Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/14138#discussion_r70366108
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/CallMethodViaReflection.scala
 ---
    @@ -0,0 +1,166 @@
    +/*
    + * 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.catalyst.expressions
    +
    +import java.lang.reflect.{Method, Modifier}
    +
    +import org.apache.spark.sql.catalyst.InternalRow
    +import org.apache.spark.sql.catalyst.analysis.TypeCheckResult
    +import 
org.apache.spark.sql.catalyst.analysis.TypeCheckResult.{TypeCheckFailure, 
TypeCheckSuccess}
    +import org.apache.spark.sql.catalyst.expressions.codegen.CodegenFallback
    +import org.apache.spark.sql.types._
    +import org.apache.spark.unsafe.types.UTF8String
    +import org.apache.spark.util.Utils
    +
    +/**
    + * An expression that invokes a method on a class via reflection.
    + *
    + * For now, only types defined in `Reflect.typeMapping` are supported 
(basically primitives
    + * and string) as input types, and the output is turned automatically to a 
string.
    + *
    + * Note that unlike Hive's reflect function, this expression calls only 
static methods
    + * (i.e. does not support calling non-static methods).
    + *
    + * We should also look into how to consolidate this expression with
    + * [[org.apache.spark.sql.catalyst.expressions.objects.StaticInvoke]] in 
the future.
    + *
    + * @param children the first element should be a literal string for the 
class name,
    + *                 and the second element should be a literal string for 
the method name,
    + *                 and the remaining are input arguments to the Java 
method.
    + */
    +// scalastyle:off line.size.limit
    +@ExpressionDescription(
    +  usage = "_FUNC_(class,method[,arg1[,arg2..]]) calls method with 
reflection",
    +  extended = "> SELECT _FUNC_('java.util.UUID', 'randomUUID');\n 
c33fb387-8500-4bfa-81d2-6e0e3e930df2")
    +// scalastyle:on line.size.limit
    +case class CallMethodViaReflection(children: Seq[Expression])
    +  extends Expression with CodegenFallback {
    +
    +  override def prettyName: String = "reflect"
    +
    +  override def checkInputDataTypes(): TypeCheckResult = {
    +    if (children.size < 2) {
    +      TypeCheckFailure("requires at least two arguments")
    +    } else if (!children.take(2).forall(e => e.dataType == StringType && 
e.foldable)) {
    +      // The first two arguments must be string type.
    +      TypeCheckFailure("first two arguments should be string literals")
    +    } else if (!classExists) {
    +      TypeCheckFailure(s"class $className not found")
    +    } else if (method == null) {
    +      TypeCheckFailure(s"cannot find a static method that matches the 
argument types in $className")
    +    } else {
    +      TypeCheckSuccess
    +    }
    +  }
    +
    +  override def deterministic: Boolean = false
    +  override def nullable: Boolean = true
    +  override val dataType: DataType = StringType
    +
    +  override def eval(input: InternalRow): Any = {
    +    var i = 0
    +    while (i < argExprs.length) {
    +      buffer(i) = argExprs(i).eval(input).asInstanceOf[Object]
    +      // Convert if necessary. Based on the types defined in typeMapping, 
string is the only
    +      // type that needs conversion. If we support timestamps, dates, 
decimals, arrays, or maps
    +      // in the future, proper conversion needs to happen here too.
    +      if (buffer(i).isInstanceOf[UTF8String]) {
    +        buffer(i) = buffer(i).toString
    +      }
    +      i += 1
    +    }
    +    val ret = method.invoke(null, buffer : _*)
    +    UTF8String.fromString(String.valueOf(ret))
    +  }
    +
    +  @transient private lazy val argExprs: Array[Expression] = 
children.drop(2).toArray
    +
    +  /** Name of the class -- this has to be called after we verify children 
has at least two exprs. */
    +  @transient private lazy val className = 
children(0).eval().asInstanceOf[UTF8String].toString
    --- End diff --
    
    can we just write something like this?
    ```
    @transient private lazy val cls: Class[_] = ... // null if not found
    
    @transient lazy val method: Method = ...
    ```
    This looks more straightforward and I don't think it's hard to test.


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