Index: oak-jcr/src/main/java/org/apache/jackrabbit/oak/jcr/SessionImpl.java
===================================================================
--- oak-jcr/src/main/java/org/apache/jackrabbit/oak/jcr/SessionImpl.java	(revision 1462128)
+++ oak-jcr/src/main/java/org/apache/jackrabbit/oak/jcr/SessionImpl.java	(working copy)
@@ -19,7 +19,8 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
-import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Locale;
 import java.util.Map;
@@ -33,6 +34,7 @@
 import javax.jcr.Item;
 import javax.jcr.ItemNotFoundException;
 import javax.jcr.NamespaceException;
+import javax.jcr.NamespaceRegistry;
 import javax.jcr.Node;
 import javax.jcr.PathNotFoundException;
 import javax.jcr.Property;
@@ -80,6 +82,12 @@
     private final SessionDelegate sd;
 
     /**
+     * A snapshot of the namespace registry on first usage. See
+     * <a href="http://www.day.com/specs/jcr/2.0/3_Repository_Model.html#3.5.1%20Namespace%20Registry">JCR 2.0 (3.5.1)</a>
+     */
+    private Map<String, String> namespacesSnapshot;
+
+    /**
      * Local namespace remappings. Prefixes as keys and namespace URIs as values.
      * <p/>
      * This map is only accessed from synchronized methods (see
@@ -642,11 +650,12 @@
     public String[] getNamespacePrefixes() throws RepositoryException {
         synchronized (namespaces) {
             if (namespaces.isEmpty()) {
-                return getWorkspace().getNamespaceRegistry().getPrefixes();
+                Set<String> prefixes = getNamespaceSnapshot().keySet();
+                return prefixes.toArray(new String[prefixes.size()]);
             }
         }
         Set<String> uris = new HashSet<String>();
-        uris.addAll(Arrays.asList(getWorkspace().getNamespaceRegistry().getURIs()));
+        uris.addAll(getNamespaceSnapshot().values());
         synchronized (namespaces) {
             // Add namespace uris only visible to session
             uris.addAll(namespaces.values());
@@ -664,12 +673,16 @@
             String uri = namespaces.get(prefix);
 
             if (uri == null) {
-                // Not in local mappings, try the global ones
-                uri = getWorkspace().getNamespaceRegistry().getURI(prefix);
-                if (namespaces.containsValue(uri)) {
-                    // The global URI is locally mapped to some other prefix,
-                    // so there are no mappings for this prefix
-                    throw new NamespaceException("Namespace not found: " + prefix);
+                // Not in local mappings, try snapshot ones
+                uri = getNamespaceSnapshot().get(prefix);
+                if (uri == null) {
+                    // Not in snapshot mappings, try the global ones
+                    uri = getWorkspace().getNamespaceRegistry().getURI(prefix);
+                    if (namespaces.containsValue(uri)) {
+                        // The global URI is locally mapped to some other prefix,
+                        // so there are no mappings for this prefix
+                        throw new NamespaceException("Namespace not found: " + prefix);
+                    }
                 }
             }
 
@@ -686,6 +699,13 @@
                 }
             }
 
+            // try namespace snapshot
+            for (Map.Entry<String, String> entry : getNamespaceSnapshot().entrySet()) {
+                if (entry.getValue().equals(uri)) {
+                    return entry.getKey();
+                }
+            }
+
             // The following throws an exception if the URI is not found, that's OK
             String prefix = getWorkspace().getNamespaceRegistry().getPrefix(uri);
 
@@ -702,6 +722,19 @@
         }
     }
 
+    private Map<String, String> getNamespaceSnapshot()
+            throws RepositoryException {
+        if (namespacesSnapshot == null) {
+            Map<String, String> map = new HashMap<String, String>();
+            NamespaceRegistry registry = getWorkspace().getNamespaceRegistry();
+            for (String prefix : registry.getPrefixes()) {
+                map.put(prefix, registry.getURI(prefix));
+            }
+            namespacesSnapshot = Collections.unmodifiableMap(map);
+        }
+        return namespacesSnapshot;
+    }
+
     //--------------------------------------------------< JackrabbitSession >---
 
     @Override