JPA ArrayList OneToMany UnsupportedOperationException 異常

在使用JPA的OneToMany註解，並使用List聲明類型，在更新的操做時，會遇到UnsupportedOperationException異常。

示例

好比有如下Teacher，對應多個班級

public class Teacher {
    @OneToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REMOVE})
    private List<Klass> klassList = new ArrayList<>();
}

而後咱們執行以下代碼，則會產生UnsupportedOperationException異常.

Teacher teacher = teacherRepository.findOne(id);
teacherRepository.save(teacher);

緣由

1.findByOne返回的實體中， klassList的類型是PersistentBag，而非ArrayList。

public class PersistentBag extends AbstractPersistentCollection implements List {

2.@OneToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REMOVE})註解，使得jpa須要對klassList進行如下操做：

at java.util.AbstractList.clear(AbstractList.java:234)

而
java.util.AbstractList.clear中，並無具體實現，直接跑出了UnsupportedOperationException

* @throws UnsupportedOperationException if the {@code remove}
     *         operation is not supported by this list iterator
     * @throws IllegalStateException if neither {@code next} nor
     *         {@code previous} have been called, or {@code remove} or
     *         {@code add} have been called after the last call to
     *         {@code next} or {@code previous}
     */
    void remove();

解決方案

在保存前，強制轉換爲ArrayList

Teacher teacher = teacherRepository.findOne(id);
teacher.setKlassList(new ArrayList(teacher.getKlassList()));
teacherRepository.save(teacher);