[core] Ignore missing files form auxclasspath

- Resolves #988

pmd-core/src/main/java/net/sourceforge/pmd/cache/AbstractAnalysisCache.java

@@ -5,6 +5,7 @@
 package net.sourceforge.pmd.cache;
 
 import java.io.File;
+import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.net.URL;
 import java.net.URLClassLoader;
@@ -184,6 +185,8 @@ public abstract class AbstractAnalysisCache implements AnalysisCache {
                 while (IOUtils.skip(inputStream, Long.MAX_VALUE) == Long.MAX_VALUE) {
                     // just loop
                 }
+            } catch (final FileNotFoundException ignored) {
+                LOG.warning("Auxclasspath entry " + url.toString() + " doesn't exist, ignoring it");
             } catch (final IOException e) {
                 // Can this even happen?
                 LOG.log(Level.SEVERE, "Incremental analysis can't check auxclasspath contents", e);

pmd-core/src/test/java/net/sourceforge/pmd/cache/FileAnalysisCacheTest.java

@@ -138,6 +138,21 @@ public class FileAnalysisCacheTest {
                 reloadedCache.isUpToDate(sourceFile));
     }
     
+    @Test
+    public void testAuxClasspathNonExistingAuxclasspathEntriesIgnored() throws MalformedURLException, IOException {
+        final RuleSets rs = mock(RuleSets.class);
+        final URLClassLoader cl = mock(URLClassLoader.class);
+        when(cl.getURLs()).thenReturn(new URL[] { new File(tempFolder.getRoot(), "non-existing-dir").toURI().toURL(), });
+        
+        setupCacheWithFiles(newCacheFile, rs, cl, sourceFile);
+        
+        final FileAnalysisCache analysisCache = new FileAnalysisCache(newCacheFile);
+        when(cl.getURLs()).thenReturn(new URL[] {});
+        analysisCache.checkValidity(rs, cl);
+        assertTrue("Cache believes unmodified file is not up to date after non-existing auxclasspath entry removed",
+                analysisCache.isUpToDate(sourceFile));
+    }
+    
     @Test
     public void testAuxClasspathChangeWithoutDFAorTypeResolutionDoesNotInvalidatesCache() throws MalformedURLException, IOException {
         final RuleSets rs = mock(RuleSets.class);