java - JPA从反面删除双向关联

Question

在我的域模型中，有很多双向关联(OneToMany 和 ManyToMany)

我已阅读 this article并根据样本模式进行所有关联。 (ManyToMany 关联有一个两侧的 addXY 方法，遵循该模式)

使用the pattern in this article问题是，从反面删除呢？

示例:

public class Customer implements Serializable {

...

@ManyToOne()
private CustomerStatus customerStatus;


@PreRemove
public void preRemove(){
    setCustomerStatus(null);
}

public void setCustomerStatus(CustomerStatus customerStatus) {
    if(this.customerStatus != null) { this.customerStatus.internalRemoveCustomer(this); }
    this.customerStatus = customerStatus;
    if(customerStatus != null) { customerStatus.internalAddCustomer(this); }
}

另一边:

public class CustomerStatus implements Serializable {

private static final long serialVersionUID = 1L;

@OneToMany(mappedBy="customerStatus")
private List<Customer> customers;

@PreRemove
public void preRemove(){
    for(Customer c : customers){
        c.setCustomerStatus(null); // this causes ConcurrentException
    }

}

public List<Customer> getCustomers() {
    return Collections.unmodifiableList(this.customers);
}

public void addCustomer(Customer c){
    c.setCustomerStatus(this);
}

public void removeCustomer(Customer c){
    c.setCustomerStatus(null);
}

void internalAddCustomer(Customer c){
    this.customers.add(c);
}

void internalRemoveCustomer(Customer c){
    this.customers.remove(c);
}

问题是 preRemove 方法导致 ConcurrentException。这要怎么处理呢？ 目标是删除 CustomerStatus，并将所有具有该状态的客户设置为 NULL。

更新

如果没有 preRemove 方法，我会遇到 MySQLIntegrityConstraintViolationException:无法删除或更新父行:外键约束失败

Answer 1

当您迭代客户集合时，无法调用 this.customers.remove(c)。这个问题之前已经出现过，所以您可能会找到其他解决方案，如下所示: How to avoid ConcurrentModificationException when iterating over a map and changing values?

但一个简单的解决方案是从旧列表创建一个新列表以迭代 preRemove:

public void preRemove(){
    List<Customer> tempList = new ArrayList(customers);
    for(Customer c : tempList){
        c.setCustomerStatus(null); 
    }
}