When using the @Lob annotation on a java.util.Map I find that both the value and key of the Map are mapped to a BLOB. This is in violation of the JPA 2.0 specification which states:

The Lob annotation may be used in conjunction with the Basic annotation or with the ElementCollection[100] annotation when the element collection value is of basic type.
...
[100] If the element collection is a Map, this applies to the map value.

To reproduce:

@Entity
public class Entry

Unknown macro: { @Id @GeneratedValue private long identifier; @ElementCollection @Column(nullable = false) private Map<String, String> titles; @ElementCollection @Column(nullable = false) @Lob private Map<String, String> contents; // Getters and setters, other fields and methods }

This produces the following error during schema creation:

Unsuccessful: create table myblog.Entry_contents (Entry_identifier bigint not null, contents longtext not null, contents_KEY longtext, primary key (Entry_identifier, contents_KEY)) type=InnoDB
BLOB/TEXT column 'contents_KEY' used in key specification without a key length

Workarounds include defining column type with @MapKeyColumn(columnDefinition = "...") or using @Embeddable as a wrapper for values. (thanks to axtavt and Sean Patrick Floyd)