Creating objects using Reflection

[Senthil Kumar Sekar]

Why is that a constructor is made mandatory for POJO-->For instance hibernate asks the developer to have a constructor in the POJOs.

Can anybody what are the other ways a f/w or any container does to create instances other than below

       Class clazz = SampleClass.class;
       Object obj = clazz.newInstance();

and my POJO is

  class SampleClass{
	private int empID;
	private String empName;
	
	public int getEmpID() {
		return empID;
	}
	public void setEmpID(int empID) {
		this.empID = empID;
	}
	public String getEmpName() {
		return empName;
	}
	public void setEmpName(String empName) {
		this.empName = empName;
	}
}

In this case i do not hav a default constructor and iam able to create instances as shown above without any issues.

Then why we should have a contructor as a mandatory?

Thanks.

Regards
Senthil Kumar Sekar

[Paul Clapham]

No XML relevance. Moving.

[John Jai]

It seems that Hibernate uses Reflection Constructor methodology to create an instance. Try the below code and check for the error with your POJO.

public static void main(String...args){		
		try {
			Class<SampleClass> clazz = SampleClass.class;
			SampleClass obj = clazz.newInstance();
			obj.setEmpID(381422);
			System.out.println(obj.getEmpID());
			
			Constructor<SampleClass> constructor = SampleClass.class.getConstructor();
			SampleClass obj2 = constructor.newInstance();
			obj2.setEmpID(112342);
			System.out.println(obj2.getEmpID());			
		} catch(Exception e){
			e.printStackTrace();
		}
	}

The code fails when the getConstructor() method is called with the Pojo class

[Rob Spoor]

The reason being that the automatically generated constructor has the same visibility as the class itself, in this case default visibility (no access modifier). Make the class public and that code will work.

[John Jai]

Yes Rob. it worked perfect. there should be some other reason for Hibernate forcing to have default constructors in POJO then.