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   <a href="../index.html" class="el_report">JaCoCo</a> &gt;
   <a href="index.html" class="el_group">Documentation</a> &gt;
   <span class="el_source">Implementation Design</span>
 </div>
 <div id="content">

 <h1>Implementation Design</h1>

 <p>
   This is a unordered list of implementation design decisions. Each topic tries
   to follow this structure:
 </p>

 <ul>
   <li>Problem statement</li>
   <li>Proposed Solution</li>
   <li>Alternatives and Discussion</li>
 </ul>


 <h2>Coverage Analysis Mechanism</h2>

 <p class="intro">
   Coverage information has to be collected at runtime. For this purpose JaCoCo
   creates instrumented versions of the original class definitions. The
   instrumentation process happens on-the-fly during class loading using so
   called Java agents.
 </p>

 <p>
   There are several different approaches to collect coverage information. For
   each approach different implementation techniques are known. The following
   diagram gives an overview with the techniques used by JaCoCo highlighted:
 </p>

 <img src="resources/implementation.png" alt="Coverage Implementation Techniques"/>

 <p>
   Byte code instrumentation is very fast, can be implemented in pure Java and
   works with every Java VM. On-the-fly instrumentation with the Java agent
   hook can be added to the JVM without any modification of the target
   application.
 </p>

 <p>
   The Java agent hook requires at least 1.5 JVMs. Class files compiled with
   debug information (line numbers) allow for source code highlighting. Unluckily
   some Java language constructs get compiled to byte code that produces
   unexpected highlighting results, especially in case of implicitly generated
   code like default constructors or control structures for finally statements.
 </p>


 <h2>Coverage Agent Isolation</h2>

 <p class="intro">
   The Java agent is loaded by the application class loader. Therefore the
   classes of the agent live in the same name space like the application classes
   which can result in clashes especially with the third party library ASM. The
   JoCoCo build therefore moves all agent classes into a unique package.
 </p>

 <p>
   The JaCoCo build renames all classes contained in the
   <code>jacocoagent.jar</code> into classes with a
   <code>org.jacoco.agent.rt_&lt;randomid&gt;</code> prefix, including the
   required ASM library classes. The identifier is created from a random number.
   As the agent does not provide any API, no one should be affected by this
   renaming. This trick also allows that JaCoCo tests can be verified with
   JaCoCo.
 </p>


 <h2>Minimal Java Version</h2>

 <p class="intro">
   JaCoCo requires Java 1.5.
 </p>

 <p>
   The Java agent mechanism used for on-the-fly instrumentation became available
   with Java 1.5 VMs. Coding and testing with Java 1.5 language level is more
   efficient, less error-prone &ndash; and more fun than with older versions.
   JaCoCo will still allow to run against Java code compiled for these.
 </p>


 <h2>Byte Code Manipulation</h2>

 <p class="intro">
   Instrumentation requires mechanisms to modify and generate Java byte code.
   JaCoCo uses the ASM library for this purpose internally.
 </p>

 <p>
   Implementing the Java byte code specification would be an extensive and
   error-prone task. Therefore an existing library should be used. The
   <a href="http://asm.objectweb.org/">ASM</a> library is lightweight, easy to
   use and very efficient in terms of memory and CPU usage. It is actively
   maintained and includes as huge regression test suite. Its simplified BSD
   license is approved by the Eclipse Foundation for usage with EPL products.
 </p>

 <h2>Java Class Identity</h2>

 <p class="intro">
   Each class loaded at runtime needs a unique identity to associate coverage data with.
   JaCoCo creates such identities by a CRC64 hash code of the raw class definition.
 </p>

 <p>
   In multi-classloader environments the plain name of a class does not
   unambiguously identify a class. For example OSGi allows to use different
   versions of the same class to be loaded within the same VM. In complex
   deployment scenarios the actual version of the test target might be different
   from current development version. A code coverage report should guarantee that
   the presented figures are extracted from a valid test target. A hash code of
   the class definitions allows to differentiate between classes and versions of
   classes. The CRC64 hash computation is simple and fast resulting in a small 64
   bit identifier.
 </p>

 <p>
   The same class definition might be loaded by class loaders which will result
   in different classes for the Java runtime system. For coverage analysis this
   distinction should be irrelevant. Class definitions might be altered by other
   instrumentation based technologies (e.g. AspectJ). In this case the hash code
   will change and identity gets lost. On the other hand code coverage analysis
   based on classes that have been somehow altered will produce unexpected
   results. The CRC64 code might produce so called <i>collisions</i>, i.e.
   creating the same hash code for two different classes. Although CRC64 is not
   cryptographically strong and collision examples can be easily computed, for
   regular class files the collision probability is very low.
 </p>

 <h2>Coverage Runtime Dependency</h2>

 <p class="intro">
   Instrumented code typically gets a dependency to a coverage runtime which is
   responsible for collecting and storing execution data. JaCoCo uses JRE types
   only in generated instrumentation code.
 </p>

 <p>
   Making a runtime library available to all instrumented classes can be a
   painful or impossible task in frameworks that use their own class loading
   mechanisms. Since Java 1.6 <code>java.lang.instrument.Instrumentation</code>
   has an API to extends the bootsstrap loader. As our minimum target is Java 1.5
   JaCoCo decouples the instrumented classes and the coverage runtime through
   official JRE API types only. The instrumented classes communicate through the
   <code>Object.equals(Object)</code> method with the runtime. A instrumented
   class can retrieve its probe array instance with the following code. Note
   that only JRE APIs are used:
 </p>


 <pre class="source lang-java linenums">
 Object access = ...                          // Retrieve instance

 Object[] args = new Object[3];
 args[0] = Long.valueOf(8060044182221863588); // class id
 args[1] = "com/example/MyClass";             // class name
 args[2] = Integer.valueOf(24);               // probe count

 access.equals(args);

 boolean[] probes = (boolean[]) args[0];
 </pre>

 <p>
   The most tricky part takes place in line 1 and is not shown in the snippet
   above. The object instance providing access to the coverage runtime through
   its <code>equals()</code> method has to be obtained. Different approaches have
   been implemented and tested so far:
 </p>

 <ul>
   <li><b><code>SystemPropertiesRuntime</code></b>: This approach stores the
     object instance under a system property. This solution breaks the contract
     that system properties must only contain <code>java.lang.String</code>
     values and therefore causes trouble in applications that rely on this
     definition (e.g. Ant).</li>
   <li><b><code>LoggerRuntime</code></b>: Here we use a shared
     <code>java.util.logging.Logger</code> and communicate through the logging
     parameter array instead of a <code>equals()</code> method. The coverage
     runtime registers a custom <code>Handler</code> to receive the parameter
     array. This approach might break environments that install their own log
     managers (e.g. Glassfish).</li>
   <li><b><code>URLStreamHandlerRuntime</code></b>: This runtime registers a
     <code>URLStreamHandler</code> for a "jacoco-xxxxx" protocol. Instrumented
     classes open a connection on this protocol. The returned connection object
     is the one that provides access to the coverage runtime through its
     <code>equals()</code> method. However to register the protocol the runtime
     needs to access internal members of the <code>java.net.URL</code> class.</li>
   <li><b><code>ModifiedSystemClassRuntime</code></b>: This approach adds a
     public static field to an existing JRE class through instrumentation. Unlike
     the other methods above this is only possible for environments where a Java
     agent is active.</li>
 </ul>

 <p>
   The current JaCoCo Java agent implementation uses the
   <code>ModifiedSystemClassRuntime</code> adding a field to the class
   <code>java.sql.Types</code>.
 </p>


 <h2>Memory Usage</h2>

 <p class="intro">
   Coverage analysis for huge projects with several thousand classes or hundred
   thousand lines of code should be possible. To allow this with reasonable
   memory usage the coverage analysis is based on streaming patterns and
   "depth first" traversals.
 </p>

 <p>
   The complete data tree of a huge coverage report is too big to fit into a
   reasonable heap memory configuration. Therefore the coverage analysis and
   report generation is implemented as "depth first" traversals. Which means that
   at any point in time only the following data has to be held in working memory:
 </p>

 <ul>
   <li>A single class which is currently processed.</li>
   <li>The summary information of all parents of this class (package, groups).</li>
 </ul>

 <h2>Java Element Identifiers</h2>

 <p class="intro">
   The Java language and the Java VM use different String representation formats
   for Java elements. For example while a type reference in Java reads like
   <code>java.lang.Object</code>, the VM references the same type as
   <code>Ljava/lang/Object;</code>. The JaCoCo API is based on VM identifiers only.
 </p>

 <p>
   Using VM identifiers directly does not cause any transformation overhead at
   runtime. There are several programming languages based on the Java VM that
   might use different notations. Specific transformations should therefore only
   happen at the user interface level, for example during report generation.
 </p>

 <h2>Modularization of the JaCoCo implementation</h2>

 <p class="intro">
   JaCoCo is implemented in several modules providing different functionality.
   These modules are provided as OSGi bundles with proper manifest files. But
   there are no dependencies on OSGi itself.
 </p>

 <p>
   Using OSGi bundles allows well defined dependencies at development time and
   at runtime in OSGi containers. As there are no dependencies on OSGi, the
   bundles can also be used like regular JAR files.
 </p>

 </div>
 <div class="footer">
   <span class="right"><a href="@jacoco.home.url@">JaCoCo</a> @qualified.bundle.version@</span>
   <a href="license.html">Copyright</a> &copy; @copyright.years@ Mountainminds GmbH &amp; Co. KG and Contributors
 </div>

 </body>
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	<link rel="shortcut icon" href="resources/report.gif" type="image/gif" />
	<script type="text/javascript" src="../coverage/jacoco-resources/prettify.js"></script>
	<title>JaCoCo - Implementation Design</title>
	</head>
	<body onload="prettyPrint()">

	<div class="breadcrumb">
	<a href="../index.html" class="el_report">JaCoCo</a> >
	<a href="index.html" class="el_group">Documentation</a> >
	<span class="el_source">Implementation Design</span>
	</div>
	<div id="content">

	<h1>Implementation Design</h1>

	<p>
	This is a unordered list of implementation design decisions. Each topic tries
	to follow this structure:
	</p>

	<ul>
	<li>Problem statement</li>
	<li>Proposed Solution</li>
	<li>Alternatives and Discussion</li>
	</ul>


	<h2>Coverage Analysis Mechanism</h2>

	<p class="intro">
	Coverage information has to be collected at runtime. For this purpose JaCoCo
	creates instrumented versions of the original class definitions. The
	instrumentation process happens on-the-fly during class loading using so
	called Java agents.
	</p>

	<p>
	There are several different approaches to collect coverage information. For
	each approach different implementation techniques are known. The following
	diagram gives an overview with the techniques used by JaCoCo highlighted:
	</p>

	<img src="resources/implementation.png" alt="Coverage Implementation Techniques"/>

	<p>
	Byte code instrumentation is very fast, can be implemented in pure Java and
	works with every Java VM. On-the-fly instrumentation with the Java agent
	hook can be added to the JVM without any modification of the target
	application.
	</p>

	<p>
	The Java agent hook requires at least 1.5 JVMs. Class files compiled with
	debug information (line numbers) allow for source code highlighting. Unluckily
	some Java language constructs get compiled to byte code that produces
	unexpected highlighting results, especially in case of implicitly generated
	code like default constructors or control structures for finally statements.
	</p>


	<h2>Coverage Agent Isolation</h2>

	<p class="intro">
	The Java agent is loaded by the application class loader. Therefore the
	classes of the agent live in the same name space like the application classes
	which can result in clashes especially with the third party library ASM. The
	JoCoCo build therefore moves all agent classes into a unique package.
	</p>

	<p>
	The JaCoCo build renames all classes contained in the
	<code>jacocoagent.jar</code> into classes with a
	<code>org.jacoco.agent.rt_<randomid></code> prefix, including the
	required ASM library classes. The identifier is created from a random number.
	As the agent does not provide any API, no one should be affected by this
	renaming. This trick also allows that JaCoCo tests can be verified with
	JaCoCo.
	</p>


	<h2>Minimal Java Version</h2>

	<p class="intro">
	JaCoCo requires Java 1.5.
	</p>

	<p>
	The Java agent mechanism used for on-the-fly instrumentation became available
	with Java 1.5 VMs. Coding and testing with Java 1.5 language level is more
	efficient, less error-prone – and more fun than with older versions.
	JaCoCo will still allow to run against Java code compiled for these.
	</p>


	<h2>Byte Code Manipulation</h2>

	<p class="intro">
	Instrumentation requires mechanisms to modify and generate Java byte code.
	JaCoCo uses the ASM library for this purpose internally.
	</p>

	<p>
	Implementing the Java byte code specification would be an extensive and
	error-prone task. Therefore an existing library should be used. The
	<a href="http://asm.objectweb.org/">ASM</a> library is lightweight, easy to
	use and very efficient in terms of memory and CPU usage. It is actively
	maintained and includes as huge regression test suite. Its simplified BSD
	license is approved by the Eclipse Foundation for usage with EPL products.
	</p>

	<h2>Java Class Identity</h2>

	<p class="intro">
	Each class loaded at runtime needs a unique identity to associate coverage data with.
	JaCoCo creates such identities by a CRC64 hash code of the raw class definition.
	</p>

	<p>
	In multi-classloader environments the plain name of a class does not
	unambiguously identify a class. For example OSGi allows to use different
	versions of the same class to be loaded within the same VM. In complex
	deployment scenarios the actual version of the test target might be different
	from current development version. A code coverage report should guarantee that
	the presented figures are extracted from a valid test target. A hash code of
	the class definitions allows to differentiate between classes and versions of
	classes. The CRC64 hash computation is simple and fast resulting in a small 64
	bit identifier.
	</p>

	<p>
	The same class definition might be loaded by class loaders which will result
	in different classes for the Java runtime system. For coverage analysis this
	distinction should be irrelevant. Class definitions might be altered by other
	instrumentation based technologies (e.g. AspectJ). In this case the hash code
	will change and identity gets lost. On the other hand code coverage analysis
	based on classes that have been somehow altered will produce unexpected
	results. The CRC64 code might produce so called <i>collisions</i>, i.e.
	creating the same hash code for two different classes. Although CRC64 is not
	cryptographically strong and collision examples can be easily computed, for
	regular class files the collision probability is very low.
	</p>

	<h2>Coverage Runtime Dependency</h2>

	<p class="intro">
	Instrumented code typically gets a dependency to a coverage runtime which is
	responsible for collecting and storing execution data. JaCoCo uses JRE types
	only in generated instrumentation code.
	</p>

	<p>
	Making a runtime library available to all instrumented classes can be a
	painful or impossible task in frameworks that use their own class loading
	mechanisms. Since Java 1.6 <code>java.lang.instrument.Instrumentation</code>
	has an API to extends the bootsstrap loader. As our minimum target is Java 1.5
	JaCoCo decouples the instrumented classes and the coverage runtime through
	official JRE API types only. The instrumented classes communicate through the
	<code>Object.equals(Object)</code> method with the runtime. A instrumented
	class can retrieve its probe array instance with the following code. Note
	that only JRE APIs are used:
	</p>


	<pre class="source lang-java linenums">
	Object access = ... // Retrieve instance

	Object[] args = new Object[3];
	args[0] = Long.valueOf(8060044182221863588); // class id
	args[1] = "com/example/MyClass"; // class name
	args[2] = Integer.valueOf(24); // probe count

	access.equals(args);

	boolean[] probes = (boolean[]) args[0];
	</pre>

	<p>
	The most tricky part takes place in line 1 and is not shown in the snippet
	above. The object instance providing access to the coverage runtime through
	its <code>equals()</code> method has to be obtained. Different approaches have
	been implemented and tested so far:
	</p>

	<ul>
	<li><b><code>SystemPropertiesRuntime</code></b>: This approach stores the
	object instance under a system property. This solution breaks the contract
	that system properties must only contain <code>java.lang.String</code>
	values and therefore causes trouble in applications that rely on this
	definition (e.g. Ant).</li>
	<li><b><code>LoggerRuntime</code></b>: Here we use a shared
	<code>java.util.logging.Logger</code> and communicate through the logging
	parameter array instead of a <code>equals()</code> method. The coverage
	runtime registers a custom <code>Handler</code> to receive the parameter
	array. This approach might break environments that install their own log
	managers (e.g. Glassfish).</li>
	<li><b><code>URLStreamHandlerRuntime</code></b>: This runtime registers a
	<code>URLStreamHandler</code> for a "jacoco-xxxxx" protocol. Instrumented
	classes open a connection on this protocol. The returned connection object
	is the one that provides access to the coverage runtime through its
	<code>equals()</code> method. However to register the protocol the runtime
	needs to access internal members of the <code>java.net.URL</code> class.</li>
	<li><b><code>ModifiedSystemClassRuntime</code></b>: This approach adds a
	public static field to an existing JRE class through instrumentation. Unlike
	the other methods above this is only possible for environments where a Java
	agent is active.</li>
	</ul>

	<p>
	The current JaCoCo Java agent implementation uses the
	<code>ModifiedSystemClassRuntime</code> adding a field to the class
	<code>java.sql.Types</code>.
	</p>


	<h2>Memory Usage</h2>

	<p class="intro">
	Coverage analysis for huge projects with several thousand classes or hundred
	thousand lines of code should be possible. To allow this with reasonable
	memory usage the coverage analysis is based on streaming patterns and
	"depth first" traversals.
	</p>

	<p>
	The complete data tree of a huge coverage report is too big to fit into a
	reasonable heap memory configuration. Therefore the coverage analysis and
	report generation is implemented as "depth first" traversals. Which means that
	at any point in time only the following data has to be held in working memory:
	</p>

	<ul>
	<li>A single class which is currently processed.</li>
	<li>The summary information of all parents of this class (package, groups).</li>
	</ul>

	<h2>Java Element Identifiers</h2>

	<p class="intro">
	The Java language and the Java VM use different String representation formats
	for Java elements. For example while a type reference in Java reads like
	<code>java.lang.Object</code>, the VM references the same type as
	<code>Ljava/lang/Object;</code>. The JaCoCo API is based on VM identifiers only.
	</p>

	<p>
	Using VM identifiers directly does not cause any transformation overhead at
	runtime. There are several programming languages based on the Java VM that
	might use different notations. Specific transformations should therefore only
	happen at the user interface level, for example during report generation.
	</p>

	<h2>Modularization of the JaCoCo implementation</h2>

	<p class="intro">
	JaCoCo is implemented in several modules providing different functionality.
	These modules are provided as OSGi bundles with proper manifest files. But
	there are no dependencies on OSGi itself.
	</p>

	<p>
	Using OSGi bundles allows well defined dependencies at development time and
	at runtime in OSGi containers. As there are no dependencies on OSGi, the
	bundles can also be used like regular JAR files.
	</p>

	</div>
	<div class="footer">
	<span class="right"><a href="@jacoco.home.url@">JaCoCo</a> @qualified.bundle.version@</span>
	<a href="license.html">Copyright</a> © @copyright.years@ Mountainminds GmbH & Co. KG and Contributors
	</div>

	</body>
	</html>