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<!DOCTYPE document [
<!ENTITY project SYSTEM "project.xml">
]>
<document url="graal.html">
&project;
<properties>
<title>Ahead of Time compilation support</title>
</properties>
<body>
<section name="Table of Contents">
<toc/>
</section>
<section name="Introduction">
<p>
Tomcat supports using the GraalVM/Mandrel Native Image tool to produce
a native binary including the container. This documentation page
describes the build process of such an image.
</p>
</section>
<section name="Setup">
<p>
The native image tool is much easier to use with single JARs, as a result
the process will use the Maven shade plugin JAR packaging. The
idea is to produce a single JAR that contains all necessary classes from
Tomcat, the webapps and all additional dependencies. Although Tomcat has
received compatibility fixes to support native images, other libraries
may not be compatible and may require replacement code (the GraalVM
documentation has more details about this).
</p>
<p>
Download and install GraalVM or Mandrel.
</p>
<p>
Download the Tomcat Stuffed module from
<code>https://github.com/apache/tomcat/tree/main/modules/stuffed</code>.
For convenience, an env property can be set:
<source>export TOMCAT_STUFFED=/absolute...path...to/stuffed</source>
The build process now requires both Apache Ant and Maven.
</p>
</section>
<section name="Packaging and Building">
<p>
Inside the <code>$TOMCAT_STUFFED</code> folder, the directory structure
is the same as for regular Tomcat. The main configuration files
are placed in the <code>conf</code> folder, and if using the default
<code>server.xml</code> the webapps are placed in the <code>webapps</code>
folder.
</p>
<p>
All the webapp classes need to be made available to the Maven shade plugin
as well as the compiler during the JSP precompilation step.
Any JARs that are present in <code>/WEB-INF/lib</code>
need to be made available as Maven dependencies.
The <code>webapp-jspc.ant.xml</code> script will copy classes from the
<code>/WEB-INF/classes</code> folder of the webapp
to the <code>target/classes</code> path that Maven uses as the compilation
target, but if any of the JSP sources use them, then they need to be
packaged as JARs instead.
</p>
<p>
The first step is to build the shaded Tomcat JAR with all dependencies.
Any JSP in the webapp must all be precompiled and packaged (assuming
that the <code>webapps</code> contains a <code>$WEBAPPNAME</code> webapp):
<source>cd $TOMCAT_STUFFED
mvn package
ant -Dwebapp.name=$WEBAPPNAME -f webapp-jspc.ant.xml</source>
Dependencies for the webapp should now be added to the main
<code>$TOMCAT_STUFFED/pom.xml</code>,
following by building the shaded JAR:
<source>mvn package</source>
</p>
<p>
As it is best to avoid using reflection whenever possible with Ahead of
Time compilation, it can be a good idea to generate and compile Tomcat
Embedded code out of the main server.xml configuration as well as the
context.xml files used to configure the contexts.
<source>$JAVA_HOME/bin/java\
-Dcatalina.base=. -Djava.util.logging.config.file=conf/logging.properties\
-jar target/tomcat-stuffed-1.0.jar --catalina -generateCode src/main/java</source>
Then stop Tomcat and use the following command to include the generated
embedded code:
<source>mvn package</source>
The rest of the process described here will assume this step was done and
the <code>--catalina -useGeneratedCode</code> arguments are added to the
command lines. If this was not the case, they should be removed.
</p>
</section>
<section name="Native image configuration">
<p>
Native images do not support any form of dynamic classloading or
reflection unless it is defined explicitly in descriptors. Generating
them uses a tracing agent from the GraalVM, and needs additional manual
configuration in some cases.
</p>
<p>
Run Tomcat using the GraalVM substrate VM and its trace agent:
<source>$JAVA_HOME/bin/java\
-agentlib:native-image-agent=config-output-dir=$TOMCAT_STUFFED/target/\
-Dorg.graalvm.nativeimage.imagecode=agent\
-Dcatalina.base=. -Djava.util.logging.config.file=conf/logging.properties\
-jar target/tomcat-stuffed-1.0.jar --catalina -useGeneratedCode</source>
</p>
<p>
Now all paths from the webapp that lead to dynamic classloading
(ex: Servlet access, websockets, etc) need to be accessed using a script
that will exercise the webapp. Servlets may be loaded on startup
instead of needing an actual access. Listeners may also be used to load
additional classes on startup. When that is done, Tomcat can be stopped.
</p>
<p>
The descriptors have now been generated in the agent output directory.
At this point, further configuration must be made to add items that are
not traced, including: base interfaces, resource bundles, BeanInfo based
reflection, etc. Please refer to the Graal documentation for more
information on this process.
</p>
<p>
Even though all classes that are used have to be complied AOT into the
native image, webapps must still be left unchanged, and continue including
all needed classes and JARs in the <code>WEB-INF</code> folder. Although
these classes will not actually be run or loaded, access to them is
required.
</p>
</section>
<section name="Building the native image">
<p>
If everything has been done properly, the native image can now be built
using the native-image tool.
<source>$JAVA_HOME/bin/native-image --report-unsupported-elements-at-runtime\
--enable-http --enable-https --enable-url-protocols=http,https,jar,jrt\
--initialize-at-build-time=org.eclipse.jdt,org.apache.el.parser.SimpleNode,jakarta.servlet.jsp.JspFactory,org.apache.jasper.servlet.JasperInitializer,org.apache.jasper.runtime.JspFactoryImpl\
-H:+UnlockExperimentalVMOptions\
-H:+JNI -H:+ReportExceptionStackTraces\
-H:ConfigurationFileDirectories=$TOMCAT_STUFFED/target/\
-H:ReflectionConfigurationFiles=$TOMCAT_STUFFED/tomcat-reflection.json\
-H:ResourceConfigurationFiles=$TOMCAT_STUFFED/tomcat-resource.json\
-H:JNIConfigurationFiles=$TOMCAT_STUFFED/tomcat-jni.json\
-jar $TOMCAT_STUFFED/target/tomcat-stuffed-1.0.jar</source>
The additional <code>--static</code> parameter enables static linking of
glibc, zlib and libstd++ in the generated binary.
</p>
<p>
Running the native image is then:
<source>./tomcat-stuffed-1.0 -Dcatalina.base=. -Djava.util.logging.config.file=conf/logging.properties --catalina -useGeneratedCode</source>
</p>
</section>
<section name="Compatibility">
<p>
Servlets, JSPs, EL, websockets, the Tomcat container, tomcat-native, HTTP/2
are all supported out of the box in a native image.
</p>
<p>
At the time of writing this documentation, JULI is not supported as the
log manager configuration property is not supported by Graal, in addition
to some static initializer problems, and the regular java.util.logging
loggers and implementation should be used instead.
</p>
<p>
If using the default server.xml file, some Server listeners have
to be removed from the configuration as they are not compatible with native
images, such as a JMX listener (JMX is unsupported) and leak prevention
listeners (use of internal code that does not exist in Graal).
</p>
<p>
Missing items for better Tomcat functionality:
<ul>
<li>java.util.logging LogManager: Configuration through a system property
is not implemented, so standard java.util.logging must be used instead
of JULI</li>
<li>Static linking configuration: tomcat-native cannot be statically
linked</li>
</ul>
</p>
</section>
</body>
</document>