Add ability to render thymeleaf fragments

views-thymeleaf/src/main/java/io/micronaut/views/thymeleaf/ThymeleafViewsRenderer.java

@@ -30,13 +30,18 @@
 import io.micronaut.views.exceptions.ViewRenderingException;
 import jakarta.inject.Singleton;
 import org.thymeleaf.TemplateEngine;
+import org.thymeleaf.context.ExpressionContext;
 import org.thymeleaf.context.IContext;
 import org.thymeleaf.exceptions.TemplateEngineException;
+import org.thymeleaf.exceptions.TemplateProcessingException;
+import org.thymeleaf.standard.expression.FragmentExpression;
+import org.thymeleaf.standard.expression.StandardExpressions;
 import org.thymeleaf.templateresolver.AbstractConfigurableTemplateResolver;
 import org.thymeleaf.templateresolver.ClassLoaderTemplateResolver;
 
 import java.io.Writer;
 import java.util.Locale;
+import java.util.Set;
 
 /**
  * Renders templates Thymeleaf Java template engine.
@@ -81,11 +86,30 @@ public Writable render(@NonNull String viewName,
         ArgumentUtils.requireNonNull("viewName", viewName);
         return (writer) -> {
             IContext context = new WebContext(request, request != null ? httpLocaleResolver.resolveOrDefault(request) : Locale.getDefault(),
-                    ViewUtils.modelOf(data));
-            render(viewName, context, writer);
+                ViewUtils.modelOf(data));
+
+            var templateAndFragment = resolveTemplate(viewName);
+
+            render(templateAndFragment.templateName, templateAndFragment.fragmentSelectors, context, writer);
         };
     }
 
+    /**
+     * Passes the arguments as is to {@link TemplateEngine#process(String, IContext, Writer)}.
+     *
+     * @param viewName          The view name
+     * @param fragmentSelectors Fragment selectors
+     * @param context           The context
+     * @param writer            The writer
+     */
+    public void render(String viewName, Set<String> fragmentSelectors, IContext context, Writer writer) {
+        try {
+            engine.process(viewName, fragmentSelectors, context, writer);
+        } catch (TemplateEngineException e) {
+            throw new ViewRenderingException("Error rendering Thymeleaf view [" + viewName + "]: " + e.getMessage(), e);
+        }
+    }
+
     /**
      * Passes the arguments as is to {@link TemplateEngine#process(String, IContext, Writer)}.
      *
@@ -103,7 +127,8 @@ public void render(String viewName, IContext context, Writer writer) {
 
     @Override
     public boolean exists(@NonNull String viewName) {
-        String location = viewLocation(viewName);
+        var templateAndFragment = resolveTemplate(viewName);
+        String location = viewLocation(templateAndFragment.templateName);
         return resourceLoader.getResourceAsStream(location).isPresent();
     }
 
@@ -131,8 +156,30 @@ private ClassLoaderTemplateResolver initializeTemplateResolver(ViewsConfiguratio
 
     private String viewLocation(final String name) {
         return templateResolver.getPrefix() +
-                ViewUtils.normalizeFile(name, templateResolver.getSuffix()) +
-                templateResolver.getSuffix();
+            ViewUtils.normalizeFile(name, templateResolver.getSuffix()) +
+            templateResolver.getSuffix();
     }
 
+    private record TemplateAndFragment(String templateName, Set<String> fragmentSelectors) {
+    }
+
+    private TemplateAndFragment resolveTemplate(String viewName) {
+        if (!viewName.contains("::")) {
+            return new TemplateAndFragment(viewName, null);
+        }
+
+        var expressionContext = new ExpressionContext(engine.getConfiguration());
+        var parser = StandardExpressions.getExpressionParser(engine.getConfiguration());
+        FragmentExpression fragmentExpression;
+        try {
+            fragmentExpression = (FragmentExpression) parser.parseExpression(expressionContext, "~{" + viewName + "}");
+        } catch (TemplateProcessingException e) {
+            throw new IllegalArgumentException("Invalid template name specification: '" + viewName + "'");
+        }
+        var fragment = FragmentExpression.createExecutedFragmentExpression(expressionContext, fragmentExpression);
+        var templateName = FragmentExpression.resolveTemplateName(fragment);
+        var fragmentSelectors = FragmentExpression.resolveFragments(fragment);
+
+        return new TemplateAndFragment(templateName, fragmentSelectors);
+    }
 }

views-thymeleaf/src/test/groovy/io/micronaut/views/thymeleaf/ThymeleafViewRenderFragment.groovy

@@ -0,0 +1,53 @@
+package io.micronaut.views.thymeleaf
+
+import io.micronaut.core.io.Writable
+import io.micronaut.test.extensions.spock.annotation.MicronautTest
+import jakarta.inject.Inject
+import spock.lang.Specification
+
+@MicronautTest(startApplication = false)
+class ThymeleafViewRenderFragment extends Specification {
+
+    @Inject
+    ThymeleafViewsRenderer<?> viewRenderer
+
+    void "can render fragment"() {
+        when:
+        Writable writeable = viewRenderer.render("fragment :: thefragment", ["some": "data"], null)
+        String result = new StringWriter().with {
+            writeable.writeTo(it)
+            it.toString()
+        }
+
+        then:
+        result == "<div>FRAGMENT</div>"
+    }
+
+    void "can render main body"() {
+        when:
+        Writable writeable = viewRenderer.render("fragment", ["some": "data"], null)
+        String result = new StringWriter().with {
+            writeable.writeTo(it)
+            it.toString()
+        }
+
+        then:
+        result.contains("MAIN") && result.contains("FRAGMENT")
+    }
+
+    void "exists is successful when using fragments"() {
+        when:
+        var result = viewRenderer.exists("fragment :: thefragment")
+
+        then:
+        result
+    }
+
+    void "exists is successful when using regular view name"() {
+        when:
+        var result = viewRenderer.exists("fragment")
+
+        then:
+        result
+    }
+}

views-thymeleaf/src/test/resources/views/fragment.html

@@ -0,0 +1,10 @@
+<!DOCTYPE html>
+<html lang="en" xmlns:th="http://www.thymeleaf.org">
+<head>
+    <title>Home</title>
+</head>
+<body>
+<div>MAIN</div>
+<div th:fragment="thefragment">FRAGMENT</div>
+</body>
+</html>