ClassNotFoundException when dynamically loading a Java class executed by exec-maven-plugin

Minimal reproducer project: https://github.com/Kirchoffs/exec-plugin-issue

What I am trying to do, is to create a java class (PrimitiveServlet.class) file by javac, and then use URLClassLoader to load this class file dynamically.

To reproduce the bug using exec-maven-plugin, run the following command:

>> javac -cp webroot/javax.servlet-api-4.0.1.jar webroot/PrimitiveServlet.java
>> mvn clean install
>> mvn exec:java

The first command is to create the class file, the third command is to run the whole program.

Error Information:

java.lang.NoClassDefFoundError: javax/servlet/Servlet
    at java.lang.ClassLoader.defineClass1 (Native Method)
    at java.lang.ClassLoader.defineClass (ClassLoader.java:1016)
    at java.security.SecureClassLoader.defineClass (SecureClassLoader.java:174)
    at java.net.URLClassLoader.defineClass (URLClassLoader.java:550)
    at java.net.URLClassLoader$1.run (URLClassLoader.java:458)
    at java.net.URLClassLoader$1.run (URLClassLoader.java:452)
    at java.security.AccessController.doPrivileged (Native Method)
    at java.net.URLClassLoader.findClass (URLClassLoader.java:451)
    at java.lang.ClassLoader.loadClass (ClassLoader.java:588)
    at java.lang.ClassLoader.loadClass (ClassLoader.java:521)
    at org.syh.demo.App.main (App.java:25)
    at org.codehaus.mojo.exec.ExecJavaMojo$1.run (ExecJavaMojo.java:279)
    at java.lang.Thread.run (Thread.java:834)
Caused by: java.lang.ClassNotFoundException: javax.servlet.Servlet
    at java.net.URLClassLoader.findClass (URLClassLoader.java:471)
    at java.lang.ClassLoader.loadClass (ClassLoader.java:588)
    at java.lang.ClassLoader.loadClass (ClassLoader.java:521)
    at java.lang.ClassLoader.defineClass1 (Native Method)
    at java.lang.ClassLoader.defineClass (ClassLoader.java:1016)
    at java.security.SecureClassLoader.defineClass (SecureClassLoader.java:174)
    at java.net.URLClassLoader.defineClass (URLClassLoader.java:550)
    at java.net.URLClassLoader$1.run (URLClassLoader.java:458)
    at java.net.URLClassLoader$1.run (URLClassLoader.java:452)
    at java.security.AccessController.doPrivileged (Native Method)
    at java.net.URLClassLoader.findClass (URLClassLoader.java:451)
    at java.lang.ClassLoader.loadClass (ClassLoader.java:588)
    at java.lang.ClassLoader.loadClass (ClassLoader.java:521)
    at org.syh.demo.App.main (App.java:25)
    at org.codehaus.mojo.exec.ExecJavaMojo$1.run (ExecJavaMojo.java:279)
    at java.lang.Thread.run (Thread.java:834)
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  1.111 s
[INFO] Finished at: 2022-08-01T21:21:09-07:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:3.1.0:java (default-cli) on project exec-plugin-issue: An exception occurred while executing the Java class. javax/servlet/Servlet: javax.servlet.Servlet

However, if we use pure java command to run the program, the code can work without any error:

>> javac -cp webroot/javax.servlet-api-4.0.1.jar webroot/PrimitiveServlet.java
>> mvn clean install
>> java -cp webroot/javax.servlet-api-4.0.1.jar:target/exec-plugin-issue-1.0-SNAPSHOT.jar org.syh.demo.App

Reproduce the failure with the listed javac, mvn clean install, and mvn exec:java commands, using the linked minimal project and org.syh.demo.App at App.java:25 as the entry point. Then inspect the exec-maven-plugin Java goal's classpath and class-loader behavior, comparing it with the working java command. Done means the dynamically loaded PrimitiveServlet resolves javax.servlet.Servlet under mvn exec:java.