Add DEBUG level logging and report classpath and JDK type to allow debugging of classpath/JDK related problems

src/main/java/de/thetaphi/forbiddenapis/Checker.java

@@ -101,6 +101,7 @@ public Checker(Logger logger, ClassLoader loader, EnumSet<Option> options) {
       method_Module_getName = method_Class_getModule
           .getReturnType().getMethod("getName");
       isSupportedJDK = true;
+      logger.debug("Detected Java 9 or later with module system.");
     } catch (NoSuchMethodException e) {
       method_Class_getModule = method_Module_getName = null;
     }
@@ -116,6 +117,7 @@ public Checker(Logger logger, ClassLoader loader, EnumSet<Option> options) {
         if (objectClassURL != null && "jrt".equalsIgnoreCase(objectClassURL.getProtocol())) {
           // this is Java 9+ allowing direct access to .class file resources - we do not need to deal with modules!
           isSupportedJDK = true;
+          logger.debug("Detected Java 9 or later with JRT file system.");
         } else {
           String javaHome = System.getProperty("java.home");
           if (javaHome != null) {
@@ -146,7 +148,9 @@ public Checker(Logger logger, ClassLoader loader, EnumSet<Option> options) {
             }
           }
           isSupportedJDK = !runtimePaths.isEmpty();
-          if (!isSupportedJDK) {
+          if (isSupportedJDK) {
+            logger.debug("Detected classical classpath-based JDK @ " + runtimePaths);
+          } else {
             logger.warn("Boot classpath appears to be empty or ${java.home} not defined; marking runtime as not suppported.");
           }
         }

src/main/java/de/thetaphi/forbiddenapis/Logger.java

@@ -20,4 +20,5 @@ public interface Logger {
   void error(String msg);
   void warn(String msg);
   void info(String msg);
+  void debug(String msg);
 }

src/main/java/de/thetaphi/forbiddenapis/StdIoLogger.java

@@ -38,4 +38,9 @@ public void info(String msg) {
     System.out.println(msg);
   }
 
+  @Override
+  public void debug(String msg) {
+    // no reporting of debug messages
+  }
+
 }

src/main/java/de/thetaphi/forbiddenapis/ant/AntTask.java

@@ -91,6 +91,11 @@ public void warn(String msg) {
       public void info(String msg) {
         log(msg, Project.MSG_INFO);
       }
+
+      @Override
+      public void debug(String msg) {
+        log(msg, Project.MSG_DEBUG);
+      }
     };
 
     AntClassLoader antLoader = null;
@@ -100,6 +105,7 @@ public void info(String msg) {
         classpath.setProject(getProject());
         loader = antLoader = getProject().createClassLoader(ClassLoader.getSystemClassLoader(), classpath);
         antLoader.setParentFirst(true); // use default classloader delegation
+        log.debug("Classpath: " + classpath.toString());
       } else {
         loader = ClassLoader.getSystemClassLoader();
       }

src/main/java/de/thetaphi/forbiddenapis/gradle/CheckForbiddenApis.java

@@ -498,21 +498,32 @@ public void warn(String msg) {
       public void info(String msg) {
         getLogger().info(msg);
       }
+
+      @Override
+      public void debug(String msg) {
+        getLogger().debug(msg);
+      }
     };
 
     final Set<File> cpElements = new LinkedHashSet<>();
     cpElements.addAll(classpath.getFiles());
     cpElements.addAll(classesDirs.getFiles());
     final URL[] urls = new URL[cpElements.size()];
+    final StringBuilder humanClasspath = new StringBuilder();
     try {
       int i = 0;
       for (final File cpElement : cpElements) {
         urls[i++] = cpElement.toURI().toURL();
+        if (humanClasspath.length() > 0) {
+          humanClasspath.append(File.pathSeparatorChar);
+        }
+        humanClasspath.append(cpElement);
       }
       assert i == urls.length;
     } catch (MalformedURLException mfue) {
       throw new InvalidUserDataException("Failed to build classpath URLs.", mfue);
     }
+    log.debug("Classpath: " + humanClasspath);
 
     URLClassLoader urlLoader = null;
     final ClassLoader loader = (urls.length > 0) ?

src/main/java/de/thetaphi/forbiddenapis/maven/AbstractCheckMojo.java

@@ -304,6 +304,11 @@ public void warn(String msg) {
       public void info(String msg) {
         getLog().info(msg);
       }
+
+      @Override
+      public void debug(String msg) {
+        getLog().debug(msg);
+      }
     };
 
     if (skip) {
@@ -323,15 +328,21 @@ public void info(String msg) {
 
     final List<String> cp = getClassPathElements();
     final URL[] urls = new URL[cp.size()];
+    final StringBuilder humanClasspath = new StringBuilder();
     try {
       int i = 0;
       for (final String cpElement : cp) {
         urls[i++] = new File(cpElement).toURI().toURL();
+        if (humanClasspath.length() > 0) {
+          humanClasspath.append(File.pathSeparatorChar);
+        }
+        humanClasspath.append(cpElement);
       }
       assert i == urls.length;
     } catch (MalformedURLException e) {
       throw new MojoExecutionException("Failed to build classpath.", e);
     }
+    log.debug("Classpath: " + humanClasspath);
 
     URLClassLoader urlLoader = null;
     final ClassLoader loader = (urls.length > 0) ?