Github user marmbrus commented on a diff in the pull request:

    https://github.com/apache/spark/pull/9766#discussion_r83057793
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/UDFRegistration.scala ---
    @@ -414,6 +418,84 @@ class UDFRegistration private[sql] (functionRegistry: 
FunctionRegistry) extends
       
//////////////////////////////////////////////////////////////////////////////////////////////
     
       /**
    +   * Register a Java UDF class using reflection, for use from pyspark
    +   *
    +   * @param name   udf name
    +   * @param className   fully qualified class name of udf
    +   * @param returnDataType  return type of udf. If it is null, spark would 
try to infer
    +   *                        via reflection.
    +   */
    +  def registerJava(name: String, className: String, returnDataType: 
DataType): Unit = {
    +
    +    try {
    +      val clazz = Utils.classForName(className)
    +      val udfInterfaces = clazz.getGenericInterfaces
    +        .filter(_.isInstanceOf[ParameterizedType])
    +        .map(_.asInstanceOf[ParameterizedType])
    +        .filter(e => e.getRawType.isInstanceOf[Class[_]] && 
e.getRawType.asInstanceOf[Class[_]].getCanonicalName.startsWith("org.apache.spark.sql.api.java.UDF"))
    +      if (udfInterfaces.length == 0) {
    +        throw new IOException(s"UDF class ${className} doesn't implement 
any UDF interface")
    +      } else if (udfInterfaces.length > 1) {
    +        throw new IOException(s"It is invalid to implement multiple UDF 
interfaces, UDF class ${className}")
    +      } else {
    +        try {
    +          val udf = clazz.newInstance()
    +          val udfReturnType = udfInterfaces(0).getActualTypeArguments.last
    +          var returnType = returnDataType
    +          if (returnType == null) {
    +            if (udfReturnType.isInstanceOf[Class[_]]) {
    +              returnType = 
udfReturnType.asInstanceOf[Class[_]].getCanonicalName match {
    --- End diff --
    
    Can we use 
[`JavaTypeInference`](https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/JavaTypeInference.scala)
 here?


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