Java 11: Implementation of JAXB-API has not been found on module path or classpath

The solution is very easy. First, use jaxb 2.3.1. The beta-stuff is for testing only. The real problem was the module-info.java. It is required to have 2 entries for jaxb:

requires java.xml.bind;
requires com.sun.xml.bind;

The first requires defines the implementation, the second the API.

The only two dependencies required are:

compile group: 'javax.xml.bind', name: 'jaxb-api', version: '2.3.1'
compile group: 'org.glassfish.jaxb', name: 'jaxb-runtime', version: '2.3.1'

When upgrading to SpringBoot 2.5, I received the same exception. Adding the following dependency (Maven) solved my issues:

<dependency>
    <groupId>org.glassfish.jaxb</groupId>
    <artifactId>jaxb-runtime</artifactId>
</dependency>

In order to complete the other answers:

JAXB uses the classloader bound to current thread to find the JAXB ContextFactory class (Thread.currentThread().getContextClassLoader()), and in some cases (for instance with jetty threads), the bound classloader is a PlatformClassLoader, which has the particularity to not being aware of class-path, contrary to the AppClassLoader.

To solve this issue JAXB API and runtime must be in the module-path (which is accessible by PlatformClassLoader).

For instance app is packaged with following dependencies:

<dependency>
    <groupId>jakarta.xml.bind</groupId>
    <artifactId>jakarta.xml.bind-api</artifactId>
    <version>2.3.3</version>
</dependency>
<dependency>
    <groupId>org.glassfish.jaxb</groupId>
    <artifactId>jaxb-runtime</artifactId>
    <version>2.3.3</version>
</dependency>

So the application should be started with jaxb-runtime and its module dependencies, as below:

### Set the module-path
MODULEPATH="./lib/jaxb-runtime-2.3.3.jar"
MODULEPATH="${MODULEPATH}:./lib/jakarta.activation-1.2.2.jar"
MODULEPATH="${MODULEPATH}:./lib/jakarta.xml.bind-api-2.3.3.jar"
MODULEPATH="${MODULEPATH}:./lib/txw2-2.3.3.jar"
MODULEPATH="${MODULEPATH}:./lib/istack-commons-runtime-3.0.11.jar"

java --module-path ${MODULEPATH} --add-modules ALL-MODULE-PATH [...]

Note: ALL-MODULE-PATH could be replaced by specific module names

I found there to be a ClassLoader bug, especially if you're doing work within a thread via a ForkJoinPool or similar. The bug is listed here: https://github.com/eclipse-ee4j/jaxb-api/issues/78

Creating the JaxbContext statically will force the correct ClassLoader to be used, but it's hacky. :/

I believe java.lang.ClassNotFoundException is coming because of multiple versions not able to define definition.

Solution: delete com.sun.xml folder from your local m2 repository, and download only one dependency.

javax.xml.bind:jaxb-api:"latest-version" in pom/gradle.

Removed maven-dependency com.sun.xml.bind from my external library which is not modular and a lot of errors like the unnamed module reads package com.sun.xml.bind.util has gone.

I was using the Moxy and I was getting this error even after adding the jaxb.properties file. So I had to make sure followings:

Following are dependency:

    <dependency>
      <groupId>jakarta.xml.bind</groupId>
      <artifactId>jakarta.xml.bind-api</artifactId>
      <version>3.0.1</version>
    </dependency>

    <dependency>
      <groupId>org.eclipse.persistence</groupId>
      <artifactId>eclipselink</artifactId>
      <version>3.0.0</version>
    </dependency>

Along with dependency its important to make sure that Moxy uses the jaxb.properties file during the compilatation which is present within the same package. So add the following above the dependencies within the pom.xml:

  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <configuration>
          <source>8</source>
          <target>8</target>
        </configuration>
      </plugin>
    </plugins>
    <resources>
      <resource>
        <directory>src/main/java</directory>
        <excludes>
          <exclude>**/*.java</exclude>
        </excludes>
      </resource>
    </resources>
  </build>

Evertything together would be something like this:

  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <configuration>
          <source>8</source>
          <target>8</target>
        </configuration>
      </plugin>
    </plugins>
    <resources>
      <resource>
        <directory>src/main/java</directory>
        <excludes>
          <exclude>**/*.java</exclude>
        </excludes>
      </resource>
    </resources>
  </build>

<dependencies>
    <dependency>
      <groupId>jakarta.xml.bind</groupId>
      <artifactId>jakarta.xml.bind-api</artifactId>
      <version>3.0.1</version>
    </dependency>

    <dependency>
      <groupId>org.eclipse.persistence</groupId>
      <artifactId>eclipselink</artifactId>
      <version>3.0.0</version>
    </dependency>

</dependencies>

For the implementation of OAuth2 with Spring 2.5.3, along with the dependency of OAuth2

<dependency>
    <groupId>org.springframework.security.oauth</groupId>
    <artifactId>spring-security-oauth2</artifactId>
    <version>2.5.1.RELEASE</version>
</dependency>

use of following dependency will remove the error Caused by: java.lang.ClassNotFoundException: com.sun.xml.internal.bind.v2.ContextFactory at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:822) ~[na:na] at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:182) ~[na:na]

<dependency>
    <groupId>jakarta.xml.bind</groupId>
    <artifactId>jakarta.xml.bind-api</artifactId>
    <version>2.3.3</version>
</dependency>
<dependency>
    <groupId>org.glassfish.jaxb</groupId>
    <artifactId>jaxb-runtime</artifactId>
    <version>2.3.3</version>
</dependency>

I was facing the same issue for Java 11 - Gradle and below dependencies resolved it

compileOnly 'javax.xml.bind:jaxb-api:2.3.1'
compileOnly 'org.glassfish.jaxb:jaxb-runtime:2.3.1' code here