apache
@@ -109,10 +109,6 @@ public void copyAsField(String name, ${name}Writer writer) {
 
   </#list></#list>
 
-  public void copyAsValue(StructWriter writer, ExtensionTypeWriterFactory writerFactory) {
-    fail("CopyAsValue StructWriter");
-  }
-
   public void read(ExtensionHolder holder) {
     fail("Extension");
   }
@@ -147,4 +143,5 @@ public int size() {
   private void fail(String name) {
     throw new IllegalArgumentException(String.format("You tried to read a [%s] type when you are using a field reader of type [%s].", name, this.getClass().getSimpleName()));
   }
+
 }
@@ -107,14 +107,17 @@ public void endEntry() {
     throw new IllegalStateException(String.format("You tried to end a map entry when you are using a ValueWriter of type %s.", this.getClass().getSimpleName()));
   }
 
+  @Override
   public void write(ExtensionHolder var1)  {
-    this.fail("ExtensionType");
+    this.fail("Cannot write ExtensionHolder");
   }
+  @Override
   public void writeExtension(Object var1)  {
-    this.fail("ExtensionType");
+    this.fail("Cannot write extension object");
   }
-  public void addExtensionTypeWriterFactory(ExtensionTypeWriterFactory var1) {
-    this.fail("ExtensionType");
+  @Override
+  public void writeExtension(Object var1, ArrowType type)  {
+    this.fail("Cannot write extension with type " + type);
   }
 
   <#list vv.types as type><#list type.minor as minor><#assign name = minor.class?cap_first />

@@ -27,8 +27,10 @@
 
 import org.apache.arrow.flatbuf.Type;
 import org.apache.arrow.memory.BufferAllocator;
+import org.apache.arrow.vector.complex.writer.FieldWriter;
 import org.apache.arrow.vector.types.*;
 import org.apache.arrow.vector.FieldVector;
+import org.apache.arrow.vector.ValueVector;
 
 import com.fasterxml.jackson.annotation.JsonCreator;
 import com.fasterxml.jackson.annotation.JsonIgnore;
@@ -331,6 +333,10 @@ public boolean equals(Object obj) {
     public <T> T accept(ArrowTypeVisitor<T> visitor) {
       return visitor.visit(this);
     }
+
+    public FieldWriter getNewFieldWriter(ValueVector vector) {
+      throw new UnsupportedOperationException("WriterImpl not yet implemented.");
+    }
   }
 
   private static final int defaultDecimalBitWidth = 128;

@@ -49,7 +49,6 @@ public interface RepeatedStructReader extends StructReader{
     boolean next();
     int size();
     void copyAsValue(StructWriter writer);
-    void copyAsValue(StructWriter writer, ExtensionTypeWriterFactory writerFactory);
   }
 
   public interface ListReader extends BaseReader{
@@ -60,7 +59,6 @@ public interface RepeatedListReader extends ListReader{
     boolean next();
     int size();
     void copyAsValue(ListWriter writer);
-    void copyAsValue(ListWriter writer, ExtensionTypeWriterFactory writerFactory);
   }
 
   public interface MapReader extends BaseReader{
@@ -71,7 +69,6 @@ public interface RepeatedMapReader extends MapReader{
     boolean next();
     int size();
     void copyAsValue(MapWriter writer);
-    void copyAsValue(MapWriter writer, ExtensionTypeWriterFactory writerFactory);
   }
 
   public interface ScalarReader extends

@@ -125,11 +125,12 @@ public interface ExtensionWriter extends BaseWriter {
     void writeExtension(Object value);
 
     /**
-     * Adds the given extension type factory. This factory allows configuring writer implementations for specific ExtensionTypeVector.
+     * Writes the given extension type value.
      *
-     * @param factory the extension type factory to add
+     * @param value the extension type value to write
+     * @param type of the extension
      */
-    void addExtensionTypeWriterFactory(ExtensionTypeWriterFactory factory);
+    void writeExtension(Object value, ArrowType type);
   }
 
   public interface ScalarWriter extends

@@ -41,15 +41,8 @@ public class ComplexCopier {
    * @param input field to read from
    * @param output field to write to
    */
-  public static void copy(FieldReader input, FieldWriter output) {
-    writeValue(input, output, null);
-  }
-
-  public static void copy(FieldReader input, FieldWriter output, ExtensionTypeWriterFactory extensionTypeWriterFactory) {
-    writeValue(input, output, extensionTypeWriterFactory);
-  }
+  public static void copy(FieldReader reader, FieldWriter writer) {
 
-  private static void writeValue(FieldReader reader, FieldWriter writer, ExtensionTypeWriterFactory extensionTypeWriterFactory) {
     final MinorType mt = reader.getMinorType();
 
       switch (mt) {
@@ -65,7 +58,7 @@ private static void writeValue(FieldReader reader, FieldWriter writer, Extension
             FieldReader childReader = reader.reader();
             FieldWriter childWriter = getListWriterForReader(childReader, writer);
             if (childReader.isSet()) {
-              writeValue(childReader, childWriter, extensionTypeWriterFactory);
+              copy(childReader, childWriter);
             } else {
               childWriter.writeNull();
             }
@@ -83,8 +76,8 @@ private static void writeValue(FieldReader reader, FieldWriter writer, Extension
             FieldReader structReader = reader.reader();
             if (structReader.isSet()) {
               writer.startEntry();
-              writeValue(mapReader.key(), getMapWriterForReader(mapReader.key(), writer.key()), extensionTypeWriterFactory);
-              writeValue(mapReader.value(), getMapWriterForReader(mapReader.value(), writer.value()), extensionTypeWriterFactory);
+              copy(mapReader.key(), getMapWriterForReader(mapReader.key(), writer.key()));
+              copy(mapReader.value(), getMapWriterForReader(mapReader.value(), writer.value()));
               writer.endEntry();
             } else {
               writer.writeNull();
@@ -103,7 +96,7 @@ private static void writeValue(FieldReader reader, FieldWriter writer, Extension
             if (childReader.getMinorType() != Types.MinorType.NULL) {
               FieldWriter childWriter = getStructWriterForReader(childReader, writer, name);
               if (childReader.isSet()) {
-                writeValue(childReader, childWriter, extensionTypeWriterFactory);
+                copy(childReader, childWriter);
               } else {
                 childWriter.writeNull();
               }
@@ -115,14 +108,10 @@ private static void writeValue(FieldReader reader, FieldWriter writer, Extension
         }
         break;
       case EXTENSIONTYPE:
-        if (extensionTypeWriterFactory == null) {
-          throw new IllegalArgumentException("Must provide ExtensionTypeWriterFactory");
-        }
         if (reader.isSet()) {
           Object value = reader.readObject();
           if (value != null) {
-            writer.addExtensionTypeWriterFactory(extensionTypeWriterFactory);
-            writer.writeExtension(value);
+            writer.writeExtension(value, reader.getField().getType());
           }
         } else {
           writer.writeNull();

@@ -86,7 +86,6 @@ public void read(int arrayIndex, Nullable${name}Holder holder){
   }
   </#list></#list>
 
-  public void copyAsValue(StructWriter writer, ExtensionTypeWriterFactory writerFactory){}
   public void read(ExtensionHolder holder) {
     holder.isSet = 0;
   }

@@ -286,7 +286,7 @@ protected void setWriter(ValueVector v) {
         writer = new UnionWriter((UnionVector) vector, nullableStructWriterFactory);
         break;
       case EXTENSIONTYPE:
-        writer = new UnionExtensionWriter((ExtensionTypeVector) vector);
+        writer = ((ExtensionType) vector.getField().getType()).getNewFieldWriter(vector);
         break;
       default:
         writer = type.getNewFieldWriter(vector);
@@ -541,17 +541,13 @@ public void writeLargeVarChar(String value) {
   }
 
   @Override
-  public void writeExtension(Object value) {
-    getWriter(MinorType.EXTENSIONTYPE).writeExtension(value);
+  public void writeExtension(Object value, ArrowType arrowType) {
+    getWriter(MinorType.EXTENSIONTYPE, arrowType).writeExtension(value, arrowType);
   }
 
   @Override
-  public void addExtensionTypeWriterFactory(ExtensionTypeWriterFactory factory) {
-    getWriter(MinorType.EXTENSIONTYPE).addExtensionTypeWriterFactory(factory);
-  }
-
-  public void addExtensionTypeWriterFactory(ExtensionTypeWriterFactory factory, ArrowType arrowType) {
-    getWriter(MinorType.EXTENSIONTYPE, arrowType).addExtensionTypeWriterFactory(factory);
+  public void write(ExtensionHolder holder) {
+    getWriter(MinorType.EXTENSIONTYPE, holder.type()).write(holder);
   }
 
   @Override

@@ -204,13 +204,13 @@ public MapWriter map(String name, boolean keysSorted) {
 
   @Override
   public ExtensionWriter extension(ArrowType arrowType) {
-    this.extensionType = arrowType;
+    extensionType = arrowType;
     return this;
   }
+
   @Override
   public ExtensionWriter extension(String name, ArrowType arrowType) {
-    ExtensionWriter extensionWriter = writer.extension(name, arrowType);
-    return extensionWriter;
+    return writer.extension(name, arrowType);
   }
 
   <#if listName == "LargeList">
@@ -337,13 +337,13 @@ public void writeNull() {
 
   @Override
   public void writeExtension(Object value) {
-    writer.writeExtension(value);
+    writer.writeExtension(value, extensionType);
     writer.setPosition(writer.idx() + 1);
   }
 
   @Override
-  public void addExtensionTypeWriterFactory(ExtensionTypeWriterFactory var1) {
-    writer.addExtensionTypeWriterFactory(var1, extensionType);
+  public void writeExtension(Object value, ArrowType type) {
+    writeExtension(value);
   }
 
   public void write(ExtensionHolder var1) {

@@ -79,6 +79,10 @@ public void read(int index, UnionHolder holder) {
   }
 
   private FieldReader getReaderForIndex(int index) {
+    return getReaderForIndex(index, null);
+  }
+
+  private FieldReader getReaderForIndex(int index, ArrowType type) {
     int typeValue = data.getTypeValue(index);
     FieldReader reader = (FieldReader) readers[typeValue];
     if (reader != null) {
@@ -105,11 +109,26 @@ private FieldReader getReaderForIndex(int index) {
         </#if>
       </#list>
     </#list>
+    case EXTENSIONTYPE:
+      if(type == null) {
+        throw new RuntimeException("Cannot get Extension reader without an ArrowType");
+      }
+      return (FieldReader) getExtension(type);
     default:
       throw new UnsupportedOperationException("Unsupported type: " + MinorType.values()[typeValue]);
     }
   }
 
+  private ExtensionReader extensionReader;
+
+  private ExtensionReader getExtension(ArrowType type) {
+    if (extensionReader == null) {
+      extensionReader = data.getExtension(type).getReader();
+      extensionReader.setPosition(idx());
+    }
+    return extensionReader;
+  }
+
   private SingleStructReaderImpl structReader;
 
   private StructReader getStruct() {
@@ -240,4 +259,8 @@ public FieldReader reader() {
   public boolean next() {
     return getReaderForIndex(idx()).next();
   }
+
+  public void read(ExtensionHolder holder){
+    getReaderForIndex(idx(), holder.type()).read(holder);
+  }
 }
@@ -379,6 +379,22 @@ public MapVector getMap(String name, ArrowType arrowType) {
     return mapVector;
   }
 
+  private ExtensionTypeVector extensionVector;
+
+  public ExtensionTypeVector getExtension(ArrowType arrowType) {
+    if (extensionVector == null) {
+      int vectorCount = internalStruct.size();
+      extensionVector = addOrGet(null, MinorType.EXTENSIONTYPE, arrowType, ExtensionTypeVector.class);
+      if (internalStruct.size() > vectorCount) {
+        extensionVector.allocateNew();
+        if (callBack != null) {
+          callBack.doWork();
+        }
+      }
+    }
+    return extensionVector;
+  }
+
   public int getTypeValue(int index) {
     return typeBuffer.getByte(index * TYPE_WIDTH);
   }
@@ -725,6 +741,8 @@ public ValueVector getVectorByType(int typeId, ArrowType arrowType) {
           return getListView();
         case MAP:
           return getMap(name, arrowType);
+        case EXTENSIONTYPE:
+          return getExtension(arrowType);
         default:
           throw new UnsupportedOperationException("Cannot support type: " + MinorType.values()[typeId]);
       }

@@ -28,6 +28,8 @@
 package org.apache.arrow.vector.complex.impl;
 
 <#include "/@includes/vv_imports.ftl" />
+import java.util.HashMap;
+
 import org.apache.arrow.vector.complex.writer.BaseWriter;
 import org.apache.arrow.vector.types.Types.MinorType;
 
@@ -213,8 +215,31 @@ public MapWriter asMap(ArrowType arrowType) {
     return getMapWriter(arrowType);
   }
 
+  private java.util.Map<ArrowType, ExtensionWriter> extensionWriters = new HashMap<>();
+
   private ExtensionWriter getExtensionWriter(ArrowType arrowType) {
-    throw new UnsupportedOperationException("ExtensionTypes are not supported yet.");
+    ExtensionWriter w = extensionWriters.get(arrowType);
+    if (w == null) {
+      w = ((ExtensionType) arrowType).getNewFieldWriter(data.getExtension(arrowType));
+      w.setPosition(idx());
+      extensionWriters.put(arrowType, w);
+    }
+    return w;
+  }
+
+  public void writeExtension(Object value, ArrowType type)  {
+    data.setType(idx(), MinorType.EXTENSIONTYPE);
+    ExtensionWriter w = getExtensionWriter(type);
+    w.setPosition(idx());
+    w.writeExtension(value);
+  }
+
+  @Override
+  public void write(ExtensionHolder holder)  {
+    data.setType(idx(), MinorType.EXTENSIONTYPE);
+    ExtensionWriter w = getExtensionWriter(holder.type());
+    w.setPosition(idx());
+    w.write(holder);
   }
 
   BaseWriter getWriter(MinorType minorType) {

@@ -22,7 +22,6 @@
 import org.apache.arrow.memory.BufferAllocator;
 import org.apache.arrow.memory.ReferenceManager;
 import org.apache.arrow.util.Preconditions;
-import org.apache.arrow.vector.complex.impl.ExtensionTypeWriterFactory;
 import org.apache.arrow.vector.complex.reader.FieldReader;
 import org.apache.arrow.vector.util.DataSizeRoundingUtil;
 import org.apache.arrow.vector.util.TransferPair;
@@ -261,18 +260,6 @@ public void copyFromSafe(int fromIndex, int thisIndex, ValueVector from) {
     throw new UnsupportedOperationException();
   }
 
-  @Override
-  public void copyFrom(
-      int fromIndex, int thisIndex, ValueVector from, ExtensionTypeWriterFactory writerFactory) {
-    throw new UnsupportedOperationException();
-  }
-
-  @Override
-  public void copyFromSafe(
-      int fromIndex, int thisIndex, ValueVector from, ExtensionTypeWriterFactory writerFactory) {
-    throw new UnsupportedOperationException();
-  }
-
   /**
    * Transfer the validity buffer from `validityBuffer` to the target vector's `validityBuffer`.
    * Start at `startIndex` and copy `length` number of elements. If the starting index is 8 byte