UniqueConstraint and Column unique true in hibernate annotation

Question

What is difference between  UniqueConstraint and  Column unique   true    For example    Table     name    product serial group mask       uniqueConstraints     UniqueConstraint columnNames     mask    group         And   Column unique   true   ManyToOne optional   false  fetch   FetchType EAGER  private ProductSerialMask mask    Column unique   true   ManyToOne optional   false  fetch   FetchType EAGER  private Group group

User · Answer

In addition to Boaz s answer        UniqueConstraint allows you to name the constraint  while  Column unique   true  generates a random name  e g  UK 3u5h7y36qqa13y3mauc5xxayq    Sometimes it can be helpful to know what table a constraint is associated with  E g     Table     name    product serial group mask       uniqueConstraints            UniqueConstraint            columnNames     mask    group              name  uk product serial group mask

User · Answer

From the Java EE documentation    public abstract boolean unique        Optional  Whether the property is a unique key  This is a shortcut for the    UniqueConstraint annotation at the table level and is useful for when the unique key    constraint is only a single field  This constraint applies in addition to any constraint    entailed by primary key mapping and to constraints specified at the table level    See doc

User · Answer

As said before   Column unique   true  is a shortcut to UniqueConstraint when it is only a single field   From the example you gave  there is a huge difference between both    Column unique   true   ManyToOne optional   false  fetch   FetchType EAGER  private ProductSerialMask mask    Column unique   true   ManyToOne optional   false  fetch   FetchType EAGER  private Group group    This code implies that both mask and group have to be unique  but separately  That means that if  for example  you have a record with a mask id   1 and tries to insert another record with mask id   1  you ll get an error  because that column should have unique values  The same aplies for group   On the other hand    Table     name    product serial group mask       uniqueConstraints     UniqueConstraint columnNames     mask    group         Implies that the values of mask   group combined should be unique  That means you can have  for example  a record with mask id   1 and group id   1  and if you try to insert another record with mask id   1 and group id   2  it ll be inserted successfully  whereas in the first case it wouldn t   If you d like to have both mask and group to be unique separately and to that at class level  you d have to write the code as following    Table          name    product serial group mask           uniqueConstraints                      UniqueConstraint columnNames    mask                     UniqueConstraint columnNames    group                 This has the same effect as the first code block

User · Answer

In addition to  Boaz s and  vegemite4me s answers      By implementing ImplicitNamingStrategy you may create rules for automatically naming the constraints  Note you add your naming strategy to the metadataBuilder during Hibernate s initialization   metadataBuilder applyImplicitNamingStrategy new MyImplicitNamingStrategy       It works for  UniqueConstraint  but not for  Column unique   true   which always generates a random name  e g  UK 3u5h7y36qqa13y3mauc5xxayq    There is a bug report to solve this issue  so if you can  please vote there to have this implemented  Here  https   hibernate atlassian net browse HHH-11586  Thanks

[java] @UniqueConstraint and @Column(unique = true) in hibernate annotation

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