Refactoring Graphs: Assessing Refactoring over Time

Refactoring is an essential activity during software evolution. Frequently, practitioners rely on such transformations to improve source code maintainability and quality. As a consequence, this process may produce new source code entities or change the structure of existing ones. Sometimes, the transformations are atomic, i.e., performed in a single commit. In other cases, they generate sequences of modifications performed over time. To study and reason about refactorings over time, in this paper, we propose a novel concept called refactoring graphs and provide an algorithm to build such graphs. Then, we investigate the history of 10 popular open-source Java-based projects. After eliminating trivial graphs, we characterize a large sample of 1,150 refactoring graphs, providing quantitative data on their size, commits, age, refactoring composition, and developers. We conclude by discussing applications and implications of refactoring graphs, for example, to improve code comprehension, detect refactoring patterns, and support software evolution studies.

Transcript

1. Refactoring Graphs: Assessing Refactoring over Time Aline Brito, Andre Hora, Marco Tulio Valente IEEE SANER 2020
2. Motivation Refactoring is an essential activity during software evolution 2 Refactoring engines Motivation Benefits and challenges Refactoring over time
3. Motivation Refactoring is an essential activity during software evolution 3 Refactoring engines Motivation Benefits and challenges Refactoring over time
4. Motivation Refactoring is an essential activity during software evolution 4 Refactoring engines Motivation Benefits and challenges Refactoring over time
5. Motivation Refactoring is an essential activity during software evolution 5 Refactoring engines Motivation Benefits and challenges Refactoring over time
6. Motivation Refactoring is an essential activity during software evolution 6 Refactoring engines Motivation Benefits and challenges Refactoring over time
7. Motivation Refactoring is an essential activity during software evolution 7 Refactoring engines Motivation Benefits and challenges Refactoring over time
8. “Refactoring takes time...” 8 Fowler, 1999
9. Refactoring Graph 9
10. A refactoring graph is a set of disconnected subgraphs 10 Refactoring Graph Software history
11. Example of Refactoring Subgraph 11
12. Example of Refactoring Subgraph class Foo{ A(){…} } Method A() from class Foo 12
13. Example of Refactoring Subgraph class Foo{ A(){…} } Method A() from class Foo Alice Bob Two developers 13
14. Example of Refactoring Subgraph class Foo{ A(){…} } class Bar{ A(){…} } MOVE Alice moved method A() from class Foo to Bar 14
15. Example of Refactoring Subgraph class Foo{ A(){…} } class Bar{ A(){…} } class Bar{ B(){…} } MOVE RENAME Six days later, Bob renamed method A() to B() 15
16. Example of Refactoring Subgraph class Foo{ A(){…} } class Bar{ A(){…} } class Bar{ B(){…} } MOVE These operations create a refactoring subgraph over time RENAME 16
17. Example of Refactoring Subgraph class Foo{ A(){…} } class Bar{ A(){…} } class Bar{ B(){…} } MOVE The refactoring subgraph contains three vertices RENAME 17
18. Example of Refactoring Subgraph MOVE RENAME The refactoring subgraph contains two edges class Foo{ A(){…} } class Bar{ A(){…} } class Bar{ B(){…} } 18
19. Example of Refactoring Subgraph class Foo{ A(){…} } class Bar{ A(){…} } class Bar{ B(){…} } MOVE The edge represents the refactoring operation RENAME 19
20. Example of Refactoring Subgraph MOVE util.Foo#A() util.Bar#A() util.Bar#B() The vertices are the full signature of methods RENAME 20
21. Example of Refactoring Subgraph class Bar{ A(){…} } class Bar{ B(){…} } MOVE Method A() from class Foo and package util util.Foo#A() RENAME 21
22. Example of Refactoring Subgraph class Foo{ A(){…} } class Bar{ A(){…} } class Bar{ B(){…} } MOVE A refactoring subgraph can include refactorings performed by one or more developers RENAME 22
23. Example of Refactoring Subgraph class Foo{ A(){…} } class Bar{ A(){…} } class Bar{ B(){…} } MOVE The subgraph contains refactorings performed by two authors RENAME 23
24. Outline 1. RefDiff Tool 2. Dataset 3. Building refactoring graphs 4. Results 24
25. RefDiff Tool 25
26. RefDiff A multi-language refactoring detection tool Refactorings between two versions of a git-based project 26 RefDiff 2.0: A Multi-language Refactoring Detection Tool TSE, 2020
27. RefDiff Rename Extract Move Extract and Move Rename and Move Push Down Inline Pull Up We center on eight refactorings at the method level 27
28. Rename Method util.Foo#a() util.Foo#b() util.Foo#m() util.Bar#m() util.Foo#a() util.Bar#b() Move and Rename Method Move Method Most trivial operations 28
29. Rename Method util.Foo#a() util.Foo#b() util.Foo#m() util.Bar#m() util.Foo#a() util.Bar#b() Move and Rename Method Move Method Change in method’s name 29
30. Rename Method util.Foo#a() util.Foo#b() util.Foo#a() util.Bar#b() Move and Rename Method Change in method’s class util.Foo#m() util.Bar#m() Move Method 30
31. util.Foo#m() util.Bar#m() Move Method Change in method’s name and class Rename Method util.Foo#a() util.Foo#b() util.Foo#a() util.Bar#b() Move and Rename Method 31
32. Push Down Method util.SubFoo2#m() util.SuperFoo#m() util.SubFoo1#m() util.SubFooi#m() Pull up Method util.SubFoo2#m() util.SuperFoo#m() util.SubFoo1#m() util.SubFooi#m() 32
33. Push Down Method util.SubFoo2#m() util.SuperFoo#m() util.SubFoo1#m() util.SubFooi#m() Moving a method from a superclass to one or more subclasses 33
34. Pull up Method util.SubFoo2#m() util.SuperFoo#m() util.SubFoo1#m() util.SubFooi#m() Moving one or more methods from subclasses to a superclass 34
35. Extract Method util.Foo#m2() util.Foo#m() util.Foo#m1() util.Foo#mi() util.Foo#m2() util.Foo#m() util.Foo#m1() util.Foo#mi() 35
36. Extract Method util.Foo#m2() util.Foo#m() util.Foo#m1() util.Foo#mi() Extracting multiple methods from a single method 36
37. Extract Method util.Foo#m2() util.Foo#m() util.Foo#m1() util.Foo#mi() Extracting a single method from multiple methods 37
38. Inline Method util.Bar2#m2() util.Foo#m() util.Bar1#m1() util.Bari#mi() Extract and Move Method util.Foo#m2() util.Bar#m() util.Foo#m1() util.Foo#mi() 38
39. Extract and Move Method util.Foo#m2() util.Bar#m() util.Foo#m1() util.Foo#mi() Extracting a method to a distinct class 39
40. Inline Method util.Bar2#m2() util.Foo#m() util.Bar1#m1() util.Bari#mi() Removal of trivial elements and replacement of the respective calls 40
41. Dataset 41
42. Dataset 10 popular Java projects in terms of stars on GitHub 42
43. Dataset 10 popular Java projects in terms of stars on GitHub 43
44. Dataset + 100 Java files + 1K commits 10 popular Java projects in terms of stars on GitHub 44
45. Building Refactoring Graphs 45
46. Building Refactoring Graphs 46 Scripts INPUT OUTPUT We implement a set of scripts to build refactoring graphs
47. Building Refactoring Graphs 47 Algorithm INPUT OUTPUT The input comprises a list of refactorings
48. Building Refactoring Graphs 48 INPUT OUTPUT Identification of each refactoring and the two methods involved Algorithm
49. Building Refactoring Graphs 49 INPUT OUTPUT Creation of a directed edge representing this refactoring Algorithm
50. Building Refactoring Graphs 50 Algorithm INPUT OUTPUT The output includes sets of refactoring subgraphs in text format
51. Building Refactoring Graphs 51 We detected a total of 8,926 refactoring subgraphs
52. Building Refactoring Graphs 52 We assess 1,150 (13%) refactoring subgraphs with more than one commit
53. Results 53
54. RQ1: What is the size of refactoring subgraphs? 54
55. Number of vertices by refactoring subgraph 55
56. Number of vertices by refactoring subgraph Number of vertices ranges from two to four (85%) 56
57. Number of vertices by refactoring subgraph The most frequent cases are subgraphs with three vertices (639 occurrences, 56%) 57
58. Number of edges by refactoring subgraph 58
59. Number of edges by refactoring subgraph Number of edges ranges between two and three (83%) 59
60. Number of edges by refactoring subgraph The most frequent cases are subgraphs with two edges (772 occurrences, 67%) 60
61. Refactoring subgraph from MPAndroidChart RENAME EXTRACT EXTRACT AND MOVE EXTRACT AND MOVE 61
62. Refactoring subgraph from MPAndroidChart RENAME EXTRACT EXTRACT AND MOVE EXTRACT AND MOVE A developer renamed drawYLegend() to drawYLabels() 62
63. Refactoring subgraph from MPAndroidChart EXTRACT EXTRACT AND MOVE EXTRACT AND MOVE RENAME 13 days later The same developer extracted a new method 63
64. Refactoring subgraph from MPAndroidChart RENAME EXTRACT EXTRACT AND MOVE EXTRACT AND MOVE Two days later The developer made new extractions to another class 64
65. Refactoring subgraph from MPAndroidChart RENAME EXTRACT EXTRACT AND MOVE EXTRACT AND MOVE Only one developer 65
66. Refactoring subgraph from MPAndroidChart RENAME EXTRACT EXTRACT AND MOVE EXTRACT AND MOVE Five vertices 66
67. Refactoring subgraph from MPAndroidChart RENAME EXTRACT EXTRACT AND MOVE EXTRACT AND MOVE Four edges 67
68. Refactoring subgraph from MPAndroidChart RENAME EXTRACT EXTRACT AND MOVE EXTRACT AND MOVE Three commits Commit C1 Commit C2 Commit C3 68
69. RQ2: How many commits are there in refactoring subgraphs? 69
70. Number of commits by refactoring subgraph 70
71. Number of commits by refactoring subgraph Most refactoring subgraphs are created in two or three commits (95%) 71
72. Number of commits by refactoring subgraph Most recurrent case has two commits (81%) 72
73. Refactoring subgraph from Elasticsearch MOVE MOVE EXTRACT EXTRACT EXTRACT Commit C1 Commit C2 73
74. Refactoring subgraph from Elasticsearch MOVE MOVE EXTRACT EXTRACT EXTRACT Commit C1 A developer moved two methods to another class 74
75. Refactoring subgraph from Elasticsearch MOVE MOVE EXTRACT EXTRACT EXTRACT A second developer extracted duplicated code from three methods Three months later Commit C2 75
76. Refactoring subgraph from Elasticsearch MOVE MOVE EXTRACT EXTRACT EXTRACT Two methods are the ones moved early 76
77. Refactoring subgraph from Elasticsearch MOVE MOVE EXTRACT EXTRACT EXTRACT Two authors were responsible for this refactoring subgraph 77
78. Refactoring subgraph from Elasticsearch MOVE MOVE EXTRACT EXTRACT EXTRACT Commit C1 Commit C2 Two commits 78
79. RQ3: What is the age of refactoring subgraphs? 79
80. Age of the refactoring subgraphs Most recent commit Oldest commit Age 80
81. Age of the refactoring subgraphs 81
82. Age of the refactoring subgraphs 82 67% of the refactoring subgraphs have more than one month
83. Age of the refactoring subgraphs Some subgraphs have few days 83
84. Age of the refactoring subgraphs Most subgraphs have weeks or even months 84
85. Refactoring subgraph from Spring Framework RENAME RENAME 85
86. Refactoring subgraph from Spring Framework RENAME RENAME Commit C1 A developer renamed method before(...) to filterBefore(...) 86
87. Refactoring subgraph from Spring Framework RENAME The same developer reverted the operation, renaming filterBefore(...) to before(...) RENAME 87 Six days later
88. Refactoring subgraph from Spring Framework RENAME RENAME A single developer was responsible for this refactoring subgraph 88
89. Refactoring subgraph from Spring Framework RENAME RENAME Two commits Commit C1 Commit C2 89
90. RQ4: Which refactorings compose the refactoring subgraphs? 90
91. Frequency of refactoring operations 91
92. Frequency of refactoring operations Most common refactoring operations include rename (21%), extract and move (19%), and extract (17%) 92
93. Frequency of refactoring operations We detected only 83 occurrences of move and rename operations 93
94. Frequency of refactoring operations There are also few inheritance-based refactorings 94
95. Homogeneous: Subgraphs with a single refactoring operation Heterogeneous: Subgraphs with two or more distinct refactoring operations Two groups: 95
96. Heterogeneous vs Homogeneous subgraphs 96
97. Heterogeneous x Homogeneous subgraphs Most refactoring subgraphs include more than one refactoring type (72%) 97
98. Number of distinct refactoring operations Most heterogeneous subgraphs includes two distinct refactoring types (84%) 98
99. Homogeneous refactoring subgraph from Facebook Fresco EXTRACT EXTRACT EXTRACT EXTRACT 99
100. Homogeneous refactoring subgraph from Facebook Fresco EXTRACT EXTRACT EXTRACT EXTRACT Four extract method operations 100
101. Homogeneous refactoring subgraph from Facebook Fresco EXTRACT EXTRACT EXTRACT EXTRACT A developer extracted method fetchDecodedImage(...) 101
102. Homogeneous refactoring subgraph from Facebook Fresco EXTRACT EXTRACT EXTRACT years later EXTRACT A second developer made two new extract operations 102
103. Homogeneous refactoring subgraph from Facebook Fresco EXTRACT EXTRACT EXTRACT EXTRACT Two developers 103
104. Homogeneous refactoring subgraph from Facebook Fresco EXTRACT EXTRACT EXTRACT EXTRACT Commit C1 Commit C2 Commit C3 Three commits 104
105. RQ5: Are the refactoring subgraphs created by the same or multiple developers? 105
106. Subgraphs performed by a single developer Subgraphs created by multiple developers Two groups: 106
107. Developers of refactoring subgraphs 107
108. Developers of refactoring subgraphs Most refactoring subgraphs are created by a single developer (60%) 108
109. Refactoring subgraph from Square Okhttp RENAME RENAME RENAME MOVE EXTRACT EXTRACT EXTRACT 109
110. Refactoring subgraph from Square Okhttp RENAME RENAME RENAME MOVE EXTRACT EXTRACT EXTRACT Commit C1 A developer renamed three methods 110
111. Refactoring subgraph from Square Okhttp RENAME RENAME RENAME MOVE EXTRACT EXTRACT EXTRACT Commit C2 A second developer extracted method checkDuration(...) 111
112. Refactoring subgraph from Square Okhttp MOVE EXTRACT EXTRACT EXTRACT Commit C2 Commit C3 … moving to a new class named Util RENAME RENAME RENAME 112
113. Refactoring subgraph from Square Okhttp RENAME RENAME RENAME MOVE EXTRACT EXTRACT EXTRACT Two developers 113
114. Refactoring subgraph from Square Okhttp RENAME RENAME RENAME MOVE EXTRACT EXTRACT EXTRACT Seven refactoring operations 114
115. Refactoring subgraph from Square Okhttp RENAME RENAME RENAME MOVE EXTRACT EXTRACT EXTRACT Commit C3 Three commits Commit C2 Commit C1 115
116. Large Subgraph Example 116
117. Large refactoring subgraph from Square Okhttp 117 37 vertices
118. Large refactoring subgraph from Square Okhttp Push down and move method operations 118
119. Large refactoring subgraph from Square Okhttp 24 extract and move method operations 119
120. Large refactoring subgraph from Square Okhttp 21 extract and move operations to a single method 120
121. Large refactoring subgraph from Square Okhttp public int readInt() throws IOException { require(4, Deadline.NONE); return buffer.readInt(); } A developer performed 21 extract method operations to move this duplicated code to a single method 121
122. Implications and Conclusions 122
123. Refactoring-aware Software Evolution Refactoring Graphs is a key data structure to improve the results of current software evolution tools 123
124. Example: Git Blame Show the last author that changed each line of a file 124
125. Example: Git Blame Bob creates a method to calculate the area of a square class Math{ } float squareArea(float l){ + return l * l * l; } + float squareArea(float l){ + return l * l; + } 125
126. Example: Git Blame Git-blame shows Bob as a creator Bob class Math{ } float squareArea(float l){ + return l * l * l; } + float squareArea(float l){ + return l * l; + } 126
127. Example: Git Blame class Math{ } float squareArea(float l){ + return l * l * l; } Bob introduces a bug in a second commit + return l * l * l; l 127
128. Example: Git Blame class Math{ } float squareArea(float l){ return l * l; } + return l * l * l; Git-blame shows Bob as responsible for the last change (bug) Bob 128
129. Example: Git Blame class Math{ } - float squareArea(float l){ - return l * l * l; - } Alice moves the method to a utility class class Utility{ } + float squareArea(float l){ + return l * l * l; + } 129
130. Example: Git Blame Git-blame shows Alice as creator of method squareArea Alice class Utility{ } float squareArea(float l){ + return l * l * l; } + float squareArea(float l){ + return l * l * l; + } 130
131. Example: Git Blame class Math{ float squareArea(float l){ return l * l; } } class Math{ float squareArea(float l){ return l * l * l; } } class Utility{ float squareArea(float l){ return l * l * l; } } Bob is the real creator of squareArea() 131
132. Example: Git Blame class Math{ float squareArea(float l){ return l * l; } } class Math{ float squareArea(float l){ return l * l * l; } } class Utility{ float squareArea(float l){ return l * l * l; } } Bob is responsible for the bug 132
133. Example: Git Blame class Math{ float squareArea(float l){ return l * l; } } class Math{ float squareArea(float l){ return l * l * l; } } class Utility{ float squareArea(float l){ return l * l * l; } } git-blame may miss relevant data due to refactoring operations 133
134. Example: Git Blame class Math{ float squareArea(float l){ return l * l; } } class Math{ float squareArea(float l){ return l * l * l; } } class Utility{ float squareArea(float l){ return l * l * l; } } A refactoring history can improve existing tools and techniques 134
135. 135 Future Studies Other popular programming languages and ecosystems Refactoring graphs based on class and package level
136. Refactoring subgraphs... … are small … have up to three commits … are often heterogeneous … are mostly created by a single developer … span from a few days to months RQ1 RQ2 RQ3 RQ4 RQ5 136
137. Refactoring Graphs: Assessing Refactoring over Time Aline Brito, Andre Hora, Marco Tulio Valente IEEE SANER 2020