动态类加载器的 IllegalAccessError

Question

我创建了一个自定义ParquetOutputFormat（org.apache.parquet.hadoop 中的类）来覆盖getRecordWriter 方法。在getRecordWriter 方法中，它访问CodecFactory，这会导致IllegalAccessError。为了解决这个问题，我尝试创建自己的类加载器，但这并没有帮助。我关注了这篇博文http://techblog.applift.com/upgrading-spark#advanced-case-parquet-writer

在创建自定义类加载器之前，我使用的是 CustomParquetOutputFormat，如下所示：

override def createOutputFormat: OutputFormat[Void, InternalRow] with Ext = new CustomParquetOutputFormat[InternalRow]() with Ext {
 ...
}

当调用getRecordWriter 时CustomParquetOutputFormat 尝试在第274 行访问CodecFactory 时会出现此问题：

  CodecFactory codecFactory = new CodecFactory(conf);

（这是 CustomParquetOutputFormat 访问的 ParquetOutputFormat 的第 274 行）

CodecFactory 是包私有的。

自定义类加载器：

class CustomClassLoader(urls: Array[URL], parent: ClassLoader, whiteList: List[String])
  extends ChildFirstURLClassLoader(urls, parent) {
  override def  loadClass(name: String) = {
    if (whiteList.exists(name.startsWith)) {
      super.loadClass(name)
    } else {
      parent.loadClass(name)
    }
  }
}

用法：

val sc: SparkContext = SparkContext.getOrCreate()
val cl: CustomClassLoader = new CustomClassLoader(sc.jars.map(new URL(_)).toArray,
  Thread.currentThread.getContextClassLoader, List(
    "org.apache.parquet.hadoop.CustomParquetOutputFormat",
    "org.apache.parquet.hadoop.CodecFactory",
    "org.apache.parquet.hadoop.ParquetFileWriter",
    "org.apache.parquet.hadoop.ParquetRecordWriter",
    "org.apache.parquet.hadoop.InternalParquetRecordWriter",
    "org.apache.parquet.hadoop.ColumnChunkPageWriteStore",
    "org.apache.parquet.hadoop.MemoryManager"
  ))


cl.loadClass("org.apache.parquet.hadoop.CustomParquetOutputFormat")
  .getConstructor(classOf[String], classOf[TaskAttemptContext])
  .newInstance(fullPathWithoutExt, taskAttemptContext)
  .asInstanceOf[OutputFormat[Void, InternalRow] with ProvidesExtension]

错误：

java.lang.IllegalAccessError: tried to access class org.apache.parquet.hadoop.CodecFactory from class org.apache.parquet.hadoop.customParquetOutputFormat
        at org.apache.parquet.hadoop.CustomParquetOutputFormat.getRecordWriter(CustomParquetOutputFormat.scala:40)
        at org.apache.parquet.hadoop.ParquetOutputFormat.getRecordWriter(ParquetOutputFormat.java:262)
        at org.apache.spark.custom.hadoop.HadoopWriter.<init>(HadoopWriter.scala:35)
        at org.apache.spark.sql.execution.datasources.parquet.ParquetWriter.<init>(ParquetWriter.scala:16)
        at org.apache.spark.sql.execution.datasources.parquet.ParquetWriterFactory.createWriter(ParquetWriterFactory.scala:71)
        at com.abden.custom.index.IndexBuilder$$anonfun$4.apply(IndexBuilder.scala:55)
        at com.abden.custom.index.IndexBuilder$$anonfun$4.apply(IndexBuilder.scala:54)
        at scala.collection.immutable.Stream.map(Stream.scala:418)
        at com.abden.custom.index.IndexBuilder.generateTiles(IndexBuilder.scala:54)
        at com.abden.custom.index.IndexBuilder.generateLayer(IndexBuilder.scala:155)
        at com.abden.custom.index.IndexBuilder.appendLayer(IndexBuilder.scala:184)
        at com.abden.custom.index.IndexBuilder$$anonfun$appendLayers$1$$anonfun$apply$1.apply(IndexBuilder.scala:213)
        at com.abden.custom.index.IndexBuilder$$anonfun$appendLayers$1$$anonfun$apply$1.apply(IndexBuilder.scala:210)
        at scala.collection.Iterator$class.foreach(Iterator.scala:742)
        at com.abden.custom.util.SplittingByKeyIterator.foreach(SplittingByKeyIterator.scala:3)
        at com.abden.custom.index.IndexBuilder$$anonfun$appendLayers$1.apply(IndexBuilder.scala:210)
        at com.abden.custom.index.IndexBuilder$$anonfun$appendLayers$1.apply(IndexBuilder.scala:209)
        at org.apache.spark.rdd.RDD$$anonfun$foreachPartition$1$$anonfun$apply$33.apply(RDD.scala:920)
        at org.apache.spark.rdd.RDD$$anonfun$foreachPartition$1$$anonfun$apply$33.apply(RDD.scala:920)
        at org.apache.spark.SparkContext$$anonfun$runJob$5.apply(SparkContext.scala:1858)
        at org.apache.spark.SparkContext$$anonfun$runJob$5.apply(SparkContext.scala:1858)
        at org.apache.spark.scheduler.ResultTask.runTask(ResultTask.scala:66)
        at org.apache.spark.scheduler.Task.run(Task.scala:89)
        at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:227)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:745)

错误发生在getRecordWriter的这一行：

val codecFactory = new CodecFactory(conf)

CodecFactory 没有修饰符，因此仅限于其包。即使使用动态类加载器从同一个类加载器加载所有类，我仍然得到IllegalAccessError

Answer 1

所以你试图做的就是打破 Java 的工作方式！您想通过实现您自己的允许破坏 JVM 保护规则的类加载器来访问在其包之外是包私有的类（因此您想破坏 Java 语言规范！）。

我的答案很简单：不要这样做！

如果它是包私有的，您将无法访问它。期间！

我认为最好的办法是考虑您需要什么功能，并使用当前的 API 来实现它，而不是试图强行进入。因此，与其问如何做一些技术黑客，最好是解释什么你想做的（为什么你要实现自己的getRecordWriter 方法。

我已经在这个 SOW 问题中给出了关于如何在纯 java 中读/写 parquet 文件的答案：Write Parquet format to HDFS using Java API with out using Avro and MR

问候，

洛伊克