giao thc TCP1. M hnh client/serverM hnh c ph
bin nht v c chp nhn rng ri trong cc h thng phn tn l m hnh
client/server. Trong m hnh ny s c mt tp cc tin trnh m mi tin trnh
ng vai tr nh l mt trnh qun l ti nguyn cho mt tp hp cc ti nguyn cho
trc v mt tp hp cc tin trnh client trong mi tin trnh thc hin mt tc v
no cn truy xut ti ti nguyn phn cng hoc phn mm dng chung. Bn thn cc
trnh qun l ti nguyn cn phi truy xut ti cc ti nguyn dng chung c qun
l bi mt tin trnh khc, v vy mt s tin trnh va l tin trnh client va l
tin trnh server. Cc tin trnh pht ra cc yu cu ti cc server bt k khi
no chng cn truy xut ti mt trong cc ti nguyn ca cc server. Nu yu cu
l ng n th server s thc hin hnh ng c yu cu v gi mt p ng tr li ti tin
trnh client. M hnh client/server cung cp mt cch tip cn tng qut chia
s ti nguyn trong cc h thng phn tn. M hnh ny c th c ci t bng rt nhiu
mi trng phn cng v phn mm khc nhau. Cc my tnh c s dng chy cc tin
trnh client/server c nhiu kiu khc nhau v khng cn thit phi phn bit
gia chng; c tin trnh client v tin trnh server u c th chy trn cng mt
my tnh. Mt tin trnh server c th s dng dch v ca mt server khc. M hnh
truyn tin client/server hng ti vic cung cp dch v. Qu trnh trao i d
liu bao gm: 1. Truyn mt yu cu t tin trnh client ti tin trnh server
2. Yu cu c server x l 3. Truyn p ng cho client M hnh truyn tin ny
lin quan n vic truyn hai thng ip v mt dng ng b ha c th gia client v
server. Tin trnh server phi nhn thc c thng ip c yu cu bc mt ngay
khi n n v hnh ng pht ra yu cu trong client phi c tm dng (b phong
ta) v buc tin trnh client trng thi ch cho t khi n nhn c p ng do
server gi v bc ba. M hnh client/server thng c ci t da trn cc thao
tc c bn l gi (send) v nhn (receive).
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Client Server
Request message
Wait
Reply Execution
Request message
Tin trnh ang x l
Tin trnh ang phong taHnh 4.1 Qu trnh giao tip client v server c
th din ra theo mt t rong hai ch : b phong ta (blocked) v khng b
phong ta (non-blocked). Ch b phong ta (blocked): Trong ch b phong
ta, khi tin trnh client hoc server pht ra lnh gi d liu (send), vic
thc thi ca tin trnh s b tm ngng cho ti khi tin trnh nhn pht ra lnh
nhn d liu (receive). Tng t i vi tin trnh nhn d liu, nu tin trnh no
(client hoc server) pht ra lnh nhn d liu, m ti thi im cha c d liu
gi ti th vic thc thi ca tin trnh cng s b tm ngng cho ti khi c d liu
gi ti. Ch khng b phong ta (non-blocked) Trong ch ny, khi tin trnh
client hay server pht ra lnh gi d liu thc s, vic thc thi ca tin
trnh vn c tin hnh m khng quan tm n vic c tin trnh no pht ra lnh nhn
d liu hay khng. Tng t cho trng hp nhn d liu, khi tin trnh pht ra
lnh nhn d liu, n s nhn d liu hin c, vic thc thi ca tin trnh vn c
tin hnh m khng quan tm n vic c tin trnh no pht ra lnh gi d liu tip
theo hay khng.
2. Cc kin trc Client/Server2.1. Client/Server hai tng (two-tier
client/server) Kin trc client/server n gin nht l kin trc hai tng.
Trong thc t hu ht cc kin trc client/server l kin trc hai tng. Mt ng
dng hai tng cung cp nhiu trm lm vic vi mt tng trnh din thng nht,
tng ny truyn tin vi tng lu tr d liu tp trung. Tng trnh din thng
thng l client, v tng lu tr d liu l server. Hu ht cc ng dng Internet
nh l email, telnet, ftp thm ch l c Web l cc ng dng hai tng. Phn ln
cc lp trnh vin trnh ng dng vit cc ng dng client/server c xu th s
dng kin trc ny.
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Trong ng dng hai tng truyn thng, khi lng cng vic x l c dnh cho
pha client trong khi server ch n gin ng vai tr nh l chng trnh kim
sot lung vo ra gia ng dng v d liu. Kt qu l khng ch hiu nng ca ng
dng b gim i do ti nguyn hn ch ca PC, m khi lng d liu truyn i trn
mng cng tng theo. Khi ton b ng dng c x l trn mt PC, ng dng bt buc
phi yu cu nhiu d liu trc khi a ra bt k kt qu x l no cho ngi dng.
Nhiu yu cu d liu cng lm gim hiu nng ca mng. Mt vn thng gp khc i vi
ng dng hai tng l vn bo tr. Ch cn mt thay i nh i vi ng dng cng cn
phi thay i li ton b ng dng client v server.
Hnh 4.2 2.2. Client/Server ba tng Ta c th trnh c cc vn ca kin
trc client/server hai tng bng cch m rng kin trc thnh ba tng. Mt kin
trc ba tng c thm mt tng mi tc bit vic x l d liu v tr trung tm.
Hnh 4.3
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Theo kin trc ba tng, mt ng dng c chia thnh ba tng tch bit nhau v
mt logic. Tng u tin l tng trnh din thng bao gm cc giao din ha. Tng
th hai, cn c gi l tng trung gian hay tng tc nghip. Tng th ba cha d
liu cn cho ng dng. Tng th ba v c bn l chng trnh thc hin cc li gi hm
tm kim d liu cn thit. Tng trnh din nhn d liu v nh dng n hin th. S
tch bit gia chc nng x l vi giao din to nn s linh hot cho vic thit k
ng dng. Nhiu giao din ngi dng c xy dng v trin khai m khng lm thay i
logic ng dng. Tng th ba cha d liu cn thit cho ng dng. D liu ny c th
bao gm bt k ngun thng tin no, bao gm c s d liu nh Oracale, SQL
Server hoc ti liu XML. 2.3. Kin trc n-tng Kin trc n-tng c chia thnh
cc tng nh sau: Tng giao din ngi dng: qun l tng tc ca ngi dng vi ng
dng Tng logic trnh din: Xc nh cch thc hin th giao din ngi dng v cc
yu cu ca ngi dng c qun l nh th no. Tng logic tc nghip: M hnh ha cc
quy tc tc nghip, Tng cc dch v h tng: Cung cp mt chc nng b tr cn
thit cho ng dng nh cc thnh phn (truyn thng ip, h tr giao tc).
3. M hnh truyn tin socketServer Socket() 1 Client Socket()
Bind()
2
Bind()
Listen()
3 4 Connect()
Accept()
5
Cc chc nng gi v nhn
6
Cc chc nng gi v nhn
Close()
7
Close()
Hnh 4.4
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Khi lp trnh, ta cn quan tm n ch b phong ta, v n c th dn n tnh
hung mt tin trnh no s ri vo vng lp v hn ca qu trnh gi hoc nhn.
Trong chng 1 chng ta bit hai giao thc TCP v UDP l cc giao thc tng
giao vn truyn d liu. Mi giao thc c nhng u v nhc im ring. Chng hn,
giao thc TCP c tin cy truyn tin cao, nhng tc truyn tin b hn ch do
phi c giai on thit lp v gii phng lin kt khi truyn tin, khi gi tin c
li hay b tht lc th giao thc TCP phi c trch nhim truyn li,Ngc li,
giao thc UDP c tc truyn tin rt nhanh v n ch c mt c ch truyn tin rt
n gin: khng cn phi thit lp v gii phng lin kt. Khi lp trnh cho TCP
ta s dng cc socket lung, cn i vi giao thc UDP ta s s dng lp
DatagramSocket v DatagramPacket. Truyn tin hng lin kt ngha l cn c
giai on thit lp lin kt v gii phng lin kt trc khi truyn tin. D liu c
truyn trn mng Internet di dng cc gi (packet) c kch thc hu hn c gi l
datagram. Mi datagram cha mt header v mt payload. Header cha a ch v
cng cn truyn gi tin n, cng nh a ch v cng xut pht ca gi tin, v cc
thng tin khc c s dng m bo tin cy truyn tin, payload cha d liu. Tuy
nhin do cc datagram c chiu di hu hn nn thng phi phn chia d liu thnh
nhiu gi v khi phc li d liu ban u t cc gi ni nhn. Trong qu trnh
truyn tin c th c th c mt hay nhiu gi b mt hay b hng v cn phi truyn
li hoc cc gi tin n khng theo ng trnh t. trnh nhng iu ny, vic phn
chia d liu thnh cc gi, to cc header, phn tch header ca cc gi n, qun
l danh sch cc gi nhn c v cc gi cha nhn c, ... rt nhiu cng vic cn
phi thc hin, v i hi rt nhiu phn mm phc tp. Tht may mn, ta khng cn
phi t thc hin cng vic ny. Socket l mt cuc cch mng ca Berkeley UNIX.
Chng cho php ngi lp trnh xem mt lin kt mng nh l mt lung m c th c d
liu ra hay ghi d liu vo t lung ny. V mt lch s Socket l mt s m rng
ca mt trong nhng tng quan trng nht ca UNIX: tt c cc thao tc vo/ra
ging nh vo ra tp tin i vi ngi lp trnh, cho d ta ang lm vic vi bn
phm, mn hnh ha, mt file thng thng, hay mt lin kt mng. Cc Socket che
du ngi lp trnh khi cc chi tit mc thp ca mng nh mi kiu ng truyn, cc
kch thc gi, yu cu truyn li gi, cc a ch mng... Mt socket c th thc
hin by thao tc c bn: Kt ni vi mt my xa (v d, chun b gi v nhn d liu)
Gi d liu Nhn d liu Ngt lin kt Gn cng Nghe d liu n Chp nhn lin kt t
cc my xa trn cng c gn
Lp Socket ca Java c s dng bi c client v server, c cc phng thc
tng ng vi bn thao tc u tin. Ba thao tc cui ch cn cho server ch cc
client lin kt vi chng. Cc thao tc ny c ci t bi lp ServerSocket. Cc
socket cho client thng c s dng theo m hnh sau: Mt socket mi c to ra
bng cch s dng hm Socket(). Socket c gng lin kt vi mt host xa. Mi
khi lin kt c thit lp, cc host xa nhn cc lung vo v lung ra t socket,
v s dng cc lung ny gi d liu cho nhau. Kiu lin kt ny c gi
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l song cng (full-duplex)-cc host c th nhn v gi d liu ng thi.
ngha ca d liu ph thuc vo giao thc. Khi vic truyn d liu hon thnh, mt
hoc c hai pha ngt lin kt. Mt s giao thc, nh HTTP, i hi mi lin kt
phi b ng sau mi khi yu cu c phc v. Cc giao thc khc, chng hn FTP,
cho php nhiu yu cu c x l trong mt lin kt n.
4. Socket cho Client 4.1. Cc constructor public Socket(String
host, int port) throws UnknownHostException, IOException Hm ny to
mt socket TCP vi host v cng xc nh, v thc hin lin kt vi host xa. V
d: try{ Socket s = new Socket( www.vnn.vn,80); }
catch(UnknownHostException e){ System.err.println(e); }
catch(IOException e){ System.err.println(e); } Trong hm ny tham s
host l hostname kiu String, nu host khng xc nh hoc my ch tn min
khng hot ng th constructor a ra ngoi l UnknownHostException. V mt l
do no m khng th m c socket th constructor s a ra ngoi l
IOException. C nhiu nguyn nhn khin cho mt lin kt tht bi: host m ta
ang c gng kt ni ti khng chp nhn lin kt, kt ni Internet c th b ngt,
hoc vn nh tuyn c th ngn nga cc gi tin ca ta ti ch. V d: Vit chng
trnh kim tra trn 1024 cng u tin nhng cng no ang c server hot ng
import java.net.*; import java.io.*; class PortScanner { public
static void main(String[] args) { String host="localhost";
if(args.length>0){ host=args[0]; } for(int i=0;istart java
EchoClient1
Hnh 4.7
8. ng dng a tuyn on trong lp trnh JavaCc server nh vit trn rt n
gin nhng nhc im ca n l b hn ch v mt hiu nng v n ch qun l c mt
client ti mt thi im. Khi khi lng cng vic m server cn x l mt yu cu
ca client l qu ln v khng bit trc c thi im hon thnh cng vic x l th
cc server ny l khng th chp nhn c. khc phc iu ny, ngi ta qun l mi
phin ca client bng mt tuyn on ring, cho php cc server lm vic vi
nhiu client ng thi. Server ny c gi l server tng tranh (concurrent
server)-server to ra mt tuyn on qun l tng yu cu, sau tip tc lng
nghe cc client khc. Chng trnh client/server chng ta xt m mc 6 v mc
7 l chng trnh client/server n tuyn on. Cc server n tuyn on ch qun l
c mt lin kt ti mt thi im. Trong thc t mt server c th phi qun l nhiu
lin kt cng mt lc. thc hin iu ny server chp nhn cc lin kt v chuyn cc
lin kt ny cho tng tuyn on x l. Trong phn di y chng ta s xem xt cch
tin hnh ci t mt chng trnh client/server a tuyn on. Chng trnh pha
server import java.io.*; import java.net.*; class EchoServe extends
Thread
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{ private Socket socket; private BufferedReader in; private
PrintWriter out; public EchoServe (Socket s) throws IOException {
socket = s; System.out.println("Serving: "+socket); in = new
BufferedReader(new InputStreamReader(socket.getInputStream())); //
Cho php auto-flush: out = new PrintWriter(new BufferedWriter(new
OutputStreamWriter( socket.getOutputStream())), true); // Nu bt k
li gi no trn a ra ngoi l // th chng trnh gi c trch nhim ng socket.
Ngc li tuyn on s // s ng socket start(); } public void run() { try
{ while (true) { System.out.println("....Server is waiting...");
String str = in.readLine(); if (str.equals(exit) ) break;
System.out.println("Received: " + str); System.out.println("From:
"+ socket); String upper=str.toUpperCase(); // gi li cho client
out.println(upper); } System.out.println("Disconnected
with.."+socket); } catch (IOException e) {} finally { try {
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socket.close(); } catch(IOException e) {} } } } public class
TCPServer1 { static int PORT=0; . public static void main(String[]
args) throws IOException { if (args.length == 1) {
PORT=Integer.parseInt(args[0]); // Nhp s hiu cng t i dng lnh } //
To mt i tng Server Socket ServerSocket s = new ServerSocket(PORT);
InetAddress addrs= InetAddress.getLocalHost();
System.out.println("TCP/Server running on : "+ addrs +" ,Port
"+s.getLocalPort()); try { while(true) { // Phong ta cho ti khi c
mt lin kt n Socket socket = s.accept(); try { new
EchoServe(socket); // To mt tuyn on qun l ring tng lin kt }
catch(IOException e) { socket.close(); } } } finally {
s.close();
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} } } Chng trnh pha client import java.net.*; import java.io.*;
public class TCPClient1 { public static void main(String[] args)
throws IOException { if (args.length != 2) { System.out.println("S
dng: java TCPClient hostid port#"); System.exit(0); } try {
InetAddress addr = InetAddress.getByName(args[0]); Socket socket =
new Socket(addr, Integer.parseInt(args[1])); try {
System.out.println("socket = " + socket); BufferedReader in = new
BufferedReader(new InputStreamReader( socket.getInputStream())); //
Output is automatically flushed by PrintWriter: PrintWriter out
=new PrintWriter(new BufferedWriter( new
OutputStreamWriter(socket.getOutputStream())),true); // c dng k t t
bn phm DataInputStream myinput = new DataInputStream(new
BufferedInputStream(System.in)); try {
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for(;;) { System.out.println("Type anything followed by RETURN,
or Exit to terminate the program."); String
strin=myinput.readLine(); // Quit if the user typed ctrl+D if
(strin.equals("exit")) break; else out.println(strin); String
strout = in.readLine(); { // Compare Both Strings
System.out.println("Received: "+strout); } else
System.out.println("Echo bad -- string unequal"+ strout); } // of
for ;; } catch (IOException e) { e.printStackTrace(); } // User is
exiting } finally { System.out.println("EOF...exit");
socket.close(); } } catch(UnknownHostException e) {
System.err.println("Can't find host"); System.exit(1); } catch
(SocketException e) // Send the message // Recive it back
if ( strin.length()==strout.length())
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{ System.err.println("Can't open socket"); e.printStackTrace();
System.exit(1); } } }
9. Kt lunChng ta tm hiu cch lp trnh mng cho giao thc TCP. Cc
Socket cn c gi l socket lung v gi v nhn d liu u c tin hnh thng qua
vic c ghi cc lung. Ta c cng tm hiu c ch hot ng ca socket v cch thc
lp cc chng trnh server v client. Ngoi ra, chng ny cng gii thch to
sao cn c ci t server a tuyn on v tm hiu cch thc lp cc chng trnh
client/server a tuyn on. Trong chng tip theo chng ta s hc cch xy
dng mt chng trnh client/server cho giao thc UDP, mt giao thc gn vi
giao thc TCP.
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