我正在尝试使用 sparkjava.com 框架为我的 apache spark 作业构建一个 web api。我的代码是:
@Override
public void init() {
get("/hello",
(req, res) -> {
String sourcePath = "hdfs://spark:54310/input/*";
SparkConf conf = new SparkConf().setAppName("LineCount");
conf.setJars(new String[] { "/home/sam/resin-4.0.42/webapps/test.war" });
File configFile = new File("config.properties");
String sparkURI = "spark://hamrah:7077";
conf.setMaster(sparkURI);
conf.set("spark.driver.allowMultipleContexts", "true");
JavaSparkContext sc = new JavaSparkContext(conf);
@SuppressWarnings("resource")
JavaRDD<String> log = sc.textFile(sourcePath);
JavaRDD<String> lines = log.filter(x -> {
return true;
});
return lines.count();
});
}
如果我删除 lambda 表达式或将其放入一个简单的 jar 而不是 Web 服务(某种 servlet)中,它将运行而不会出现任何错误。但是在 servlet 中使用 lambda 表达式会导致这个异常:
15/01/28 10:36:33 WARN TaskSetManager: Lost task 0.0 in stage 0.0 (TID 0, hamrah): java.lang.ClassCastException: cannot assign instance of java.lang.invoke.SerializedLambda to field org.apache.spark.api.java.JavaRDD$$anonfun$filter$1.f$1 of type org.apache.spark.api.java.function.Function in instance of org.apache.spark.api.java.JavaRDD$$anonfun$filter$1
at java.io.ObjectStreamClass$FieldReflector.setObjFieldValues(ObjectStreamClass.java:2089)
at java.io.ObjectStreamClass.setObjFieldValues(ObjectStreamClass.java:1261)
at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1999)
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1918)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1801)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1351)
at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1993)
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1918)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1801)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1351)
at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1993)
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1918)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1801)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1351)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:371)
at org.apache.spark.serializer.JavaDeserializationStream.readObject(JavaSerializer.scala:62)
at org.apache.spark.serializer.JavaSerializerInstance.deserialize(JavaSerializer.scala:87)
at org.apache.spark.scheduler.ResultTask.runTask(ResultTask.scala:57)
at org.apache.spark.scheduler.Task.run(Task.scala:56)
at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:196)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
P.S:我尝试将 jersey 和 javaspark 与 jetty、tomcat 和树脂结合使用,所有这些都让我得到了相同的结果。
最佳答案
您在这里看到的是一个后续错误,它掩盖了原始错误。
当 lambda 实例被序列化时,它们使用 writeReplace
来分解它们特定的 JRE
从持久形式实现,即 SerializedLambda
实例。当 SerializedLambda
实例恢复后,将调用其 readResolve
方法
重构适当的 lambda 实例。正如文档所说,它将通过调用定义原始 lambda 的类的特殊方法来实现(另见 this answer )。重要的一点是原始类是必需的,而这正是您的情况所缺少的。
但是 ObjectInputStream
有一个……特殊的……行为。当它遇到异常时,它不会立即退出。它将记录异常并继续该过程,标记当前正在读取的所有对象,因此也将错误对象视为错误对象。只有在进程结束时,它才会抛出它遇到的原始异常。让它如此奇怪的是它还会继续尝试设置这些对象的字段。但是当您查看方法 ObjectInputStream.readOrdinaryObject
第 1806 行时:
…
if (obj != null &&
handles.lookupException(passHandle) == null &&
desc.hasReadResolveMethod())
{
Object rep = desc.invokeReadResolve(obj);
if (unshared && rep.getClass().isArray()) {
rep = cloneArray(rep);
}
if (rep != obj) {
handles.setObject(passHandle, obj = rep);
}
}
return obj;
}
当 lookupException
报告非 null
异常时,您会看到它没有调用 readResolve
方法。但是当替换没有发生时,继续尝试设置引用者的字段值不是一个好主意,但这正是这里发生的事情,因此产生了 ClassCastException
。
您可以轻松重现问题:
public class Holder implements Serializable {
Runnable r;
}
public class Defining {
public static Holder get() {
final Holder holder = new Holder();
holder.r=(Runnable&Serializable)()->{};
return holder;
}
}
public class Writing {
static final File f=new File(System.getProperty("java.io.tmpdir"), "x.ser");
public static void main(String... arg) throws IOException {
try(FileOutputStream os=new FileOutputStream(f);
ObjectOutputStream oos=new ObjectOutputStream(os)) {
oos.writeObject(Defining.get());
}
System.out.println("written to "+f);
}
}
public class Reading {
static final File f=new File(System.getProperty("java.io.tmpdir"), "x.ser");
public static void main(String... arg) throws IOException, ClassNotFoundException {
try(FileInputStream is=new FileInputStream(f);
ObjectInputStream ois=new ObjectInputStream(is)) {
Holder h=(Holder)ois.readObject();
System.out.println(h.r);
h.r.run();
}
System.out.println("read from "+f);
}
}
编译这四个类并运行Writing
。然后删除类文件Defining.class
并运行Reading
。然后你会得到一个
Exception in thread "main" java.lang.ClassCastException: cannot assign instance of java.lang.invoke.SerializedLambda to field test.Holder.r of type java.lang.Runnable in instance of test.Holder
at java.io.ObjectStreamClass$FieldReflector.setObjFieldValues(ObjectStreamClass.java:2089)
at java.io.ObjectStreamClass.setObjFieldValues(ObjectStreamClass.java:1261)
(用 1.8.0_20 测试)
最重要的是,一旦了解发生了什么,您可能会忘记这个序列化问题,解决问题所需要做的就是确保定义 lambda 表达式的类在运行时中也可用lambda 被反序列化。
Spark Job 直接从 IDE 运行的示例(spark-submit 默认分发 jar):
SparkConf sconf = new SparkConf()
.set("spark.eventLog.dir", "hdfs://nn:8020/user/spark/applicationHistory")
.set("spark.eventLog.enabled", "true")
.setJars(new String[]{"/path/to/jar/with/your/class.jar"})
.setMaster("spark://spark.standalone.uri:7077");
关于java.lang.ClassCastException 在远程服务器上的 spark 作业中使用 lambda 表达式,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/28186607/