[MJAVADOC-722] Log javadoc warnings and errors to file

src/main/java/org/apache/maven/plugins/javadoc/AbstractJavadocMojo.java

@@ -203,6 +203,14 @@ public abstract class AbstractJavadocMojo
      */
     protected static final String FILES_FILE_NAME = "files";
 
+    /**
+     * The <code>errors</code> file name in the output directory containing the errors and warnings returned by
+     * <code>javadoc.exe(or .sh)</code>.
+     * <br />
+     * This won't exist if <code>javadoc.exe(or .sh)</code> didn't return any warnings or errors.
+     */
+    protected static final String ERRORS_FILE_NAME = "errors";
+
     /**
      * Default css file name, used as file name in the output directory for the temporary custom stylesheet file
      * loaded from classloader resources.
@@ -5848,6 +5856,11 @@ && isJavadocVMInitError( output ) )
                     getLog().info( output );
                 }
 
+                if ( StringUtils.isNotEmpty( err.getOutput() ) )
+                {
+                    writeErrorFile( err.getOutput(), javadocOutputDirectory );
+                }
+
                 StringBuilder msg = new StringBuilder( "\nExit code: " );
                 msg.append( exitCode );
                 if ( StringUtils.isNotEmpty( err.getOutput() ) )
@@ -6702,6 +6715,28 @@ private void writeDebugJavadocScript( String cmdLine, File javadocOutputDirector
         }
     }
 
+    /**
+     * Write a files containing the javadoc errors and warnings.
+     *
+     * @param errorsAndWarnings      the javadoc errors and warnings as string, not null.
+     * @param javadocOutputDirectory the output dir, not null.
+     * @since 3.4.2-SNAPSHOT
+     */
+    private void writeErrorFile( String errorsAndWarnings, File javadocOutputDirectory )
+    {
+        File commandLineFile = new File( javadocOutputDirectory, ERRORS_FILE_NAME );
+        commandLineFile.getParentFile().mkdirs();
+
+        try
+        {
+            FileUtils.fileWrite( commandLineFile.getAbsolutePath(), null /* platform encoding */, errorsAndWarnings );
+        }
+        catch ( IOException e )
+        {
+            logError( "Unable to write '" + commandLineFile.getName() + "' errors file", e );
+        }
+    }
+
     /**
      * Check if the Javadoc JVM is correctly started or not.
      *

src/main/java/org/apache/maven/plugins/javadoc/JavadocJar.java

@@ -67,10 +67,11 @@ public class JavadocJar
      * @see AbstractJavadocMojo#PACKAGES_FILE_NAME
      * @see AbstractJavadocMojo#ARGFILE_FILE_NAME
      * @see AbstractJavadocMojo#FILES_FILE_NAME
+     * @see AbstractJavadocMojo#ERRORS_FILE_NAME
      */
     private static final String[] DEFAULT_EXCLUDES =
         new String[]{ DEBUG_JAVADOC_SCRIPT_NAME, OPTIONS_FILE_NAME, PACKAGES_FILE_NAME, ARGFILE_FILE_NAME,
-            FILES_FILE_NAME };
+            FILES_FILE_NAME, ERRORS_FILE_NAME };
 
     // ----------------------------------------------------------------------
     // Mojo components

src/test/java/org/apache/maven/plugins/javadoc/JavadocJarTest.java

@@ -42,7 +42,7 @@
 public class JavadocJarTest
     extends AbstractMojoTestCase
 {
-    
+
     private JavadocJar lookupMojo( File testPom )
                     throws Exception
     {
@@ -51,13 +51,13 @@ private JavadocJar lookupMojo( File testPom )
         MojoExecution mojoExec = new MojoExecution( new Plugin(), "javadoc", null );
 
         setVariableValueToObject( mojo, "mojo", mojoExec );
-        
+
         MavenProject currentProject = new MavenProjectStub();
         currentProject.setGroupId( "GROUPID" );
         currentProject.setArtifactId( "ARTIFACTID" );
-        
+
         setVariableValueToObject( mojo, "session", newMavenSession( currentProject ) );
-        
+
         return mojo;
     }
 
@@ -126,6 +126,7 @@ else if ( javadocVersion.isBefore( "1.8" ) )
         assertFalse( set.contains( AbstractJavadocMojo.FILES_FILE_NAME ) );
         assertFalse( set.contains( AbstractJavadocMojo.OPTIONS_FILE_NAME ) );
         assertFalse( set.contains( AbstractJavadocMojo.PACKAGES_FILE_NAME ) );
+        assertFalse( set.contains( AbstractJavadocMojo.ERRORS_FILE_NAME ) );
 
         //check if the javadoc files were created
         generatedFile =