Question

There was method setWrappedInstance in org.springframework.beans.BeanWrapper in 2.5.6 and was removed in 3.0.0. As i am in the process of migrating my project from 2.5 to 3.0 I get errors. I investigated and the implementation class org.springframework.beans.BeanWrapperImpl still has the method setWrappedInstance implemented.

below is the piece of code from my project which is causing trouble.

public FieldComparator(String fieldName, Class clazz) {
    _fieldName = fieldName;
    _bw = new BeanWrapperImpl(clazz);
}    

public int compare(Object o1, Object o2) {
    if (o1 == null && o2 == null) return 0;
    else if (o1 == null) return -1;
    else if (o2 == null) return 1;
    // otherwise
    _bw.setWrappedInstance(o1);
    Comparable v1 = (Comparable) _bw.getPropertyValue(_fieldName);

    _bw.setWrappedInstance(o2);
    Comparable v2 = (Comparable) _bw.getPropertyValue(_fieldName);
    return NullsLowComparator.INSTANCE.compare(v1, v2);
}

So would it be ok if I just replace _bw implementation with BeanWrapperImpl. I am in the learning stage and I believe that spring strongly suggests to use interfaces rather than implementation classes itself.

Is this change against standard practices or can I just move on with the simple change?

Answer 1

The BeanWrapper.setWrappedInstance method was marked as deprecated in Spring 2.5, and removed altogether in 3.0. Unlike deprecations in the JRE (which never get removed), deprecated APIs in Spring do get removed, so you’re well advised to avoid them.

The 2.5.6 Javadoc for setWrappedInstance says:

Deprecated. as of Spring 2.5, in favor of recreating a BeanWrapper per target instance

In other words, instead of reusing instances of BeanWrapper, you should create new BeanWrapperImpl instances as required. There’s no performance penalty for this – the BeanWrapperImpl javadoc says that it “Caches introspection results for efficiency.”

So replace this:

_bw.setWrappedInstance(o1);
Comparable v1 = (Comparable) _bw.getPropertyValue(_fieldName);

with this:

Comparable v1 = (Comparable) new BeanWrapperImpl(o1).getPropertyValue(_fieldName);

and get rid of the _bw field altogether.

I believe that spring strongly suggests to use interfaces rather than implementation classes itself

As a general rule of thumb, yes. However, try to apply some practicality to this. Your use of BeanWrapperImpl is restricted entirely to internal implementation detail of your comparator, so there’s no real harm in using it directly. If your comparator were to expose the BeanWrapper in a public method signature, for some reason, then that would be best done using the interface rather than the implementation.