我正在尝试使用JDBC在Presto上处理查询，并将结果集传递回Spark以在其上创建临时表。我的结果集在列表中
我正在从kafka生产者那里以json消息的形式获取查询。因此创建了kafka消费者来获取消息并进行进一步处理。
下面是我的主要功能：
public static void main(String[] args) throws InterruptedException {
SparkConf conf = new SparkConf();
conf.setAppName("Wordcount Background");
conf.setMaster("local");
//SparkContext sc = SparkContext.getOrCreate(conf);
SparkSession spark =
SparkSession.builder().config(conf).getOrCreate();
JavaSparkContext sc = new
JavaSparkContext(spark.sparkContext());
JavaStreamingContext ssc = new JavaStreamingContext(sc,
Durations.seconds(5));
SQLContext sqc = new SQLContext(sc);
Set<String> topics = Collections.singleton("TestTopic");
Map<String, String> kafkaParams = new HashMap<>();
kafkaParams.put("metadata.broker.list", "172.20.3.189:9092");
JavaPairInputDStream<String, String> directKafkaStream =
KafkaUtils.createDirectStream(ssc,
String.class, String.class, StringDecoder.class,
StringDecoder.class, kafkaParams, topics);
directKafkaStream.foreachRDD(rdd -> {
//System.out.println("--- New RDD with " +
rdd.partitions().size()
// + " partitions and " + rdd.count() + "
records");
rdd.foreach(record -> {
SparkkafkaJson sk = new SparkkafkaJson();
Dataset<String> dfrdd =
spark.createDataset(sk.process_query(record._2), Encoders.STRING());
System.out.print(dfrdd);
//Dataset<Row> df = spark.read().json(dfrdd);
//df.show();
});
});
ssc.start();
ssc.awaitTermination();
}
下面是process_query方法，该方法将结果集返回到主函数：
public List<String> process_query(String queryjson) {
String resstr="";
String columnValue="";
List<String> jsonList = new ArrayList<>();
//List<String> list=new ArrayList<String>();
try {
Class.forName(JDBC_DRIVER);
//Open a connection
conn = DriverManager.getConnection(DB_URL, USER, PASS);
//Execute a query
stmt = conn.createStatement();
String sql = process_json(queryjson);
ResultSet res = stmt.executeQuery(sql);
ResultSetMetaData rsmd = res.getMetaData();
int columnsNumber = rsmd.getColumnCount();
//Extract data from result set
while (res.next()) {
//System.out.println(res.getString(""));
Gson userGson=new GsonBuilder().create();
JsonObject params = new JsonObject();
for (int i = 1; i <= columnsNumber; i++) {
String ColName = rsmd.getColumnName(i);
String ColVal = res.getString(i);
params.addProperty(ColName, ColVal);
}
resstr = userGson.toJson(params);
jsonList.add(resstr);
}
//Clean-up environment
res.close();
stmt.close();
conn.close();
} catch (SQLException se) {
//Handle errors for JDBC
se.printStackTrace();
} catch (Exception e) {
//Handle errors for Class.forName
e.printStackTrace();
} finally {
//finally block used to close resources
try {
if (stmt != null) stmt.close();
} catch (SQLException sqlException) {
sqlException.printStackTrace();
}
try {
if (conn != null) conn.close();
} catch (SQLException se) {
se.printStackTrace();
}
}
return jsonList;
}
但我仍然收到此错误输出
2019-05-30 13:17:41 INFO ContextCleaner:54 - Cleaned accumulator 42
2019-05-30 13:17:41 INFO ContextCleaner:54 - Cleaned accumulator 109
2019-05-30 13:17:43 INFO CodeGenerator:54 - Code generated in 216.222798
ms
2019-05-30 13:17:43 ERROR Executor:91 - Exception in task 1.0 in stage 9.0
(TID 19)
java.lang.NullPointerException
at
org.apache.spark.sql.SparkSession.sessionState$lzycompute(SparkSession.scala:143)
at org.apache.spark.sql.SparkSession.sessionState(SparkSession.scala:141)
at org.apache.spark.sql.Dataset.<init>(Dataset.scala:183)
at org.apache.spark.sql.Dataset$.apply(Dataset.scala:65)
at org.apache.spark.sql.SparkSession.createDataset(SparkSession.scala:474)
at org.apache.spark.sql.SparkSession.createDataset(SparkSession.scala:511)
at SparkkafkaJson.SparkkafkaJson.lambda$1(SparkkafkaJson.java:213)
at org.apache.spark.api.java.JavaRDDLike$$anonfun$foreach$1.apply(JavaRDDLike.scala:351)
at org.apache.spark.api.java.JavaRDDLike$$anonfun$foreach$1.apply(JavaRDDLike.scala:351)
at scala.collection.Iterator$class.foreach(Iterator.scala:891)
at org.apache.spark.util.NextIterator.foreach(NextIterator.scala:21)
at org.apache.spark.rdd.RDD$$anonfun$foreach$1$$anonfun$apply$27.apply(RDD.scala:927)
at org.apache.spark.rdd.RDD$$anonfun$foreach$1$$anonfun$apply$27.apply(RDD.scala:927)
at org.apache.spark.SparkContext$$anonfun$runJob$5.apply(SparkContext.scala:2101)
at org.apache.spark.SparkContext$$anonfun$runJob$5.apply(SparkContext.scala:2101)
at org.apache.spark.scheduler.ResultTask.runTask(ResultTask.scala:90)
at org.apache.spark.scheduler.Task.run(Task.scala:121)
at org.apache.spark.executor.Executor$TaskRunner$$anonfun$10.apply(Executor.scala:402)
at org.apache.spark.util.Utils$.tryWithSafeFinally(Utils.scala:1360)
at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:408)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
2019-05-30 13:17:43 WARN TaskSetManager:66 - Lost task 1.0 in stage 9.0 (TID 19, localhost, executor driver): java.lang.NullPointerException
at org.apache.spark.sql.SparkSession.sessionState$lzycompute(SparkSession.scala:143)
at org.apache.spark.sql.SparkSession.sessionState(SparkSession.scala:141)
at org.apache.spark.sql.Dataset.<init>(Dataset.scala:183)
at org.apache.spark.sql.Dataset$.apply(Dataset.scala:65)
at org.apache.spark.sql.SparkSession.createDataset(SparkSession.scala:474)
at org.apache.spark.sql.SparkSession.createDataset(SparkSession.scala:511)
at SparkkafkaJson.SparkkafkaJson.lambda$1(SparkkafkaJson.java:213)
at org.apache.spark.api.java.JavaRDDLike$$anonfun$foreach$1.apply(JavaRDDLike.scala:351)
at org.apache.spark.api.java.JavaRDDLike$$anonfun$foreach$1.apply(JavaRDDLike.scala:351)
请帮助
答案 0 :(得分：0)
按如下逻辑解决了上述问题：
directKafkaStream.foreachRDD(rdd -> {
//System.out.println("--- New RDD with " + rdd.partitions().size()
// + " partitions and " + rdd.count() + " records");
rdd.collect().forEach(record -> {
List<String> jsonList2 = new ArrayList<>();
//System.out.print(sk.process_query(record._2,sk,spark));
jsonList2 = sk.process_query(record._2,sk,spark);
if(jsonList2.size() > 0) {
//System.out.print("came here");
sk.jsonList3 = jsonList2;
String JsonStr = jsonList2.toString();
//System.out.print(JsonStr);
System.out.print("Sending Data to API");
System.out.print(" ");
try {
sk.Send_query_data_SCDF(JsonStr);
} catch (IOException e) {
e.printStackTrace();
}
}else {
System.out.print("came into else");
sk.jsonList3= new ArrayList<String>();
}
});
我试图在RDD的foreach下创建一个数据集，这就是为什么给nullpointer异常的原因，因为数据集的创建需要在驱动程序端而不是执行者端进行。因此，收集了RDD结果，并在后边的foreach中使用它来使其持久化
Dataset<String> dfrdd = spark.createDataset(sk.jsonList3,
Encoders.STRING());
Dataset<Row> wordsDataFrame = spark.read().json(dfrdd);
wordsDataFrame.createOrReplaceTempView("words");
Dataset<Row> wordCountsDataFrame =
spark.sql("select * from words limit 10");
wordCountsDataFrame.show();
String jsonToReturn =
wordCountsDataFrame.toJSON().collectAsList().toString();
System.out.print(jsonToReturn);
sk.jsonList3= new ArrayList<String>();
rdd.unpersist();
});