[JAVA] If you delete an immutable collection with JPA, you will naturally get an UnsupportedOperationException.

Normally, you rarely see UnsupportedOperationException. If you inadvertently do a add or `` `remove``` on an immutable collection, that's probably the case.

However, I encountered UnsupportedOperationException in JPA and was addicted to it for a while, so I will leave a note.

What I want to do is to persist a 1: N entity and then delete the child collection for that entity.

	@Transactional
	@Override
	public void run(String... args) throws Exception {
		Parent parent = new Parent(5L, "name", List.of(
				new Child(new ChildId(101L, 1L), "aaa", null),
				new Child(new ChildId(102L, 1L), "bbb", null)
				));
		em.persist(parent);
		
		Parent parent2 = pr.getOne(5L);
		System.out.println(parent == parent2); // true
		List<Child> children = parent2.getChildren();
		parent2.getChildren().clear();
		em.persist(parent2);
	}

Caused by: java.lang.UnsupportedOperationException: null
	at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:71) ~[na:na]
	at java.base/java.util.ImmutableCollections$AbstractImmutableCollection.clear(ImmutableCollections.java:77) ~[na:na]
	at org.hibernate.collection.internal.PersistentBag.clear(PersistentBag.java:495) ~[hibernate-core-5.4.20.Final.jar:5.4.20.Final]

With the above code, the same instance is returned for `parent``` and parent2```. Then ``` getChildren``` will return an immutable collection of List.of```, so clear``` will result in `ʻUnsupportedOperationException```.

I didn't immediately notice that the same instance was returned because the actual code was separated from each other in the persistence context. I was unknowingly thinking that if I put it in a persistence context and retrieve it, it would be repacked into a mutable collection such as ArrayList, so the above code would return the same instance. I was addicted to it without noticing it.