Operating Systems Lecture Notes

27 February 2012 • Pop Quiz 2

A Question

findbugs flagged several solutions to assignment 1 with the warning
```
In os.java at line 485:
  Write to static field os.moo 
  from instance method os.cow()
```
What is findbugs complaining about?

Static Fields

There is only ever one copy of a static field, and it’s associated with the class.

Static Field References

A class instance reference to a static field refers to the class.

Add Threads for Fun

A thread per instance causes unmediated access to a shared resource.

The Answer

In os.java at line 485:
  Write to static field os.moo 
  from instance method os.cow()

Findbugs is pointing out a potential race condition.
There are several solutions:
- Get rid of the static field.
- Enforce mutual exclusion for static-field access.
- Don’t use threads.

Postscript

From: rclayton@monmouth.edu (R. Clayton)
To: s0------@monmouth.edu
Subject: Pop quiz 2 answer.
This is what the FindBugs site definition says, “ST: Write to static field from instance method (ST_WRITE_TO_STATIC_FROM_INSTANCE_METHOD) This instance method writes to a static field. This is tricky to get correct if multiple instances are being manipulated, and generally bad practice.”, a close match of what my answers was on the question.

First, there’s no arguing pop-quiz grades.

Second, the findbugs explanation doesn’t make sense. To what does “multiple instances” refer? Not the static field, because there’s only one instance of a static field. Not to the class instances because, while there are several of them, they are manipulating the static field; they are not being manipulated themselves. What else is there to refer to?

Third, assuming we can assign some kind of meaning to the findbugs explanation, why is it “tricky to get correct?” (And what does it mean to be “correct,” which turns out to be a hard question, one that Java gets mostly wrong). Why is it considered bad practice? Used in the way (we imagine) findbugs describes, a static field is effectively a limited-scope global variable, and that’s usually considered both bad style and bad practice, but so what? There’s an answer more directly relevant to the class, which is why I gave the verbal clue about covering the answer to the pop quiz in the lectures, and why I gave the pop quiz at all.

This page last modified on 2012 February 27.