What are the "env" options (and their purpose) when creating a Java NIO filesystem?

There are lots of examples on the net about creating Java NIO Filesystem instances using code like this

    Map<String, String> env = new HashMap<String, String>();
    env.put("create", "true");

    FileSystem zipfs = FileSystems.newFileSystem(zipUri, env);

But what are the supported options that can be placed within the "env" map? And what are they good for?

Solution

The Javadoc for FileSystem.newFileSystem(URI uri, Map<String,?> env) specifies

env - a map of provider specific properties to configure the file system; may be empty

So these depend on the type of provider that will create the new FileSystem for the given uri.

In your example, you've provided a URI presumably representing a ZIP file. This is served by a Zip File System Provider whose properties are defined here. Those are

"create": true / false

The value should be of type java.lang.String. The default value is false. If the value is true, the zip file system provider creates a new zip file if it does not exist.
"encoding": String indicating the encoding scheme

The value should be of type java.lang.String. The value of the property indicates the encoding scheme for the names of the entries in the zip or JAR file. The default value is UTF-8.