Question

I have the following requirement.

Introduction

The system is report/content management system. It allows user to do CRUD operations on reports.

Business Logic/UI Component

When a user is editing a report, other users can’t edit the report but view only.

It contains a page with a table that monitor the locked report for view.

Challenger

1) How should I implement this “locking” Mechanism ?
2) What are the design pattern and API that will assist me?

My current implementation

I will have a report service class
It will contain a hashmap of all the reports that are locked(with information of user for lock management)

I have completed SCJD and was considering using my lock mechanism, but I realize I do not need the wait “locking”.

The only problem I worried is concurrency issue when “locking” the report(adding the lock into map), I believe that can be easily solve by using synchronization.

For the monitoring of locked report table, I plan to implement observer pattern in the report service class. For each user/backingbean it will “subscribe” to the report service.

Any inputs? ????

Answer 1

Answer is simple… We can manage this problem with 2 classes.

Features of each class is given below

ReportUtil:
(1) track whether any report is open in write mode
(2) create object of report based on access mode available

Report:
(1) open read only or writable report based on access given
(2) While closing, reset the flag in ReportUtil class, if the current report was open in write mode.

Client:
To test ReportUtil and Report classes.

---

import java.util.LinkedList;

public class ReportUtil {

    private static boolean bIsWriteLockAvaialable = true;

    public static synchronized Report getReport() {
        Report reportObj = new Report(bIsWriteLockAvaialable);
        if(true == bIsWriteLockAvaialable) {
            bIsWriteLockAvaialable = false;
        }
        return reportObj;
    }   

    public static void resetLock() {
        bIsWriteLockAvaialable = true;
    }
}

---

public class Report {
    private boolean bICanWrite = false;

    public Report(boolean WriteAccess) {
        bICanWrite = WriteAccess;
    }

    public void open() {
        if(bICanWrite == true) {
            //Open in write mode
            System.out.println("Report open in Write mode");
        }
        else {
            //Open in readonly mode
            System.out.println("Report open in Read only mode");
        }
    }

    public synchronized void close() {
        if(bICanWrite == true) {
            ReportUtil.resetLock();
        }
    }
}

---

public class Client {

    public static void main(String[] args) {
        Report report1 = ReportUtil.getReport();
        report1.open(); //First time open in writable mode

        Report report2 = ReportUtil.getReport();
        report2.open(); //Opens in readonly mode

        Report report3 = ReportUtil.getReport();
        report3.open(); //Opens in readonly mode

        report1.close(); //close the write mode

        Report report4 = ReportUtil.getReport();
        report4.open(); //Opens in writable mode since the first writeable report was closed
    }

}

---

Output:
Report open in Write mode
Report open in Read only mode
Report open in Read only mode
Report open in Write mode

---

I don’t know why we want to use Hash table here. May be I didn’t understand your requirement. Also, I have used synchronized methods to escape from synchronization problems.

If your requirement was to track all the users who are using the report, please let me know.

Happy Learning!!!