Fix: JAXB + Java >= 9 Class Loader Bug & Ensure Tomcat Deployment

- Problem: Jaxb + JAVA >= 9 Runtime + Tomcat = ClassNotFoundException:
  ContextFactory.

  Reason: Apparently, jaxb uses the "wrong" classloader (the system
  classloader via the thread classloader) and this classloader does
  not know about jaxb api's and implementations at runtime since
  oracle decided to move jaxb* out of JRE.

  Solution: create a new thread, override the thread's "thread"
  classloader with the "class" classloader and do all jaxb
  interactions in that thread. See:
  https://sjhannah.com/blog/2018/11/21/jaxb-hell-on-jdk-9/

- Move "run jaxb interactions in own thread with class class loader"
  code into own component (JaxbClassNotFoundFix,
  runInTheadWithClassClassLoader) and wrap ClientFactory and
  Marshaller code into the "runInTheadWithClassClassLoader".

- Ensure that app can be deployed in a Tomcat container (by following
  this guide: https://www.baeldung.com/spring-boot-war-tomcat-deploy)

1 files changed, 55 insertions, 0 deletions

diff --git a/src/main/java/at/gv/egiz/moazs/util/JAXBClassNotFoundFix.java b/src/main/java/at/gv/egiz/moazs/util/JAXBClassNotFoundFix.java
new file mode 100644
index 0000000..9e72286
--- /dev/null
+++ b/src/main/java/at/gv/egiz/moazs/util/JAXBClassNotFoundFix.java
@@ -0,0 +1,55 @@
+package at.gv.egiz.moazs.util;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.stereotype.Component;
+
+import java.util.function.Supplier;
+
+import static at.gv.egiz.moazs.MoaZSException.moaZSException;
+
+@Component
+public class JAXBClassNotFoundFix {
+
+    private static final Logger log = LoggerFactory.getLogger(JAXBClassNotFoundFix.class);
+
+    /**
+     * Solves JAXB's "ClassNotFoundException: com.sun.xml.internal.bind.v2.ContextFactory" in Java >= 9
+     * by running func in a dedicated thread which has its context classloader replaced with a classloader that finds
+     * the ContextFactory.
+     *
+     * See https://sjhannah.com/blog/2018/11/21/jaxb-hell-on-jdk-9/
+     * Note: "runInTheadWithClassClassLoader" is not a typo.
+     * @param func function that is called in a dedicated thread.
+     * @param <A> return value of func.
+     * @return
+     */
+    public static <A> A runInTheadWithClassClassLoader(Supplier<A> func) {
+
+        final Object[] result = new Object[1];
+        final Exception[] error = new Exception[1];
+
+        Thread t = new Thread(() -> {
+            try {
+                result[0] = func.get();
+            } catch (Exception ex) {
+                error[0] = ex;
+            }
+        });
+
+        t.setContextClassLoader(JAXBClassNotFoundFix.class.getClassLoader());
+        t.start();
+
+        try {
+            t.join();
+        } catch (InterruptedException ex) {
+            log.error("Interrupted. ", ex);
+        }
+
+        if (error[0] != null) {
+            throw moaZSException("Could not run function.", error[0]);
+        }
+
+        return (A)result[0];
+    }
+}