hadoop No FileSystem for scheme: file

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This is a typical case of the maven-assembly plugin breaking things.

Why this happened to us

Different JARs (hadoop-commons for LocalFileSystem, hadoop-hdfs for DistributedFileSystem) each contain a different file called org.apache.hadoop.fs.FileSystem in their META-INFO/services directory. This file lists the canonical classnames of the filesystem implementations they want to declare (This is called a Service Provider Interface implemented via java.util.ServiceLoader, see org.apache.hadoop.FileSystem#loadFileSystems).

When we use maven-assembly-plugin, it merges all our JARs into one, and all META-INFO/services/org.apache.hadoop.fs.FileSystem overwrite each-other. Only one of these files remains (the last one that was added). In this case, the FileSystem list from hadoop-commons overwrites the list from hadoop-hdfs, so DistributedFileSystem was no longer declared.

How we fixed it

After loading the Hadoop configuration, but just before doing anything FileSystem-related, we call this:

    hadoopConfig.set("fs.hdfs.impl", 
        org.apache.hadoop.hdfs.DistributedFileSystem.class.getName()
    );
    hadoopConfig.set("fs.file.impl",
        org.apache.hadoop.fs.LocalFileSystem.class.getName()
    );

Update: the correct fix

It has been brought to my attention by krookedking that there is a configuration-based way to make the maven-assembly use a merged version of all the FileSystem services declarations, check out his answer below.

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