diff --git a/java/src/main/java/org/softwareheritage/graph/SwhBidirectionalGraph.java b/java/src/main/java/org/softwareheritage/graph/SwhBidirectionalGraph.java
--- a/java/src/main/java/org/softwareheritage/graph/SwhBidirectionalGraph.java
+++ b/java/src/main/java/org/softwareheritage/graph/SwhBidirectionalGraph.java
@@ -139,7 +139,7 @@
 
     @Override
     public SwhBidirectionalGraph copy() {
-        return new SwhBidirectionalGraph(forwardGraph.copy(), backwardGraph.copy(), this.properties);
+        return new SwhBidirectionalGraph(forwardGraph.copy(), backwardGraph.copy(), this.properties.copy());
     }
 
     @Override
diff --git a/java/src/main/java/org/softwareheritage/graph/SwhGraphProperties.java b/java/src/main/java/org/softwareheritage/graph/SwhGraphProperties.java
--- a/java/src/main/java/org/softwareheritage/graph/SwhGraphProperties.java
+++ b/java/src/main/java/org/softwareheritage/graph/SwhGraphProperties.java
@@ -18,6 +18,7 @@
 import it.unimi.dsi.fastutil.longs.LongMappedBigList;
 import it.unimi.dsi.fastutil.shorts.ShortBigList;
 import it.unimi.dsi.fastutil.shorts.ShortMappedBigList;
+import it.unimi.dsi.lang.FlyweightPrototype;
 import it.unimi.dsi.sux4j.util.EliasFanoLongBigList;
 import org.apache.commons.configuration2.ex.ConfigurationException;
 import org.softwareheritage.graph.maps.NodeIdMap;
@@ -43,7 +44,7 @@
  * @see NodeIdMap
  * @see NodeTypesMap
  */
-public class SwhGraphProperties {
+public class SwhGraphProperties implements FlyweightPrototype<SwhGraphProperties> {
     private final String path;
 
     private final NodeIdMap nodeIdMap;
@@ -68,6 +69,66 @@
         this.nodeTypesMap = nodeTypesMap;
     }
 
+    protected SwhGraphProperties(String path, NodeIdMap nodeIdMap, NodeTypesMap nodeTypesMap,
+            LongBigList authorTimestamp, ShortBigList authorTimestampOffset, LongBigList committerTimestamp,
+            ShortBigList committerTimestampOffset, LongBigList contentLength, LongArrayBitVector contentIsSkipped,
+            IntBigList authorId, IntBigList committerId, ByteBigList messageBuffer, LongBigList messageOffsets,
+            ByteBigList tagNameBuffer, LongBigList tagNameOffsets, MappedFrontCodedStringBigList edgeLabelNames) {
+        this.path = path;
+        this.nodeIdMap = nodeIdMap;
+        this.nodeTypesMap = nodeTypesMap;
+        this.authorTimestamp = authorTimestamp;
+        this.authorTimestampOffset = authorTimestampOffset;
+        this.committerTimestamp = committerTimestamp;
+        this.committerTimestampOffset = committerTimestampOffset;
+        this.contentLength = contentLength;
+        this.contentIsSkipped = contentIsSkipped;
+        this.authorId = authorId;
+        this.committerId = committerId;
+        this.messageBuffer = messageBuffer;
+        this.messageOffsets = messageOffsets;
+        this.tagNameBuffer = tagNameBuffer;
+        this.tagNameOffsets = tagNameOffsets;
+        this.edgeLabelNames = edgeLabelNames;
+    }
+
+    public SwhGraphProperties copy() {
+        return new SwhGraphProperties(path, nodeIdMap.copy(), nodeTypesMap.copy(),
+                ((authorTimestamp instanceof LongMappedBigList)
+                        ? ((LongMappedBigList) authorTimestamp).copy()
+                        : authorTimestamp),
+                ((authorTimestampOffset instanceof ShortMappedBigList)
+                        ? ((ShortMappedBigList) authorTimestampOffset).copy()
+                        : authorTimestampOffset),
+                ((committerTimestamp instanceof LongMappedBigList)
+                        ? ((LongMappedBigList) committerTimestamp).copy()
+                        : committerTimestamp),
+                ((committerTimestampOffset instanceof ShortMappedBigList)
+                        ? ((ShortMappedBigList) committerTimestampOffset).copy()
+                        : committerTimestampOffset),
+                ((contentLength instanceof LongMappedBigList)
+                        ? ((LongMappedBigList) contentLength).copy()
+                        : contentLength),
+                (contentIsSkipped != null) ? contentIsSkipped.copy() : null,
+                ((authorId instanceof IntMappedBigList) ? ((IntMappedBigList) authorId).copy() : authorId),
+                ((committerId instanceof IntMappedBigList) ? ((IntMappedBigList) committerId).copy() : committerId),
+                ((messageBuffer instanceof ByteMappedBigList)
+                        ? ((ByteMappedBigList) messageBuffer).copy()
+                        : messageBuffer),
+                ((messageOffsets instanceof LongMappedBigList)
+                        ? ((LongMappedBigList) messageOffsets).copy()
+                        : messageOffsets),
+                ((tagNameBuffer instanceof ByteMappedBigList)
+                        ? ((ByteMappedBigList) tagNameBuffer).copy()
+                        : tagNameBuffer),
+                ((tagNameOffsets instanceof LongMappedBigList)
+                        ? ((LongMappedBigList) tagNameOffsets).copy()
+                        : tagNameOffsets),
+                // TODO: not thread safe!! see https://github.com/vigna/dsiutils/issues/5
+                // Once https://github.com/vigna/dsiutils/pull/6 is merged, add a .copy() here:
+                edgeLabelNames);
+    }
+
     public static SwhGraphProperties load(String path) throws IOException {
         return new SwhGraphProperties(path, new NodeIdMap(path), new NodeTypesMap(path));
     }
diff --git a/java/src/main/java/org/softwareheritage/graph/SwhUnidirectionalGraph.java b/java/src/main/java/org/softwareheritage/graph/SwhUnidirectionalGraph.java
--- a/java/src/main/java/org/softwareheritage/graph/SwhUnidirectionalGraph.java
+++ b/java/src/main/java/org/softwareheritage/graph/SwhUnidirectionalGraph.java
@@ -150,7 +150,7 @@
     @Override
     public SwhUnidirectionalGraph copy() {
         return new SwhUnidirectionalGraph(this.graph.copy(),
-                this.labelledGraph != null ? this.labelledGraph.copy() : null, this.properties);
+                this.labelledGraph != null ? this.labelledGraph.copy() : null, this.properties.copy());
     }
 
     @Override
diff --git a/java/src/main/java/org/softwareheritage/graph/maps/NodeIdMap.java b/java/src/main/java/org/softwareheritage/graph/maps/NodeIdMap.java
--- a/java/src/main/java/org/softwareheritage/graph/maps/NodeIdMap.java
+++ b/java/src/main/java/org/softwareheritage/graph/maps/NodeIdMap.java
@@ -14,6 +14,7 @@
 import it.unimi.dsi.fastutil.longs.LongBigList;
 import it.unimi.dsi.fastutil.longs.LongMappedBigList;
 import it.unimi.dsi.fastutil.objects.Object2LongFunction;
+import it.unimi.dsi.lang.FlyweightPrototype;
 import org.softwareheritage.graph.SWHID;
 import org.softwareheritage.graph.compress.NodeMapBuilder;
 
@@ -35,7 +36,7 @@
  * @see NodeMapBuilder
  */
 
-public class NodeIdMap implements Size64 {
+public class NodeIdMap implements Size64, FlyweightPrototype<NodeIdMap> {
     /** Fixed length of binary SWHID buffer */
     public static final int SWHID_BIN_SIZE = 22;
 
@@ -73,6 +74,29 @@
         }
     }
 
+    protected NodeIdMap(String graphPath, ByteBigList nodeToSwhMap, Object2LongFunction<byte[]> mph,
+            LongBigList orderMap) {
+        this.graphPath = graphPath;
+        this.nodeToSwhMap = nodeToSwhMap;
+        this.mph = mph;
+        this.orderMap = orderMap;
+    }
+
+    /** Return the number of nodes in the map. */
+    @Override
+    public long size64() {
+        return nodeToSwhMap.size64();
+    }
+
+    @Override
+    public NodeIdMap copy() {
+        return new NodeIdMap(graphPath,
+                ((nodeToSwhMap instanceof ByteMappedBigList)
+                        ? ((ByteMappedBigList) nodeToSwhMap).copy()
+                        : nodeToSwhMap),
+                mph, ((orderMap instanceof LongMappedBigList) ? ((LongMappedBigList) orderMap).copy() : orderMap));
+    }
+
     @SuppressWarnings("unchecked")
     public static Object2LongFunction<byte[]> loadMph(String path) throws IOException {
         Object obj;
@@ -187,10 +211,4 @@
         nodeToSwhMap.getElements(nodeId * SWHID_BIN_SIZE, swhid, 0, SWHID_BIN_SIZE);
         return SWHID.fromBytes(swhid);
     }
-
-    /** Return the number of nodes in the map. */
-    @Override
-    public long size64() {
-        return nodeToSwhMap.size64();
-    }
 }
diff --git a/java/src/main/java/org/softwareheritage/graph/maps/NodeTypesMap.java b/java/src/main/java/org/softwareheritage/graph/maps/NodeTypesMap.java
--- a/java/src/main/java/org/softwareheritage/graph/maps/NodeTypesMap.java
+++ b/java/src/main/java/org/softwareheritage/graph/maps/NodeTypesMap.java
@@ -9,6 +9,8 @@
 
 import it.unimi.dsi.fastutil.io.BinIO;
 import it.unimi.dsi.fastutil.longs.LongBigList;
+import it.unimi.dsi.fastutil.longs.LongMappedBigList;
+import it.unimi.dsi.lang.FlyweightPrototype;
 import org.softwareheritage.graph.SwhType;
 
 import java.io.IOException;
@@ -26,7 +28,7 @@
  * @author The Software Heritage developers
  */
 
-public class NodeTypesMap {
+public class NodeTypesMap implements FlyweightPrototype<NodeTypesMap> {
     /** File extension for the long node id to node type map */
     public static final String NODE_TO_TYPE = ".node2type.map";
 
@@ -48,6 +50,16 @@
         }
     }
 
+    public NodeTypesMap(LongBigList nodeTypesMap) {
+        this.nodeTypesMap = nodeTypesMap;
+    }
+
+    @Override
+    public NodeTypesMap copy() {
+        return new NodeTypesMap(
+                (nodeTypesMap instanceof LongMappedBigList) ? ((LongMappedBigList) nodeTypesMap).copy() : nodeTypesMap);
+    }
+
     /**
      * Returns node type from a node long id.
      *