diff --git a/runtime/src/main/java/org/capnproto/BuilderArena.java b/runtime/src/main/java/org/capnproto/BuilderArena.java
index 0323fad..02d4396 100644
--- a/runtime/src/main/java/org/capnproto/BuilderArena.java
+++ b/runtime/src/main/java/org/capnproto/BuilderArena.java
@@ -64,6 +64,30 @@ public final class BuilderArena implements Arena {
         this.allocator = allocator;
     }
 
+     /**
+     * Constructs a BuilderArena from a ReaderArena and uses the size of the largest segment
+     * as the next allocation size.
+     */
+    BuilderArena(ReaderArena arena) {
+        this.segments = new ArrayList<SegmentBuilder>();
+        int largestSegment = SUGGESTED_FIRST_SEGMENT_WORDS*Constants.BYTES_PER_WORD;
+        for (int ii = 0; ii < arena.segments.size(); ++ii) {
+            SegmentReader segment = arena.segments.get(ii);
+            SegmentBuilder segmentBuilder = new SegmentBuilder(segment.buffer, this);
+            segmentBuilder.id = ii;
+            segmentBuilder.pos = segmentBuilder.capacity(); // buffer is pre-filled
+            segments.add(segmentBuilder);
+
+            // Find the largest segment for the allocation strategy.
+            largestSegment = Math.max(largestSegment, segment.buffer.capacity());
+        }
+        DefaultAllocator defaultAllocator = new DefaultAllocator(SUGGESTED_ALLOCATION_STRATEGY);
+
+        // Use largest segment as next size.
+        defaultAllocator.setNextAllocationSizeBytes(largestSegment);
+        this.allocator = defaultAllocator;
+    }
+
     @Override
     public final SegmentReader tryGetSegment(int id) {
         return this.segments.get(id);
diff --git a/runtime/src/main/java/org/capnproto/MessageBuilder.java b/runtime/src/main/java/org/capnproto/MessageBuilder.java
index 40d714d..8d75b6d 100644
--- a/runtime/src/main/java/org/capnproto/MessageBuilder.java
+++ b/runtime/src/main/java/org/capnproto/MessageBuilder.java
@@ -71,6 +71,22 @@ public final class MessageBuilder {
         this.arena = new BuilderArena(new DefaultAllocator(), firstSegment);
     }
 
+   /**
+    * Constructs a MessageBuilder from a MessageReader. This constructor is private
+    * because it is unsafe. To use it, you must call `unsafeConstructFromMessageReader()`.
+    */
+    private MessageBuilder(MessageReader messageReader) {
+        this.arena = new BuilderArena(messageReader.arena);
+    }
+
+    /**
+     * Constructs a MessageBuilder from a MessageReader without copying the segments.
+     * This method should only be used on trusted data. Otherwise you may observe infinite
+     * loops or large memory allocations or index-out-of-bounds errors.
+     */
+    public static MessageBuilder unsafeConstructFromMessageReader(MessageReader messageReader) {
+        return new MessageBuilder(messageReader);
+    }
 
     private AnyPointer.Builder getRootInternal() {
         if (this.arena.segments.isEmpty()) {
diff --git a/runtime/src/main/java/org/capnproto/SegmentBuilder.java b/runtime/src/main/java/org/capnproto/SegmentBuilder.java
index e9ac57a..9727b58 100644
--- a/runtime/src/main/java/org/capnproto/SegmentBuilder.java
+++ b/runtime/src/main/java/org/capnproto/SegmentBuilder.java
@@ -35,7 +35,7 @@ public final class SegmentBuilder extends SegmentReader {
     }
 
     // the total number of words the buffer can hold
-    private final int capacity() {
+    final int capacity() {
         this.buffer.rewind();
         return this.buffer.remaining() / 8;
     }