diff -r 14e098407bb0 -r ad69fd32778e src/jdk.compiler/share/classes/com/sun/tools/javac/util/MandatoryWarningHandler.java
--- a/src/jdk.compiler/share/classes/com/sun/tools/javac/util/MandatoryWarningHandler.java	Mon Oct 21 15:11:44 2019 +0200
+++ b/src/jdk.compiler/share/classes/com/sun/tools/javac/util/MandatoryWarningHandler.java	Mon Oct 21 15:38:26 2019 +0200
@@ -186,18 +186,18 @@
     /**
      * The log to which to report warnings.
      */
-    private Log log;
+    private final Log log;
 
     /**
      * Whether or not to report individual warnings, or simply to report a
      * single aggregate warning at the end of the compilation.
      */
-    private boolean verbose;
+    private final boolean verbose;
 
     /**
      * The common prefix for all I18N message keys generated by this handler.
      */
-    private String prefix;
+    private final String prefix;
 
     /**
      * A set containing the names of the source files for which specific
@@ -262,4 +262,11 @@
         else
             log.note(file, new Note("compiler", msg, args));
     }
+
+    public void clear() {
+        sourcesWithReportedWarnings = null;
+        deferredDiagnosticKind = null;
+        deferredDiagnosticSource = null;
+        deferredDiagnosticArg = null;
+    }
 }