Context
I want to iterate over a Spark Dataset and update a HashMap for each row.
Here is the code I have:
// At this point, I have a my_dataset variable containing 300 000 rows and 10 columns
// - my_dataset.count() == 300 000
// - my_dataset.columns().length == 10
// Declare my HashMap
HashMap<String, Vector<String>> my_map = new HashMap<String, Vector<String>>();
// Initialize the map
for(String col : my_dataset.columns())
{
my_map.put(col, new Vector<String>());
}
// Iterate over the dataset and update the map
my_dataset.foreach( (ForeachFunction<Row>) row -> {
for(String col : my_map.KeySet())
{
my_map.get(col).add(row.get(row.fieldIndex(col)).toString());
}
});
Issue
My issue is that the foreach doesn't iterate at all, the lambda is never executed and I don't know why.
I implemented it as indicated here: How to traverse/iterate a Dataset in Spark Java?
At the end, all the inner Vectors remain empty (as they were initialized) despite the Dataset is not (Take a look to the first comments in the given code sample).
I know that the foreach never iterates because I did two tests:
- Add an
AtomicInteger
to count the iterations, increment it right in the beginning of the lambda withincrementAndGet()
method. => The counter value remains0
at the end of the process. - Print a debug message right in the beginning of the lambda. => The message is never displayed.
I'm not used of Java (even less with Java lambdas) so maybe I missed an important point but I can't find what.
CodePudding user response:
I am probably a little old school, but I never like lambdas too much, as it can get pretty complicated.
Here is a full example of a foreach()
:
package net.jgp.labs.spark.l240_foreach.l000;
import java.io.Serializable;
import org.apache.spark.api.java.function.ForeachFunction;
import org.apache.spark.sql.Dataset;
import org.apache.spark.sql.Row;
import org.apache.spark.sql.SparkSession;
public class ForEachBookApp implements Serializable {
private static final long serialVersionUID = -4250231621481140775L;
private final class BookPrinter implements ForeachFunction<Row> {
private static final long serialVersionUID = -3680381094052442862L;
@Override
public void call(Row r) throws Exception {
System.out.println(r.getString(2) " can be bought at " r.getString(
4));
}
}
public static void main(String[] args) {
ForEachBookApp app = new ForEachBookApp();
app.start();
}
private void start() {
SparkSession spark = SparkSession.builder().appName("For Each Book").master(
"local").getOrCreate();
String filename = "data/books.csv";
Dataset<Row> df = spark.read().format("csv").option("inferSchema", "true")
.option("header", "true")
.load(filename);
df.show();
df.foreach(new BookPrinter());
}
}
As you can see, this example reads a CSV file and prints a message from the data. It is fairly simple.
The foreach()
instantiates a new class, where the work is done.
df.foreach(new BookPrinter());
The work is done in the call()
method of the class:
private final class BookPrinter implements ForeachFunction<Row> {
@Override
public void call(Row r) throws Exception {
...
}
}
As you are new to Java, make sure you have the right signature (for classes and methods) and the right imports.
You can also clone the example from https://github.com/jgperrin/net.jgp.labs.spark/tree/master/src/main/java/net/jgp/labs/spark/l240_foreach/l000. This should help you with foreach()
.