Create n instances of bean in spring

Question

I got a question in one of interview to create only n (Fixed number) instance of any bean. If more than n instances are tried to create at runtime then throw any exception or print a message. Thank in advance

Answer 1

Its easy to control over object creation of beans by providing custom bean factory. Where you can configure restriction like fixed number of object creation.

Example Code.

Answer 2

You just need to manage how many instances are created, you can do this in same bean using a list and a not standard constructor, but I will use a factory pattern:

Gived this bean

class BeanTest {
    String name;

    protected BeanTest(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return this.name;
    }

}

You create a Factory like this (in same package!)

class BeanFactory {
    private static final int LIMIT = 5;
    private static List<BeanTest> list = new ArrayList<BeanTest>();

    public static synchronized BeanTest getInstance(String name) {
        if (list.size() < LIMIT) {
            BeanTest beanTest = new BeanTest(name);
            list.add(beanTest);
            return beanTest;
        }
        System.out.println("Not giving instance");
        return null;
    }

}

Test:

public static void main(String[] args) {
    BeanTest a1 = BeanFactory.getInstance("a1");
    System.out.println(a1);
    BeanTest a2 = BeanFactory.getInstance("a2");
    System.out.println(a2);
    BeanTest a3 = BeanFactory.getInstance("a3");
    System.out.println(a3);
    BeanTest a4 = BeanFactory.getInstance("a4");
    System.out.println(a4);
    BeanTest a5 = BeanFactory.getInstance("a5");
    System.out.println(a5);
    BeanTest a6 = BeanFactory.getInstance("a6");
    System.out.println(a6);
}

OUTPUT:

a1
a2
a3
a4
a5
Not giving instance
null

NOTES:

• If you need more security just add getInstance method of the bean constructor into the BeanTest class itself and make constructor private instead of protected

• You can also make a method destroyInstance or removeInstance in order to make more dynamic the Factory :

   public static synchronized boolean removeInstance(BeanTest toRemove) { if (list.contains(toRemove)) { return list.remove(toRemove); } return false; }