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copied!<p>This is not well documented, many of the solutions on here and other sites relate to older versions of hibernate where the HiLo sequenceGenerator was the default. But after investigation I found the underlying cause is that JBoss EAP 6 sets</p> <pre><code>hibernate.id.new_generator_mappings=true </code></pre> <p>by default, which uses an org.org.hibernate.id.enhanced.SequenceStyleGenerator instead of the older version.</p> <p>The Hibernate SequenceStyleGenerator default increment is 1 (check the code!), however JPA overrides the increment value in this generator to 50. This means that the Generator looks at the sequence nextval and keeps a cache of 50 ids to use, starting from nextval - 49. When these are exhausted the generator reads the next sequence from oracle, and repeats the process. So once the first series of ids are exhausted we start seeing duplicate keys.</p> <p>So the resolution is:</p> <p>1) Either define your Oracle sequence(s) with an increment value of 50 to match the JPA default</p> <pre><code>CREATE SEQUENCE MY_SEQ START WITH 50 MAXVALUE 9999999999999999999 INCREMENT BY 50 NOCYCLE; </code></pre> <p>or</p> <p>2) Add allocationSize=1 to the @SequenceGenerator annotation - this forces the SequenceGenerator to go back to read the next value from the oracle sequence for each ID it requires (with a potential performance impact)</p> <pre><code> @SequenceGenerator(name = "USERS_ID_GENERATOR", sequenceName = "MY_SEQ", allocationSize = 1) </code></pre> <p>, or</p> <p>3) define the Oracle sequence INCREMENT BY some other value, and ensure the allocationSize matches.</p> <p>Answered my own question in the hope of helping others that strike this issue.</p>

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