Fix: Remove race condition in Schematron thread

CityDoctorParent/CityDoctorValidation/src/main/java/de/hft/stuttgart/citydoctor2/check/Checker.java

@@ -81,8 +81,6 @@ import java.io.InputStream;
 import java.io.UncheckedIOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
-import java.text.ChoiceFormat;
-import java.text.MessageFormat;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
@@ -195,26 +193,31 @@ public class Checker {
         if (config == null) {
             config = ValidationConfiguration.loadStandardValidationConfig();
         }
-        Thread schematronThread = new Thread(() -> {
-            SvrlContentHandler handler = executeSchematronValidationIfAvailable(config, model.getFile());
-            if (handler != null) {
-                handleSchematronResults(handler);
-            }
-        });
-        schematronThread.start();
+
+        Callable<SvrlContentHandler> schematronThread = () -> executeSchematronValidationIfAvailable(config, model.getFile());
+        ExecutorService executor = Executors.newSingleThreadExecutor();
+        Future<SvrlContentHandler> handlerFuture =  executor.submit(schematronThread);
+        executor.shutdown();
+
         checkCityModel(model, l);
-        if (schematronThread.isAlive()) {
+        if (!handlerFuture.isDone()) {
+            if (logger.isInfoEnabled()) {
                 logger.info(Localization.getText("Checker.schematronStillRunning"));
+            }
             if(l !=null){
                 l.updateProgress(-0.5f);
             }
-            try{
-                schematronThread.join();
-                logger.info(Localization.getText("Checker.schematronCompleted"));
+        }
+        try {
+            SvrlContentHandler handler = handlerFuture.get();
+            if (handler != null) {
+                handleSchematronResults(handler);
+            }
+        } catch (ExecutionException e) {
+            logger.error(e);
         } catch (InterruptedException e) {
             logger.error(Localization.getText("Checker.schematronInterrupted"), e);
-                schematronThread.interrupt();
-            }
+            Thread.currentThread().interrupt();
         }
         CacheManager.synchronizeCache();
         model.setValidated(createValidationPlan());