Save default params separately in JSON.

mllib/src/main/scala/org/apache/spark/ml/classification/DecisionTreeClassifier.scala

@@ -278,7 +278,7 @@ object DecisionTreeClassificationModel extends MLReadable[DecisionTreeClassifica
       val numClasses = (metadata.metadata \ "numClasses").extract[Int]
       val root = loadTreeNodes(path, metadata, sparkSession)
       val model = new DecisionTreeClassificationModel(metadata.uid, root, numFeatures, numClasses)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/classification/GBTClassifier.scala

@@ -379,14 +379,14 @@ object GBTClassificationModel extends MLReadable[GBTClassificationModel] {
         case (treeMetadata, root) =>
           val tree =
             new DecisionTreeRegressionModel(treeMetadata.uid, root, numFeatures)
-          DefaultParamsReader.getAndSetParams(tree, treeMetadata)
+          treeMetadata.getAndSetParams(tree)
           tree
       }
       require(numTrees == trees.length, s"GBTClassificationModel.load expected $numTrees" +
         s" trees based on metadata but found ${trees.length} trees.")
       val model = new GBTClassificationModel(metadata.uid,
         trees, treeWeights, numFeatures)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/classification/LinearSVC.scala

@@ -377,7 +377,7 @@ object LinearSVCModel extends MLReadable[LinearSVCModel] {
       val Row(coefficients: Vector, intercept: Double) =
         data.select("coefficients", "intercept").head()
       val model = new LinearSVCModel(metadata.uid, coefficients, intercept)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala

@@ -1267,7 +1267,7 @@ object LogisticRegressionModel extends MLReadable[LogisticRegressionModel] {
           numClasses, isMultinomial)
       }
 
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/classification/MultilayerPerceptronClassifier.scala

@@ -388,7 +388,7 @@ object MultilayerPerceptronClassificationModel
       val weights = data.getAs[Vector](1)
       val model = new MultilayerPerceptronClassificationModel(metadata.uid, layers, weights)
 
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/classification/NaiveBayes.scala

@@ -407,7 +407,7 @@ object NaiveBayesModel extends MLReadable[NaiveBayesModel] {
         .head()
       val model = new NaiveBayesModel(metadata.uid, pi, theta)
 
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/classification/OneVsRest.scala

@@ -257,7 +257,7 @@ object OneVsRestModel extends MLReadable[OneVsRestModel] {
         DefaultParamsReader.loadParamsInstance[ClassificationModel[_, _]](modelPath, sc)
       }
       val ovrModel = new OneVsRestModel(metadata.uid, labelMetadata, models)
-      DefaultParamsReader.getAndSetParams(ovrModel, metadata)
+      metadata.getAndSetParams(ovrModel)
       ovrModel.set("classifier", classifier)
       ovrModel
     }
@@ -448,7 +448,7 @@ object OneVsRest extends MLReadable[OneVsRest] {
     override def load(path: String): OneVsRest = {
       val (metadata, classifier) = OneVsRestParams.loadImpl(path, sc, className)
       val ovr = new OneVsRest(metadata.uid)
-      DefaultParamsReader.getAndSetParams(ovr, metadata)
+      metadata.getAndSetParams(ovr)
       ovr.setClassifier(classifier)
     }
   }

mllib/src/main/scala/org/apache/spark/ml/classification/RandomForestClassifier.scala

@@ -319,14 +319,14 @@ object RandomForestClassificationModel extends MLReadable[RandomForestClassifica
         case (treeMetadata, root) =>
           val tree =
             new DecisionTreeClassificationModel(treeMetadata.uid, root, numFeatures, numClasses)
-          DefaultParamsReader.getAndSetParams(tree, treeMetadata)
+          treeMetadata.getAndSetParams(tree)
           tree
       }
       require(numTrees == trees.length, s"RandomForestClassificationModel.load expected $numTrees" +
         s" trees based on metadata but found ${trees.length} trees.")
 
       val model = new RandomForestClassificationModel(metadata.uid, trees, numFeatures, numClasses)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/clustering/BisectingKMeans.scala

@@ -188,7 +188,7 @@ object BisectingKMeansModel extends MLReadable[BisectingKMeansModel] {
       val dataPath = new Path(path, "data").toString
       val mllibModel = MLlibBisectingKMeansModel.load(sc, dataPath)
       val model = new BisectingKMeansModel(metadata.uid, mllibModel)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/clustering/GaussianMixture.scala

@@ -233,7 +233,7 @@ object GaussianMixtureModel extends MLReadable[GaussianMixtureModel] {
       }
       val model = new GaussianMixtureModel(metadata.uid, weights, gaussians)
 
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/clustering/KMeans.scala

@@ -248,7 +248,7 @@ object KMeansModel extends MLReadable[KMeansModel] {
         sparkSession.read.parquet(dataPath).as[OldData].head().clusterCenters
       }
       val model = new KMeansModel(metadata.uid, new MLlibKMeansModel(clusterCenters))
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/clustering/LDA.scala

@@ -366,7 +366,7 @@ private[clustering] trait LDAParams extends Params with HasFeaturesCol with HasM
 private object LDAParams {
 
   /**
-   * Equivalent to [[DefaultParamsReader.getAndSetParams()]], but handles [[LDA]] and [[LDAModel]]
+   * Equivalent to [[Metadata.getAndSetParams()]], but handles [[LDA]] and [[LDAModel]]
    * formats saved with Spark 1.6, which differ from the formats in Spark 2.0+.
    *
    * @param model    [[LDA]] or [[LDAModel]] instance.  This instance will be modified with
@@ -391,7 +391,7 @@ private object LDAParams {
               s"Cannot recognize JSON metadata: ${metadata.metadataJson}.")
         }
       case _ => // 2.0+
-        DefaultParamsReader.getAndSetParams(model, metadata)
+        metadata.getAndSetParams(model)
     }
   }
 }

mllib/src/main/scala/org/apache/spark/ml/feature/BucketedRandomProjectionLSH.scala

@@ -230,7 +230,7 @@ object BucketedRandomProjectionLSHModel extends MLReadable[BucketedRandomProject
       val model = new BucketedRandomProjectionLSHModel(metadata.uid,
         randUnitVectors.rowIter.toArray)
 
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/Bucketizer.scala

@@ -217,8 +217,6 @@ final class Bucketizer @Since("1.4.0") (@Since("1.4.0") override val uid: String
   override def copy(extra: ParamMap): Bucketizer = {
     defaultCopy[Bucketizer](extra).setParent(parent)
   }
-
-  override def write: MLWriter = new Bucketizer.BucketizerWriter(this)
 }
 
 @Since("1.6.0")
@@ -296,28 +294,6 @@ object Bucketizer extends DefaultParamsReadable[Bucketizer] {
     }
   }
 
-
-  private[Bucketizer] class BucketizerWriter(instance: Bucketizer) extends MLWriter {
-
-    override protected def saveImpl(path: String): Unit = {
-      // SPARK-23377: The default params will be saved and loaded as user-supplied params.
-      // Once `inputCols` is set, the default value of `outputCol` param causes the error
-      // when checking exclusive params. As a temporary to fix it, we skip the default value
-      // of `outputCol` if `inputCols` is set when saving the metadata.
-      // TODO: If we modify the persistence mechanism later to better handle default params,
-      // we can get rid of this.
-      var paramWithoutOutputCol: Option[JValue] = None
-      if (instance.isSet(instance.inputCols)) {
-        val params = instance.extractParamMap().toSeq
-        val jsonParams = params.filter(_.param != instance.outputCol).map { case ParamPair(p, v) =>
-          p.name -> parse(p.jsonEncode(v))
-        }.toList
-        paramWithoutOutputCol = Some(render(jsonParams))
-      }
-      DefaultParamsWriter.saveMetadata(instance, path, sc, paramMap = paramWithoutOutputCol)
-    }
-  }
-
   @Since("1.6.0")
   override def load(path: String): Bucketizer = super.load(path)
 }

mllib/src/main/scala/org/apache/spark/ml/feature/ChiSqSelector.scala

@@ -334,7 +334,7 @@ object ChiSqSelectorModel extends MLReadable[ChiSqSelectorModel] {
       val selectedFeatures = data.getAs[Seq[Int]](0).toArray
       val oldModel = new feature.ChiSqSelectorModel(selectedFeatures)
       val model = new ChiSqSelectorModel(metadata.uid, oldModel)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/CountVectorizer.scala

@@ -361,7 +361,7 @@ object CountVectorizerModel extends MLReadable[CountVectorizerModel] {
         .head()
       val vocabulary = data.getAs[Seq[String]](0).toArray
       val model = new CountVectorizerModel(metadata.uid, vocabulary)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/IDF.scala

@@ -182,7 +182,7 @@ object IDFModel extends MLReadable[IDFModel] {
         .select("idf")
         .head()
       val model = new IDFModel(metadata.uid, new feature.IDFModel(OldVectors.fromML(idf)))
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/Imputer.scala

@@ -262,7 +262,7 @@ object ImputerModel extends MLReadable[ImputerModel] {
       val dataPath = new Path(path, "data").toString
       val surrogateDF = sqlContext.read.parquet(dataPath)
       val model = new ImputerModel(metadata.uid, surrogateDF)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/MaxAbsScaler.scala

@@ -172,7 +172,7 @@ object MaxAbsScalerModel extends MLReadable[MaxAbsScalerModel] {
         .select("maxAbs")
         .head()
       val model = new MaxAbsScalerModel(metadata.uid, maxAbs)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/MinHashLSH.scala

@@ -197,7 +197,7 @@ object MinHashLSHModel extends MLReadable[MinHashLSHModel] {
         .map(tuple => (tuple(0), tuple(1))).toArray
       val model = new MinHashLSHModel(metadata.uid, randCoefficients)
 
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/MinMaxScaler.scala

@@ -243,7 +243,7 @@ object MinMaxScalerModel extends MLReadable[MinMaxScalerModel] {
           .select("originalMin", "originalMax")
           .head()
       val model = new MinMaxScalerModel(metadata.uid, originalMin, originalMax)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/OneHotEncoderEstimator.scala

@@ -386,7 +386,7 @@ object OneHotEncoderModel extends MLReadable[OneHotEncoderModel] {
         .head()
       val categorySizes = data.getAs[Seq[Int]](0).toArray
       val model = new OneHotEncoderModel(metadata.uid, categorySizes)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/PCA.scala

@@ -220,7 +220,7 @@ object PCAModel extends MLReadable[PCAModel] {
         new PCAModel(metadata.uid, pc.asML,
           Vectors.dense(Array.empty[Double]).asInstanceOf[DenseVector])
       }
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/QuantileDiscretizer.scala

@@ -253,35 +253,11 @@ final class QuantileDiscretizer @Since("1.6.0") (@Since("1.6.0") override val ui
 
   @Since("1.6.0")
   override def copy(extra: ParamMap): QuantileDiscretizer = defaultCopy(extra)
-
-  override def write: MLWriter = new QuantileDiscretizer.QuantileDiscretizerWriter(this)
 }
 
 @Since("1.6.0")
 object QuantileDiscretizer extends DefaultParamsReadable[QuantileDiscretizer] with Logging {
 
-  private[QuantileDiscretizer]
-  class QuantileDiscretizerWriter(instance: QuantileDiscretizer) extends MLWriter {
-
-    override protected def saveImpl(path: String): Unit = {
-      // SPARK-23377: The default params will be saved and loaded as user-supplied params.
-      // Once `inputCols` is set, the default value of `outputCol` param causes the error
-      // when checking exclusive params. As a temporary to fix it, we skip the default value
-      // of `outputCol` if `inputCols` is set when saving the metadata.
-      // TODO: If we modify the persistence mechanism later to better handle default params,
-      // we can get rid of this.
-      var paramWithoutOutputCol: Option[JValue] = None
-      if (instance.isSet(instance.inputCols)) {
-        val params = instance.extractParamMap().toSeq
-        val jsonParams = params.filter(_.param != instance.outputCol).map { case ParamPair(p, v) =>
-          p.name -> parse(p.jsonEncode(v))
-        }.toList
-        paramWithoutOutputCol = Some(render(jsonParams))
-      }
-      DefaultParamsWriter.saveMetadata(instance, path, sc, paramMap = paramWithoutOutputCol)
-    }
-  }
-
   @Since("1.6.0")
   override def load(path: String): QuantileDiscretizer = super.load(path)
 }

mllib/src/main/scala/org/apache/spark/ml/feature/RFormula.scala

@@ -445,7 +445,7 @@ object RFormulaModel extends MLReadable[RFormulaModel] {
 
       val model = new RFormulaModel(metadata.uid, resolvedRFormula, pipelineModel)
 
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }
@@ -509,7 +509,7 @@ private object ColumnPruner extends MLReadable[ColumnPruner] {
       val columnsToPrune = data.getAs[Seq[String]](0).toSet
       val pruner = new ColumnPruner(metadata.uid, columnsToPrune)
 
-      DefaultParamsReader.getAndSetParams(pruner, metadata)
+      metadata.getAndSetParams(pruner)
       pruner
     }
   }
@@ -601,7 +601,7 @@ private object VectorAttributeRewriter extends MLReadable[VectorAttributeRewrite
       val prefixesToRewrite = data.getAs[Map[String, String]](1)
       val rewriter = new VectorAttributeRewriter(metadata.uid, vectorCol, prefixesToRewrite)
 
-      DefaultParamsReader.getAndSetParams(rewriter, metadata)
+      metadata.getAndSetParams(rewriter)
       rewriter
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/StandardScaler.scala

@@ -212,7 +212,7 @@ object StandardScalerModel extends MLReadable[StandardScalerModel] {
         .select("std", "mean")
         .head()
       val model = new StandardScalerModel(metadata.uid, std, mean)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/StringIndexer.scala

@@ -315,7 +315,7 @@ object StringIndexerModel extends MLReadable[StringIndexerModel] {
         .head()
       val labels = data.getAs[Seq[String]](0).toArray
       val model = new StringIndexerModel(metadata.uid, labels)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/VectorIndexer.scala

@@ -537,7 +537,7 @@ object VectorIndexerModel extends MLReadable[VectorIndexerModel] {
       val numFeatures = data.getAs[Int](0)
       val categoryMaps = data.getAs[Map[Int, Map[Double, Int]]](1)
       val model = new VectorIndexerModel(metadata.uid, numFeatures, categoryMaps)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/feature/Word2Vec.scala

@@ -410,7 +410,7 @@ object Word2VecModel extends MLReadable[Word2VecModel] {
       }
 
       val model = new Word2VecModel(metadata.uid, oldModel)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/fpm/FPGrowth.scala

@@ -335,7 +335,7 @@ object FPGrowthModel extends MLReadable[FPGrowthModel] {
       val dataPath = new Path(path, "data").toString
       val frequentItems = sparkSession.read.parquet(dataPath)
       val model = new FPGrowthModel(metadata.uid, frequentItems)
-      DefaultParamsReader.getAndSetParams(model, metadata)
+      metadata.getAndSetParams(model)
       model
     }
   }

mllib/src/main/scala/org/apache/spark/ml/param/params.scala

@@ -791,7 +791,7 @@ trait Params extends Identifiable with Serializable {
    *               this method gets called.
    * @param value  the default value
    */
-  protected final def setDefault[T](param: Param[T], value: T): this.type = {
+  private[ml] final def setDefault[T](param: Param[T], value: T): this.type = {
     defaultParamMap.put(param -> value)
     this
   }
@@ -865,10 +865,10 @@ trait Params extends Identifiable with Serializable {
   }
 
   /** Internal param map for user-supplied values. */
-  private val paramMap: ParamMap = ParamMap.empty
+  private[ml] val paramMap: ParamMap = ParamMap.empty
 
   /** Internal param map for default values. */
-  private val defaultParamMap: ParamMap = ParamMap.empty
+  private[ml] val defaultParamMap: ParamMap = ParamMap.empty
 
   /** Validates that the input param belongs to this instance. */
   private def shouldOwn(param: Param[_]): Unit = {