No default constructor for entity for inner class in Hibernate

Question

I have two classes. One is the entity class, the other serves as a composite key class.

The code is as followed.

@Entity public class Supply {      @Embeddable     class Id implements Serializable {          @Column(name = "supplier_id")         private long supplierId;         @Column(name = "merchandise_id")         private long merchandiseId;          public Id() {         }          public Id(long sId, long mId) {             this.supplierId = sId;             this.merchandiseId = mId;         }     }      @EmbeddedId     private Id id = new Id(); } 

If I use try to find

from Supply where merchandise_id=%d and supplier_id=%d 

Hibernate will throw an exception,namely:

No default constructor for entity: com.entity.Supply$Id; nested exception is org.hibernate.InstantiationException: No default constructor for entity: com.entity.Supply$Id 

However, I found that if I change class Id to static. Everything will be fine.

I am just curious about how all these stuff can happen.

Answer 1

If the class is not static, it requires an instance of the outer class in order to be instantiated - so there will be no default constructor. You'd have to use syntax similar to:

new Supply().new Id(); 

If the Id class is static, you can just call:

new Id();