Add STErrorListener to SLF4J bridge.

`StTemplateRenderer` uses `StringTemplate` as the template rendering
engine. By default, StringTemplate use
`org.stringtemplate.v4.misc.ErrorManager#DEFAULT_ERROR_LISTENER` as the
error listener for rendering exceptions, which outputs errors via
System.err.println. This may cause users to miss important information
about rendering errors. This commit introduces a custom `STErrorListener`
that logs exceptions to the application's logging system instead.

Signed-off-by: Sun Yuhan <[email protected]>
Signed-off-by: Eric Bottard <[email protected]>

Fixes #3604

Author: sunyuhan1998

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

@@ -0,0 +1,62 @@
+/*
+ * 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.stringtemplate.v4.STErrorListener;
+import org.stringtemplate.v4.misc.ErrorType;
+import org.stringtemplate.v4.misc.STMessage;
+
+/**
+ * An {@link STErrorListener} that delegates to {@link Logger slf4j}.
+ *
+ * @author Sun Yuhan
+ */
+public class Slf4jStErrorListener implements STErrorListener {
+
+	private final Logger logger;
+
+	/* package */ Slf4jStErrorListener(Logger logger) {
+		this.logger = logger;
+	}
+
+	@Override
+	public void compileTimeError(STMessage msg) {
+		logger.error(msg.toString());
+	}
+
+	@Override
+	public void runTimeError(STMessage msg) {
+		if (msg.error != ErrorType.NO_SUCH_PROPERTY) { // ignore these
+			logger.error(msg.toString());
+		}
+		else {
+			logger.warn(msg.toString());
+		}
+	}
+
+	@Override
+	public void IOError(STMessage msg) {
+		logger.error(msg.toString());
+	}
+
+	@Override
+	public void internalError(STMessage msg) {
+		logger.error(msg.toString());
+	}
+
+}

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

@@ -25,6 +25,7 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import org.stringtemplate.v4.ST;
+import org.stringtemplate.v4.STGroup;
 import org.stringtemplate.v4.compiler.Compiler;
 import org.stringtemplate.v4.compiler.STLexer;
 
@@ -113,7 +114,9 @@ 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(new Slf4jStErrorListener(logger));
+			return new ST(group, template);
 		}
 		catch (Exception ex) {
 			throw new IllegalArgumentException("The template string is not valid.", ex);