Why do we need a default no argument constructor in Java?

Question

Why do we need a default no argument constructor in many Java related APIs? Like as a general rule all java bean classes or entity classes (JPA etc) or JAX-WS implementation classes require a explicit no argument constructor.

If by default Java provides a no argument constructor then why most of these standards require a explicit constructor?

Answer 1

Java only provides a default no-argument constructor if no other constructors are defined. Thus, if you have other constructors you must explicitly define a no-arg constructor yourself.

These frameworks use the reflection API and look at method names to determine how to set properties. The arguments of a constructor can only be found by type, not by name, so there is no way for the framework to reliably match properties to constructor args. Therefore, they require a no-arg constructor to create the object, then can use the setter methods to initialise the data.

Some frameworks may support @ConstructorProperties as an alternative.

Answer 2

I believe frameworks that require public nullary constructors do so because they use reflection to instantiate types, e.g. through Class.newInstance().

As to why the default constructor may not work for this case, here's the relevant JLS section:

JLS 8.8.9 Default Constructor

If a class contains no constructor declarations, then a default constructor that takes no parameters is automatically provided:

• if the class is declared public, then the default constructor is implicitly given the access modifier public;
• if the class is declared protected, then the default constructor is implicitly given the access modifier protected;
• if the class is declared private, then the default constructor is implicitly given the access modifier private;
• otherwise, the default constructor has the default access implied by no access modifier.

So in a public class, the default constructor would have the right visibility, but otherwise an explicitly public one must be provided.