You will get the ConcurrentModificationException exception (stacktrace http://pastebin.com/21qJpLHe) if you try create mapping for the entity with list of embeddable with map of enum-string

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@Entity
@Table(name = "ITEM")
public class Item {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "ID")
protected long id;

@ElementCollection(fetch = FetchType.EAGER)
@CollectionTable(name = "IMAGE", joinColumns = @JoinColumn(name = "ITEM_ID"))
private List<Image> images;
Item() {}

}
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@Embeddable
public class Image {
@ElementCollection(fetch = FetchType.EAGER)
@CollectionTable(name = "IMAGE_METADATA", joinColumns = @JoinColumn(name = "IMAGE_ID"))
@MapKeyColumn(name = "IMAGE_PROPERTY")
@MapKeyEnumerated(EnumType.STRING)
@Column(name = "PROPERTY_VALUE")
private Map<ImageProperty, String> metadata;

Image() {}
}
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public enum ImageProperty

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You may get the bug example on the next project (https://bitbucket.org/senleft/hibernate4tutorial) on the model.dao.mappings.collections.nestedmapofembedables.bug.MappingTest.java