UnsupportedOperationException merge-saving many-to-many relation with hibernate and JPA

I've set up a simple many-to-many relationship account : role with Hibernate but when I try to save an account in a unit test after it has had its role added I get an UnsupportedOperationException:

java.lang.UnsupportedOperationException
	at java.util.AbstractList.remove(AbstractList.java:144)
	at java.util.AbstractList$Itr.remove(AbstractList.java:360)
	at java.util.AbstractList.removeRange(AbstractList.java:559)
	at java.util.AbstractList.clear(AbstractList.java:217)
	at org.hibernate.type.CollectionType.replaceElements(CollectionType.java:502)
	at org.hibernate.type.CollectionType.replace(CollectionType.java:582)
	at org.hibernate.type.TypeHelper.replace(TypeHelper.java:178)
	at org.hibernate.event.def.DefaultMergeEventListener.copyValues(DefaultMergeEventListener.java:563)
	at org.hibernate.event.def.DefaultMergeEventListener.entityIsPersistent(DefaultMergeEventListener.java:288)
	at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:261)
	at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:84)
	at org.hibernate.impl.SessionImpl.fireMerge(SessionImpl.java:867)
	at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:851)
	at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:855)
	at org.hibernate.ejb.AbstractEntityManagerImpl.merge(AbstractEntityManagerImpl.java:686)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:240)
	at $Proxy33.merge(Unknown Source)
	at org.springframework.data.jpa.repository.support.SimpleJpaRepository.save(SimpleJpaRepository.java:360)
	at ....JpaProvider.save(JpaProvider.java:161)
	at ....DataModelTest.testAccountRole(DataModelTest.java:47)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
	at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:74)
	at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:82)
	at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:72)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:240)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:49)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
	at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
	at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
	at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:180)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

What's going wrong here? Is my entity setup faulty or is this a hibernate or JPA limitation forcing me to split apart my m:m relationship into 3 1:n relations modeling the m:n relationship table as well (which I wanted to avoid since it does not have any additional information). I've modeled other 1:n entities in my prototype and that seemed to work out nicely so far...

Here's my setup, any thoughts whether it might be faulty are appreciated.

Entities (simplified):

@Entity
@Table(name="account")
public class Account extends AbstractPersistable<Long> {

	private static final long serialVersionUID = 627519641892468240L;

	private String username;

    
    @ManyToMany
    @JoinTable(	name = "account_roles", 
    			joinColumns = { @JoinColumn(name = "account_id")}, 
    			inverseJoinColumns={@JoinColumn(name="role_id")})  
    private List<Role> roles;   
	   
   
	public List<Role> getRoles() {
		return roles;
	}
	public void setRoles(List<Role> roles) {
		this.roles = roles;
	}



    @Entity
    @Table(name="role")
    public class Role extends AbstractPersistable<Long> {
    
    	private static final long serialVersionUID = 8127092070228048914L;
    	
    	private String name;
    
    	@ManyToMany
    	@JoinTable(	name = "account_roles",   
    				joinColumns={@JoinColumn(name="role_id")},   
    				inverseJoinColumns={@JoinColumn(name="account_id")})  
    	private List<Account> accounts;
    	
    
    	public List<Account> getAccounts() {
    		return accounts;
    	}
    
    	public void setAccounts(List<Account> accounts) {
    		this.accounts = accounts;
    	}

Unit Test:

@TransactionConfiguration
@ContextConfiguration({"classpath:dw-security-context-test.xml"})
@Transactional
@RunWith(SpringJUnit4ClassRunner.class)
public class DataModelTest {

	@Inject
	private AccountProvider accountProvider;	
	
	@Inject 
	private RoleProvider roleProvider;
	
	@Before
	public void mockAccountRolePermission(){
		Account account = MockAccount.getSavedInstance(accountProvider);
		Role role = MockRole.getSavedInstance(roleProvider);
	}

	@Test
	public void testAccountRole(){		
		Account returnedAccount = accountProvider.findAll().get(0);
		returnedAccount.setRoles(Arrays.asList(roleProvider.findAll().get(0)));
		accountProvider.save(returnedAccount);

	}
}

MockAccount (same for MockRole):

public class MockAccount {
	
	public static Account getInstance(){
		Account account = new Account();
		account.setUsername(RandomData.rndStr("userName-", 5));
		return account;
	}
	
	public static Account getSavedInstance(AccountProvider accountProvider){
		Account account = getInstance();
		accountProvider.save(account);
		return account;
	}
	
}

And finally the Provider:

@Repository
public class AccountProvider extends JpaProvider<Account, Long> {
	
}

where JPAProvider just wraps a lot of JPARepository methods (at least as far as it is important in this case):

public abstract class JpaProvider<T extends Object, ID extends Serializable> implements JpaRepository<T, ID>, JpaSpecificationExecutor<T>, QueryDslPredicateExecutor<T> {
...
}

Any Ideas on why the save might be an UnsupportedOperation?

It is because of your

Arrays.asList(roleProvider.findAll().get(0))

This creates an unmodifiable list (in fact, a non-resizable list). Hibernate seems to expect a modifiable list. Try using this instead:

public void testAccountRole(){      
    Account returnedAccount = accountProvider.findAll().get(0);

    List<Role> list = new ArrayList<Role>();
    list.add(roleProvider.findAll().get(0));    
    returnedAccount.setRoles(list);  
      
    accountProvider.save(returnedAccount);
}

This solution won't explain why exactly you got the other exception (might be documented in the Hibernate docs), but it might be a valid workaround.

Hibernate's persistent variant of the Collection in question attempts to delegate to an abstract base class (PersistenceBag) that doesn't implement the add method.