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Hi today in interview they asked a multi threading question that create a procedure in which there is a method getDBConnect() so that atmost 3 threads can access it concurrently. if 4th thraed try to access getDBConnect() method then 4th thread will go on wait state if anyone of 3 thread release the method getDBConnect() then 4th thread will access the getDBConnect() method using simple thread or executor thread.
Please help me to understand how can i make program of multi threading so that above criteria should be satisfied.
I'm not going to write code for you. But I can hint what this question is about. There is very interesting primitive of multithreading synchronization called Semaphore. JDK contains detailed description and sample of use https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Semaphore.html
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This question was asked in an interview for the Senior Developer Role.
As I don't have any exposure to Java multi-threading and concurrency. So, I could not answer this.
Every thread we create is part of a ThreadGroup. Then how do we create
a thread without ThreadGroup?
It is not possible to create a thread without ThreadGroup.
For more details look here: https://www.eg.bucknell.edu/~mead/Java-tutorial/essential/threads/group.html
"If you create a new Thread without specifying its group in the
constructor, the runtime system automatically places the new thread in
the same group as the thread that created it (known as the current
thread group and the current thread, respectively). So, if you leave
the thread group unspecified when you create your thread, what group
contains your thread?
When a Java application first starts up, the Java runtime system
creates a ThreadGroup named main. Unless specified otherwise, all new
threads that you create become members of the main thread group."
And here: http://www.java2s.com/example/java-book/thread-group.html
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I want to run simultaneously two different methods in two different classes
Is run method is the only way?
If it is Why??
Your code snippet does not show how you actually start these threads, I am assuming something like new Thread2().start(). What this does is create a new thread and the new thread proceeds (in the background) to execute its run method (and then terminates when that method returns).
Every thread runs code that is contained with a run() method. That is just how threads work. There has to be some convention to let the JVM know where the code for the thread is.
From within that method, you can call any other method on any other object that you have access to.
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I have learnt the concepts of Future ,Fork and Join. Can someone explain differences/similarities between these two. Where to apply which concept.
It's not really comparable.
A Future represents the result of an asynchronous computation and enables you to access the result of that computation when it's done (possibly in the future if the computation takes some time, hence the name).
A Fork/Join pool is a form of ExecutorService (task execution system) that executes its tasks using multiple threads and uses a work stealing algorithm. The result of a fork/join task is a Future.
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How can I know when the thread was stoped and the processor moves to another thread..
Transition between threads harms my calculations, there is some way to know if the thread left and returned to my function?
You can not know when your thread was stopped and the system rescheduled another thread.
But - you can minimize the number of times this event happens by setting the affinity of your thread to one of the processors, and the affinities of the other threads to the other processors.
If you are using Linux, you can use taskset for each thread in the system (get the list by "ps -e") to set the affinities of the other threads to other processors.
This will decrease the load on the processor and will cause it to context-switch less times.
The simple answer is - you can't. Even if you could detect thread context switches it happens far too often to usefully logged.
A better question would be to look at why you need to know. If there is a problem there post it as a question and we can solve the real issue.
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My understanding is that it creates another thread and runs compute() in another thread.
then join fetches the result once recursively got it.
I would like to know whether fork() calls compute() or not. Thanks in advance.
The fork method will not call compute. It will push the forked task to a work queue in which the running thread determines if it should eventually call compute itself or notify other threads to steal that task and invoke compute.