JPA实体默认值

Question

I have an entity with default values and a calculated field as follow:

public class Target{
    @Transient
    public Long       total;

    @Min(0)
    @Column(columnDefinition="default 0")
    public Long       val1 = 0L;
    @Min(0)
    @Column(columnDefinition="default 0")
    public Long       val2 = 0L;

    public Target() {
        this.total = Long.valueOf(0L);
        this.val1 = Long.valueOf(0L);
        this.val2 = Long.valueOf(0L);
    }

    public Long calcTotal() {
        return val1 + val2 ;
    }

    public void setVal1(Long val) {
        this.val1 = checkNotNull(val);
        total = calcTotal();
    }

    public void setVal2(Long val) {
        this.val2 = checkNotNull(val);
        total = calcTotal();
    }
}

However whenever the entity is loaded by JPA, the setters are called and a NullPointerException is thrown in calc. Is there anyway to default the values before JPA calls the setters?

Answer 1

First of all, given your mapping, the JPA engine should not call the setters at all, because you chose field access by placing the annotations on the field.

Second, there is no total field in the code.

Third, this field should not exist at all, since it can be computed from two other fields. Just let other classes call calcTotal() to access its value. And rename this method getTotal() .

Oh, and the fields should be private, not public.

If you really want to store the result for reuse, then compute it lazily, and reset it to null when one of the operands is modified:

public Long getTotal() {
    if (total == null) {
        total = val1 + val2;
    }
    return total;
}

public void setVal1(Long val1) {
    this.val1 = val1;
    this.total = null;
}

public void setVal2(Long val2) {
    this.val2 = val2;
    this.total = null;
}