Merge branch 'master' into 10.3.x

src/main/java/org/codelibs/fess/thumbnail/impl/WebDriverGenerator.java

@@ -94,9 +94,11 @@ public class WebDriverGenerator extends BaseThumbnailGenerator {
     @Override
     public void destroy() {
         if (webDriver != null) {
+            synchronized (this) {
                 webDriver.quit();
             }
         }
+    }
 
     @Override
     public boolean generate(final String url, final File outputFile) {
@@ -121,10 +123,12 @@ public class WebDriverGenerator extends BaseThumbnailGenerator {
         }
 
         if (webDriver instanceof TakesScreenshot) {
+            synchronized (this) {
                 webDriver.get(url);
                 final File thumbnail = ((TakesScreenshot) webDriver).getScreenshotAs(OutputType.FILE);
                 convert(thumbnail, outputFile);
                 return true;
+            }
         } else {
             logger.warn("WebDriver is not instance of TakesScreenshot: " + webDriver);
             return false;