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Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]10 <title>JaCoCo - Implementation Design</title>
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12<body onload="prettyPrint()">
13
14<div class="breadcrumb">
15 <a href="../index.html" class="el_report">JaCoCo</a> &gt;
16 <a href="index.html" class="el_group">Documentation</a> &gt;
17 <span class="el_source">Implementation Design</span>
18</div>
19<div id="content">
20
21<h1>Implementation Design</h1>
22
23<p>
24 This is a unordered list of implementation design decisions. Each topic tries
25 to follow this structure:
26</p>
27
28<ul>
29 <li>Problem statement</li>
30 <li>Proposed Solution</li>
31 <li>Alternatives and Discussion</li>
32</ul>
33
34
35<h2>Coverage Analysis Mechanism</h2>
36
37<p class="intro">
38 Coverage information has to be collected at runtime. For this purpose JaCoCo
39 creates instrumented versions of the original class definitions. The
40 instrumentation process happens on-the-fly during class loading using so
41 called Java agents.
42</p>
43
44<p>
45 There are several different approaches to collect coverage information. For
46 each approach different implementation techniques are known. The following
47 diagram gives an overview with the techniques used by JaCoCo highlighted:
48</p>
49
Evgeny Mandrikov8b212982016-06-12 17:55:49 +0200[diff] [blame^]50<img src="resources/implementation.png" alt="Coverage Implementation Techniques"/>
Evgeny Mandrikov82a92ca2012-01-15 20:25:48 +0000[diff] [blame]51
52<p>
53 Byte code instrumentation is very fast, can be implemented in pure Java and
54 works with every Java VM. On-the-fly instrumentation with the Java agent
55 hook can be added to the JVM without any modification of the target
56 application.
57</p>
58
59<p>
60 The Java agent hook requires at least 1.5 JVMs. Class files compiled with
61 debug information (line numbers) allow for source code highlighting. Unluckily
62 some Java language constructs get compiled to byte code that produces
63 unexpected highlighting results, especially in case of implicitly generated
64 code like default constructors or control structures for finally statements.
65</p>
66
67
68<h2>Coverage Agent Isolation</h2>
69
70<p class="intro">
71 The Java agent is loaded by the application class loader. Therefore the
72 classes of the agent live in the same name space like the application classes
73 which can result in clashes especially with the third party library ASM. The
74 JoCoCo build therefore moves all agent classes into a unique package.
75</p>
76
77<p>
78 The JaCoCo build renames all classes contained in the
79 <code>jacocoagent.jar</code> into classes with a
80 <code>org.jacoco.agent.rt_&lt;randomid&gt;</code> prefix, including the
81 required ASM library classes. The identifier is created from a random number.
82 As the agent does not provide any API, no one should be affected by this
83 renaming. This trick also allows that JaCoCo tests can be verified with
84 JaCoCo.
85</p>
86
87
88<h2>Minimal Java Version</h2>
89
90<p class="intro">
91 JaCoCo requires Java 1.5.
92</p>
93
94<p>
95 The Java agent mechanism used for on-the-fly instrumentation became available
96 with Java 1.5 VMs. Coding and testing with Java 1.5 language level is more
97 efficient, less error-prone &ndash; and more fun than with older versions.
98 JaCoCo will still allow to run against Java code compiled for these.
99</p>
100
101
102<h2>Byte Code Manipulation</h2>
103
104<p class="intro">
105 Instrumentation requires mechanisms to modify and generate Java byte code.
106 JaCoCo uses the ASM library for this purpose internally.
107</p>
108
109<p>
110 Implementing the Java byte code specification would be an extensive and
111 error-prone task. Therefore an existing library should be used. The
112 <a href="http://asm.objectweb.org/">ASM</a> library is lightweight, easy to
113 use and very efficient in terms of memory and CPU usage. It is actively
114 maintained and includes as huge regression test suite. Its simplified BSD
115 license is approved by the Eclipse Foundation for usage with EPL products.
116</p>
117
118<h2>Java Class Identity</h2>
119
120<p class="intro">
121 Each class loaded at runtime needs a unique identity to associate coverage data with.
122 JaCoCo creates such identities by a CRC64 hash code of the raw class definition.
123</p>
124
125<p>
126 In multi-classloader environments the plain name of a class does not
127 unambiguously identify a class. For example OSGi allows to use different
128 versions of the same class to be loaded within the same VM. In complex
129 deployment scenarios the actual version of the test target might be different
130 from current development version. A code coverage report should guarantee that
131 the presented figures are extracted from a valid test target. A hash code of
132 the class definitions allows to differentiate between classes and versions of
133 classes. The CRC64 hash computation is simple and fast resulting in a small 64
134 bit identifier.
135</p>
136
137<p>
138 The same class definition might be loaded by class loaders which will result
139 in different classes for the Java runtime system. For coverage analysis this
140 distinction should be irrelevant. Class definitions might be altered by other
141 instrumentation based technologies (e.g. AspectJ). In this case the hash code
142 will change and identity gets lost. On the other hand code coverage analysis
143 based on classes that have been somehow altered will produce unexpected
144 results. The CRC64 code might produce so called <i>collisions</i>, i.e.
145 creating the same hash code for two different classes. Although CRC64 is not
146 cryptographically strong and collision examples can be easily computed, for
147 regular class files the collision probability is very low.
148</p>
149
150<h2>Coverage Runtime Dependency</h2>
151
152<p class="intro">
153 Instrumented code typically gets a dependency to a coverage runtime which is
154 responsible for collecting and storing execution data. JaCoCo uses JRE types
155 only in generated instrumentation code.
156</p>
157
158<p>
159 Making a runtime library available to all instrumented classes can be a
160 painful or impossible task in frameworks that use their own class loading
161 mechanisms. Since Java 1.6 <code>java.lang.instrument.Instrumentation</code>
162 has an API to extends the bootsstrap loader. As our minimum target is Java 1.5
163 JaCoCo decouples the instrumented classes and the coverage runtime through
164 official JRE API types only. The instrumented classes communicate through the
165 <code>Object.equals(Object)</code> method with the runtime. A instrumented
166 class can retrieve its probe array instance with the following code. Note
167 that only JRE APIs are used:
168</p>
169
170
171<pre class="source lang-java linenums">
172Object access = ... // Retrieve instance
173
174Object[] args = new Object[3];
175args[0] = Long.valueOf(8060044182221863588); // class id
176args[1] = "com/example/MyClass"; // class name
177args[2] = Integer.valueOf(24); // probe count
178
179access.equals(args);
180
181boolean[] probes = (boolean[]) args[0];
182</pre>
183
184<p>
185 The most tricky part takes place in line 1 and is not shown in the snippet
186 above. The object instance providing access to the coverage runtime through
187 its <code>equals()</code> method has to be obtained. Different approaches have
188 been implemented and tested so far:
189</p>
190
191<ul>
192 <li><b><code>SystemPropertiesRuntime</code></b>: This approach stores the
193 object instance under a system property. This solution breaks the contract
194 that system properties must only contain <code>java.lang.String</code>
195 values and therefore causes trouble in applications that rely on this
196 definition (e.g. Ant).</li>
197 <li><b><code>LoggerRuntime</code></b>: Here we use a shared
198 <code>java.util.logging.Logger</code> and communicate through the logging
199 parameter array instead of a <code>equals()</code> method. The coverage
200 runtime registers a custom <code>Handler</code> to receive the parameter
201 array. This approach might break environments that install their own log
202 managers (e.g. Glassfish).</li>
203 <li><b><code>URLStreamHandlerRuntime</code></b>: This runtime registers a
204 <code>URLStreamHandler</code> for a "jacoco-xxxxx" protocol. Instrumented
205 classes open a connection on this protocol. The returned connection object
206 is the one that provides access to the coverage runtime through its
207 <code>equals()</code> method. However to register the protocol the runtime
208 needs to access internal members of the <code>java.net.URL</code> class.</li>
209 <li><b><code>ModifiedSystemClassRuntime</code></b>: This approach adds a
210 public static field to an existing JRE class through instrumentation. Unlike
211 the other methods above this is only possible for environments where a Java
212 agent is active.</li>
213</ul>
214
215<p>
216 The current JaCoCo Java agent implementation uses the
217 <code>ModifiedSystemClassRuntime</code> adding a field to the class
218 <code>java.sql.Types</code>.
219</p>
220
221
222<h2>Memory Usage</h2>
223
224<p class="intro">
225 Coverage analysis for huge projects with several thousand classes or hundred
226 thousand lines of code should be possible. To allow this with reasonable
227 memory usage the coverage analysis is based on streaming patterns and
228 "depth first" traversals.
229</p>
230
231<p>
232 The complete data tree of a huge coverage report is too big to fit into a
233 reasonable heap memory configuration. Therefore the coverage analysis and
234 report generation is implemented as "depth first" traversals. Which means that
235 at any point in time only the following data has to be held in working memory:
236</p>
237
238<ul>
239 <li>A single class which is currently processed.</li>
240 <li>The summary information of all parents of this class (package, groups).</li>
241</ul>
242
243<h2>Java Element Identifiers</h2>
244
245<p class="intro">
246 The Java language and the Java VM use different String representation formats
247 for Java elements. For example while a type reference in Java reads like
248 <code>java.lang.Object</code>, the VM references the same type as
249 <code>Ljava/lang/Object;</code>. The JaCoCo API is based on VM identifiers only.
250</p>
251
252<p>
253 Using VM identifiers directly does not cause any transformation overhead at
254 runtime. There are several programming languages based on the Java VM that
255 might use different notations. Specific transformations should therefore only
256 happen at the user interface level, for example during report generation.
257</p>
258
259<h2>Modularization of the JaCoCo implementation</h2>
260
261<p class="intro">
262 JaCoCo is implemented in several modules providing different functionality.
263 These modules are provided as OSGi bundles with proper manifest files. But
264 there are no dependencies on OSGi itself.
265</p>
266
267<p>
268 Using OSGi bundles allows well defined dependencies at development time and
269 at runtime in OSGi containers. As there are no dependencies on OSGi, the
270 bundles can also be used like regular JAR files.
271</p>
272
273</div>
274<div class="footer">
275 <span class="right"><a href="@jacoco.home.url@">JaCoCo</a> @qualified.bundle.version@</span>
276 <a href="license.html">Copyright</a> &copy; @copyright.years@ Mountainminds GmbH &amp; Co. KG and Contributors
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