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Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]8 <title>JaCoCo - Coverage Counter</title>
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11
12<div class="breadcrumb">
13 <a href="../index.html" class="el_report">JaCoCo</a> &gt;
14 <a href="index.html" class="el_group">Documentation</a> &gt;
15 <span class="el_source">Coverage Counters</span>
16</div>
17<div id="content">
18
19<h1>Coverage Counters</h1>
20
21<p>
22 JaCoCo uses a set of different counters to calculate coverage metrics. All
23 these counters are derived from information contained in Java class files
24 which basically are Java byte code instructions and debug information
25 optionally embedded in class files. This approach allows efficient on-the-fly
26 instrumentation and analysis of applications even when no source code is
27 available. In most cases the collected information can be mapped back to
28 source code and visualized down to line level granularity. Anyhow there are
29 limitations to this approach. The class files have to be compiled with debug
30 information to calculate line level coverage and provide source highlighting.
31 Not all Java language constructs can be directly compiled to corresponding
32 byte code. In such cases the Java compiler creates so called <i>synthetic</i>
33 code which sometimes results in unexpected code coverage results.
34</p>
35
36<h2>Instructions (C0 Coverage)</h2>
37
38<p>
39 The smallest unit JaCoCo counts are single Java byte code instructions.
40 <i>Instruction coverage</i> provides information about the amount of code that
41 has been executed or missed. This metric is completely independent from source
42 formatting and always available, even in absence of debug information in the
43 class files.
44</p>
45
46<h2>Branches (C1 Coverage)</h2>
47
48<p>
49 JaCoCo also calculates <i>branch coverage</i> for all <code>if</code> and
50 <code>switch</code> statements. This metric counts the total number of such
51 branches in a method and determines the number of executed or missed branches.
52 Branch coverage is always available, even in absence of debug information in
53 the class files. Note that exception handling is not considered as branches
54 in the context of this counter definition.
55</p>
56
57<p>
58 If the class files haven been compiled with debug information decision points
59 can be mapped to source lines and highlighted accordingly:
60</p>
61
62<ul>
63 <li>No coverage: No branches in the line has been executed (red diamond)</li>
64 <li>Partial coverage: Only a part of the branches in the line have been
65 executed (yellow diamond)</li>
66 <li>Full coverage: All branches in the line have been executed (green diamond)</li>
67</ul>
68
69<h2>Cyclomatic Complexity</h2>
70
71<p>
72 JaCoCo also calculates cyclomatic complexity for each non-abstract method and
73 summarizes complexity for classes, packages and groups. According to its
74 definition by
75 <a href="http://hissa.nist.gov/HHRFdata/Artifacts/ITLdoc/235/title.htm">McCabe1996</a>
76 cyclomatic complexity is the minimum number of paths that can, in (linear)
77 combination, generate all possible paths through a method. Thus the
78 complexity value can serve as an indication for the number of unit test cases
79 to fully cover a certain piece of software. Complexity figures can always be
80 calculated, even in absence of debug information in the class files.
81</p>
82
83<p>
84 The formal definition of the cyclomatic complexity v(G) is based on the
85 representation of a method's control flow graph as a directed graph:
86</p>
87
88<blockquote>
Marc R. Hoffmann1fadbfb2014-05-16 15:43:29 +0200[diff] [blame]89 <p>
Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]90 v(G) = E - N + 2
Marc R. Hoffmann1fadbfb2014-05-16 15:43:29 +0200[diff] [blame]91 </p>
Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]92</blockquote>
93
94<p>
95 Where E is the number of edges and N the number of nodes. JaCoCo calculates
96 cyclomatic complexity of a method with the following equivalent equation based
97 on the number of branches (B) and the number of decision points (D):
98</p>
99
100<blockquote>
Marc R. Hoffmann1fadbfb2014-05-16 15:43:29 +0200[diff] [blame]101 <p>
Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]102 v(G) = B - D + 1
Marc R. Hoffmann1fadbfb2014-05-16 15:43:29 +0200[diff] [blame]103 </p>
Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]104</blockquote>
105
106<p>
107 Based on the coverage status of each branch JaCoCo also calculates covered and
108 missed complexity for each method. Missed complexity again is an indication
109 for the number of test cases missing to fully cover a module. Note that as
110 JaCoCo does not consider exception handling as branches try/catch blocks will
111 also not increase complexity.
112</p>
113
114<h2>Lines</h2>
115
116<p>
117 For all class files that have been compiled with debug information, coverage
118 information for individual lines can be calculated. A source line is
119 considered executed when at least one instruction that is assigned to this
120 line has been executed.
121</p>
122
123<p>
124 Due to the fact that a single line typically compiles to multiple byte code
125 instructions the source code highlighting shows three different status for
126 each line containing source code:
127</p>
128
129<ul>
130 <li>No coverage: No instruction in the line has been executed (red
131 background)</li>
132 <li>Partial coverage: Only a part of the instruction in the line have been
133 executed (yellow background)</li>
134 <li>Full coverage: All instructions in the line have been executed (green
135 background)</li>
136</ul>
137
Marc R. Hoffmann1fadbfb2014-05-16 15:43:29 +0200[diff] [blame]138<p>
139 Depending on source formatting a single line of a source code may refer to
140 multiple methods or multiple classes. Therefore the line count of methods
141 cannot be simply added to obtain the total number for the containing class.
142 The same holds true for the lines of multiple classes within a single source
143 file. JaCoCo calculates line coverage for classes and source file based on the
144 actual source lines covered.
145</p>
146
Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]147<h2>Methods</h2>
148
149<p>
150 Each non-abstract method contains at least one instruction. A method is
151 considered as executed when at least one instruction has been executed. As
152 JaCoCo works on byte code level also constructors and static initializers are
153 counted as methods. Some of these methods may not have a direct correspondence
154 in Java source code, like implicit and thus generated default constructors or
155 initializers for constants.
156</p>
157
158<h2>Classes</h2>
159
160<p>
161 A class is considered as executed when at least one of its methods has been
162 executed. Note that JaCoCo considers constructors as well as static
163 initializers as methods. As Java interface types may contain static
164 initializers such interfaces are also considered as executable classes.
165</p>
166
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