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Question:

1. What is the reason for setting MappedBy in bidirectional many-to-many relationships?

2. When one table has significant amount of records, while other has a few, which side is better to put mappedBy

Answer:

It's actually a good question, and it helps to understand the concept of an "owning" entity. If you want to prevent both sides (in a bidirectional relationship) from having join tables, a good idea, then you need to have a mappedBy= element on one side.

Whether or not there is a join table is controlled by the mappedBy="name" element of the @ManyToMany annotation. The Javadoc for mappedBy for the ManyToMany annotation says:

      The field that owns the relationship. Required unless the relationship is unidirectional.

For your (bidirectional) example, if there were only two @ManyToMany annotations and no mappedBy= element, the default will have two Entity tables and two Join Tables:

Hibernate: create table SideA (id bigint not null, primary key (id))
Hibernate: create table SideA_SideB (sidea_id bigint not null, sidebs_id bigint not null, primary key (sidea_id, sidebs_id))
Hibernate: create table SideB (id bigint not null, primary key (id))
Hibernate: create table SideB_SideA (sideb_id bigint not null, sideas_id bigint not null, primary key (sideb_id, sideas_id))

While this is saying that each Entity "owns" its ManyToMany relationship, the extra join table is redundant in the typical use case, and the Javadoc says you need a mappedBy annotation. If I decide to have SideA "own" the relationship, then I add the mappedBy= element to the SideB entity to specify that it doesn't own the relationship:

@Entity
public class SideA {
    @ManyToMany
    Set<SideB> sidebs;
}
@Entity
public class SideB {
    @ManyToMany(mappedBy="sidebs")
    Set<SideA> sideas;
}

Since the SideB entity no longer owns its ManyToMany relationship, the extra JoinTable will not be created:

Hibernate: create table SideA (id bigint not null, primary key (id))
Hibernate: create table SideB (id bigint not null, primary key (id))
Hibernate: create table SideA_SideB (sideas_id bigint not null, sidebs_id bigint not null, primary key (sideas_id, sidebs_id))

This is important to the developer because he or she must understand that no relationship is persisted unless it's added to the owning entity, in this case the SideA entity. However, since this a bidirectional relationship, the developer should be adding both a SideA to SideB.sideas and a SideB to SideA.sidebs anyway.

So, if you have a bidirectional ManyToMany relationship, which means you have ManyToMany on both entities involved, then you should add a mappedBy="name" on one of them as per the Javadoc and to avoid having a redundant join table. Since it's bidirectional, I don't think it matters which side you make the owning entity - it depends more your application preferences. As always, it's always a good idea to enable the sql logs and see what's going on in the database.