Meh*_*ban 22 java lambda java-8 spark-java
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表达式或将其放在一个简单的jar而不是web服务(不知何故是一个servlet)中,它将运行而没有任何错误.但是在servlet中使用lambda表达式将导致此异常:
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)
PS:我尝试过jerseypark与jetty,tomcat和resin的组合,所有这些都让我得到了同样的结果.
Hol*_*ger 41
你有什么,是一个掩盖原始错误的后续错误.
当lambda实例被序列化时,它们用于writeReplace从作为SerializedLambda
实例的持久形式中解散其JRE特定实现.当SerializedLambda实例已恢复,它的readResolve方法将被调用来重建适当的拉姆达实例.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(请参阅此答案).重要的是,需要原始课程,这就是你的案例中缺少的课程.
但是......有......特别......行为ObjectInputStream.遇到异常时,它不会立即纾困.它将记录异常并继续进程,标记当前正在读取的所有对象,因此依赖于错误对象也是错误的.只有在流程结束时才会抛出它遇到的原始异常.令它如此奇怪的是,它还将继续尝试设置这些对象的字段.但是当你看到方法ObjectInputStream.readOrdinaryObject行1806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;
}
您会看到readResolve在lookupException报告非null异常时它不会调用该方法.但是当替换没有发生时,继续尝试设置引用者的字段值并不是一个好主意,但这正是在这里发生的事情,因此产生了一个ClassCastException.
您可以轻松地重现问题:
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$FieldReflector.setObjFieldValues(ObjectStreamClass.java:2089)
at java.io.ObjectStreamClass.setObjFieldValues(ObjectStreamClass.java:1261)
(经过1.8.0_20测试)
最重要的是,一旦了解了正在发生的事情,你可能会忘记这个序列化问题,你要解决的问题就是确保定义lambda表达式的类在lambda所在的运行时也可用.反序列化.
Spark Job直接从IDE运行的示例(默认情况下spark-submit分配jar):
SparkConf sconf = new SparkConf()
.set("spark.eventLog.dir", "hdfs://nn:8020/user/spark/applicationHistory")
.set("spark.eventLog.enabled", "true")
.setJars(new String[]{"/path/to/jar/with/your/class.jar"})
.setMaster("spark://spark.standalone.uri:7077");
我遇到了同样的错误，我用内部类替换了 lambda，然后它就工作了。我真的不明白为什么，并且重现这个错误非常困难（我们有一台服务器表现出这种行为，而没有其他服务器）。
不起作用
this.variable = () -> { ..... }
产量java.lang.ClassCastException: cannot assign instance of java.lang.invoke.SerializedLambda to field MyObject.val$variable
作品
this.variable = new MyInterface() {
public void myMethod() {
.....
}
};