Studying software design patterns is an effective way to learn from the experience of others

Transcription

1 Studying software design patterns is an effective way to learn from the experience of others Design Pattern allows the requester of a particular action to be decoupled from the object that performs the action. 1

2 Consider an application that gets commands from the Standard Input device, and then executes those commands in an application specific way. The requester gets a command from an actor and sends the command to the executor, which executes the command according to the application requirements. We want to separate the requester from the executor. We want to design the requester for reuse in other applications. We want to minimize the amount of code changes for adding, removing, or changing commands. 2

3 How to Dispatch and Execute s Efficiently The command design pattern solves these problems. It provides a way for executing commands in a nonapplication specific way. It manages new or changing commands very efficiently. But, before we can look at the pattern design: We need to understand the core idea behind how to process a received command to determine which command to execute in a non-application specific way. 3

4 A command invoker sends the string name of the command to execute to the command executor. Typically, a user enters the command string through a user interface and is received by the command invoker. The command executor converts the string name of the command to a function pointer. The function pointer will point to a function of the same name as the command string. The command executor runs the command by running the function pointed to by the function pointer. There would be a function for every command string sent by the command invoker. Adding, deleting, or changing commands to execute is handled efficiently. For example, to add a command called dothis, just create a function with the name dothis(), and put the code to implement the command in the function. Therefore, the switch (or if then else) constructs are not needed! The dispatcher can invoke a command in a non-application specific way, because the dispatcher does not know the name of the commands or the names of the functions, since the command names are input as strings by the user and command invoker. 4

5 A command is received from the command invoker, in String format: commandname <arg1> <arg2> We append a security string to the command For example: for the #login command, and using as the security string, the secured command name is: #login We sometimes calls this type of security, security by obscurity. We predefine a class called #login,and ensure it has been compiled. Load the class into memory, and initialize it Class cls = Class.forName(#login); Instantiate the class commandexecutor = () cls.newinstance() Run the execute method on the object commandexecutor.; 5

6 <<commandinvoker>> Keyboard setexecutor() run() <<enter commands>> User Sub-classes (s) can be added at runtime, without stopping the application. User 1 2 N 6

7 <<commandinvoker>> Keyboard setexecutor() run() User 1 N 7

8 Java Code and UML Diagram package commandinterface; public interface { public abstract void ; Package name commandinterface UML Package diagram UML Interface diagram 8

9 PatternTest CLASS package commandpatterntest; public class PatternTest { public static void main(string[] args) { commandinterface. theinterface; standardin.keyboard thekeyboard = new standardin.keyboard(); theinterface = new usercommand.user(); thekeyboard.setexecutor(theinterface); Thread thread = new Thread(theKeyboard); thread.start(); <<commandinvoker>> Keyboard setexecutor() run() <<enter commands>> User User 9

10 package standardin; import java.io.bufferedreader; import java.io.ioexception; import java.io.inputstreamreader; public class Keyboard implements Runnable { commandinterface. ; public void setexecutor(commandinterface. ){ this. = ; public void run() { try { final InputStreamReader isr = new InputStreamReader(System.in); final BufferedReader br = new BufferedReader(isr); while (true) { String the = br.readline();.execute(the); catch (IOException e) { e.printstacktrace(); User <<enter commands>> <<commandinvoker>> Keyboard setexecutor() run() 10

11 User CLASS package usercommand; import java.util.logging.level; import java.util.logging.logger; public class User implements commandinterface. { User ; public void { String secure = "usercommand." + the; try { Class cls = Class.forName(secure); = (User) cls.newinstance();.execute(secure); <<commandinvoker>> Keyboard setexecutor() run() <<enter commands>> catch (ClassNotFoundException InstantiationException IllegalAccessException ex) { System.out.print("Sorry, your command not found.\n"); User User N 11

12 N Subclass public class N extends User { public void execute (String the){ System.out.println("Executed N/n"); User 1 N 12

13 UML Model of Pattern Using Java Interface <<commandinvoker>> LineUI commandinterface setexecutor() run() User commandexecutor Sub-classes (s) can be added at runtime, without stopping the application. 1 2 N 13