# Process data with a custom JAR
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A custom JAR runs a compiled Java program that you can upload to Amazon S3. You should compile the program against the version of Hadoop you want to launch, and submit a `CUSTOM_JAR` step to your Amazon EMR cluster. For more information about how to compile a JAR file, see [Build binaries using Amazon EMR](emr-build-binaries.md).

For more information about building a Hadoop MapReduce application, see the [MapReduce Tutorial](http://hadoop.apache.org/docs/stable/hadoop-mapreduce-client/hadoop-mapreduce-client-core/MapReduceTutorial.html) in the Apache Hadoop documentation.

**Topics**
+ [Submit a custom JAR step](emr-launch-custom-jar-cli.md)