add test demonstrating that FlyingSaucer can be used in parallel

NB! ITextRenderer and SharedContext should not be shared between threads. They are NOT thread-safe.

flying-saucer-core/src/main/java/org/xhtmlrenderer/resource/FSCatalog.java

@@ -81,9 +81,9 @@ public Map<String, String> parseCatalog(String catalogURI) {
             URL url = requireNonNull(FSCatalog.class.getClassLoader().getResource(catalogURI),
                     () -> "Catalog not found in classpath: " + catalogURI);
             return parseCatalog(url);
-        } catch (Exception ex) {
+        } catch (IOException ex) {
             XRLog.xmlEntities(Level.WARNING, "Could not open XML catalog from URI '" + catalogURI + "'", ex);
-            return new HashMap<>();
+            throw new IllegalStateException("Cannot find " + catalogURI + " in classpath", ex);
         }
     }
 

flying-saucer-core/src/main/java/org/xhtmlrenderer/resource/FSEntityResolver.java

@@ -56,7 +56,7 @@ public class FSEntityResolver implements EntityResolver2 {
     /**
      * Singleton instance, use {@link #instance()} to retrieve.
      */
-    private static FSEntityResolver instance;
+    private static final FSEntityResolver instance = new FSEntityResolver();
 
     private final Map<String, String> entities = new HashMap<>();
 
@@ -150,10 +150,7 @@ private static InputSource newEmptySource() {
      *
      * @return An instance of .
      */
-    public static synchronized FSEntityResolver instance() {
-        if (instance == null) {
-            instance = new FSEntityResolver();
-        }
+    public static FSEntityResolver instance() {
         return instance;
     }
 

flying-saucer-examples/src/test/java/org/xhtmlrenderer/pdf/ConcurrentPdfGenerationTest.java

@@ -0,0 +1,92 @@
+package org.xhtmlrenderer.pdf;
+
+import com.codeborne.pdftest.PDF;
+import com.lowagie.text.DocumentException;
+import junit.framework.TestCase;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.w3c.dom.Document;
+import org.xhtmlrenderer.resource.FSEntityResolver;
+import org.xml.sax.SAXException;
+
+import javax.xml.parsers.DocumentBuilder;
+import javax.xml.parsers.DocumentBuilderFactory;
+import javax.xml.parsers.ParserConfigurationException;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.net.URL;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.ScheduledExecutorService;
+
+import static com.codeborne.pdftest.assertj.Assertions.assertThat;
+import static java.util.Objects.requireNonNull;
+import static java.util.concurrent.Executors.newFixedThreadPool;
+import static java.util.concurrent.Executors.newScheduledThreadPool;
+import static java.util.concurrent.TimeUnit.MILLISECONDS;
+import static java.util.concurrent.TimeUnit.SECONDS;
+
+public class ConcurrentPdfGenerationTest extends TestCase {
+    private static final Logger log = LoggerFactory.getLogger(ConcurrentPdfGenerationTest.class);
+
+    public void testSamplePdf() {
+        byte[] pdf = generatePdf("sample.html");
+        verifyPdf(pdf);
+    }
+
+    public void testConcurrentPdfGeneration() throws InterruptedException {
+        ScheduledExecutorService timer = newScheduledThreadPool(1);
+        timer.scheduleWithFixedDelay(System::gc, 0, 15, MILLISECONDS);
+
+        ExecutorService pool = newFixedThreadPool(20);
+        for (int j = 0; j < 2000; j++) {
+            final int i = j;
+            pool.submit(() -> {
+                try {
+                    byte[] pdf = generatePdf("sample.html");
+                    verifyPdf(pdf);
+                    log.info("Check #{} ok", i);
+                }
+                catch (Throwable e) {
+                    log.error("Check #{} failed: ", i, e);
+                }
+            });
+        }
+
+        pool.shutdown();
+        assert pool.awaitTermination(250, SECONDS) : "Timeout!";
+    }
+
+    private void verifyPdf(byte[] pdfBytes) {
+        PDF pdf = new PDF(pdfBytes);
+        assertThat(pdf).containsText("Bill To:", "John doe", "[email protected]");
+        assertThat(pdf).containsText("Invoice #:", "INV-666");
+        assertThat(pdf).containsText("Invoice Date:", "Sep 27, 2023");
+    }
+
+    private byte[] generatePdf(String htmlPath) {
+        ITextRenderer renderer = new ITextRenderer();
+        renderer.getSharedContext().setMedia("pdf");
+        renderer.getSharedContext().setInteractive(false);
+        renderer.getSharedContext().getTextRenderer().setSmoothingThreshold(0);
+
+        URL htmlUrl = requireNonNull(Thread.currentThread().getContextClassLoader().getResource(htmlPath), () -> "Test resource not found: " + htmlPath);
+
+        try {
+            DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
+            builder.setEntityResolver(FSEntityResolver.instance());
+
+            Document doc = builder.parse(htmlUrl.openStream());
+
+            renderer.setDocument(doc, htmlUrl.toString());
+            renderer.layout();
+
+            ByteArrayOutputStream bos = new ByteArrayOutputStream();
+            renderer.createPDF(bos);
+            return bos.toByteArray();
+        }
+        catch (DocumentException | IOException | SAXException | ParserConfigurationException e) {
+            throw new IllegalArgumentException(e);
+        }
+    }
+
+}