Add Spring SpEL implementation of TemplateRenderer

pom.xml

@@ -34,6 +34,7 @@
 		<module>spring-ai-bom</module>
 		<module>spring-ai-commons</module>
 		<module>spring-ai-template-st</module>
+		<module>spring-ai-template-spel</module>
 		<module>spring-ai-client-chat</module>
 		<module>spring-ai-model</module>
 		<module>spring-ai-test</module>

spring-ai-bom/pom.xml

@@ -101,6 +101,12 @@
 				<version>${project.version}</version>
 			</dependency>
 
+			<dependency>
+				<groupId>org.springframework.ai</groupId>
+				<artifactId>spring-ai-template-spel</artifactId>
+				<version>${project.version}</version>
+			</dependency>
+
 			<!-- Spring AI model -->
 
 			<dependency>

spring-ai-model/pom.xml

@@ -53,6 +53,12 @@
 			<version>${project.parent.version}</version>
 		</dependency>
 
+		<dependency>
+			<groupId>org.springframework.ai</groupId>
+			<artifactId>spring-ai-template-spel</artifactId>
+			<version>${project.parent.version}</version>
+		</dependency>
+
 		<dependency>
 			<groupId>io.micrometer</groupId>
 			<artifactId>micrometer-observation</artifactId>

spring-ai-template-spel/pom.xml

@@ -0,0 +1,69 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  ~ Copyright 2023-2025 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.
+  -->
+
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>org.springframework.ai</groupId>
+        <artifactId>spring-ai-parent</artifactId>
+        <version>1.1.0-SNAPSHOT</version>
+    </parent>
+    <artifactId>spring-ai-template-spel</artifactId>
+    <packaging>jar</packaging>
+    <name>Spring AI Template SpEL</name>
+    <description>SpEL implementation for Spring AI templating</description>
+    <url>https://github.com/spring-projects/spring-ai</url>
+
+    <scm>
+        <url>https://github.com/spring-projects/spring-ai</url>
+        <connection>git://github.com/spring-projects/spring-ai.git</connection>
+        <developerConnection>[email protected]:spring-projects/spring-ai.git</developerConnection>
+    </scm>
+
+    <properties>
+        <maven.compiler.target>17</maven.compiler.target>
+        <maven.compiler.source>17</maven.compiler.source>
+    </properties>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.springframework.ai</groupId>
+            <artifactId>spring-ai-commons</artifactId>
+            <version>${project.parent.version}</version>
+        </dependency>
+
+        <dependency>
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-expression</artifactId>
+        </dependency>
+
+        <!-- Logging -->
+        <dependency>
+            <groupId>org.slf4j</groupId>
+            <artifactId>slf4j-api</artifactId>
+        </dependency>
+
+        <!-- test dependencies -->
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-test</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+</project>

spring-ai-template-spel/src/main/java/org/springframework/ai/template/spel/SpelTemplateRenderer.java

@@ -0,0 +1,220 @@
+/*
+ * Copyright 2023-2025 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.ai.template.spel;
+
+import java.util.LinkedHashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import org.springframework.ai.template.TemplateRenderer;
+import org.springframework.ai.template.ValidationMode;
+import org.springframework.context.expression.MapAccessor;
+import org.springframework.expression.EvaluationContext;
+import org.springframework.expression.Expression;
+import org.springframework.expression.common.CompositeStringExpression;
+import org.springframework.expression.common.TemplateParserContext;
+import org.springframework.expression.spel.standard.SpelExpression;
+import org.springframework.expression.spel.standard.SpelExpressionParser;
+import org.springframework.expression.spel.support.DataBindingPropertyAccessor;
+import org.springframework.expression.spel.support.StandardEvaluationContext;
+import org.springframework.util.Assert;
+
+/**
+ * Renders a template using the Spring SpEL.
+ *
+ * <p>
+ * This renderer allows customization of delimiters, validation behavior when template
+ * variables are missing.
+ *
+ * <p>
+ * Use the {@link #builder()} to create and configure instances.
+ *
+ * <p>
+ * <b>Thread safety:</b> This class is safe for concurrent use. Each call to
+ * {@link #apply(String, Map)} creates a new SpEL Expression instance, and no mutable
+ * state is shared between threads.
+ *
+ * @author Yanming Zhou
+ * @since 1.1.0
+ */
+public class SpelTemplateRenderer implements TemplateRenderer {
+
+	private static final Logger logger = LoggerFactory.getLogger(SpelTemplateRenderer.class);
+
+	private static final String VALIDATION_MESSAGE = "Not all variables were replaced in the template. Missing variable names are: %s.";
+
+	private static final char DEFAULT_START_DELIMITER_TOKEN = '{';
+
+	private static final char DEFAULT_END_DELIMITER_TOKEN = '}';
+
+	private static final ValidationMode DEFAULT_VALIDATION_MODE = ValidationMode.THROW;
+
+	private final char startDelimiterToken;
+
+	private final char endDelimiterToken;
+
+	private final ValidationMode validationMode;
+
+	private final EvaluationContext evaluationContext;
+
+	/**
+	 * Constructs a new {@code SpelTemplateRenderer} with the specified delimiter tokens,
+	 * validation mode.
+	 * @param startDelimiterToken the character used to denote the start of a template
+	 * variable (e.g., '{')
+	 * @param endDelimiterToken the character used to denote the end of a template
+	 * variable (e.g., '}')
+	 * @param validationMode the mode to use for template variable validation; must not be
+	 * null template
+	 */
+	public SpelTemplateRenderer(char startDelimiterToken, char endDelimiterToken, ValidationMode validationMode) {
+		Assert.notNull(validationMode, "validationMode cannot be null");
+		this.startDelimiterToken = startDelimiterToken;
+		this.endDelimiterToken = endDelimiterToken;
+		this.validationMode = validationMode;
+
+		StandardEvaluationContext ctx = new StandardEvaluationContext();
+		ctx.setPropertyAccessors(List.of(new MapAccessor(false), DataBindingPropertyAccessor.forReadOnlyAccess()));
+		this.evaluationContext = ctx;
+	}
+
+	@Override
+	public String apply(String template, Map<String, Object> variables) {
+		Assert.hasText(template, "template cannot be null or empty");
+		Assert.notNull(variables, "variables cannot be null");
+		Assert.noNullElements(variables.keySet(), "variables keys cannot be null");
+
+		Expression expression = parseExpression(template);
+
+		if (this.validationMode != ValidationMode.NONE) {
+			validate(expression, variables);
+		}
+
+		return String.valueOf(expression.getValue(this.evaluationContext, variables));
+	}
+
+	private Expression parseExpression(String template) {
+		SpelExpressionParser parser = new SpelExpressionParser();
+		return parser.parseExpression(template, new TemplateParserContext(String.valueOf(this.startDelimiterToken),
+				String.valueOf(this.endDelimiterToken)));
+	}
+
+	/**
+	 * Validates that all required template variables are provided in the model. Returns
+	 * the set of missing variables for further handling or logging.
+	 * @param expression the Expression instance
+	 * @param templateVariables the provided variables
+	 * @return set of missing variable names, or empty set if none are missing
+	 */
+	private Set<String> validate(Expression expression, Map<String, Object> templateVariables) {
+		Set<String> templateTokens = getInputVariables(expression);
+		Set<String> modelKeys = templateVariables.keySet();
+		Set<String> missingVariables = new LinkedHashSet<>(templateTokens);
+		missingVariables.removeAll(modelKeys);
+
+		if (!missingVariables.isEmpty()) {
+			if (this.validationMode == ValidationMode.WARN) {
+				logger.warn(VALIDATION_MESSAGE.formatted(missingVariables));
+			}
+			else if (this.validationMode == ValidationMode.THROW) {
+				throw new IllegalStateException(VALIDATION_MESSAGE.formatted(missingVariables));
+			}
+		}
+		return missingVariables;
+	}
+
+	public Set<String> getInputVariables(Expression expression) {
+		Set<String> inputVariables = new LinkedHashSet<>();
+		if (expression instanceof CompositeStringExpression cse) {
+			for (Expression ex : cse.getExpressions()) {
+				if (ex instanceof SpelExpression se) {
+					inputVariables.add(se.getExpressionString());
+				}
+			}
+		}
+
+		return inputVariables;
+	}
+
+	public static Builder builder() {
+		return new Builder();
+	}
+
+	/**
+	 * Builder for configuring and creating {@link SpelTemplateRenderer} instances.
+	 */
+	public static final class Builder {
+
+		private char startDelimiterToken = DEFAULT_START_DELIMITER_TOKEN;
+
+		private char endDelimiterToken = DEFAULT_END_DELIMITER_TOKEN;
+
+		private ValidationMode validationMode = DEFAULT_VALIDATION_MODE;
+
+		private Builder() {
+		}
+
+		/**
+		 * Sets the character used as the start delimiter for template expressions.
+		 * Default is '{'.
+		 * @param startDelimiterToken The start delimiter character.
+		 * @return This builder instance for chaining.
+		 */
+		public Builder startDelimiterToken(char startDelimiterToken) {
+			this.startDelimiterToken = startDelimiterToken;
+			return this;
+		}
+
+		/**
+		 * Sets the character used as the end delimiter for template expressions. Default
+		 * is '}'.
+		 * @param endDelimiterToken The end delimiter character.
+		 * @return This builder instance for chaining.
+		 */
+		public Builder endDelimiterToken(char endDelimiterToken) {
+			this.endDelimiterToken = endDelimiterToken;
+			return this;
+		}
+
+		/**
+		 * Sets the validation mode to control behavior when the provided variables do not
+		 * match the variables required by the template. Default is
+		 * {@link ValidationMode#THROW}.
+		 * @param validationMode The desired validation mode.
+		 * @return This builder instance for chaining.
+		 */
+		public Builder validationMode(ValidationMode validationMode) {
+			this.validationMode = validationMode;
+			return this;
+		}
+
+		/**
+		 * Builds and returns a new {@link SpelTemplateRenderer} instance with the
+		 * configured settings.
+		 * @return A configured {@link SpelTemplateRenderer}.
+		 */
+		public SpelTemplateRenderer build() {
+			return new SpelTemplateRenderer(this.startDelimiterToken, this.endDelimiterToken, this.validationMode);
+		}
+
+	}
+
+}

spring-ai-template-spel/src/main/java/org/springframework/ai/template/spel/package-info.java

@@ -0,0 +1,22 @@
+/*
+ * Copyright 2023-2025 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.
+ */
+
+@NonNullApi
+@NonNullFields
+package org.springframework.ai.template.spel;
+
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;