If FAIL_ON_MISSING_CLASSES is disabled only log a summary of classes with WARN level

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

@@ -23,6 +23,7 @@
 import java.net.URL;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
+import java.util.Collection;
 import java.util.Locale;
 import java.util.Objects;
 import java.util.regex.Pattern;
@@ -190,5 +191,23 @@ public static boolean isExceptionInAsmClassReader(RuntimeException re) {
     final StackTraceElement[] stack = re.getStackTrace();
     return stack.length > 0 && Objects.equals(ClassReader.class.getName(), stack[0].getClassName());
   }
+
+  /** Formats a list of classes, abbreviated, with 2 spaces in front (for logging) */
+  public static String formatClassesAbbreviated(Collection<String> missingClasses) {
+    final StringBuilder sb = new StringBuilder();
+    int count = 0;
+    for (String s : missingClasses) {
+      sb.append(count == 0 ? "  " : ", ").append(s);
+      count++;
+      if (sb.length() >= 70) {
+        int remaining = missingClasses.size() - count;
+        if (remaining > 0) {
+          sb.append(",... (and ").append(remaining).append(" more).");
+        }
+        break;
+      }
+    }
+    return sb.toString();
+  }
 
 }

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

@@ -74,6 +74,10 @@ public static enum Option {
   /** Cache of loaded classes: key is the binary name (dotted) */
   final Map<String,ClassMetadata> classpathClassCache = new HashMap<>();
 
+  /** Related classes (binary name, dotted) which were not found while looking up
+   * class metadata [referenced (super)classes, interfaces,...] */
+  final Set<String> missingClasses = new TreeSet<>();
+
   final Signatures forbiddenSignatures;
 
   /** descriptors (not internal names) of all annotations that suppress */
@@ -305,10 +309,7 @@ public ClassMetadata lookupRelatedClass(String internalName, String internalName
       if (options.contains(Option.FAIL_ON_MISSING_CLASSES)) {
         throw new RelatedClassLoadingException(cnfe, origClassName);
       } else {
-        logger.warn(String.format(Locale.ENGLISH,
-          "Class '%s' cannot be loaded (while looking up details about referenced class '%s'). Please fix the classpath!",
-          type.getClassName(), origClassName
-        ));
+        missingClasses.add(type.getClassName());
         return null;
       }
     } catch (IOException ioe) {
@@ -464,6 +465,11 @@ public void run() throws ForbiddenApiException {
       errors += checkClass(c, suppressAnnotationsPattern);
     }
 
+    if (!missingClasses.isEmpty() ) {
+      logger.warn("While scanning classes to check, the following referenced classes were not found on classpath (this may miss some violations):");
+      logger.warn(AsmUtils.formatClassesAbbreviated(missingClasses));
+    }
+
     final String message = String.format(Locale.ENGLISH, 
         "Scanned %d class file(s) for forbidden API invocations (in %.2fs), %d error(s).",
         classesToCheck.size(), (System.currentTimeMillis() - start) / 1000.0, errors);

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

@@ -232,20 +232,7 @@ private void reportMissingSignatureClasses(Set<String> missingClasses) {
       return;
     }
     logger.warn("Some signatures were ignored because the following classes were not found on classpath:");
-    final StringBuilder sb = new StringBuilder();
-    int count = 0;
-    for (String s : missingClasses) {
-      sb.append(count == 0 ? "  " : ", ").append(s);
-      count++;
-      if (sb.length() >= 70) {
-        int remaining = missingClasses.size() - count;
-        if (remaining > 0) {
-          sb.append(",... (and ").append(remaining).append(" more).");
-        }
-        break;
-      }
-    }
-    logger.warn(sb.toString());
+    logger.warn(AsmUtils.formatClassesAbbreviated(missingClasses));
   }
 
   private void addBundledSignatures(String name, String jdkTargetVersion, boolean logging, Set<String> missingClasses) throws IOException,ParseException {