diff --git a/source/net/yacy/kelondro/logging/GuiHandler.java b/source/net/yacy/kelondro/logging/GuiHandler.java
index f88695383..70d4df144 100644
--- a/source/net/yacy/kelondro/logging/GuiHandler.java
+++ b/source/net/yacy/kelondro/logging/GuiHandler.java
@@ -208,8 +208,8 @@ public class GuiHandler extends Handler {
         
     }
 
-    public void close() throws SecurityException {
-        
+    public synchronized void close() throws SecurityException {
+        // Nothing implement here
     }
 
 }
diff --git a/source/net/yacy/kelondro/workflow/InstantBusyThread.java b/source/net/yacy/kelondro/workflow/InstantBusyThread.java
index b5d48328f..7cbbcd51c 100644
--- a/source/net/yacy/kelondro/workflow/InstantBusyThread.java
+++ b/source/net/yacy/kelondro/workflow/InstantBusyThread.java
@@ -179,7 +179,17 @@ public final class InstantBusyThread extends AbstractBusyThread implements BusyT
 
     public static WorkflowThread oneTimeJob(final Runnable thread, final long startupDelay, final int maxJobs) {
         while (instantThreadCounter >= maxJobs) try {Thread.sleep(100);} catch (final InterruptedException e) {break;}
-        return oneTimeJob( thread, startupDelay);
+        return oneTimeJob( thread, startupDelay);
+    }
+
+    @Override
+    public void open() {
+        // Not implemented in this thread
+    }
+
+    @Override
+    public synchronized void close() {
+     // Not implemented in this thread
     }
 
 }
diff --git a/source/net/yacy/search/index/DocumentMetadata.java b/source/net/yacy/search/index/DocumentMetadata.java
index cefeb8359..56f4f773f 100644
--- a/source/net/yacy/search/index/DocumentMetadata.java
+++ b/source/net/yacy/search/index/DocumentMetadata.java
@@ -50,7 +50,7 @@ public class DocumentMetadata implements Metadata {
 	}
 
 	@Override
-	public void close() {
+	public synchronized void close() {
 		// TODO Auto-generated method stub
 		
 	}
diff --git a/source/net/yacy/search/index/DocumentReference.java b/source/net/yacy/search/index/DocumentReference.java
index 51906c65e..0f2b7c1d1 100644
--- a/source/net/yacy/search/index/DocumentReference.java
+++ b/source/net/yacy/search/index/DocumentReference.java
@@ -42,7 +42,7 @@ public class DocumentReference {
     	return data.size();
     }
 
-    public void close() {
+    public synchronized void close() {
     	if (data != null) {
     		data.close();
     	}