Index: CHANGES.txt
===================================================================
--- CHANGES.txt	(revision 833547)
+++ CHANGES.txt	(working copy)
@@ -12,6 +12,10 @@
 
 * oal.Lock.isLocked is now allowed to throw an IOException
 
+* LUCENE-2030: CachingWrapperFilter and CachingSpanFilter now hide
+  the internal cache implementation for thread safety, before it was
+  declared protected.  (Peter Lenahan, Uwe Schindler, Simon Willnauer)
+
 Changes in runtime behavior
 
 * LUCENE-1677: Remove the system property to set SegmentReader class
Index: src/java/org/apache/lucene/search/CachingSpanFilter.java
===================================================================
--- src/java/org/apache/lucene/search/CachingSpanFilter.java	(revision 833547)
+++ src/java/org/apache/lucene/search/CachingSpanFilter.java	(working copy)
@@ -28,13 +28,16 @@
  * filters to simply filter, and then wrap with this class to add caching.
  */
 public class CachingSpanFilter extends SpanFilter {
-  protected SpanFilter filter;
+  private SpanFilter filter;
 
   /**
-   * A transient Filter cache.
+   * A transient Filter cache (package private because of test)
    */
-  protected transient Map<IndexReader,SpanFilterResult> cache;
+  private transient Map<IndexReader,SpanFilterResult> cache;
 
+  // serializable, should normally be Object:
+  private final String lock = new String();
+
   /**
    * @param filter Filter to cache results of
    */
@@ -49,18 +52,18 @@
   }
   
   private SpanFilterResult getCachedResult(IndexReader reader) throws IOException {
-    SpanFilterResult result = null;
-    if (cache == null) {
-      cache = new WeakHashMap<IndexReader,SpanFilterResult>();
-    }
-
-    synchronized (cache) {  // check cache
-      result = cache.get(reader);
-      if (result == null) {
-        result = filter.bitSpans(reader);
-        cache.put(reader, result);
+    synchronized(lock) {
+      if (cache == null) {
+        cache = new WeakHashMap<IndexReader,SpanFilterResult>();
       }
+      final SpanFilterResult cached = cache.get(reader);
+      if (cached != null) return cached;
     }
+    
+    final SpanFilterResult result = filter.bitSpans(reader);
+    synchronized(lock) {
+      cache.put(reader, result);
+    }
     return result;
   }
 
Index: src/java/org/apache/lucene/search/CachingWrapperFilter.java
===================================================================
--- src/java/org/apache/lucene/search/CachingWrapperFilter.java	(revision 833547)
+++ src/java/org/apache/lucene/search/CachingWrapperFilter.java	(working copy)
@@ -29,12 +29,15 @@
  * filters to simply filter, and then wrap with this class to add caching.
  */
 public class CachingWrapperFilter extends Filter {
-  protected Filter filter;
+  Filter filter;
 
   /**
-   * A transient Filter cache.
+   * A transient Filter cache (package private because of test)
    */
-  protected transient Map<IndexReader, DocIdSet> cache;
+  transient Map<IndexReader, DocIdSet> cache;
+  
+  // serializable, should normally be Object:
+  private final String lock = new String();
 
   /**
    * @param filter Filter to cache results of
@@ -63,27 +66,21 @@
   
   @Override
   public DocIdSet getDocIdSet(IndexReader reader) throws IOException {
-    if (cache == null) {
-      cache = new WeakHashMap<IndexReader, DocIdSet>();
-    }
+    synchronized(lock) {
+      if (cache == null) {
+        cache = new WeakHashMap<IndexReader,DocIdSet>();
+      }
 
-    DocIdSet cached = null;
-    synchronized (cache) {  // check cache
-      cached = cache.get(reader);
+      final DocIdSet cached = cache.get(reader);
+      if (cached != null) return cached;
     }
 
-    if (cached != null) {
-      return cached;
-    }
-
     final DocIdSet docIdSet = docIdSetToCache(filter.getDocIdSet(reader), reader);
-
     if (docIdSet != null) {
-      synchronized (cache) {  // update cache
+      synchronized(lock) {
         cache.put(reader, docIdSet);
       }
     }
-
     return docIdSet;
   }