public class TestEntity {
    @Id
    private int id;
    @ElementCollection
    private Map<String, Emb> embs1;
}

@Embeddable
public class Emb {
   private String value;
   public Emb(value) {
        this.value = value;
   }
}

TestEntity e = new TestEntity();
e.id = 1;
e.embs1 = new HashMap<>();
e.embs1.put("a", new Emb("value1"));
e.embs1.put("b", new Emb("value2"));
em.persist(e);

TestEntity e = em.find(TestEntity.class, 1);
e.put("a", "value3")

[
    {
        id: 1
        embs1: {
            a: "value3"
        }
    },
    {
        id: 1
        embs1: {
            a: "value1",
            b: "value2"
        }
    }
]