ARROW-207: Extend BufferAllocator interface to allow decorators around BufferAllocator

java/memory/src/main/java/org/apache/arrow/memory/AllocationManager.java

@@ -332,7 +332,7 @@ public int decrement(int decrement) {
      * @return The ledger associated with the BufferAllocator.
      */
     public BufferLedger getLedgerForAllocator(BufferAllocator allocator) {
-      return associate((BaseAllocator) allocator);
+      return associate(allocator.unwrap(BaseAllocator.class));
     }
 
     /**

java/memory/src/main/java/org/apache/arrow/memory/BaseAllocator.java

@@ -783,6 +783,16 @@ public static enum Verbosity {
     }
   }
 
+  @SuppressWarnings("unchecked")
+  @Override
+  public <T> T unwrap(Class<T> c) {
+    if (BaseAllocator.class.isAssignableFrom(c)) {
+      return (T) this;
+    }
+
+    throw new UnsupportedOperationException("Unable to unwrap type to class: " + c.getName());
+  }
+
   public static boolean isDebug() {
     return DEBUG;
   }

java/memory/src/main/java/org/apache/arrow/memory/BufferAllocator.java

@@ -79,6 +79,14 @@ public interface BufferAllocator extends AutoCloseable {
   @Override
   public void close();
 
+  /**
+   * Unwrap the class so that exposes the provided interface, if possible. Otherwise, throw Exception.
+   * @param c
+   *          The class or interface that you want this class to implement/extend.
+   * @return The instance of that class related to 'this'
+   */
+  public <T> T unwrap(Class<T> c);
+
   /**
    * Returns the amount of memory currently allocated from this allocator.
    *