title: Is prepare-agent goal is necessary in jacoco-maven-plugin?
body: <p>In My project, if I write pom like this:</p>
  
  <pre><code>...
          &lt;plugin&gt;
              &lt;groupId&gt;org.jacoco&lt;/groupId&gt;
              &lt;artifactId&gt;jacoco-maven-plugin&lt;/artifactId&gt;
              &lt;version&gt;0.7.9&lt;/version&gt;
              &lt;executions&gt; 
                  &lt;execution&gt;  
                    &lt;id&gt;post-test&lt;/id&gt;  
                    &lt;phase&gt;test&lt;/phase&gt;  
                    &lt;goals&gt;  
                          &lt;goal&gt;report&lt;/goal&gt;  
                    &lt;/goals&gt;  
                  &lt;/execution&gt;  
             &lt;/executions&gt;  
  
          &lt;/plugin&gt;
  ...
  </code></pre>
  
  <p>it will not generate report in my project after i run mvn install.</p>
  
  <p>but i change it to </p>
  
  <pre><code>&lt;plugin&gt;
              &lt;groupId&gt;org.jacoco&lt;/groupId&gt;
              &lt;artifactId&gt;jacoco-maven-plugin&lt;/artifactId&gt;
              &lt;version&gt;0.7.9&lt;/version&gt;
              &lt;executions&gt; 
                  &lt;execution&gt;  
                      &lt;id&gt;pre-test&lt;/id&gt;  
                      &lt;goals&gt;  
                        &lt;goal&gt;prepare-agent&lt;/goal&gt;  
                      &lt;/goals&gt;  
                    &lt;/execution&gt; 
                  &lt;execution&gt;  
                    &lt;id&gt;post-test&lt;/id&gt;  
                    &lt;phase&gt;test&lt;/phase&gt;  
                    &lt;goals&gt;  
                          &lt;goal&gt;report&lt;/goal&gt;  
                    &lt;/goals&gt;  
                  &lt;/execution&gt;  
             &lt;/executions&gt;  
  
          &lt;/plugin&gt;
  </code></pre>
  
  <p>it worked!!</p>
  
  <p>I want to know what was the different?</p>
  
  <p>I read the offical document here: <a href="http://www.jacoco.org/jacoco/trunk/doc/prepare-agent-mojo.html" rel="noreferrer">http://www.jacoco.org/jacoco/trunk/doc/prepare-agent-mojo.html</a></p>
  
  <p>the goal prepare-agent is just for set property for jvm agent, not start jvm agent, why it is necessary?</p>