在使用JPA的OneToMany注解，并使用List声明类型，在更新的操作时，会遇到UnsupportedOperationException异常。

示例

比如有以下Teacher，对应多个班级

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

然后我们执行如下代码，则会产生UnsupportedOperationException异常.

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

原因

1.findByOne返回的实体中， klassList的类型是PersistentBag，而非ArrayList。

public class PersistentBag extends AbstractPersistentCollection implements List {

2.@OneToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REMOVE})注解，使得jpa需要对klassList进行以下操作：

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

而

* @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();

解决方案

在保存前，强制转换为ArrayList

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