Spark hive类总是优先记载应用里面的jar包,跟spark.{driver/executor}.userClassPathFirst无关

原创已于 2023-08-13 16:00:58 修改 · 753 阅读

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#大数据 #spark

于 2021-08-09 18:01:49 首次发布

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背景

最近在弄spark on k8s的时候，要集成同事的一些功能，其实这并没有什么，但是里面涉及到了hive的类问题（具体指这个org.apache.hadoop.hive.包下的类）。之后发现hive类总是优先加载应用jar包里的类，而忽略掉spark自带的系统jars包，这给我带了了很大的困扰，大约花了一两周的时间，终于把这个问题排查清楚了。

问题分析

直接分析：
我们知道在spark提交的时候，会获取URLClassLoader去加载类，如下：

private def runMain(args: SparkSubmitArguments, uninitLog: Boolean): Unit = {
    val (childArgs, childClasspath, sparkConf, childMainClass) = prepareSubmitEnvironment(args)
    // Let the main class re-initialize the logging system once it starts.
    if (uninitLog) {
      Logging.uninitialize()
    }

    if (args.verbose) {
      logInfo(s"Main class:\n$childMainClass")
      logInfo(s"Arguments:\n${childArgs.mkString("\n")}")
      // sysProps may contain sensitive information, so redact before printing
      logInfo(s"Spark config:\n${Utils.redact(sparkConf.getAll.toMap).mkString("\n")}")
      logInfo(s"Classpath elements:\n${childClasspath.mkString("\n")}")
      logInfo("\n")
    }
    val loader = getSubmitClassLoader(sparkConf)
    for (jar <- childClasspath) {
      addJarToClasspath(jar, loader)
    }

    var mainClass: Class[_] = null

    try {
      mainClass = Utils.classForName(childMainClass)
    } catch {
      ....

这里的getSubmitClassLoader方法就是获得URLClassloader：

private def getSubmitClassLoader(sparkConf: SparkConf): MutableURLClassLoader = {
    val loader =
      if (sparkConf.get(DRIVER_USER_CLASS_PATH_FIRST)) {
        new ChildFirstURLClassLoader(new Array[URL](0),
          Thread.currentThread.getContextClassLoader)
      } else {
        new MutableURLClassLoader(new Array[URL](0),
          Thread.currentThread.getContextClassLoader)
      }
    Thread.currentThread.setContextClassLoader(loader)
    loader
  }

区别就是ChildFirstURLClassLoader自定义了类加载的顺序，也就是会优先加载应用jar包里的顺序，可是我们的应用的并没有设置spark.{driver/executor}.userClassPathFirst,所以该hive类是跟这个加载器无关的。
就在百思不得其解的时候，突然想到了spark 对于hive metastore的兼容性随笔–通过classloader实现,这里的实现，这里就不得不分析一下IsolatedClientLoader这个类的细节：

 private[hive] val classLoader: MutableURLClassLoader = {
    val isolatedClassLoader =
      if (isolationOn) {
        if (allJars.isEmpty) {
          // See HiveUtils; this is the Java 9+ + builtin mode scenario
          baseClassLoader
        } else {
          val rootClassLoader: ClassLoader =
            if (SystemUtils.JAVA_VERSION.split("\\.")(1).toInt>=9) {
              // In Java 9, the boot classloader can see few JDK classes. The intended parent
              // classloader for delegation is now the platform classloader.
              // See http://java9.wtf/class-loading/
              val platformCL =
              classOf[ClassLoader].getMethod("getPlatformClassLoader").
                invoke(null).asInstanceOf[ClassLoader]
              // Check to make sure that the root classloader does not know about Hive.
              assert(Try(platformCL.loadClass("org.apache.hadoop.hive.conf.HiveConf")).isFailure)
              platformCL
            } else {
              // The boot classloader is represented by null (the instance itself isn't accessible)
              // and before Java 9 can see all JDK classes
              null
            }
          new URLClassLoader(allJars, rootClassLoader) {
            override def loadClass(name: String, resolve: Boolean): Class[_] = {
              val loaded = findLoadedClass(name)
              if (loaded == null) doLoadClass(name, resolve) else loaded
            }
            def doLoadClass(name: String, resolve: Boolean): Class[_] = {
              val classFileName = name.replaceAll("\\.", "/") + ".class"
              if (isBarrierClass(name)) {
                // For barrier classes, we construct a new copy of the class.
                val bytes = IOUtils.toByteArray(baseClassLoader.getResourceAsStream(classFileName))
                logDebug(s"custom defining: $name - ${util.Arrays.hashCode(bytes)}")
                defineClass(name, bytes, 0, bytes.length)
              } else if (!isSharedClass(name)) {
                logDebug(s"hive class: $name - ${getResource(classToPath(name))}")
                super.loadClass(name, resolve)
              } else {
                // For shared classes, we delegate to baseClassLoader, but fall back in case the
                // class is not found.
                logDebug(s"shared class: $name")
                try {
                  baseClassLoader.loadClass(name)
                } catch {
                  case _: ClassNotFoundException =>
                    super.loadClass(name, resolve)
                }
              }
            }
          }
        }
      } else {
        baseClassLoader
      }

如果是在JDK8的情况下rootClassLoader是为null的，这就导致了在加载hive相关的类的时候，super.loadClass方法就会直接执行URLClassLoader的findClass方法，进而从URL（也就是通过addURL方法加载进来的jar）查着相关的类
而在spark中，最终的任务提交是通过SparkSubmit的runMain方法来提交的，代码如第一块代码：

 val loader = getSubmitClassLoader(sparkConf)
    for (jar <- childClasspath) {
      addJarToClasspath(jar, loader)
    }

注意childClasspath这个是怎么来着，我们在提交任务的时候，是可以看到Classpath elements输出的，也就是会包括–jars指定的jar包和因公的jar包，所以在加载hive相关的类的时候，就会优先从childClassPath中加载对应的class，这是通过IolatedClassLoader实现。