ClassNotFoundException with custom xml serializer for javadoc doclet

Question

tl;dr; the class loader used by the XmlOutputFactory complains it can't find Woodstox, even though it can be found by the doclet's own classloader.

If running the javadoc target in our sample project where we have set the xml factory to use the Woodstox serializer the class loader used by XmlOutputFactory complains it can't find the file, even though it can be found in the doclet.

Constructing Javadoc information...
java.net.URLClassLoader@20fa23c1
Check that we can get hold of class: com.ctc.wstx.stax.WstxOutputFactory
Setting javax.xml.stream.XMLOutputFactory to com.ctc.wstx.stax.WstxOutputFactory
Trying to instantiate a new instance from XMLOutputFactory

...
[ERROR] Exit code: 4 - javadoc: error - fatal error encountered: javax.xml.stream.FactoryConfigurationError: Provider com.ctc.wstx.stax.WstxOutputFactory not found
[ERROR] javadoc: error - Please file a bug against the javadoc tool via the Java bug reporting page
[ERROR] (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com)
[ERROR] for duplicates. Include error messages and the following diagnostic in your report. Thank you.
[ERROR] javax.xml.stream.FactoryConfigurationError: Provider com.ctc.wstx.stax.WstxOutputFactory not found
[ERROR]     at java.xml/javax.xml.stream.FactoryFinder.newInstance(FactoryFinder.java:196)
[ERROR]     at java.xml/javax.xml.stream.FactoryFinder.newInstance(FactoryFinder.java:148)
[ERROR]     at java.xml/javax.xml.stream.FactoryFinder.find(FactoryFinder.java:260)
[ERROR]     at java.xml/javax.xml.stream.FactoryFinder.find(FactoryFinder.java:222)
[ERROR]     at java.xml/javax.xml.stream.XMLOutputFactory.newInstance(XMLOutputFactory.java:138)
[ERROR]     at docs.JacksonWriter.write(JacksonWriter.java:161)
[ERROR]     at docs.TestSheetDoclet.run(TestSheetDoclet.java:98)
[ERROR]     at jdk.javadoc/jdk.javadoc.internal.tool.Start.parseAndExecute(Start.java:588)
[ERROR]     at jdk.javadoc/jdk.javadoc.internal.tool.Start.begin(Start.java:432)
[ERROR]     at jdk.javadoc/jdk.javadoc.internal.tool.Start.begin(Start.java:345)
[ERROR]     at jdk.javadoc/jdk.javadoc.internal.tool.Main.execute(Main.java:63)
[ERROR]     at jdk.javadoc/jdk.javadoc.internal.tool.Main.main(Main.java:52)
[ERROR] Caused by: java.lang.ClassNotFoundException: com/ctc/wstx/stax/WstxOutputFactory

So although our JacksonWriter clearly finds the class (see the output), it seems that the XmlOutputFactory does not, indicating it has some kind of other class loader that does not see the provided classpath.

You can see the full classpaths used in target/site/testapidocs/options:

    -classpath
    '/home/myuser/dev/tmp/server-javadoc-repro/target/classes:/home/myuser/dev/tmp/server-javadoc-repro/target/test-classes:/home/myuser/.m2/repository/org/jetbrains/annotations/17.0.0/annotations-17.0.0.jar:/home/myuser/.m2/repository/com/fasterxml/jackson/module/jackson-module-parameter-names/2.9.7/jackson-module-parameter-names-2.9.7.jar:/home/myuser/.m2/repository/org/hamcrest/hamcrest-core/1.3/hamcrest-core-1.3.jar:/home/myuser/.m2/repository/ch/qos/logback/logback-core/1.2.3/logback-core-1.2.3.jar:/home/myuser/.m2/repository/com/fasterxml/jackson/core/jackson-databind/2.9.7/jackson-databind-2.9.7.jar:/home/myuser/.m2/repository/com/fasterxml/jackson/core/jackson-annotations/2.9.0/jackson-annotations-2.9.0.jar:/home/myuser/.m2/repository/org/assertj/assertj-core/3.11.1/assertj-core-3.11.1.jar:/home/myuser/.m2/repository/org/codehaus/woodstox/stax2-api/3.1.4/stax2-api-3.1.4.jar:/home/myuser/.m2/repository/com/fasterxml/jackson/dataformat/jackson-dataformat-xml/2.9.7/jackson-dataformat-xml-2.9.7.jar:/home/myuser/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar:/home/myuser/.m2/repository/org/slf4j/slf4j-api/1.7.25/slf4j-api-1.7.25.jar:/home/myuser/.m2/repository/junit/junit/4.12/junit-4.12.jar:/home/myuser/.m2/repository/com/fasterxml/jackson/core/jackson-core/2.9.7/jackson-core-2.9.7.jar:/home/myuser/.m2/repository/com/fasterxml/jackson/module/jackson-module-jaxb-annotations/2.9.7/jackson-module-jaxb-annotations-2.9.7.jar:/home/myuser/.m2/repository/com/fasterxml/woodstox/woodstox-core/5.0.3/woodstox-core-5.0.3.jar:/home/myuser/.m2/repository/com/fasterxml/jackson/dataformat/jackson-dataformat-xml/2.9.7/jackson-dataformat-xml-2.9.7.jar:/home/myuser/.m2/repository/com/fasterxml/woodstox/woodstox-core/5.0.3/woodstox-core-5.0.3.jar'
    -doclet
    'docs.TestSheetDoclet'
    -docletpath
    '/home/myuser/dev/tmp/server-javadoc-repro/target/acme-server-with-javadoc-issues-0.0.1-SNAPSHOT.jar:/home/myuser/.m2/repository/com/fasterxml/jackson/core/jackson-databind/2.9.7/jackson-databind-2.9.7.jar:/home/myuser/.m2/repository/com/fasterxml/jackson/core/jackson-annotations/2.9.0/jackson-annotations-2.9.0.jar:/home/myuser/.m2/repository/com/fasterxml/jackson/core/jackson-core/2.9.7/jackson-core-2.9.7.jar:/home/myuser/.m2/repository/com/fasterxml/jackson/module/jackson-module-parameter-names/2.9.7/jackson-module-parameter-names-2.9.7.jar:/home/myuser/.m2/repository/com/fasterxml/jackson/dataformat/jackson-dataformat-xml/2.9.7/jackson-dataformat-xml-2.9.7.jar:/home/myuser/.m2/repository/com/fasterxml/jackson/module/jackson-module-jaxb-annotations/2.9.7/jackson-module-jaxb-annotations-2.9.7.jar:/home/myuser/.m2/repository/org/codehaus/woodstox/stax2-api/3.1.4/stax2-api-3.1.4.jar:/home/myuser/.m2/repository/com/fasterxml/woodstox/woodstox-core/5.0.3/woodstox-core-5.0.3.jar:/home/myuser/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar:/home/myuser/.m2/repository/ch/qos/logback/logback-core/1.2.3/logback-core-1.2.3.jar:/home/myuser/.m2/repository/org/slf4j/slf4j-api/1.7.25/slf4j-api-1.7.25.jar:/home/myuser/.m2/repository/org/jetbrains/annotations/17.0.0/annotations-17.0.0.jar:/home/myuser/.m2/repository/com/fasterxml/woodstox/woodstox-core/5.0.3/woodstox-core-5.0.3.jar:/home/myuser/.m2/repository/org/codehaus/woodstox/stax2-api/3.1.4/stax2-api-3.1.4.jar'
    -encoding
    'UTF-8'
    -protected
    -sourcepath
    '/home/myuser/dev/tmp/server-javadoc-repro/src/test/java:/home/myuser/dev/tmp/server-javadoc-repro/target/generated-test-sources/test-annotations'

How to reproduce

git clone https://github.com/fatso83/server-javadoc-repro
cd server-javadoc-repro
mvn install org.apache.maven.plugins:maven-javadoc-plugin:3.0.1:test-javadoc

Answer 1

The documentation of javax.xml.datatype.FactoryFinder.getProviderClass() (per OpenJDK 12+33) states:

    /**
     * Attempt to load a class using the class loader supplied. If that fails
     * and fall back is enabled, the current (i.e. bootstrap) class loader is
     * tried.
     *
     * If the class loader supplied is <code>null</code>, first try using the
     * context class loader followed by the current (i.e. bootstrap) class
     * loader.
     *
     * Use bootstrap classLoader if cl = null and useBSClsLoader is true
     */

So if you override the threads context classloader just before calling XMLOutputFactory.newInstance() you should be good:

Thread.currentThread().setContextClassLoader(WstxOutputFactory.class.getClassLoader());
final var xmlOutputFactory = XMLOutputFactory.newInstance();

JAXP has a number of different ways of configuring which JAXP implementation (or "provider") to use in runtime, but the simplest is to set the system property javax.xml.stream.XMLOutputFactory to the name of the implementation class you want to use, com.ctc.wstx.stax.WstxOutputFactory in your case:

System.setProperty("javax.xml.stream.XMLOutputFactory", "com.ctc.wstx.stax.WstxOutputFactory");