how multiple threads invoke singleton object's method and work on them?

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I have multiple threads running which access singleton object and call its method and pass objects in it. In the method I do some calculations only on recieved object. I have heard that there would not be any problem in this case cause it is stateless and it is free for all.

My question is how it is free for all? I want to know how multiple threads can call the shared method in their own thread without overwriting the passed objects of other threads? Please explain in terms of memory allocation wise and at stack level.

class Singleton class{

    //no shared class or object variables and only one copy of this Singleton object exists.

    Object someMethod(Object o){
        //do some calculation or logic on the object o and return some string result
    }

}

Thread Safety in Java Singleton Classes, If you are in Singleton::Instance() and receive an interrupt, invoke Singleton:: Instance() from another thread, you can see how you'd get into� Right, the singleton behaves just like any other object once it is created. It has no special cases for multi-threaded applications, no protections against concurrent activities, and no inherent synchronization. You must write all those protections yourself.

Every thread has its own copy of the Object o and so even if multiple threads call the method at the same time, each method call of Thread will have its own stack allocated and the logic will apply there.

Why is it Thread Safe?

Thread Stacks are private to them and are by default thread safe as no other thread can enter in others stack.

NOTE: You can say that this Singleton of yours is Thread safe but not that the application is Thread safe as it also depends on whether the Object o being passed is itself thread safe or not. But as far as safety of Singleton is concerned it is thread safe.

Usage of Singleton pattern in multithreaded applications, To make a singleton class thread-safe, getInstance() method is made synchronized so that multiple threads can't access it simultaneously. filter_none. edit close. how multiple threads invoke limited singleton object's and work on them.In this sample I am showing n number of threads having access over 5 singleton objects. After every 5 threads it will reuse the existing sigleton objects. – Niraj Mar 3 '19 at 19:12

You can think about it in "pizza terms". You went to pizza cafe, and couple of other guys entered with you as well. Each one placed his order and then sat to his own table. Pizza boxes to be arrived then and each of you start eating his own meal.

Here, the cafe is a singleton, pizza boys are CPU cores, you and other customers are threads, orders and pizza are input and output data respectively, table is a piece of memory.

As you can see, each thread is served with his own piece of memory so CPU can distinguish your data and don't mix up. As you asked about stack, it's not a significant player here because each thread has it's own stack (as a part of your memory chunk).

Java Singleton Design Pattern Practices with Examples , In that case, multiple threads will be able to access it at the same instance of time. How do I make a Singleton Pattern Thread-Safe and Fast? What can I do to make sure all instructions in a thread are run by the CPU in a multithreaded� Multiple instances of the Singleton class can be created when the getter method is tasked with initializing the singleton when necessary, and the getter method is invoked by two or more threads simultaneously.

    import java.util.ArrayList;
    import java.util.Collections;
    import java.util.List;

    public class MainClass {
        public final static int MAX_THREAD = 20;
        public MainClass() {
            // TODO Auto-generated constructor stub
        }

        public static void main(String[] args) {

            List<Singleton> singletonList = new MainClass().createSingletonObjects();
            singletonList = Collections.synchronizedList(singletonList); 

            int index = 0;
            for (int i = 0; i < MAX_THREAD; i++) {

                Thread thread1 = new Thread(new MyThread(singletonList,index), "Thread"+i);
                thread1.start();
                index++;
            if (index == singletonList.size()) {
            index = 0;  
            }

            }

        }

    public synchronized List<Singleton> createSingletonObjects(){
        List<Singleton> listSingleton = new ArrayList<Singleton>();
            listSingleton.add(MySingleton1.getInstance());
            listSingleton.add(MySingleton2.getInstance());
            listSingleton.add(MySingleton3.getInstance());
            listSingleton.add(MySingleton4.getInstance());
            listSingleton.add(MySingleton5.getInstance());

            return listSingleton;

        }

    }


public class MySingleton1 extends Singleton{
    private static Singleton mySingleton;
    private MySingleton1() {
        super();
        // TODO Auto-generated constructor stub
    }

    public static Singleton getInstance() {
        if (mySingleton == null) {
            mySingleton = new MySingleton1();
            return mySingleton;
        }
        return mySingleton;
    }


public class MySingleton2 extends Singleton{
    private static Singleton mySingleton;
    private MySingleton2() {
        super();
        // TODO Auto-generated constructor stub
    }

    public static Singleton getInstance() {
        if (mySingleton == null) {
            mySingleton = new MySingleton2();
            return mySingleton;
        }
        return mySingleton;
    }







}


public class MySingleton3 extends Singleton{
    private static Singleton mySingleton;
    private MySingleton3() {
        super();
        // TODO Auto-generated constructor stub
    }

    public static Singleton getInstance() {
        if (mySingleton == null) {
            mySingleton = new MySingleton3();
            return mySingleton;
        }
        return mySingleton;
    }







}


public class MySingleton4 extends Singleton{
    private static Singleton mySingleton;
    private MySingleton4() {
        super();
        // TODO Auto-generated constructor stub
    }

    public static Singleton getInstance() {
        if (mySingleton == null) {
            mySingleton = new MySingleton4();
            return mySingleton;
        }
        return mySingleton;
    }







}


public class MySingleton5 extends Singleton{
    private static Singleton mySingleton;
    private MySingleton5() {
        super();
        // TODO Auto-generated constructor stub
    }

    public static Singleton getInstance() {
        if (mySingleton == null) {
            mySingleton = new MySingleton5();
            return mySingleton;
        }
        return mySingleton;
    }







}

How does singleton works in multi threading environment?, How to use Multi threads in Singleton: The lazy initialization i.e. the on-demand object creation of the singleton class works fine when we invoke the GetInstance � Using a single thread, PowerShell needs to run the code and it’s done. With multiple threads, the original thread used to run your console will be used to manage the other threads. At a certain point that original thread will be maxed out just keeping all of the other threads in line.

Thread-safe Singleton Design Pattern in C#, Employing lock mechanisms to prevent an initialization routine from being run simultaneously by multiple threads; Ensuring the class is not serializable; Ensuring� Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.

MSC07-J. Prevent multiple instantiations of singleton objects, The intent of the Singleton pattern is to ensure that a class has only one instance, you must prevent multiple threads from initializing the singleton. will run to completion before a method in another thread starts running. Thread-safe implementation for multi-threading use. A robust singleton implementation should work in any conditions. This is why we need to ensure it works when multiple threads uses it. As seen in the previous examples singletons can be used specifically in multi-threaded application to make sure the reads/writes are synchronized.

Singletons and Threads | Design Patterns in C#: Singleton, Singleton object is not instantiated until and unless GetInstance is may end up creating Duration: 8:07 Posted: Jun 5, 2017 - [Instructor] Problems can occur…when using the singleton pattern…in a multi-threaded program.…Java supports concurrency which means multiple units…of execution can run at the same time.…If a program has multiple threads…that are using the PrintSpooler class…it is possible for two different objects to be created.…In this example, the ResourceManager class…has two threads

Comments
  • So glad there was a question like this. I was going crazy
  • I haven't downcasted it.
  • can you please tell me "Methods to be called: They can be called from more than one thread. Each execution is independent."...The singelton object is shared ,so isn't its method also shared?Please clear my confusion..
  • If two objects o1 and o2 share the same member m it might no longer be thread-safe. So take care in this case.
  • @Xtreme Biker Perfectly explained..Thanks a lot
  • @XtremeBiker I have a doubt, could you please explain how spring handles this functionality? " every single thread will execute Singleton object's portion of code, wrapped in its own execution." How spring shares its object instance with different threads?
  • can you tell me how pizza store distinguish multiple orders?Well singelton class has only one method which is called by other threads and provided some data.How is this data not mixed?
  • Each pizza boy (one CPU core) has a list of his orders and iterates through it sequentially. You (as thread) has nothing to do by yourself with your order. All you need to do is been placed on that list, then a CPU core that is scheduled to serve you will do the rest: take your input data, perform all necessary calculations, deliver baked pizza then and arrange a payment finally.
  • how multiple threads invoke limited singleton object's and work on them.In this sample I am showing n number of threads having access over 5 singleton objects. After every 5 threads it will reuse the existing sigleton objects.