In JDK 11, Class Data Sharing (CDS) has been improved to support archiving classes from the module path.

To create a CDS archive using the `--module-path` VM option, the command line syntax is as follows:

    $ java -Xshare:dump -XX:SharedClassListFile=<class list file> \
        -XX:SharedArchiveFile=<shared archive file> \
        --module-path=<path to modular jar> -m <module name>

To run with a CDS archive using the `--module-path` VM option, the command line syntax is as follows:

    $ java -Xshare:on -XX:SharedArchiveFile=<shared archive file> \
        --module-path=<path to modular jar> -m <module name>

The following table describes how the VM options related to module paths may be used along with the `-Xshare` option.

<table>
  <thead>
    <tr>
      <th/>
      <th style="text-align: left; padding-left: 2em">-Xshare:dump</th>
      <th style="text-align: left; padding-left: 2em">-Xshare:{on,auto}</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td><code>--module-path</code>¹ &lt;mp&gt;</td>
      <td style="padding-left: 2em">Allowed</td>
      <td style="padding-left: 2em">Allowed²</td>
    </tr>
    <tr>
      <td><code>--module</code></td>
      <td style="padding-left: 2em">Allowed</td>
      <td style="padding-left: 2em">Allowed</td>
    </tr>
    <tr>
      <td><code>--add-module</code></td>
      <td style="padding-left: 2em">Allowed</td>
      <td style="padding-left: 2em">Allowed</td>
    </tr>
    <tr>
      <td><code>--upgrade-module-path</code>³</td>
      <td style="padding-left: 2em">Disallowed (exits if specified)</td>
      <td style="padding-left: 2em">Allowed (disables CDS)</td>
    </tr>
    <tr>
      <td><code>--patch-module</code>⁴</td>
      <td style="padding-left: 2em">Disallowed (exits if specified)</td>
      <td style="padding-left: 2em">Allowed (disables CDS)</td>
    </tr>
    <tr>
      <td><code>--limit-modules</code>⁵</td>
      <td style="padding-left: 2em">Disallowed (exits if specified)</td>
      <td style="padding-left: 2em">Allowed (disables CDS)</td>
    </tr>
  </tbody>
</table>

¹Although there are two ways of specifying a module in a `--module-path`, that is, modular `.jar` or exploded module, only modular `.jar` files are supported.

²Different &lt;mp&gt; may be specified during dump time vs run time. If an archived class `K` was loaded from `mp1.jar` at dump time, but changes in &lt;mp&gt; cause it to be available from a different `mp2.jar` at run time, then the archived version of `K` will be disregarded at run time; `K` will be loaded dynamically.

³Currently, only two system modules are upgradeable (`java.compiler` and `jdk.internal.vm.compiler`). These are seldom upgraded in production software.

⁴As documented in [JEP 261](http://openjdk.java.net/jeps/261), `--patch-module` is strongly discouraged for production use.

⁵`--limit-modules` is intended for testing purposes. It is seldom used in production software.

If any one of `--upgrade-module-path`, `--patch-module`, or `--limit-modules` is specified at dump time, the following error will be printed and the JVM will exit. For example, if the `--limit-modules` option is specified at dump time, the user will see the following error:

    Error occurred during initialization of VM
    Cannot use the following option when dumping the shared archive: --limit-modules

If any one of `--upgrade-module-path`, `--patch-module`, or `--limit-modules` is specified at run time, the following warning message will be printed indicating that CDS is disabled. For example, if the `--limit-modules` option is specified at run time, the user will see the following warning:

    Java HotSpot(TM) 64-Bit Server VM warning: CDS is disabled when the --limit-modules option is specified.

A few other noteworthy things to mention:

1. Any valid combinations of `-cp` and `--module-path` are supported.

2. A non-empty directory in the module path causes a fatal error. The user will see the following error messages:

    Error: non-empty directory &lt;directory>
    Hint: enable -Xlog:class+path=info to diagnose the failure
    Error occurred during initialization of VM
    Cannot have non-empty directory in paths

3. Unlike the class path, there's no restriction that the module path at dump time must be equal to or be a prefix of the module path at run time.

4. The archive is invalidated if an existing JAR in the module path is updated after archive generation.

5. Removing a JAR from the module path does not invalidate the shared archive. Archived classes from the removed JAR are not used at runtime.