Expose Metadata and MetadataBuilder through the public scala and java packages.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/Metadata.scala

@@ -204,6 +204,9 @@ class MetadataBuilder {
 
   private val map: mutable.Map[String, Any] = mutable.Map.empty
 
+  /** Returns the immutable version of this map.  Used for java interop. */
+  protected def getMap = map.toMap
+
   /** Include the content of an existing [[Metadata]] instance. */
   def withMetadata(metadata: Metadata): this.type = {
     map ++= metadata.map

sql/core/src/main/java/org/apache/spark/sql/api/java/DataType.java

@@ -19,8 +19,6 @@
 
 import java.util.*;
 
-import org.apache.spark.sql.catalyst.util.Metadata;
-
 /**
  * The base type of all Spark SQL data types.
  *
@@ -175,7 +173,7 @@ public static StructField createStructField(
    * @see #createStructField(String, DataType, boolean, Metadata)
    */
   public static StructField createStructField(String name, DataType dataType, boolean nullable) {
-    return createStructField(name, dataType, nullable, Metadata.empty());
+    return createStructField(name, dataType, nullable, (new MetadataBuilder()).build());
   }
 
   /**
@@ -207,5 +205,4 @@ public static StructType createStructType(StructField[] fields) {
 
     return new StructType(fields);
   }
-
 }

sql/core/src/main/java/org/apache/spark/sql/api/java/Metadata.java

@@ -0,0 +1,31 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.api.java;
+
+/**
+ * Metadata is a wrapper over Map[String, Any] that limits the value type to simple ones: Boolean,
+ * Long, Double, String, Metadata, Array[Boolean], Array[Long], Array[Double], Array[String], and
+ * Array[Metadata]. JSON is used for serialization.
+ *
+ * The default constructor is private. User should use [[MetadataBuilder]].
+ */
+class Metadata extends org.apache.spark.sql.catalyst.util.Metadata {
+  Metadata(scala.collection.immutable.Map<String, Object> map) {
+    super(map);
+  }
+}

sql/core/src/main/java/org/apache/spark/sql/api/java/MetadataBuilder.java

@@ -0,0 +1,28 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.api.java;
+
+/**
+ * Builder for [[Metadata]]. If there is a key collision, the latter will overwrite the former.
+ */
+public class MetadataBuilder extends org.apache.spark.sql.catalyst.util.MetadataBuilder {
+  @Override
+  public Metadata build() {
+    return new Metadata(getMap());
+  }
+}

sql/core/src/main/java/org/apache/spark/sql/api/java/StructField.java

@@ -19,8 +19,6 @@
 
 import java.util.Map;
 
-import org.apache.spark.sql.catalyst.util.Metadata;
-
 /**
  * A StructField object represents a field in a StructType object.
  * A StructField object comprises three fields, {@code String name}, {@code DataType dataType},

sql/core/src/main/scala/org/apache/spark/sql/SQLContext.scala

@@ -32,8 +32,6 @@ import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.optimizer.{Optimizer, DefaultOptimizer}
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.catalyst.rules.RuleExecutor
-import org.apache.spark.sql.catalyst.types.DataType
-import org.apache.spark.sql.columnar.InMemoryRelation
 import org.apache.spark.sql.execution.{SparkStrategies, _}
 import org.apache.spark.sql.json._
 import org.apache.spark.sql.parquet.ParquetRelation

sql/core/src/main/scala/org/apache/spark/sql/package.scala

@@ -125,6 +125,9 @@ package object sql {
   @DeveloperApi
   type DataType = catalyst.types.DataType
 
+  @DeveloperApi
+  val DataType = catalyst.types.DataType
+
   /**
    * :: DeveloperApi ::
    *
@@ -414,4 +417,24 @@ package object sql {
    */
   @DeveloperApi
   val StructField = catalyst.types.StructField
+
+  /**
+   * :: DeveloperApi ::
+   *
+   * Metadata is a wrapper over Map[String, Any] that limits the value type to simple ones: Boolean,
+   * Long, Double, String, Metadata, Array[Boolean], Array[Long], Array[Double], Array[String], and
+   * Array[Metadata]. JSON is used for serialization.
+   *
+   * The default constructor is private. User should use either [[MetadataBuilder]] or
+   * [[Metadata$#fromJson]] to create Metadata instances.
+   *
+   * @param map an immutable map that stores the data
+   */
+  @DeveloperApi
+  type Metadata = catalyst.util.Metadata
+
+  /**
+   * Builder for [[Metadata]]. If there is a key collision, the latter will overwrite the former.
+   */
+  type MetadataBuilder = catalyst.util.MetadataBuilder
 }

sql/core/src/main/scala/org/apache/spark/sql/types/util/DataTypeConversions.scala

@@ -18,7 +18,7 @@
 package org.apache.spark.sql.types.util
 
 import org.apache.spark.sql._
-import org.apache.spark.sql.api.java.{DataType => JDataType, StructField => JStructField}
+import org.apache.spark.sql.api.java.{DataType => JDataType, StructField => JStructField, MetadataBuilder => JMetaDataBuilder}
 
 import scala.collection.JavaConverters._
 
@@ -32,7 +32,7 @@ protected[sql] object DataTypeConversions {
       scalaStructField.name,
       asJavaDataType(scalaStructField.dataType),
       scalaStructField.nullable,
-      scalaStructField.metadata)
+      (new JMetaDataBuilder).withMetadata(scalaStructField.metadata).build())
   }
 
   /**

sql/core/src/test/scala/org/apache/spark/sql/DataTypeSuite.scala

@@ -19,9 +19,6 @@ package org.apache.spark.sql
 
 import org.scalatest.FunSuite
 
-import org.apache.spark.sql.catalyst.types.DataType
-import org.apache.spark.sql.catalyst.util.MetadataBuilder
-
 class DataTypeSuite extends FunSuite {
 
   test("construct an ArrayType") {

sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala

@@ -25,8 +25,7 @@ import org.apache.spark.sql.TestData._
 import org.apache.spark.sql.catalyst.errors.TreeNodeException
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
-import org.apache.spark.sql.execution.joins.BroadcastHashJoin
-import org.apache.spark.sql.catalyst.util.MetadataBuilder
+
 import org.apache.spark.sql.test.TestSQLContext._
 
 class SQLQuerySuite extends QueryTest with BeforeAndAfterAll {

sql/core/src/test/scala/org/apache/spark/sql/api/java/ScalaSideDataTypeConversionSuite.scala

@@ -19,9 +19,7 @@ package org.apache.spark.sql.api.java
 
 import org.scalatest.FunSuite
 
-import org.apache.spark.sql.{DataType => SDataType, StructField => SStructField,
-  StructType => SStructType}
-import org.apache.spark.sql.catalyst.util.MetadataBuilder
+import org.apache.spark.sql.{DataType => SDataType, StructField => SStructField, StructType => SStructType}
 import org.apache.spark.sql.types.util.DataTypeConversions._
 
 class ScalaSideDataTypeConversionSuite extends FunSuite {