java - Hibernate duplicates elements in parent collection while it is populating with newly persisted childs -

can explain following strange behaviour encountered? trying persist new child objects , simultaneously add them to parent collection. @ end there twice many elements in parent collection expected. let me show example:

@entity public class {      @generatedvalue(strategy = generationtype.identity)     @id     private integer id;      @onetomany(mappedby = "a")     private list<b> bs = new arraylist<>();      public integer getid() { return id; }      public void setid(integer id) { this.id = id; }      public list<b> getbs() { return bs; }      public void setbs(list<b> bs) { this.bs = bs; } }  @entity public class b {      @generatedvalue(strategy = generationtype.identity)     @id     private integer id;      @manytoone     private a;      public integer getid() { return id; }      public void setid(integer id) { this.id = id; }      public geta() { return a; }      public void seta(a a) { this.a = a; } } 

test case:

@test public void test() throws exception {     entitymanager.gettransaction().begin();      a = new a();      entitymanager.persist(a);     entitymanager.flush();     entitymanager.clear();      = entitymanager.find(a.class, a.getid());      (int = 0; < 3; i++) {         b b = new b();         b.seta(a);         entitymanager.persist(b);         a.getbs().add(b);     }      assertequals(3, a.getbs().size());      entitymanager.gettransaction().commit(); } 

result:

java.lang.assertionerror:  expected :3 actual   :6 

i don't ask how modify given code achieve expected result. want understand why given code behaves this.

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update: works fine eclipselink , datanucleus, fails hibernate.

thanks @riskop answer , further investigation can summarize:

• eclipselink 2.5.0 - it works expected.
• datanucleus 4.1.9 - it works expected.
• hibernate 5.1.0.final, 5.0.3.final 4.3.11.final, 4.2.21.final,... - it doesn't work.

unfortunately can't tell how happening

but hibernate bug. should return 3.

i checked hibernate 5.0.3.final , experienced same problem. checked exact same code datanucleus 4.1.9 , works expected (no duplicate size of collection).

however, hibernate problem goes away if of following:

• not using @generatedvalue(strategy = generationtype.identity). generationtype.identity discouraged, , can source of other problems (just google it). if change auto, hibernate works ok.
• change eager fetching on entity ( @onetomany(mappedby = "a", fetch=fetchtype.eager)
• initializes collection on after loading database (via calling a.getbs().size() example).