Wednesday, 28 June 2017

How to access websites on your Mac that requires Internet Explorer

Background

There are certain websites that can be accessed from Internet Explorer only. This happens because of the websites compatibility with IE. But this will not work on your Mac laptop or Linux machine since you cannot run IE on it. At least not in traditional way - You can always install a software like Wine and then run your windows application in that simulated environment. But there is a much simpler way.

How to access websites on your Mac that requires Internet Explorer

I will take Safari browser in our Mac as an example. 
  • Open Safari browser and open preferences from menu bar at the top.
  • Once opened go to "Advanced Tab"
  • In "Advanced Tab" select the "Show Develop menu in menu bar" check box.

  • Once done you should be able to see "Develop" menu in menubar on top. 
  • Under "Develop" menu you can select "User Agent" and then select the user agent you want. For eg - "Internet Explorer 7"

  • Once you select that your IE compatible page should load fine.

 On other operating systems and browser  - Linux/Chrome/Firefox

Above approach was specific to Safari but the solution remains same - You need to change the user agent. So you have plugins to do so -


Similar you can find a similar plugin in chrome store. 

Manual way

    FIREFOX 4.0 :  In Firefox type in the URL Address: about:config. A webpage will appear saying a warning about the use of the Config. Click on the button about you being careful. In the search bar in the Config type agent and look for the variable general.useragent.override. Double click on it and overwrite the value it has with one of the following (For the default leave the value EMPTY):

    IE6 - Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)
    IE7 - Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0)
    IE8 - Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1)


    CHROME :  Chrome has an about page to CHECK if you have changed your User Agent about: and other options like about:labs, about:memory, about:hang, about:plugins and many others that depending on your version they could be available or not. But for the question at hand this option is not yet in any of the about pages i have found. To have it manually in chrome you need to start chrome with the option user-agent. For example google-chrome --user-agent="Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)" which will open Chrome like it were IE6. The IE User Agents are from the Firefox option above.

    OPERA : In Opera type in the URL Address: about:config. A list will appear and a search address in the upper part of the list. Type in the search address user agent. The option for User Agen will appear below the search address. Click on it and depending on the Browser you want you have several options that change depending on version. For example:

    1 - Opera (this is the default user agent string used by Opera)
    2 - Mozilla (With the Opera String in it)
    3 - Internet Explorer (With the Opera String in it)
    4 - Mozilla (Without the Opera String in it. 100% Mozilla)
    5 - Internet Explorer (Without the Opera String in it. 100% IE)


    But this values could change so you would need to test each one to know what User Agent value it has.

Related Links

Saturday, 17 June 2017

Make a bootable flash drive from an ISO image on Ubuntu

Background

There are many times you download an ISO file from internet and want to install it on your machine. For eg. Windows or Ubuntu image. What you really wish is flash it iso image into a bootable drive like USB drive or a CD drive and install it from there on your machine.

 Make a bootable flash drive from an ISO image on Ubuntu

You have following options -
  1. Etcher- is a free and open-source image burner with support for Windows, OS X and GNU/Linux.
  2. Easy2Boot- Flexible and configurable USB drive multiboot solution which also supports UEFI booting.
  3. LiveUSB Install- is a free software for GNU/Linux and Windows. With LiveUSB Install you can effortlessly install various Linux distros.
  4. Multisystem- is an awesome tool created by LiveUSB.info that works similar to our Windows based MultiBootISOs USB creator.
  5. WinUSB - is a simple tool that enable you to create your own usb stick windows installer from an iso image or a real DVD.
  6. Unetbootin - UNetbootin allows you to create bootable Live USB drives for Ubuntu and other Linux distributions without burning a CD.
 Let's see some of the options -

Unetbootin

To install this run following command in your terminal -
  • sudo apt-get install unetbootin


Once installed you can  open the app. Now you can either select one of the given linux distros (those will get downloaded automatically) or  provide it an iso file from local machine.



WinUsb

To install winusb run following commands on your linux terminal -
  • sudo add-apt-repository ppa:colingille/freshlight
  • sudo apt-get update
  • sudo apt-get install winusb
Once installed you can open up the app select iso file and target usb and start creating bootable usb drive for windows.



NOTE : If you are looking for rufus then it is for Windows only. You cannot use it on your Linux machine.

Counting Semaphore, CountDownLatch, CyclicBarrier - synchronization methods for concurrency

Background

With Java 5 a lot of concurrency mechanisms were introduced for synchronization.



In one of the previous posts we saw what Reentrant locks are and how they help us achieve concurrency. -
We also saw
Along with ReentrantLock and ExecutorService there were other concurrency elements that were introduced in Java 5 like -
  • Counting Semaphore
  • CountDownLatch
  • CyclicBarrier
Today we will try to understand these. Not only their understanding helps us with multi threading they are also popular topic of Java interview question. Lets see them one by one.

Counting Semaphore

Semaphore maintains a number of permits for a resource and only that many number of threads can access the resource. If the maximum permits allowed is reached then threads will have to wait till some other thread owing a permit releases it. 

As an example lets consider a simple semaphore with 1 permit. It's called binary semaphore. It's similar to wait and notify on same object. 

    public static void main(String args[])
    {
        Semaphore binarySemaphore = new Semaphore(1);
        new Thread(() -> {
            try {
                binarySemaphore.acquire();
                System.out.println("Semaphore permit acquired by : " + Thread.currentThread().getName());
                
            } catch (Exception e) {
                e.printStackTrace();
            }
            finally {
                System.out.println("Semaphore permit getting released by : " + Thread.currentThread().getName());
                binarySemaphore.release();            }
            
        }).start();
        new Thread(() -> {
            try {
                binarySemaphore.acquire();
                System.out.println("Semaphore permit acquired by : " + Thread.currentThread().getName());
                
            } catch (Exception e) {
                e.printStackTrace();
            }
            finally {
                System.out.println("Semaphore permit getting released by : " + Thread.currentThread().getName());
                binarySemaphore.release();
            }
            
        }).start();
    }
and the output would be -
Semaphore permit acquired by : Thread-0
Semaphore permit getting released by : Thread-0
Semaphore permit acquired by : Thread-1
Semaphore permit getting released by : Thread-1


 As you can see from code above you acquire a permit using acquire() method and release a permit using release() method.


NOTES :
  1. You can also acquire permit using acquireUninterruptibly(). This is a blocking call and the thread cannot be interrupted.
  2. Now acquire() is also a blocking call however it can be interrupted unlike acquireUninterruptibly() call
  3. You can also use tryAcquire() call which will try to acquire the permit and if available will return immediately with true. If it is not available it will also return immediately with false. So this is a non blocking call.

CountDownLatch

This is another synchronization mechanism in which a resource is not allowed access till predefined number of threads don't complete their operations. So lets say there are 10 threads making a bread slice. As soon as we are ready with 5 slices we can lets say pack it together for selling. In this case we can use a CountDownLatch. Initialize one with 5 and as soon as 5 threads acknowledge they have finished making slices we can start packing (probably a new thread).

So a thread will wait for n other threads. Let's see an example -

    public static void main(String args[])
    {
        CountDownLatch countDownLatch = new CountDownLatch(2);
        new Thread(() -> {
            try {
                Thread.sleep(2000);
                System.out.println("Calling countdown by : " + Thread.currentThread().getName());
                countDownLatch.countDown();
            } catch (Exception e) {
                e.printStackTrace();
            }
           
        }).start();
        new Thread(() -> {
            try {
                Thread.sleep(2000);
                System.out.println("Calling countdown by : " + Thread.currentThread().getName());
                countDownLatch.countDown();
            } catch (Exception e) {
                e.printStackTrace();
            }
           
        }).start();
       
        try {
            System.out.println("Waiting for all other threads finish operation");
            countDownLatch.await();
            System.out.println("All other threads finish operation!");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }


Output :
Waiting for all other threads finish operation
Calling countdown by : Thread-1
Calling countdown by : Thread-0
All other threads finish operation!
As you can see main thread calls await() on the countdownlatch and wait for 2 threads to call countDown() on it. Here n is 2 but you can configure it in the constructor.

You need to use this when your use case is to wait for some other initial operations to finish before starting some other operation.

NOTE :
  1.  CountDownLatch is not reusable. So once the count reaches 0 i.e n threads have called countdown() the latch is unusable.

CyclicBarrier

CyclicBarrier is yet another synchronization mechanism. In this all n threads will wait for each other to reach the barrier. Such waiting threads are called parties. Number of parties are set in the CyclicBarrier during its creation. All parties reach the barrier and call await() which is a blocking call. Once all parties reach the barrier i.e all call await() then all threads get unblocked and proceed for next execution.

Simple usecase that you can think of is a multiple game scenario is which a game would not start untill all the players have joined. Here all the players are parties where as game start is a barrier.

Eg -

    public static void main(String args[])
    {
        CyclicBarrier cyclicBarrier = new CyclicBarrier(3);
        new Thread(() -> {
            try {
                Thread.sleep(2000);
                System.out.println("Player joining : " + Thread.currentThread().getName());
                cyclicBarrier.await();
                System.out.println("Game starting from : " + Thread.currentThread().getName());
            } catch (Exception e) {
                e.printStackTrace();
            }
           
        }).start();
        new Thread(() -> {
            try {
                Thread.sleep(2000);
                System.out.println("Player joining : " + Thread.currentThread().getName());
                cyclicBarrier.await();
                System.out.println("Game starting from : " + Thread.currentThread().getName());
            } catch (Exception e) {
                e.printStackTrace();
            }
           
        }).start();
        new Thread(() -> {
            try {
                Thread.sleep(2000);
                System.out.println("Player joining : " + Thread.currentThread().getName());
                cyclicBarrier.await();
                System.out.println("Game starting from : " + Thread.currentThread().getName());
            } catch (Exception e) {
                e.printStackTrace();
            }
           
        }).start();
       
    }

and the output is -
Player joining : Thread-0
Player joining : Thread-1
Player joining : Thread-2
Game starting from : Thread-2
Game starting from : Thread-0
Game starting from : Thread-1
As you can see all threads (3 in above case) will wait for each other to reach the barrier. Once they all reach and call await() they can all proceed to their further tasks.
NOTE :
  1. cyclicBarrier.reset() will put barrier on its initial state, other thread which is waiting or not yet reached barrier will terminate with java.util.concurrent.BrokenBarrierException. So CyclicBarrier can be reused unlike CountDownLatch.

Summary

Semaphore : Manages a fixed sized pool of resources.
CountDownLatch : One or more threads wait for a set of threads to finish operations.
CyclicBarrier : Set of threads wait for each other until they  reach a specific point.

Related Links

t> UA-39527780-1 back to top