Add support for customizing FreeMarker variables

This commit updates the auto-configuration to allow custom FreeMarker
variables to be provided programmatically. As these variables are
usually objects, they cannot be specified via properties.

Closes gh-8965

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/freemarker/AbstractFreeMarkerConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2024 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.
@@ -16,21 +16,30 @@
 
 package org.springframework.boot.autoconfigure.freemarker;
 
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
 import java.util.Properties;
 
+import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.ui.freemarker.FreeMarkerConfigurationFactory;
 
 /**
  * Base class for shared FreeMarker configuration.
  *
  * @author Brian Clozel
+ * @author Stephane Nicoll
  */
 abstract class AbstractFreeMarkerConfiguration {
 
 	private final FreeMarkerProperties properties;
 
-	protected AbstractFreeMarkerConfiguration(FreeMarkerProperties properties) {
+	private final List<FreeMarkerVariablesCustomizer> variablesCustomizers;
+
+	protected AbstractFreeMarkerConfiguration(FreeMarkerProperties properties,
+			ObjectProvider<FreeMarkerVariablesCustomizer> variablesCustomizers) {
 		this.properties = properties;
+		this.variablesCustomizers = variablesCustomizers.orderedStream().toList();
 	}
 
 	protected final FreeMarkerProperties getProperties() {
@@ -41,10 +50,23 @@ abstract class AbstractFreeMarkerConfiguration {
 		factory.setTemplateLoaderPaths(this.properties.getTemplateLoaderPath());
 		factory.setPreferFileSystemAccess(this.properties.isPreferFileSystemAccess());
 		factory.setDefaultEncoding(this.properties.getCharsetName());
+		factory.setFreemarkerSettings(createFreeMarkerSettings());
+		factory.setFreemarkerVariables(createFreeMarkerVariables());
+	}
+
+	private Properties createFreeMarkerSettings() {
 		Properties settings = new Properties();
 		settings.put("recognize_standard_file_extensions", "true");
 		settings.putAll(this.properties.getSettings());
-		factory.setFreemarkerSettings(settings);
+		return settings;
+	}
+
+	private Map<String, Object> createFreeMarkerVariables() {
+		Map<String, Object> variables = new HashMap<>();
+		for (FreeMarkerVariablesCustomizer customizer : this.variablesCustomizers) {
+			customizer.customizeFreeMarkerVariables(variables);
+		}
+		return variables;
 	}
 
 }

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/freemarker/FreeMarkerNonWebConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2024 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.
@@ -16,6 +16,7 @@
 
 package org.springframework.boot.autoconfigure.freemarker;
 
+import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnNotWebApplication;
 import org.springframework.context.annotation.Bean;
@@ -32,8 +33,9 @@ import org.springframework.ui.freemarker.FreeMarkerConfigurationFactoryBean;
 @ConditionalOnNotWebApplication
 class FreeMarkerNonWebConfiguration extends AbstractFreeMarkerConfiguration {
 
-	FreeMarkerNonWebConfiguration(FreeMarkerProperties properties) {
-		super(properties);
+	FreeMarkerNonWebConfiguration(FreeMarkerProperties properties,
+			ObjectProvider<FreeMarkerVariablesCustomizer> variablesCustomizers) {
+		super(properties, variablesCustomizers);
 	}
 
 	@Bean

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/freemarker/FreeMarkerReactiveWebConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2024 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.
@@ -16,6 +16,7 @@
 
 package org.springframework.boot.autoconfigure.freemarker;
 
+import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.autoconfigure.AutoConfigureAfter;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
@@ -38,8 +39,9 @@ import org.springframework.web.reactive.result.view.freemarker.FreeMarkerViewRes
 @AutoConfigureAfter(WebFluxAutoConfiguration.class)
 class FreeMarkerReactiveWebConfiguration extends AbstractFreeMarkerConfiguration {
 
-	FreeMarkerReactiveWebConfiguration(FreeMarkerProperties properties) {
-		super(properties);
+	FreeMarkerReactiveWebConfiguration(FreeMarkerProperties properties,
+			ObjectProvider<FreeMarkerVariablesCustomizer> variablesCustomizers) {
+		super(properties, variablesCustomizers);
 	}
 
 	@Bean

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/freemarker/FreeMarkerServletWebConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2021 the original author or authors.
+ * Copyright 2012-2024 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.
@@ -19,6 +19,7 @@ package org.springframework.boot.autoconfigure.freemarker;
 import jakarta.servlet.DispatcherType;
 import jakarta.servlet.Servlet;
 
+import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.boot.autoconfigure.AutoConfigureAfter;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
@@ -47,8 +48,9 @@ import org.springframework.web.servlet.view.freemarker.FreeMarkerViewResolver;
 @AutoConfigureAfter(WebMvcAutoConfiguration.class)
 class FreeMarkerServletWebConfiguration extends AbstractFreeMarkerConfiguration {
 
-	protected FreeMarkerServletWebConfiguration(FreeMarkerProperties properties) {
-		super(properties);
+	protected FreeMarkerServletWebConfiguration(FreeMarkerProperties properties,
+			ObjectProvider<FreeMarkerVariablesCustomizer> variablesCustomizers) {
+		super(properties, variablesCustomizers);
 	}
 
 	@Bean

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/freemarker/FreeMarkerVariablesCustomizer.java

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2012-2024 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.autoconfigure.freemarker;
+
+import java.util.Map;
+
+import freemarker.template.Configuration;
+
+import org.springframework.ui.freemarker.FreeMarkerConfigurationFactory;
+
+/**
+ * Callback interface that can be implemented by beans wishing to customize the FreeMarker
+ * variables used as {@link Configuration#getSharedVariableNames() shared variables}
+ * before it is used by an auto-configured {@link FreeMarkerConfigurationFactory}.
+ *
+ * @author Stephane Nicoll
+ * @since 3.4.0
+ */
+@FunctionalInterface
+public interface FreeMarkerVariablesCustomizer {
+
+	/**
+	 * Customize the {@code variables} to be set as well-known FreeMarker objects.
+	 * @param variables the variables to customize
+	 * @see FreeMarkerConfigurationFactory#setFreemarkerVariables(Map)
+	 */
+	void customizeFreeMarkerVariables(Map<String, Object> variables);
+
+}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/freemarker/FreeMarkerAutoConfigurationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 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.
@@ -27,8 +27,14 @@ import org.springframework.boot.test.context.runner.ApplicationContextRunner;
 import org.springframework.boot.test.system.CapturedOutput;
 import org.springframework.boot.test.system.OutputCaptureExtension;
 import org.springframework.boot.testsupport.BuildOutput;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.annotation.Order;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.BDDMockito.then;
+import static org.mockito.Mockito.mock;
 
 /**
  * Tests for {@link FreeMarkerAutoConfiguration}.
@@ -80,6 +86,24 @@ class FreeMarkerAutoConfigurationTests {
 			.run((context) -> assertThat(output).doesNotContain("Cannot find template location"));
 	}
 
+	@Test
+	void variableCustomizerShouldBeApplied() {
+		FreeMarkerVariablesCustomizer customizer = mock(FreeMarkerVariablesCustomizer.class);
+		this.contextRunner.withBean(FreeMarkerVariablesCustomizer.class, () -> customizer)
+			.run((context) -> then(customizer).should().customizeFreeMarkerVariables(any()));
+	}
+
+	@Test
+	@SuppressWarnings("unchecked")
+	void variableCustomizersShouldBeAppliedInOrder() {
+		this.contextRunner.withUserConfiguration(VariablesCustomizersConfiguration.class).run((context) -> {
+			assertThat(context).hasSingleBean(freemarker.template.Configuration.class);
+			freemarker.template.Configuration configuration = context.getBean(freemarker.template.Configuration.class);
+			assertThat(configuration.getSharedVariableNames()).contains("order", "one", "two");
+			assertThat(configuration.getSharedVariable("order")).hasToString("5");
+		});
+	}
+
 	public static class DataModel {
 
 		public String getGreeting() {
@@ -88,4 +112,27 @@ class FreeMarkerAutoConfigurationTests {
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	static class VariablesCustomizersConfiguration {
+
+		@Bean
+		@Order(5)
+		FreeMarkerVariablesCustomizer variablesCustomizer() {
+			return (variables) -> {
+				variables.put("order", 5);
+				variables.put("one", "one");
+			};
+		}
+
+		@Bean
+		@Order(2)
+		FreeMarkerVariablesCustomizer anotherVariablesCustomizer() {
+			return (variables) -> {
+				variables.put("order", 2);
+				variables.put("two", "two");
+			};
+		}
+
+	}
+
 }

spring-boot-project/spring-boot-docs/src/docs/antora/modules/how-to/pages/spring-mvc.adoc

@@ -224,6 +224,7 @@ If you add your own, you have to be aware of the order and in which position you
   The prefix is externalized to `spring.freemarker.prefix`, and the suffix is externalized to `spring.freemarker.suffix`.
   The default values of the prefix and suffix are empty and '`.ftlh`', respectively.
   You can override `FreeMarkerViewResolver` by providing a bean of the same name.
+  FreeMarker variables can be customized by defining a bean of type `FreeMarkerVariablesCustomizer`.
 * If you use Groovy templates (actually, if `groovy-templates` is on your classpath), you also have a `GroovyMarkupViewResolver` named '`groovyMarkupViewResolver`'.
   It looks for resources in a loader path by surrounding the view name with a prefix and suffix (externalized to `spring.groovy.template.prefix` and `spring.groovy.template.suffix`).
   The prefix and suffix have default values of '`classpath:/templates/`' and '`.tpl`', respectively.