WW-5690 perf(dispatcher): load the dev-mode error template on first use

core/src/main/java/org/apache/struts2/dispatcher/DefaultDispatcherErrorHandler.java

@@ -30,7 +30,6 @@
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.apache.struts2.StrutsConstants;
-import org.apache.struts2.StrutsException;
 import org.apache.struts2.views.freemarker.FreemarkerManager;
 
 import java.io.IOException;
@@ -48,8 +47,11 @@ public class DefaultDispatcherErrorHandler implements DispatcherErrorHandler {
 
     private static final Logger LOG = LogManager.getLogger(DefaultDispatcherErrorHandler.class);
 
+    private static final String ERROR_TEMPLATE = "/org/apache/struts2/dispatcher/error.ftl";
+
     private FreemarkerManager freemarkerManager;
     private boolean devMode;
+    private ServletContext servletContext;
     private Template template;
 
     @Inject
@@ -63,12 +65,26 @@ public void setDevMode(String devMode) {
     }
 
     public void init(ServletContext ctx) {
-        try {
-            freemarker.template.Configuration config = freemarkerManager.getConfiguration(ctx);
-            template = config.getTemplate("/org/apache/struts2/dispatcher/error.ftl");
-        } catch (IOException e) {
-            throw new StrutsException(e);
+        this.servletContext = ctx;
+    }
+
+    /**
+     * Loads the problem report template on first use rather than at startup: it is only ever
+     * rendered in devMode, so a production application should never build a FreeMarker
+     * configuration on its behalf.
+     * <p>
+     * Synchronised rather than lock-free: this runs only when devMode is on and a request has
+     * already failed, and {@link FreemarkerManager#getConfiguration(ServletContext)} is itself
+     * synchronised, so the lock costs nothing that this path was not paying already.
+     *
+     * @return the problem report template
+     * @throws IOException if the template cannot be loaded
+     */
+    protected synchronized Template getTemplate() throws IOException {
+        if (template == null) {
+            template = freemarkerManager.getConfiguration(servletContext).getTemplate(ERROR_TEMPLATE);
         }
+        return template;
     }
 
     public void handleError(HttpServletRequest request, HttpServletResponse response, int code, Exception e) {
@@ -115,7 +131,7 @@ protected void handleErrorInDevMode(HttpServletResponse response, int code, Exce
             } while ((cur = cur.getCause()) != null);
 
             Writer writer = new StringWriter();
-            template.process(createReportData(e, chain), writer);
+            getTemplate().process(createReportData(e, chain), writer);
 
             response.setContentType("text/html");
             response.getWriter().write(writer.toString());

core/src/test/java/org/apache/struts2/dispatcher/DefaultDispatcherErrorHandlerTest.java

@@ -18,10 +18,14 @@
  */
 package org.apache.struts2.dispatcher;
 
+import freemarker.template.Configuration;
 import java.io.IOException;
+import java.io.PrintWriter;
+import java.io.StringWriter;
 import java.util.Collections;
 import org.apache.struts2.StrutsInternalTestCase;
 
+import jakarta.servlet.ServletContext;
 import jakarta.servlet.http.HttpServletRequest;
 import jakarta.servlet.http.HttpServletResponse;
 import org.apache.struts2.views.freemarker.FreemarkerManager;
@@ -130,6 +134,61 @@ public void testHandleErrorIllegalStateExceptionDevMode() {
         defaultDispatcherErrorHandler.handleError(requestMock, responseMock, HttpServletResponse.SC_INTERNAL_SERVER_ERROR, fakeException);
     }
 
+    /**
+     * The dev-mode problem report is the only thing that needs FreeMarker here, so booting the
+     * application must not build a FreeMarker configuration just to have the template ready.
+     */
+    public void testInitDoesNotLoadErrorTemplate() {
+        RecordingFreemarkerManager freemarkerManager = createFreemarkerManager();
+        DefaultDispatcherErrorHandler defaultDispatcherErrorHandler = new DefaultDispatcherErrorHandler();
+        defaultDispatcherErrorHandler.setDevMode("true");
+        defaultDispatcherErrorHandler.setFreemarkerManager(freemarkerManager);
+
+        defaultDispatcherErrorHandler.init(dispatcher.servletContext);
+
+        assertFalse("init() must not touch FreeMarker", freemarkerManager.configurationRequested);
+    }
+
+    /**
+     * The deferred load must still happen, otherwise the problem report silently stops rendering.
+     */
+    public void testErrorTemplateLoadedOnFirstDevModeError() throws IOException {
+        RecordingFreemarkerManager freemarkerManager = createFreemarkerManager();
+        DefaultDispatcherErrorHandler defaultDispatcherErrorHandler = new DefaultDispatcherErrorHandler();
+        defaultDispatcherErrorHandler.setDevMode("true");
+        defaultDispatcherErrorHandler.setFreemarkerManager(freemarkerManager);
+        defaultDispatcherErrorHandler.init(dispatcher.servletContext);
+        Exception fakeException = new Exception("Fake Exception, devMode true");
+        responseMock.setContentType("text/html");
+        expectLastCall();
+        expect(responseMock.getWriter()).andStubReturn(new PrintWriter(new StringWriter()));
+        replay(responseMock);
+
+        defaultDispatcherErrorHandler.handleError(requestMock, responseMock, HttpServletResponse.SC_INTERNAL_SERVER_ERROR, fakeException);
+
+        assertTrue("problem report must load the template on first use", freemarkerManager.configurationRequested);
+    }
+
+    private RecordingFreemarkerManager createFreemarkerManager() {
+        RecordingFreemarkerManager freemarkerManager = new RecordingFreemarkerManager();
+        container.inject(freemarkerManager);
+        return freemarkerManager;
+    }
+
+    /**
+     * Records whether the FreeMarker configuration was ever asked for, while still delegating so the
+     * template really renders.
+     */
+    private static class RecordingFreemarkerManager extends FreemarkerManager {
+        private boolean configurationRequested;
+
+        @Override
+        public Configuration getConfiguration(ServletContext servletContext) {
+            configurationRequested = true;
+            return super.getConfiguration(servletContext);
+        }
+    }
+
     protected void setUp() {
         requestMock = (HttpServletRequest) createMock(HttpServletRequest.class);
         responseMock = (HttpServletResponse) createMock(HttpServletResponse.class);