How dynamic precondition checking can increase the correctness of the Extract Local Variable and Extract And Move Method refactorings. Masterpresentation from the SATToSE conference.

1. Making Software Refactorings Safer
Anna Maria Eilertsen
@Sowhow
Supervised by: Anya Bagge1
Volker Stolz 2
1Inst. for Informatikk, Universitetet i Bergen
2Inst. for Data- og Realfag, Høgskolen i Bergen
Norway
July 12, 2016
The results of this thesis has been accepted to the ISOLA1
conference as a paper.
1
http://www.isola-conference.org/isola2016/
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 1 / 14

2. Software Refactorings
“Behaviour preserving program
transformation”
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 2 / 14

3. Software Refactoring Tools
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 3 / 14

4. Unsafe Refactorings
“The primary risk is regression, mostly from
misunderstanding subtle corner cases in the
original code and not accounting for them
in the refactored code.”
– interviewee, Microsoft developer,
Kim et al., 2012
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 4 / 14

5. Unsafe Refactoring Example
Extract Local Variable
In Java/Eclipse:
Before
1 public void f() {
2 x.n();
3 setX();
4 x.n();
5 }
After
1 public void f() {
2 X temp = x;
3 temp.n();
4 setX();
5 temp.n();
6 }
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 5 / 14

6. An analysing problem
x = new X();
setX(); // x = new X();
1 public void f() {
2 X temp = x;
3 temp.n();
4 setX();
5 temp.n();
6 }
Solution:
assert temp == x;
∗
Dog art from Hyperbole and a Half
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 6 / 14

7. Extract Local Variable
Simplified example:
1 public class C {
2 public X x = new X();
3 {//initializer
4 x.myC = this;
5 }
6
7 public void f(){
8 x.n();
9 x.m();
0 x.n();
1 }
2 }
1 public class X{
2 public C myC;
3
4 public void m(){
5 myC.x = new X();
6 }
7
8 public void n(){
9 System.out.println(
10 this.hashCode());
11 }
12 }
Output:
1735600054
21685669 skip example
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 7 / 14

8. Extract Local Variable
Refactored:
1 public class C {
2 public X x = new X();
3 {//initializer
4 x.myC = this;
5 }
6
7 public void f(){
8 X temp = x;
9 temp.n();
0 temp.m();
1 temp.n();
2 }
3 }
1 public class X{
2 public C myC;
3
4 public void m(){
5 myC.x = new X();
6 }
7
8 public void n(){
9 System.out.println(
10 this.hashCode());
11 }
12 }
Output:
1735600054
1735600054
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 8 / 14

9. Extract Local Variable
With dynamic checks:
1 public class C {
2 public X x = new X();
3 {//initializer
4 x.myC = this;
5 }
6 public void f(){
7 X temp = x;
8 assert temp == x;
9 temp.n();
0 assert temp == x;
1 temp.m();
2 assert temp == x;
3 temp.n();
1 public class X{
2 public C myC;
3
4 public void m(){
5 myC.x = new X();
6 }
7
8 public void n(){
9 System.out.println(
10 this.hashCode());
11 }
12 }
Output:
1735600054
Exception in thread ”main” java.lang.AssertionError
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 9 / 14

10. Extract And Move Method
A similar problem:
1 public class C {
2 public X x = new X();
3 {//initializer
4 x.myC = this;
5 }
6 public void f(){
7 x.bar(this);
8 }
9 }
1 public class X{
2 ...
3 void bar(C c){
4 this.n();
5 assert this == c.x;
6 this.m();
7 assert this == c.x;
8 this.n();
9 }
10 }
Similar how?
Evaluate x once.
Refer to that value by this
Substitute every occurrence of x with this
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 10 / 14

11. Experiment: Case study
Case: Eclipse JDT UI source code
Experiment:
Execute our modified refactorings on Eclipse JDT UI project
Run Eclipse test suite
Look for triggered asserts
Profit!!
Need custom automated refactoring tool.
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 11 / 14

12. Experiment: Results
Extract Local Extract and
Variable Move Method
Executed refactorings 4538 755
Total number of asserts 7665 610
Resulting compile errors 0 14
Successful Tests 2392 2151
Unsuccessful Tests 4 245
Asserts triggered 2 / 136∗ 0
∗ 136 instances of the same 2 assert statements
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 12 / 14

13. Discussion
Take-away and questions:
Extract Local Extract and
Variable Move Method
Executed refactorings 4538 755
Total number of asserts 7665 610
Resulting compile errors 0 14
Successful Tests 2392 2151
Unsuccessful Tests 4 245
Asserts triggered 2 / 136 0
Dynamic preconditions can be useful!
Assert statements are incomplete.
Show or hide the asserts from the programmer?
Is reference equivalence too strict?
Thank you!
∗
Art with face is from Hyperbole and a Half
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 13 / 14

14. Experiment: Development
Eclipse refactoring plug-in
Modify Eclipse’s refactorings to introduce asserts
Extract Local Variable
Extract And Move Method
Automate refactoring process
Execute on Java project
One refactoring per method
Custom heuristic for finding refactoring targets
Anna Maria Eilertsen @sowhow (Inst. for Informatikk, Universitetet i Bergen, Inst. for Data-Making Software Refactorings Safer July 12, 2016 14 / 14