How to set up application class data sharing in Hotspot

For the Hotspot JVM, Application class data sharing is a feature, first released in the Oracle JVM (8u40 and later) that extends the ordinary Java class data sharing feature to also include selected classes from the application class path. In PL/Java terms, that means that not only Java’s own internal classes, but PL/Java’s also, can be saved in a preprocessed shared archive and quickly mapped when any backend starts PL/Java. For an overview, see the PL/Java VM options page.

Starting with Java 10, the feature is also available in OpenJDK with Hotspot. From Java 8 onward, a different feature with the same effect is available in OpenJDK with OpenJ9; that feature is covered on its own page.

License considerations

In Oracle Java, application class data sharing was a “commercial feature” first released in Java 8, not usable unless pljava.vmoptions also include -XX:+UnlockCommercialFeatures , with implications described in the “supplemental license terms” of the Oracle binary code license for Java SE. The license seems to impose no burden on use for internal development and testing, but requires negotiating an additional agreement with Oracle if the feature will be used “in your internal business operations or for any commercial or production purpose.” It is available to consider for any application where the additional performance margin can be given a price.

The same feature in OpenJDK with Hotspot is available from Java 10 onward, and does not require any additional license or -XX:+UnlockCommercialFeatures option.

Starting in Java 11, Oracle offers Oracle-branded downloads of both “Oracle JDK” and “Oracle’s OpenJDK builds” that are “functionally identical aside from some cosmetic and packaging differences”. “Oracle’s OpenJDK builds” may be used for production or commercial purposes with no additional licensing, while any such use of “Oracle JDK” requires a commercial license. The application class data sharing feature is available in both, and no longer requires the -XX:+UnlockCommercialFeatures option in either case (not in “Oracle’s OpenJDK builds” because their use is unrestricted, and not in “Oracle JDK” because the “commercial feature” is now, effectively, the entire JDK).

The equivalent feature in OpenJDK with OpenJ9, described separately, is available from Java 8 onward, also with no additional license or setup needed.

Setup for Hotspot, earlier than Java 13

The setup instructions on this page are for Hotspot, whether in Oracle Java or OpenJDK with Hotspot. The two differ only in that, wherever an -XX:+UnlockCommercialFeatures option is shown in the steps below, it is needed in Oracle Java 8, 9, or 10, but not in OpenJDK/Hotspot, or Oracle JDK 11 or later.

The Java version also affects the -XX:+UseAppCDS option shown below. For Java 8 through 10, the option must be used for application class data sharing to be enabled. In Java 11, the feature is enabled by default (though the shared archive must still be created as described here), and the -XX:+UseAppCDS option is no longer necessary; it will be accepted but ignored with a warning. In Java 12 and later, -XX:+UseAppCDS is not needed and will be rejected as unrecognized, making PL/Java fail to load.

Setting up PL/Java to use application class data sharing is a three-step process. Each step is done by setting a different combination of options in pljava.vmoptions. A slightly different procedure, described further below, appears in Java 13. Up through Java 12, these are the three steps in overview:

  1. Make a list of classes to be preloaded, by saving the names of classes that are loaded while executing some desired code in PL/Java.

  2. In a new session, trigger the loading of PL/Java in a dump mode, where the JVM loads the classes from the list in step 1, and writes a shared archive.

  3. Move the shared archive to a final, readable-to-postgres location, and save (with ALTER DATABASE ... SET or ALTER SYSTEM) a version of pljava.vmoptions with the final options to use the generated archive.

Before beginning, any pljava.* variable settings necessary should already have been made and saved, and basic PL/Java operation confirmed.

The steps that follow involve setting the pljava.vmoptions variable to contain various options. If other options have been set in pljava.vmoptions already, be sure to include those also when saving the final pljava.vmoptions setting at the end.

Generate the list of needed classes

Classes eligible to go in the shared archive are the Java system classes (including anything in the deprecated java.ext.dirs or java.endorsed.dirs directories), classes in the PL/Java jar itself, and any others in jars named in pljava.module_path. Classes from PL/Java application jars loaded into the database normally with sqlj.install_jar are not candidates for the shared archive. The feature will speed the startup of PL/Java itself, but application classes are still loaded from the database in PL/Java’s usual way.

The generated list will contain any such classes that Java needed to load while starting up and running some sample PL/Java code. That can be anything that exercises most features of PL/Java; the supplied examples jar will do nicely. There is no great benefit to running your specific application code, as those classes won’t end up in the archive anyway. (But see Java libraries below.)

Here, /tmp/pljava.classlist may be any file name you choose, in a location the PostgreSQL backend will be able to write and read. The URL for the pljava-examples jar is abbreviated here, standing in for its real installed location on your system. At the end of this step, use \c in psql to start a fresh new connection.

% psql
=# SET pljava.vmoptions TO
-#  '-XX:+UnlockCommercialFeatures -XX:+UseAppCDS -Xshare:off '
-#  '-XX:DumpLoadedClassList=/tmp/pljava.classlist';
=# SELECT sqlj.install_jar('file:/.../pljava-examples...jar', 'ex', true);
... lots of output from the examples tests ...
(1 row)

=# SELECT sqlj.remove_jar('ex', true);
(1 row)

=# \c
You are now connected to database "..." as ...

Process the list of classes into a shared archive

The last step ended with \c to start a new session. In this step, /tmp/pljava.classlist still represents the chosen file name that was just written, and /tmp/pljava.jsa is another chosen name where the backend will be able to write the Java shared archive.

=# SET pljava.vmoptions TO
-#  '-XX:+UnlockCommercialFeatures -XX:+UseAppCDS -Xshare:dump '
-#  '-XX:SharedArchiveFile=/tmp/pljava.jsa '
-#  '-XX:SharedClassListFile=/tmp/pljava.classlist';
=# SELECT sqlj.get_classpath('public'); -- any PL/Java function will do
INFO:  Allocated shared space: ... bytes at 0x...
INFO:  Loading classes to share ...
INFO:  Loading classes to share: done.
INFO:  Rewriting and linking classes ...
INFO:  Rewriting and linking classes: done
The connection to the server was lost. Attempting reset: Succeeded.

Java (and the postgres backend with it) exit when finished writing the shared archive; psql notices and starts a new connection, ready for the final step.

Final pljava.vmoptions settings to use the new archive

The archive just written (/tmp/pljava.jsa as illustrated) may be moved to a more permanent place, such as a system location where postgres will be able to read it, but it is protected from modification; in this example, /usr/pgsql/lib/pljava.jsa.

Be sure the final pljava.vmoptions setting also includes any other VM options you may have chosen to set.

=# SET pljava.vmoptions TO
-#  '-XX:+UnlockCommercialFeatures -XX:+UseAppCDS -Xshare:auto '
-#  '-XX:SharedArchiveFile=/usr/pgsql/lib/pljava.jsa';
=# SELECT sqlj.get_classpath('public'); -- just checking it works
(1 row)

=# ALTER DATABASE ... SET pljava.vmoptions FROM CURRENT; -- save it!

Alternatively, use ALTER SYSTEM (or edit the postgresql.conf file) to save the setting for all databases in the cluster.

The use of -Xshare:auto rather than -Xshare:on in the final production settings may be surprising, but is recommended. On operating systems with address-space layout randomization, it is possible for some backends to (randomly) fail to map the shared archive. With -Xshare:auto, they will simply proceed without sharing and with higher resource usage, which may not be ideal, but the same event with -Xshare:on would be a hard failure.

Setup for Hotspot, as of Java 13

Java 13 introduces a dynamic CDS archive feature, with fewer steps to generate a usable archive. In essence, it combines the first two earlier steps (generate a list of loaded classes from a sample run, then generate an archive from the list) into a single step: do a sample run with the option -XX:ArchiveClassesAtExit=/tmp/pljava.jsa and the archive will be written to the named file when the backend exits.

Then, as in the earlier procedure, move the archive file to a more permanent and less writable location, and name it with -XX:SharedArchiveFile= in the production pljava.vmoptions settings. That is the only option needed to enable application class data sharing as of Java 13, as -Xshare:auto is the default, and the earlier -XX:+UnlockCommercialFeatures and -XX:+UseAppCDS options are obsolete.

The dynamic CDS archive documentation covers the setup in useful detail.

Java libraries

If your own PL/Java code depends on other Java libraries distributed as jars, the usual recommendation would be to install those as well into the database with sqlj.install_jar, and use sqlj.set_classpath to make them available. That keeps everything handled uniformly within the database.

On the other hand, if you are building a shared archive, and some of the dependency libraries are substantial, you could consider instead storing those jars on the file system and naming them in pljava.classpath. Those library classes can then also be included in the shared archive.

In that case, when generating the needed class list, you should run some representative sample of your own application code, not just the PL/Java examples, so that the necessary library classes will have been exercised.

Not everything from the original jar file can go into the shared archive. After the archive has been built, the original jars still must be on the file system and named in pljava.classpath.

When generating the needed classes list, consider adding -Xverify:all to the other VM options. Java sometimes applies more relaxed verification to classes it loads from the system classpath. As you will only need to do this once and they will later be loaded quickly from the shared archive, they might as well be checked thoroughly at the start.