Schedule FreeMarker template lookup on bounded elastic scheduler

This commit makes sure that FreeMarker template lookups, which
potentially block, are scheduled on the bounded elastic scheduler.

Closes gh-30903

spring-webflux/src/main/java/org/springframework/web/reactive/result/view/AbstractUrlBasedView.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,6 +18,8 @@ package org.springframework.web.reactive.result.view;
 
 import java.util.Locale;
 
+import reactor.core.publisher.Mono;
+
 import org.springframework.beans.factory.InitializingBean;
 import org.springframework.lang.Nullable;
 
@@ -81,6 +83,18 @@ public abstract class AbstractUrlBasedView extends AbstractView implements Initi
 	 */
 	public abstract boolean checkResourceExists(Locale locale) throws Exception;
 
+	/**
+	 * Deferred check whether the resource for the configured URL actually exists.
+	 * <p>The default implementation calls {@link #checkResourceExists(Locale)}.
+	 * @param locale the desired Locale that we're looking for
+	 * @return {@code false} if the resource exists
+	 * {@code false} if we know that it does not exist
+	 * @since 6.1
+	 */
+	public Mono<Boolean> resourceExists(Locale locale) {
+		return Mono.fromCallable(() -> checkResourceExists(locale));
+	}
+
 
 	@Override
 	public String toString() {

spring-webflux/src/main/java/org/springframework/web/reactive/result/view/UrlBasedViewResolver.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -238,12 +238,8 @@ public class UrlBasedViewResolver extends ViewResolverSupport
 		}
 
 		View view = applyLifecycleMethods(viewName, urlBasedView);
-		try {
-			return (urlBasedView.checkResourceExists(locale) ? Mono.just(view) : Mono.empty());
-		}
-		catch (Exception ex) {
-			return Mono.error(ex);
-		}
+		return urlBasedView.resourceExists(locale)
+				.flatMap(exists -> exists ? Mono.just(view) : Mono.empty());
 	}
 
 	/**

spring-webflux/src/main/java/org/springframework/web/reactive/result/view/freemarker/FreeMarkerView.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -32,8 +32,10 @@ import freemarker.template.DefaultObjectWrapperBuilder;
 import freemarker.template.ObjectWrapper;
 import freemarker.template.SimpleHash;
 import freemarker.template.Template;
+import freemarker.template.TemplateException;
 import freemarker.template.Version;
 import reactor.core.publisher.Mono;
+import reactor.core.scheduler.Schedulers;
 
 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.BeanFactoryUtils;
@@ -210,6 +212,24 @@ public class FreeMarkerView extends AbstractUrlBasedView {
 		}
 	}
 
+	/**
+	 * Check that the FreeMarker template used for this view exists and is valid.
+	 * <p>Can be overridden to customize the behavior, for example in case of
+	 * multiple templates to be rendered into a single view.
+	 * @since 6.1
+	 */
+	@Override
+	public Mono<Boolean> resourceExists(Locale locale) {
+		return lookupTemplate(locale)
+				.map(template -> Boolean.TRUE)
+				.switchIfEmpty(Mono.just(Boolean.FALSE))
+				.onErrorResume(FileNotFoundException.class, t -> Mono.just(Boolean.FALSE))
+				.onErrorMap(ParseException.class, ex -> new ApplicationContextException(
+						"Failed to parse FreeMarker template for URL [" +  getUrl() + "]", ex))
+				.onErrorMap(IOException.class, ex -> new ApplicationContextException(
+						"Could not load FreeMarker template for URL [" + getUrl() + "]", ex));
+	}
+
 	/**
 	 * Prepare the model to use for rendering by potentially exposing a
 	 * {@link RequestContext} for use in Spring FreeMarker macros and then
@@ -243,7 +263,7 @@ public class FreeMarkerView extends AbstractUrlBasedView {
 			@Nullable MediaType contentType, ServerWebExchange exchange) {
 
 		return exchange.getResponse().writeWith(Mono
-				.fromCallable(() -> {
+				.defer(() -> {
 					// Expose all standard FreeMarker hash models.
 					SimpleHash freeMarkerModel = getTemplateModel(renderAttributes, exchange);
 
@@ -252,19 +272,22 @@ public class FreeMarkerView extends AbstractUrlBasedView {
 					}
 
 					Locale locale = LocaleContextHolder.getLocale(exchange.getLocaleContext());
-					FastByteArrayOutputStream bos = new FastByteArrayOutputStream();
-					try {
-						Charset charset = getCharset(contentType);
-						Writer writer = new OutputStreamWriter(bos, charset);
-						getTemplate(locale).process(freeMarkerModel, writer);
-
-						byte[] bytes = bos.toByteArrayUnsafe();
-						return exchange.getResponse().bufferFactory().wrap(bytes);
-					}
-					catch (IOException ex) {
-						String message = "Could not load FreeMarker template for URL [" + getUrl() + "]";
-						throw new IllegalStateException(message, ex);
-					}
+					return lookupTemplate(locale)
+							.flatMap(template -> {
+								try {
+									FastByteArrayOutputStream bos = new FastByteArrayOutputStream();
+									Charset charset = getCharset(contentType);
+									Writer writer = new OutputStreamWriter(bos, charset);
+									template.process(freeMarkerModel, writer);
+									byte[] bytes = bos.toByteArrayUnsafe();
+									DataBuffer buffer = exchange.getResponse().bufferFactory().wrap(bytes);
+									return Mono.just(buffer);
+								}
+								catch (TemplateException | IOException ex ) {
+									String message = "Could not load FreeMarker template for URL [" + getUrl() + "]";
+									return Mono.error(new IllegalStateException(message, ex));
+								}
+							});
 				})
 				.doOnDiscard(DataBuffer.class, DataBufferUtils::release));
 	}
@@ -302,11 +325,32 @@ public class FreeMarkerView extends AbstractUrlBasedView {
 	 * <p>By default, the template specified by the "url" bean property will be retrieved.
 	 * @param locale the current locale
 	 * @return the FreeMarker template to render
+	 * @deprecated since 6.1, in favor of {@link #lookupTemplate(Locale)}, to be
+	 * removed in 6.2
 	 */
+	@Deprecated(since = "6.1", forRemoval = true)
 	protected Template getTemplate(Locale locale) throws IOException {
 		return (getEncoding() != null ?
 				obtainConfiguration().getTemplate(getUrl(), locale, getEncoding()) :
 				obtainConfiguration().getTemplate(getUrl(), locale));
 	}
 
+	/**
+	 * Retrieve the FreeMarker template for the given locale, to be rendered by this view.
+	 * <p>By default, the template specified by the "url" bean property will be retrieved,
+	 * and the returned mono will subscribe on the
+	 * {@linkplain Schedulers#boundedElastic() bounded elastic scheduler} as template
+	 * lookups can be blocking operations.
+	 * @param locale the current locale
+	 * @return the FreeMarker template to render
+	 * @since 6.1
+	 */
+	protected Mono<Template> lookupTemplate(Locale locale) {
+		return Mono.fromCallable(() ->
+						getEncoding() != null ?
+								obtainConfiguration().getTemplate(getUrl(), locale, getEncoding()) :
+								obtainConfiguration().getTemplate(getUrl(), locale))
+				.subscribeOn(Schedulers.boundedElastic());
+	}
+
 }

spring-webflux/src/test/java/org/springframework/web/reactive/result/view/freemarker/FreeMarkerMacroTests.java

@@ -31,6 +31,7 @@ import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.condition.DisabledForJreRange;
 import org.junit.jupiter.api.condition.JRE;
 import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
 
 import org.springframework.beans.testfixture.beans.TestBean;
 import org.springframework.context.support.GenericApplicationContext;
@@ -107,80 +108,84 @@ public class FreeMarkerMacroTests {
 		view.setUrl("tmp.ftl");
 		view.setConfiguration(this.freeMarkerConfig);
 
-		view.render(singletonMap("testBean", new TestBean("Dilbert", 99)), null, this.exchange).subscribe();
+		Map<String, ?> model = singletonMap("testBean", new TestBean("Dilbert", 99));
 
-		assertThat(getOutput()).containsExactly("Hi Dilbert");
+		StepVerifier.create(view.render(model, null, this.exchange)
+						.then(Mono.fromCallable(this::getOutput)))
+				.assertNext(l -> assertThat(l).containsExactly("Hi Dilbert"));
 	}
 
 	@Test
 	public void name() throws Exception {
-		assertThat(getMacroOutput("NAME")).containsExactly("Darren");
+		testMacroOutput("NAME", "Darren");
+	}
+
+	private void testMacroOutput(String name, String... contents) throws Exception {
+		StepVerifier.create(getMacroOutput(name))
+				.assertNext(list -> assertThat(list).containsExactly(contents))
+				.verifyComplete();
+
 	}
 
 	@Test
 	@DisabledForJreRange(min = JRE.JAVA_21)
 	public void age() throws Exception {
-		assertThat(getMacroOutput("AGE")).containsExactly("99");
+		testMacroOutput("AGE", "99");
 	}
 
 	@Test
 	public void message() throws Exception {
-		assertThat(getMacroOutput("MESSAGE")).containsExactly("Howdy Mundo");
+		testMacroOutput("MESSAGE", "Howdy Mundo");
 	}
 
 	@Test
 	public void defaultMessage() throws Exception {
-		assertThat(getMacroOutput("DEFAULTMESSAGE")).containsExactly("hi planet");
+		testMacroOutput("DEFAULTMESSAGE", "hi planet");
 	}
 
 	@Test
 	public void messageArgs() throws Exception {
-		assertThat(getMacroOutput("MESSAGEARGS")).containsExactly("Howdy[World]");
+		testMacroOutput("MESSAGEARGS", "Howdy[World]");
 	}
 
 	@Test
 	public void messageArgsWithDefaultMessage() throws Exception {
-		assertThat(getMacroOutput("MESSAGEARGSWITHDEFAULTMESSAGE")).containsExactly("Hi");
+		testMacroOutput("MESSAGEARGSWITHDEFAULTMESSAGE", "Hi");
 	}
 
 	@Test
 	public void url() throws Exception {
-		assertThat(getMacroOutput("URL")).containsExactly("/springtest/aftercontext.html");
+		testMacroOutput("URL", "/springtest/aftercontext.html");
 	}
 
 	@Test
 	public void urlParams() throws Exception {
-		assertThat(getMacroOutput("URLPARAMS")).containsExactly(
-				"/springtest/aftercontext/bar?spam=bucket");
+		testMacroOutput("URLPARAMS", "/springtest/aftercontext/bar?spam=bucket");
 	}
 
 	@Test
 	public void formInput() throws Exception {
-		assertThat(getMacroOutput("FORM1")).containsExactly(
-				"<input type=\"text\" id=\"name\" name=\"name\" value=\"Darren\" >");
+		testMacroOutput("FORM1", "<input type=\"text\" id=\"name\" name=\"name\" value=\"Darren\" >");
 	}
 
 	@Test
 	public void formInputWithCss() throws Exception {
-		assertThat(getMacroOutput("FORM2")).containsExactly(
-				"<input type=\"text\" id=\"name\" name=\"name\" value=\"Darren\" class=\"myCssClass\" >");
+		testMacroOutput("FORM2", "<input type=\"text\" id=\"name\" name=\"name\" value=\"Darren\" class=\"myCssClass\" >");
 	}
 
 	@Test
 	public void formTextarea() throws Exception {
-		assertThat(getMacroOutput("FORM3")).containsExactly(
-				"<textarea id=\"name\" name=\"name\" >Darren</textarea>");
+		testMacroOutput("FORM3", "<textarea id=\"name\" name=\"name\" >Darren</textarea>");
 	}
 
 	@Test
 	public void formTextareaWithCustomRowsAndColumns() throws Exception {
-		assertThat(getMacroOutput("FORM4")).containsExactly(
-				"<textarea id=\"name\" name=\"name\" rows=10 cols=30>Darren</textarea>");
+		testMacroOutput("FORM4", "<textarea id=\"name\" name=\"name\" rows=10 cols=30>Darren</textarea>");
 	}
 
 	@Test
 	public void formSingleSelectFromMap() throws Exception {
-		assertThat(getMacroOutput("FORM5")).containsExactly(
+		testMacroOutput("FORM5",
 				"<select id=\"name\" name=\"name\" >", //
 				"<option value=\"Rob&amp;Harrop\">Rob Harrop</option>", //
 				"<option value=\"John\">John Doe</option>", //
@@ -191,7 +196,7 @@ public class FreeMarkerMacroTests {
 
 	@Test
 	public void formSingleSelectFromList() throws Exception {
-		assertThat(getMacroOutput("FORM14")).containsExactly(
+		testMacroOutput("FORM14",
 				"<select id=\"name\" name=\"name\" >", //
 				"<option value=\"Rob Harrop\">Rob Harrop</option>", //
 				"<option value=\"Darren Davison\">Darren Davison</option>", //
@@ -202,7 +207,7 @@ public class FreeMarkerMacroTests {
 
 	@Test
 	public void formMultiSelect() throws Exception {
-		assertThat(getMacroOutput("FORM6")).containsExactly(
+		testMacroOutput("FORM6",
 				"<select multiple=\"multiple\" id=\"spouses\" name=\"spouses\" >", //
 				"<option value=\"Rob&amp;Harrop\">Rob Harrop</option>", //
 				"<option value=\"John\">John Doe</option>", //
@@ -213,7 +218,7 @@ public class FreeMarkerMacroTests {
 
 	@Test
 	public void formRadioButtons() throws Exception {
-		assertThat(getMacroOutput("FORM7")).containsExactly(
+		testMacroOutput("FORM7",
 				"<input type=\"radio\" id=\"name0\" name=\"name\" value=\"Rob&amp;Harrop\" >", //
 				"<label for=\"name0\">Rob Harrop</label>", //
 				"<input type=\"radio\" id=\"name1\" name=\"name\" value=\"John\" >", //
@@ -226,28 +231,28 @@ public class FreeMarkerMacroTests {
 
 	@Test
 	public void formCheckboxForStringProperty() throws Exception {
-		assertThat(getMacroOutput("FORM15")).containsExactly(
+		testMacroOutput("FORM15",
 				"<input type=\"hidden\" name=\"_name\" value=\"on\"/>",
 				"<input type=\"checkbox\" id=\"name\" name=\"name\" />");
 	}
 
 	@Test
 	public void formCheckboxForBooleanProperty() throws Exception {
-		assertThat(getMacroOutput("FORM16")).containsExactly(
+		testMacroOutput("FORM16",
 				"<input type=\"hidden\" name=\"_jedi\" value=\"on\"/>",
 				"<input type=\"checkbox\" id=\"jedi\" name=\"jedi\" checked=\"checked\" />");
 	}
 
 	@Test
 	public void formCheckboxForNestedPath() throws Exception {
-		assertThat(getMacroOutput("FORM18")).containsExactly(
+		testMacroOutput("FORM18",
 				"<input type=\"hidden\" name=\"_spouses[0].jedi\" value=\"on\"/>",
 				"<input type=\"checkbox\" id=\"spouses0.jedi\" name=\"spouses[0].jedi\" checked=\"checked\" />");
 	}
 
 	@Test
 	public void formCheckboxForStringArray() throws Exception {
-		assertThat(getMacroOutput("FORM8")).containsExactly(
+		testMacroOutput("FORM8",
 				"<input type=\"checkbox\" id=\"stringArray0\" name=\"stringArray\" value=\"Rob&amp;Harrop\" >", //
 				"<label for=\"stringArray0\">Rob Harrop</label>", //
 				"<input type=\"checkbox\" id=\"stringArray1\" name=\"stringArray\" value=\"John\" checked=\"checked\" >", //
@@ -261,41 +266,41 @@ public class FreeMarkerMacroTests {
 
 	@Test
 	public void formPasswordInput() throws Exception {
-		assertThat(getMacroOutput("FORM9")).containsExactly(
+		testMacroOutput("FORM9",
 				"<input type=\"password\" id=\"name\" name=\"name\" value=\"\" >");
 	}
 
 	@Test
 	public void formHiddenInput() throws Exception {
-		assertThat(getMacroOutput("FORM10")).containsExactly(
+		testMacroOutput("FORM10",
 				"<input type=\"hidden\" id=\"name\" name=\"name\" value=\"Darren\" >");
 	}
 
 	@Test
 	public void formInputText() throws Exception {
-		assertThat(getMacroOutput("FORM11")).containsExactly(
+		testMacroOutput("FORM11",
 				"<input type=\"text\" id=\"name\" name=\"name\" value=\"Darren\" >");
 	}
 
 	@Test
 	public void formInputHidden() throws Exception {
-		assertThat(getMacroOutput("FORM12")).containsExactly(
+		testMacroOutput("FORM12",
 				"<input type=\"hidden\" id=\"name\" name=\"name\" value=\"Darren\" >");
 	}
 
 	@Test
 	public void formInputPassword() throws Exception {
-		assertThat(getMacroOutput("FORM13")).containsExactly(
+		testMacroOutput("FORM13",
 				"<input type=\"password\" id=\"name\" name=\"name\" value=\"\" >");
 	}
 
 	@Test
 	public void forInputWithNestedPath() throws Exception {
-		assertThat(getMacroOutput("FORM17")).containsExactly(
+		testMacroOutput("FORM17",
 				"<input type=\"text\" id=\"spouses0.name\" name=\"spouses[0].name\" value=\"Fred\" >");
 	}
 
-	private List<String> getMacroOutput(String name) throws Exception {
+	private Mono<List<String>> getMacroOutput(String name) throws Exception {
 		String macro = fetchMacro(name);
 		assertThat(macro).isNotNull();
 		storeTemplateInTempDir(macro);
@@ -336,9 +341,8 @@ public class FreeMarkerMacroTests {
 		view.setExposeSpringMacroHelpers(false);
 		view.setConfiguration(freeMarkerConfig);
 
-		view.render(model, null, this.exchange).subscribe();
-
-		return getOutput();
+		return view.render(model, null, this.exchange).
+				then(Mono.fromCallable(this::getOutput));
 	}
 
 	private static String fetchMacro(String name) throws Exception {

spring-webflux/src/test/java/org/springframework/web/reactive/result/view/freemarker/FreeMarkerViewTests.java

@@ -16,7 +16,6 @@
 
 package org.springframework.web.reactive.result.view.freemarker;
 
-import java.nio.ByteBuffer;
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
 import java.util.Locale;
@@ -28,7 +27,6 @@ import reactor.test.StepVerifier;
 
 import org.springframework.context.ApplicationContextException;
 import org.springframework.context.support.GenericApplicationContext;
-import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.http.codec.ServerCodecConfigurer;
 import org.springframework.ui.ExtendedModelMap;
 import org.springframework.ui.ModelMap;
@@ -101,6 +99,26 @@ class FreeMarkerViewTests {
 		assertThat(freeMarkerView.checkResourceExists(Locale.US)).isTrue();
 	}
 
+	@Test
+	void resourceExists() {
+		freeMarkerView.setConfiguration(this.freeMarkerConfig);
+		freeMarkerView.setUrl("test.ftl");
+
+		StepVerifier.create(freeMarkerView.resourceExists(Locale.US))
+				.assertNext(b -> assertThat(b).isTrue())
+				.verifyComplete();
+	}
+
+	@Test
+	void resourceDoesNotExists() {
+		freeMarkerView.setConfiguration(this.freeMarkerConfig);
+		freeMarkerView.setUrl("foo-bar.ftl");
+
+		StepVerifier.create(freeMarkerView.resourceExists(Locale.US))
+				.assertNext(b -> assertThat(b).isFalse())
+				.verifyComplete();
+	}
+
 	@Test
 	void render() {
 		freeMarkerView.setApplicationContext(this.context);
@@ -112,7 +130,8 @@ class FreeMarkerViewTests {
 		freeMarkerView.render(model, null, this.exchange).block(Duration.ofMillis(5000));
 
 		StepVerifier.create(this.exchange.getResponse().getBody())
-				.consumeNextWith(buf -> assertThat(asString(buf)).isEqualTo("<html><body>hi FreeMarker</body></html>"))
+				.consumeNextWith(buf -> assertThat(buf.toString(StandardCharsets.UTF_8))
+						.isEqualTo("<html><body>hi FreeMarker</body></html>"))
 				.expectComplete()
 				.verify();
 	}
@@ -138,15 +157,6 @@ class FreeMarkerViewTests {
 	}
 
 
-	private static String asString(DataBuffer dataBuffer) {
-		@SuppressWarnings("deprecation")
-		ByteBuffer byteBuffer = dataBuffer.toByteBuffer();
-		byte[] bytes = new byte[byteBuffer.remaining()];
-		byteBuffer.get(bytes);
-		return new String(bytes, StandardCharsets.UTF_8);
-	}
-
-
 	@SuppressWarnings("unused")
 	private String handle() {
 		return null;