@Entity
@EntityListeners(FooListener.class)
public class Foo {

    @Id
    private long id;

    @ElementCollection
    private Map<String, String> map = new HashMap<>();

    // getters and setters
}

public class FooListener {

    @PrePersist
    public void preUpdate(Foo foo) {
        System.out.println(foo);
        System.out.println(foo.getMap().size());
    }

}

Foo foo = new Foo();
foo.getMap().put("foo", "test");
System.out.println(foo);
System.out.println(foo.getMap().size());
System.out.println("before merge");
Foo merge = entityManager.merge(foo);
System.out.println("after merge");
System.out.println(merge);
System.out.println(merge.getMap().size());

Foo@3af17be2
1
before merge
Foo@285d851a
0
after merge
Foo@285d851a
1