1	CS110 Lecture 17 Thursday, April 1, 2004 Announcements hw7 due tonight pass/fail, withdraw deadline April 8 Agenda Questions Juno JFile system internals next week
2	Juno (Chapters 6, 7, ...) We have a (J)File system Now we provide it with a command line interface, like the one the emacs shell or a command prompt gives you to windows Use that interface to create TextFiles and Directories navigate in the Directory tree
3	Command prompt interface to windows file system
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5	Juno (Chapters 6, 7, ...) Simulates (models) that command line interface “Juno” etymology: “Juno’s Unix not” (developed under Unix) Juno UI: nested do forever loops login: login as user, register, help, exit mars:\users\eb> help, logout, mkdir, newfile, type ... cd, dir (various shell commands ) Like Bank banker command: customer, open, report, exit transaction: deposit, withdraw, …, quit,
6	Sample Juno session
7	Juno classes
8	Juno.java (version 6) fields (attributes of a Juno operating system) Map for users (line 30) Terminal for console (line 31) Directories: slash, userHomes (lines 33, 34) ShellCommandTable (line 36) main() parse command line arguments (175-186) create a Juno object (190)
9	Juno constructor (45) new Terminal for console (line50) new Map for users (line 51) new table for ShellCommands (52) file system and system administrator lines 56-59 read them next week create LoginInterpreter object to respond to user’s commands (64, 65) send it a CLIlogin message (66)
10	LoginInterpreter The object that listens for user responses to the login: prompt public void CLIlogin( ) { // (line 55) welcome(); boolean moreWork = true; while( moreWork ) { moreWork = interpret( console.readLine("Juno login: " )); } }
11	interpret (line 69) break command line into tokens (next slide) put first token into String variable visitor (77) use if - else if - else if … logic if “exit” return false! // leave loop in CLIlogin if “register” // create account for new user if “help” // give help else // input is a username look up User object in map, with username as key create a command shell for that User
12	StringTokenizer Juno user might type Juno login: register bill Bill Campbell We need to cut the blue String up into words Like an Iterator StringTokenizer st = new StringTokenizer(“register bill B C”); while (st.hasMoreTokens()) System.out.println(“\|”+st.nextToken()+“\|”); produces \|register\| \|bill\| \|B\| \|C\|
13	StringTokenizer Can change default (whitespace) delimiters StringTokenizer st = new S…T…(“x.b bz”,“.,”); while (st.hasMoreTokens()) System.out.println(st.nextToken()); produces x b b z Can change delimiters dynamically - for no delimiters at all use st.nextToken(“”)
14	“register bill Bill Campbell” LoginInterpreter lines 72, 73: create a StringTokenizer st for this String Line 74 checks for no tokens (blank line) Line 77 gets the first token for the value of the variable visitor - in this case “register” Test on 81is true so line 82 sends this LoginInterpreter a register message, passing it the rest of the StringTokenizer
15	register (line 100) Next token is userName bill Get next token using no delimiters at all - so the rest of the line. Send that String a trim message to strip off white space at beginning and end Create a new Directory with no owner Create a new User Arrange for him to own his home Directory
16	back to interpret In most common case, response to Juno login prompt is a Juno username Then that’s the first (and only) token on the line, hence the value of the variable visitor Then interpret method (88) looks up User object in map, with value of visitor String as key (89) creates a command shell for that User
17	Shell object Constructor sets some fields the Juno system that created this Shell (37) (like issuing Bank in BankAccount) the User and the console (38, 39) the current Directory (the User’s home) (40) Then invokes CLIShell (command line interface) which works just like LoginInterpreter get an input line from the user (50) invoke this Shell’s interpret method done when interpret returns false for moreWork (user has typed “logout”)
18	Shell interpret method (60) Create a StringTokenizer for the input line, after throwing away Juno comments (# …) First token is the commandName (66) If it’s “logout”, then done (return false) Replace if else if … with dispatch table (70,71) look up commandObject in command table (commandName String is key) (76) send commandObject a doIt() message Polymorphism!
19	abstract class ShellCommand Documentation managed here helpString and argstring fields (19, 20) initialized by protected constructor (31, 32) doIt() method (54): abstract public void doIt ( StringTokenizer args, Shell sh ); doIt is passed the rest of the text on the Juno command line, and the Shell it’s acting for Each concrete ShellCommand implements its own doIt() - polymorphism
20	Creating a ShellCommand object MkdirCommand extends ShellCommand (18) Constructor (24) super invokes ShellCommand constructor, telling it help string and argument string for mkdir implement abstract method doIt (37) next token on line is the name of the Directory to be made tell Directory constructor the name, owner, parent Directory constructor adds the new Directory to the parent public void doIt( StringTokenizer args, Shell sh ) { String filename = args.nextToken(); new Directory(filename, sh.getUser(), sh.getDot()); }
21	ShellCommandTable Juno constructor creates a ShellCommandTable (Juno.java line 52) ShellCommandTable.java declare and initialize a TreeMap (line 23) constructor (line 31) invokes fillTable (line 69) fillTable creates one of each concrete ShellCommand objects, invokes install (line 61) to put it in the table client (a Shell) invokes lookup (43), which wraps Map get method (and does the cast)
22	How the dispatch table works In CLIShell loop: get first token on the line: commandName lookup commandObject with commandName key send doIt() message Each particular ShellCommand extends the abstract ShellCommand class, implementing doIt() in its own way Polymorphism at work
23	How LoginInterpreter interpret works get first token on the line use if - else if - else if … logic if “exit” return false! // leave loop in CLIlogin if “register” // create account for new user if “help” // give help else // input is a username
24	Dispatch table vs if-else if-else if To add new commands just add a table entry Command semantics separate from syntax Lots of design overhead, hard to understand Good for large command sets that will grow (Juno shell commands) To add new commands must edit the main loop Command semantics and syntax in same place Quick and dirty, easy to understand and code Good for small command sets that stay put (Juno login loop)