feat(template-st): log StringTemplate compile/runtime errors via SLF4J

Fixes GH-3604 (#3604)

### Why
StringTemplate’s default ErrorManager writes diagnostics to stderr, which
is invisible in most Spring deployments. This change redirects those
messages to SLF4J so they reach Logstash / CloudWatch / etc.

### What
* Add `Slf4jStErrorListener` (STErrorListener -> SLF4J).
* Create STGroup with listener inside `StTemplateRenderer#createST`.
* JUnit `malformedTemplateShouldLogErrorViaSlf4j`.
* Javadoc updates.

### Impact
No API changes; applications automatically get structured error logs.

Signed-off-by: Your Name <[email protected]>

Author: renechoi

spring-ai-template-st/src/main/java/org/springframework/ai/template/st/Slf4jStErrorListener.java

@@ -0,0 +1,51 @@
+/*
+ * 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.st;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.stringtemplate.v4.STErrorListener;
+import org.stringtemplate.v4.misc.STMessage;
+
+/**
+ * {@link STErrorListener} implementation that logs errors using SLF4J.
+ */
+public class Slf4jStErrorListener implements STErrorListener {
+
+    private static final Logger logger = LoggerFactory.getLogger(StTemplateRenderer.class);
+
+    @Override
+    public void compileTimeError(STMessage msg) {
+        logger.error("StringTemplate compile error: {}", msg);
+    }
+
+    @Override
+    public void runTimeError(STMessage msg) {
+        logger.error("StringTemplate runtime error: {}", msg);
+    }
+
+    @Override
+    public void IOError(STMessage msg) {
+        logger.error("StringTemplate IO error: {}", msg);
+    }
+
+    @Override
+    public void internalError(STMessage msg) {
+        logger.error("StringTemplate internal error: {}", msg);
+    }
+
+}

spring-ai-template-st/src/main/java/org/springframework/ai/template/st/StTemplateRenderer.java

@@ -25,6 +25,8 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import org.stringtemplate.v4.ST;
+import org.stringtemplate.v4.STErrorListener;
+import org.stringtemplate.v4.STGroup;
 import org.stringtemplate.v4.compiler.Compiler;
 import org.stringtemplate.v4.compiler.STLexer;
 
@@ -73,6 +75,8 @@ public class StTemplateRenderer implements TemplateRenderer {
 
 	private final boolean validateStFunctions;
 
+	private final STErrorListener stErrorListener = new Slf4jStErrorListener();
+
 	/**
 	 * Constructs a new {@code StTemplateRenderer} with the specified delimiter tokens,
 	 * validation mode, and function validation flag.
@@ -112,13 +116,16 @@ public String apply(String template, Map<String, Object> variables) {
 
 	private ST createST(String template) {
 		try {
-			return new ST(template, this.startDelimiterToken, this.endDelimiterToken);
+			STGroup group = new STGroup(this.startDelimiterToken, this.endDelimiterToken);
+			group.setListener(this.stErrorListener);
+			return new ST(group, template);
 		}
 		catch (Exception ex) {
 			throw new IllegalArgumentException("The template string is not valid.", ex);
 		}
 	}
 
+
 	/**
 	 * Validates that all required template variables are provided in the model. Returns
 	 * the set of missing variables for further handling or logging.

spring-ai-template-st/src/test/java/org/springframework/ai/template/st/StTemplateRendererTests.java

@@ -16,16 +16,23 @@
 
 package org.springframework.ai.template.st;
 
+import static org.assertj.core.api.Assertions.*;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.nio.charset.StandardCharsets;
 import java.util.HashMap;
 import java.util.Map;
 
 import org.junit.jupiter.api.Test;
-
+import org.slf4j.LoggerFactory;
 import org.springframework.ai.template.ValidationMode;
 import org.springframework.test.util.ReflectionTestUtils;
 
-import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import ch.qos.logback.classic.Level;
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.read.ListAppender;
 
 /**
  * Unit tests for {@link StTemplateRenderer}.
@@ -37,8 +44,8 @@ class StTemplateRendererTests {
 	@Test
 	void shouldNotAcceptNullValidationMode() {
 		assertThatThrownBy(() -> StTemplateRenderer.builder().validationMode(null).build())
-			.isInstanceOf(IllegalArgumentException.class)
-			.hasMessageContaining("validationMode cannot be null");
+				.isInstanceOf(IllegalArgumentException.class)
+				.hasMessageContaining("validationMode cannot be null");
 	}
 
 	@Test
@@ -80,14 +87,14 @@ void shouldNotRenderEmptyTemplate() {
 		Map<String, Object> variables = new HashMap<>();
 
 		assertThatThrownBy(() -> renderer.apply("", variables)).isInstanceOf(IllegalArgumentException.class)
-			.hasMessageContaining("template cannot be null or empty");
+				.hasMessageContaining("template cannot be null or empty");
 	}
 
 	@Test
 	void shouldNotAcceptNullVariables() {
 		StTemplateRenderer renderer = StTemplateRenderer.builder().build();
 		assertThatThrownBy(() -> renderer.apply("Hello!", null)).isInstanceOf(IllegalArgumentException.class)
-			.hasMessageContaining("variables cannot be null");
+				.hasMessageContaining("variables cannot be null");
 	}
 
 	@Test
@@ -98,7 +105,7 @@ void shouldNotAcceptVariablesWithNullKeySet() {
 		variables.put(null, "Spring AI");
 
 		assertThatThrownBy(() -> renderer.apply(template, variables)).isInstanceOf(IllegalArgumentException.class)
-			.hasMessageContaining("variables keys cannot be null");
+				.hasMessageContaining("variables keys cannot be null");
 	}
 
 	@Test
@@ -108,7 +115,7 @@ void shouldThrowExceptionForInvalidTemplateSyntax() {
 		variables.put("name", "Spring AI");
 
 		assertThatThrownBy(() -> renderer.apply("Hello {name!", variables)).isInstanceOf(IllegalArgumentException.class)
-			.hasMessageContaining("The template string is not valid.");
+				.hasMessageContaining("The template string is not valid.");
 	}
 
 	@Test
@@ -118,9 +125,9 @@ void shouldThrowExceptionForMissingVariablesInThrowMode() {
 		variables.put("greeting", "Hello");
 
 		assertThatThrownBy(() -> renderer.apply("{greeting} {name}!", variables))
-			.isInstanceOf(IllegalStateException.class)
-			.hasMessageContaining(
-					"Not all variables were replaced in the template. Missing variable names are: [name]");
+				.isInstanceOf(IllegalStateException.class)
+				.hasMessageContaining(
+						"Not all variables were replaced in the template. Missing variable names are: [name]");
 	}
 
 	@Test
@@ -148,9 +155,9 @@ void shouldRenderWithoutValidationInNoneMode() {
 	@Test
 	void shouldRenderWithCustomDelimiters() {
 		StTemplateRenderer renderer = StTemplateRenderer.builder()
-			.startDelimiterToken('<')
-			.endDelimiterToken('>')
-			.build();
+				.startDelimiterToken('<')
+				.endDelimiterToken('>')
+				.build();
 		Map<String, Object> variables = new HashMap<>();
 		variables.put("name", "Spring AI");
 
@@ -162,9 +169,9 @@ void shouldRenderWithCustomDelimiters() {
 	@Test
 	void shouldHandleSpecialCharactersAsDelimiters() {
 		StTemplateRenderer renderer = StTemplateRenderer.builder()
-			.startDelimiterToken('$')
-			.endDelimiterToken('$')
-			.build();
+				.startDelimiterToken('$')
+				.endDelimiterToken('$')
+				.build();
 		Map<String, Object> variables = new HashMap<>();
 		variables.put("name", "Spring AI");
 
@@ -297,4 +304,34 @@ void shouldRenderTemplateWithBuiltInFunctions() {
 		assertThat(result).isEqualTo("Hello!");
 	}
 
+	@Test
+	void malformedTemplateShouldLogErrorViaSlf4j() {
+		Logger logger = (Logger) LoggerFactory.getLogger(StTemplateRenderer.class);
+		ListAppender<ILoggingEvent> appender = new ListAppender<>();
+		appender.start();
+		logger.addAppender(appender);
+
+		PrintStream originalErr = System.err;
+		ByteArrayOutputStream err = new ByteArrayOutputStream();
+		System.setErr(new PrintStream(err));
+		try {
+			StTemplateRenderer renderer = StTemplateRenderer.builder().build();
+			Map<String, Object> variables = new HashMap<>();
+			variables.put("name", "Spring AI");
+			assertThatThrownBy(() -> renderer.apply("Hello {name!", variables))
+					.isInstanceOf(IllegalArgumentException.class);
+		}
+		finally {
+			System.setErr(originalErr);
+			logger.detachAppender(appender);
+			appender.stop();
+		}
+
+		assertThat(appender.list).isNotEmpty();
+		ILoggingEvent event = appender.list.get(0);
+		assertThat(event.getLevel()).isEqualTo(Level.ERROR);
+		assertThat(event.getFormattedMessage()).contains("StringTemplate compile error");
+		assertThat(err.toString(StandardCharsets.UTF_8)).isEmpty();
+	}
+
 }