# java code
User user = new User();
session.save(user);

# exception
org.hibernate.PropertyValueException: not-null property references a null or transient value: User.avatar

# hibernate mapping
<class name="User" table="user" dynamic-insert="true" dynamic-update="true">
    <property name="avatarId" column="avatar_id" type="integer" not-null="true">
</class>


# table DDL 没有设置默认值，如果实体值为null依然会由db报异常
CREATE TABLE `user` (
  `avatar_id` int(11) NOT NULL
);

@RequestMapping("/queryAndUpdate")
public Result queryAndUpdate(Http http) {
	//Controller层写业务逻辑
	int id = 1; 
	//using session1
    User user = services.getDaos().getUserDao().findById(id);
	//using session2 user中加载有没有映射到的关联实体 更新时抛出异常
    lecturer = services.getUserService().updateUser(user);
    return Result.success().setBean(user);
}
//实体
@Entity
@DynamicInsert
@DynamicUpdate
@Table(name = "user")
class User{
	@Id
	@Column(name = "id")
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	private Integer id;

	@NotFound(action = NotFoundAction.IGNORE)
	@OneToOne(targetEntity = File.class,cascade = CascadeType.REFRESH)
	@JoinColumn(name = "avatar_id")
	private File avatar;  //未加载的关联实体
	...
}

@Test
public void testConcurrentModificationException() throws Exception {
  Random random = new Random();
  //初始化ArrayList
  List<Integer> dataList = new ArrayList<>();
  for (int i = 0; i < 100; i++) {
      dataList.add(random.nextInt(10));
  }
  System.out.println("initialize " + dataList.stream().map(Objects::toString).collect(Collectors.joining(",")));
  //初始化排序任务
  Runnable runnable = () -> {
      Collections.sort(dataList, Integer::compareTo);
      System.out.println("thread[" + Thread.currentThread().getId() + "] " + dataList.stream().map(Objects::toString).collect(Collectors.joining(",")));
  };
  //模拟多线程环境对同一个ArraList进行排序
  for (int i = 0; i < 1000; i++) {
      ThreadPool.execute(runnable);
  }
}

//Collections::sort
public static <T> void sort(List<T> list, Comparator<? super T> c) {
    list.sort(c);
}
//ArrayList::sort
public void sort(Comparator<? super E> c) {
    final int expectedModCount = modCount;
    Arrays.sort((E[]) elementData, 0, size, c);
    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }
    modCount++;
}

@Test
public void testConcurrentModificationException() throws Exception {
    Random random = new Random();
    List<Integer> dataList = new ArrayList<>();
    for (int i = 0; i < 100; i++) {
        dataList.add(random.nextInt(10));
    }
    System.out.println("initialize {}" + dataList.stream().map(Objects::toString).collect(Collectors.joining(",")));
    Runnable runnable = () -> {
        synchronized (random) {
            Collections.sort(dataList, Integer::compareTo);
            System.out.println("thread[" + Thread.currentThread().getId() + "] " + dataList.stream().map(Objects::toString).collect(Collectors.joining(",")));
        }
    };
    for (int i = 0; i < 1000; i++) {
        ThreadPool.execute(runnable);
    }
}

@Test
public void testConcurrentModificationException() throws Exception {
    Random random = new Random();
    List<Integer> dataList = new CopyOnWriteArrayList<>();
    for (int i = 0; i < 100; i++) {
        dataList.add(random.nextInt(10));
    }
    System.out.println("initialize {}" + dataList.stream().map(Objects::toString).collect(Collectors.joining(",")));
    Runnable runnable = () -> {
            Collections.sort(dataList, Integer::compareTo);
            System.out.println("thread[" + Thread.currentThread().getId() + "] " + dataList.stream().map(Objects::toString).collect(Collectors.joining(",")));
    };
    for (int i = 0; i < 1000; i++) {
        ThreadPool.execute(runnable);
    }
}