JAXB: Diff Results In Java7 and Java8

Diff Result In Intellij & Maven

Recently, we came across some mystrious bugs when using JAXB:
We tested our code using Intellij idea by running junit test, in which all tests passed. So we push my code to remote repos and where some build and regression test will run again. Soon, we found that the tests on remote failed. we re-tested many times in Intellij but code still works. Later, we realize that the remote testing using maven. So we test it locally, using maven. It failed also.

Analysis

My first response is the bug of either Intellij or maven, so we try to search on google, but found nothing. Soon, we find that I can get more detailed message from following command:

mvn -X test

Running it give us more configuration information that maven use. Comparing it with the setting in Intellij, we found that the main difference lay in:

• maven use Java8, whereas Intellij using Java7

So we change Intellij to use Java8 and the error also be triggered.
Debugging using Intellij, we find that the setter of a field is not called in Java8. By contrast, the setter is called in Java7 version of JAXB.

Search the rule about setter in JAXB, I got the following:

For a collection property, the set will only be called if it has a value of null when the get is first called after the object is instantiated (i.e. you are not initializing it to an empty collection).

From this rule, we found that our list are indeed initialized and setter shouldn’t be called. It turns out a bug of Java7 implementation: setter of a collection is always called.

Further

This designed behavior behind the setter is to let user control the collection more precisely. The user might prefer to use a synchronized collection which JAXB will not break it by setting a new non-synchronized one.

So we remove initialization of collection and add some null check in code (and this is the original reason we add init). But code still not working in Java8!
Our code is like following:

public void setX(X x) {
    this.x = x;
    size += x.size(); // remember the size of list
} 

Can you get where is the problem?

---

The answer is in Java8, codes like this:

if getX() == null
  List l = new List();
  setX(l); // list is still empty
  l.add(..) // add content which break the `size` counter

And under Java7, it is following way:

List l = new List();
l.add(..)
setX(l); // keep the invariant of `size`

So it turns out a code smell of our setter. We add more functionality into a setter than it should be, which causes the bug. The violation of ‘single responsibility’ is the seed of bug.

Lessons

In conclusion, we get the following lesson:

• don’t be confused by the superficial phenonmenon (Intellij runs right, maven give error);
• single responsibility law (setter should be just setter, no other logic; Even further, we should not add logic in the object which mapped to xml for which is the DTO and should not contain logic)

Ref

• Collection setting not work in Java8
• Adding logic to setter?

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