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<h1>Class Ids</h1>

<p>
  As JaCoCo's class identifiers are sometimes causing confusion this chapter
  answers the concepts and common issues with class ids in FAQ style format.
</p>

<h3>What are class ids and how are they created?</h3>
<p>
  Class ids are 64-bit integer values, for example
  <code>0x638e104737889183</code> in hex notation. Their calculation is
  considered an implementation detail of JaCoCo. Currently ids are created with
  a CRC64 checksum of the raw class file.
</p>

<h3>What are class ids used for?</h3>
<p>
  Class ids are used to unambiguously identify Java classes. At runtime execution
  data is sampled for every loaded class and typically stored to
  <code>*.exec</code> files. At analysis time &mdash; for example for report
  generation &mdash; the class ids are used to relate analyzed classes with the
  execution data.
</p>

<h3>What are the advantages of JaCoCo class ids?</h3>
<p>
  The concept of class ids allows distinguishing different versions of classes,
  for example when multiple versions of an application are deployed to an
  application server or different versions of libraries are included.
</p>
<p>
  Also class ids are the prerequisite for JaCoCo's minimal runtime-overhead and
  small <code>*.exec</code> files even for very large applications under test.
</p>

<h3>What is the disadvantage of JaCoCo class ids?</h3>
<p>
  The fact that class ids identify a specific version of a class causes problems
  in setups where different classes are used at runtime and at analysis time.
</p>

<h3>What happens if different classes are used at runtime and at analysis time?</h3>
<p>
  In this case execution data cannot be related to the analyzed classes. As a
  consequence such classes are reported with 0% coverage.
</p>

<h3>How can I detect that I have a problem with class ids?</h3>
<p>
  The typical symptom of class id mismatch is classes not shown as covered
  although they have been executed during the test. This situation can be easily
  detected e.g. in the HTML report: Open the <i>Sessions</i> page with the link
  on the top-right corner. You see a list of all classes where execution data
  has been collected for. Find the class in questions and check whether the
  entry has a link to the corresponding coverage report page. If the entry is
  not linked this means there is a class id mismatch between the class used at
  runtime and the class provided to create the report.
</p>

<h3>What can cause different class ids?</h3>
<p>
  Class ids are identical for the exact same class file only (byte-by-byte).
  There is a couple of reasons why you might get different class files. First
  compiling Java source files will result in different class files if you use
  a different tool chain:
</p>
<ul>
  <li>Different compiler vendor (e.g. Eclipse vs. Oracle JDK)</li>
  <li>Different compiler versions</li>
  <li>Different compiler settings (e.g. debug vs. non-debug) </li>
</ul>
<p>
  Also post-processing class files (obfuscation, AspectJ, etc.) will typically
  change the class files. JaCoCo will work well if you simply use the same class
  files for runtime as well as for analysis. So the tool chain to create these
  class files does not matter.
</p>
<p>
  Even if the class files on the file system are the same there is possible that
  classes seen by the JaCoCo runtime agent are different anyways. This typically
  happens when another Java agent is configured <i>before</i> the JaCoCo agent
  or special class loaders pre-process the class files. Typical candidates are:
</p>
<ul>
  <li>Mocking frameworks</li>
  <li>Application servers</li>
  <li>Persistence frameworks</li>
</ul>

<h3>What workarounds exist to deal with runtime-modified classes?</h3>
<p>
  If classes get modified at runtime in your setup there are some workarounds to
  make JaCoCo work anyways:
</p>
<ul>
  <li>If you use another Java agent make sure the <a href="agent.html">JaCoCo
      agent</a> is specified at first in the command line. This way the JaCoCo
      agent should see the original class files.</li>
  <li>Specify the <code>classdumpdir</code> option of the
      <a href="agent.html">JaCoCo agent</a> and use the dumped classes at report
      generation. Note that only loaded classes will be dumped, i.e. classes not
      executed at all will not show-up in your report as not covered.</li>
  <li>Use <a href="offline.html">offline instrumentation</a> before you run your
      tests. This way classes get instrumented by JaCoCo before any runtime
      modification can take place. Note that in this case the report has to be
      generated with the <i>original</i> classes, not with instrumented ones.</li>
</ul>

<h3>Why can't JaCoCo simply use the class name to identify classes?</h3>
<p>
  To understand why JaCoCo can't rely on class names we need to have a look at
  the way how JaCoCo measures code coverage.
</p>
<p>
  JaCoCo tracks execution with so called <i>probes</i>. Probes are additional
  byte code instructions inserted in the original class file which will note
  when they are executed and report this to the JaCoCo runtime. This process is
  called <i>instrumentation</i>. To keep the runtime overhead minimal, only a
  few probes are inserted at "strategic" places. These probe positions are
  determined by <a href="flow.html">analyzing the control flow</a> of all
  methods of a class. As a result every instrumented class produces a list of
  <code>n</code> boolean flags indicating whether the probe has been executed or
  not. A JaCoCo <code>*.exec</code> file simply stores a boolean array per
  class id.
</p>
<p>
  At analysis time, for example for report generation, the <code>*.exec</code>
  file is used to get information about probe execution status. But as probes
  are stored in a plain boolean array there is no information like corresponding
  methods or lines. To retrieve this information we need the original class
  files and perform the exact same control flow analysis than at instrumentation
  time. Because this is a deterministic process we get the same probe positions.
  With this information we can now interfere the execution status of every
  single instruction and branch of a method. Using the debug information
  embedded in the class files we can also calculate line coverage.
</p>
<p>
  If we would use just slightly different classes at analysis time than at
  runtime &mdash; e.g. different method ordering or additional branches &mdash;
  we would end-up with different probes. For example the probe at index
  <code>i</code> would be in method <code>a()</code> and not in method
  <b>b()</b>. Obviously this will create random coverage results.
</p>

<h3>Why do I get an error when I try to analyze multiple versions of the same
    class with a group?</h3>
<p>
  JaCoCo always analyzes a set of class as a group. The group is used to
  aggregate data for source files and packages (both can contain multiple
  classes). Within the reporting API classes are identified by their fully
  qualified name (e.g. to create stable file names in the HTML reports).
  Therefore it is not possible to include two different classes with the same
  name within a group. Anyhow it is possible to analyze different versions of
  class files in separate groups, for example the <a href="ant.html#report">Ant
  report task</a> can be configured with multiple groups.
</p>

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