Version: 9.4.21.v20190926 |

Startup using the Java Platform Module System (JPMS)
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## Startup using the Java Platform Module System (JPMS)

Jetty modules also act as automatic JPMS modules via the `Automatic-Module-Name` attribute in the jar’s `MANIFEST.MF` file.

This makes possible to run Jetty from the module-path, rather than the class-path.

We recommend using JDK 11 or greater due to the fact that JDK 11 removed all the "enterprise" modules from the JDK, and therefore it guarantees a more stable platform to base your application’s dependencies on. The classes in these "enterprise" modules were bundled with JDK 8, and present in "enterprise" modules in JDK 9 and JDK 10. With JDK 11, these "enterprise" classes are either not available in the JDK (because their corresponding module was removed), or they are present in a different module.

Because some of these "enterprise" classes are required by Jetty or by applications running in Jetty, it is better to use a stable source for those classes - in this case by using JDK 11 or greater, and explicitly referencing the "enterprise" classes as dependencies, rather than assuming they are bundled with the JDK.

### Starting Jetty on the module-path

To start Jetty on the module-path rather than the class-path, it is enough to add the `--jpms` option to the command line, for example:

    $ mkdir my-jetty-base
    $ cd my-jetty-base
    $ java -jar $JETTY_HOME/start.jar --add-to-start=http
    INFO  : server          transitively enabled, ini template available with --add-to-start=server
    INFO  : http            initialized in ${jetty.base}/start.ini
    INFO  : threadpool      transitively enabled, ini template available with --add-to-start=threadpool
    INFO  : Base directory was modified
    $ java -jar $JETTY_HOME/start.jar --jpms

The example above creates a Jetty base directory and enables the `http` module using the `--add-to-start` command. The server then starts Jetty on the module-path using the `--jpms` option.

    [NOTE]
    When running on the module-path using the `--jpms` option, the Jetty start mechanism will fork a second JVM passing it the right JVM options to run on the module-path.
    You will have two JVMs running: one that runs `start.jar` and one that runs Jetty on the module-path.

If you are interested in the details of how the command line to run Jetty on the module-path looks like, you can add the `--dry-run` option:

    $ java -jar $JETTY_HOME/start.jar --jpms --dry-run

This will give an output looking something like this (broken in sections for clarity):

    /opt/openjdk-11+28/bin/java
    --module-path /opt/jetty/lib/servlet-api-3.1.jar:/opt/jetty/lib/jetty-schemas-3.1.jar:/opt/jetty/lib/jetty-http-9.4.13-SNAPSHOT.jar:...
    --patch-module servlet.api=/opt/jetty/lib/jetty-schemas-3.1.jar
    --module org.eclipse.jetty.xml/org.eclipse.jetty.xml.XmlConfiguration /opt/jetty/etc/jetty-threadpool.xml /opt/jetty/etc/jetty.xml ...

The `--module-path` option specifies the list of Jetty jars. This list depends on the Jetty modules that have been enabled via the `--add-to-start` command.

The `--patch-module` option is necessary for Servlet and JSP Containers to find XML DTDs and XML Schemas required to validate the various XML files present in web applications (such as `web.xml` and others).

The `--module` option tells the JVM to run main class `XmlConfiguration` from the `org.eclipse.jetty.xml` module, with the given XML files as program arguments.

When the JVM starts, module `org.eclipse.jetty.xml` is added to the set of JPMS root modules; all other Jetty modules, being automatic, will be resolved and added to the module graph. JAR files that are not modules, such as `servlet-api-3.1.jar`, are on the module-path and therefore will be made automatic modules by the JVM (hence the derived module name `servlet.api` for this jar, referenced by the `--patch-module` command line option above).

### Advanced JPMS Configuration

Web applications may need additional services from the Servlet Container, such as JDBC `DataSource` references or JTA `UserTransaction` references.

For example, for JDBC it is typical to store, in JNDI, a reference to the connection pool’s `DataSource` (such as `com.zaxxer.hikari.HikariDataSource`) or a reference directly to the JDBC driver’s `DataSource` (`com.mysql.jdbc.jdbc2.optional.MysqlDataSource`). Jetty needs to be able to instantiate those classes and therefore needs to be able to load those classes and all their super-classes, among which includes `javax.sql.DataSource`.

When Jetty runs on the class-path, this is easily achieved by using a custom module:

mysql.mod.

    [description]
    MySQL module

    [lib]
    lib/mysql/mysql-connector-java-*.jar

However, when running on the module-path, things are quite different.

Class `javax.sql.DataSource` is in a JDK bundled module named `java.sql`, which is not automatic (it’s a proper JPMS module) and it is not in the root modules set. Because it is not an automatic module, it is not added to the module graph, and therefore needs to be added explicitly using the JVM command line `--add-modules`.

To add the JPMS module `java.sql` to the module graph, you need to modify your custom module in the following way, using our `mysql.mod` as an example:

mysql.mod.

    [description]
    MySQL module

    [lib]
    lib/mysql/mysql-connector-java-*.jar

    [jpms]
    add-modules: java.sql

The new `[jpms]` section is only used when Jetty is started on the module-path via the `--jpms` command line option.

Assuming that `mysql-connector-java-*.jar` is a non JPMS modular jar, or an automatic JPMS modular jar, the Jetty start mechanism will add `mysql-connector-java-*.jar` to the module-path, and will add the JVM command line option `--add-modules java.sql`.

If `mysql-connector-java-*.jar` were a proper JPMS modular jar with name (for example) `com.mysql.jdbc`, then it would need to be explicitly added to the module graph, in this way:

mysql.mod.

    [description]
    MySQL module

    [lib]
    lib/mysql/mysql-connector-java-*.jar

    [jpms]
    add-modules: com.mysql.jdbc

The JPMS module `java.sql` does not need to be explicitly added because it would be a dependency of the `com.mysql.jdbc` module and therefore automatically added to the module graph.

The `[jpms]` section has the following format:

    [jpms]
    add-modules: <module name>(,<module name>)*
    patch-module: <module>=<file>(:<file>)*
    add-opens: <module>/<package>=<target-module>(,<target-module>)*
    add-exports: <module>/<package>=<target-module>(,<target-module>)*
    add-reads: <module>=<target-module>(,<target-module>)*

### Alternative way to start Jetty on the module-path

The section above uses the `--jpms` command line option to start Jetty on the module-path. An alternative way of achieving the same result is to use a Jetty module, `$JETTY_BASE/modules/jpms.mod`, that specifies that you want to run using JPMS (and possibly add some JPMS specific configuration).

jpms.mod.

    [ini]
    --jpms

    [jpms]
    # Additional JPMS configuration.

The `[ini]` section is equivalent to passing the `--jpms` option to the command line. The `[jpms]` section (see also the advanced JPMS configuration section) allows you specify additional JPMS configuration.

    $ mkdir jetty-base-jpms
    $ cd jetty-base-jpms
    $ mkdir modules
    # Copy the jpms.mod file above into the $JETTY_BASE/modules/ directory.
    $ cp /tmp/jpms.mod modules/
    # Add both the http and the jpms modules.
    $ java -jar $JETTY_HOME/start.jar --add-to-start=http,jpms
    # Jetty will start on the module-path.
    $ java -jar $JETTY_HOME/start.jar

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