public static @NotNull LanguageLevel getDefaultLevel(MavenProject mavenProject) {
    MavenPlugin plugin = mavenProject.findPlugin("org.apache.maven.plugins", "maven-compiler-plugin");
    if (plugin != null && plugin.getVersion() != null) {
      //https://github.com/apache/maven-compiler-plugin/blob/master/src/main/java/org/apache/maven/plugin/compiler/AbstractCompilerMojo.java
      // consider "source" parameter documentation.
      // also note, that this are versions of plugin, not maven.
      if (VersionComparatorUtil.compare("3.11.0", plugin.getVersion()) <= 0) {
        return LanguageLevel.JDK_1_8;
      }
      if (VersionComparatorUtil.compare("3.9.0", plugin.getVersion()) <= 0) {
        return LanguageLevel.JDK_1_7;
      }
      if (VersionComparatorUtil.compare("3.8.0", plugin.getVersion()) <= 0) {
        return LanguageLevel.JDK_1_6;
      }
      else {
        return LanguageLevel.JDK_1_5;
      }
    }
    return LanguageLevel.JDK_1_5;
  }
public static @NotNull LanguageLevel getDefaultLevel(MavenProject mavenProject)