限制JPA中集合的大小

Question

Say I have an entity like this

@Entity
Class A{
//fields

@Onetomany
Set<B> b; // 
}

Now, how do I limit the number of 'B's in the collection in such a way that, when there is a new entry in the collection, the oldest one is removed, some thing like removeEldestEntry we have in a LinkedHashMap.

I am using MySQL 5.5 DB with Hibernate. Thanks in advance.

EDIT

My goal is not to have more than N number of entries in that table at any point of time. One solution I have is to use a Set and schedule a job to remove the older entries. But I find it dirty. I am looking for a cleaner solution.

Answer 1

There is one API provided by Apache Commons Collection . Here you can use the class CircularFifoBuffer for your reference of the same problem you have, if you want example shown as below that you can achive that

Buffer buf = new CircularFifoBuffer(4);
buf.add("A");
buf.add("B");
buf.add("C");
buf.add("D"); //ABCD
buf.add("E"); //BCDE

Answer 2

I would use the code to manually enforce this rule. The main idea is that the collection B should be well encapsulated such that client only can change its content by a public method (ie addB() ) . Simply ensure this rule inside this method ( addB() ) to ensure that the number of entries inside the collection B cannot larger than a value.

A:

@Entity
public class A {


    public static int MAX_NUM_B = 4;

    @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<B> b= new LinkedHashSet<B>();

    public void addB(B b) {
        if (this.b.size() == MAX_NUM_B) {
            Iterator<B> it = this.b.iterator();
            it.next();
            it.remove();
        }
        this.b.add(b);
    }

    public Set<B> getB() {
        return Collections.unmodifiableSet(this.b);
    }
}

B:

@Entity 
public class B{

    @ManyToOne
    private A a;
}

Main points:

• A should be the owner of the relationship.
• In A , do not simply return B as client can bypass the checking logic implemented in addB(B b) and change its content freely.Instead , return an unmodifiable view of B .
• In @OneToMany , set orphanRemoval to true to tell JPA to remove the B 's DB records after its corresponding instances are removed from the B collection.

Answer 3

I think you will have to do it manually.

One solution that comes to mind is using @PrePersist and @PreUpdate event listeners in entity A .

Within the method annotated with above annotations , you check if size of Set<B> , if it is above the max limit, delete the oldest B entries(which may be tracked by a created_time timestamp property of B)