README.md

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# Apache Accumulo Classloader Extras

This repository contains a variety of Java classloader-related utilities, or
libraries to support those utilities, for use with Apache Accumulo.

### CachingClassLoaderFactory

An implementation of Accumulo's [ContextClassLoaderFactory][spi] that
downloads, verifies, and locally caches classloader resources using a manifest
file at a specified URL, which itself contained a listing of resource URLs to
download, and corresponding checksums to verify their contents.

See [modules/caching-classloader/README.md](modules/caching-classloader/README.md).

### HdfsURLStreamHandlerProvider

An implementation of [URLStreamHandlerProvider][urlprovider] that supports the
`hdfs://` URL scheme, written to support using [HDFS][hadoop] for remote
resource locations in the
[CachingClassLoaderFactory](#CachingClassLoaderFactory), because that project
does not currently implement such a provider.

See [modules/hdfs-urlstreamhandler-provider/README.md](modules/hdfs-urlstreamhandler-provider/README.md).

### VFS ClassLoaderFactory (Legacy)

An experimental implementation of Accumulo's [ContextClassLoaderFactory][spi]
was developed in `modules/vfs-class-loader` in commit
2c3eb3b18d30d9fa18d744584c4595e4b4ffca9f, but was subsequently removed. It
supported using classpaths containing remote URLs, and used Apache commons-vfs2
to interpret remote locations as filesystems containing resource files. Loading
classes from VFS this way can be prone to substantial errors, and it is not
recommended.


[spi]: https://accumulo.apache.org/docs/2.x/apidocs/org/apache/accumulo/core/spi/common/ContextClassLoaderFactory.html
[urlprovider]: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/net/spi/URLStreamHandlerProvider.html
[hadoop]: https://hadoop.apache.org