fix: StTemplateRender property access syntax

  - Fixed the variable recognition issue in property access syntax within `StTemplateRenderer`
  - Added corresponding unit tests

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

Author: sunyuhan1998

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

@@ -49,6 +49,7 @@
  * is shared between threads.
  *
  * @author Thomas Vitale
+ * @author Sun Yuhan
  * @since 1.0.0
  */
 public class StTemplateRenderer implements TemplateRenderer {
@@ -165,16 +166,25 @@ private Set<String> getInputVariables(ST st) {
 			else if (token.getType() == STLexer.RDELIM) {
 				isInsideList = false;
 			}
-			// Only add IDs that are not function calls (i.e., not immediately followed by
+			// Handle regular variables - only add IDs that are at the start of an
+			// expression
 			else if (!isInsideList && token.getType() == STLexer.ID) {
+				// Check if this ID is a function call
 				boolean isFunctionCall = (i + 1 < tokens.size() && tokens.get(i + 1).getType() == STLexer.LPAREN);
-				boolean isDotProperty = (i > 0 && tokens.get(i - 1).getType() == STLexer.DOT);
-				// Only add as variable if:
-				// - Not a function call
-				// - Not a built-in function used as property (unless validateStFunctions)
-				if (!isFunctionCall && (!Compiler.funcs.containsKey(token.getText()) || this.validateStFunctions
-						|| !(isDotProperty && Compiler.funcs.containsKey(token.getText())))) {
-					inputVariables.add(token.getText());
+
+				// Check if this ID is at the beginning of an expression (not a property
+				// access)
+				boolean isAfterDot = (i > 0 && tokens.get(i - 1).getType() == STLexer.DOT);
+
+				// Only add IDs that are:
+				// 1. Not function calls
+				// 2. Not property values (not preceded by a dot)
+				// 3. Either not built-in functions or we're validating functions
+				if (!isFunctionCall && !isAfterDot) {
+					String varName = token.getText();
+					if (!Compiler.funcs.containsKey(varName) || this.validateStFunctions) {
+						inputVariables.add(varName);
+					}
 				}
 			}
 		}

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

@@ -297,4 +297,60 @@ void shouldRenderTemplateWithBuiltInFunctions() {
 		assertThat(result).isEqualTo("Hello!");
 	}
 
+	/**
+	 * Tests that property access syntax like {test.name} is correctly handled. The
+	 * top-level variable 'test' should be identified as required, but 'name' should not.
+	 */
+	@Test
+	void shouldHandlePropertyAccessSyntax() {
+		StTemplateRenderer renderer = StTemplateRenderer.builder().build();
+		Map<String, Object> variables = new HashMap<>();
+		variables.put("test", Map.of("name", "Spring AI"));
+
+		String result = renderer.apply("Hello {test.name}!", variables);
+
+		assertThat(result).isEqualTo("Hello Spring AI!");
+	}
+
+	/**
+	 * Tests that deep property access syntax like {test.tom.name} is correctly handled.
+	 * Only the top-level variable 'test' should be identified as required.
+	 */
+	@Test
+	void shouldHandleDeepPropertyAccessSyntax() {
+		StTemplateRenderer renderer = StTemplateRenderer.builder().build();
+		Map<String, Object> variables = new HashMap<>();
+		variables.put("test", Map.of("tom", Map.of("name", "Spring AI")));
+
+		String result = renderer.apply("Hello {test.tom.name}!", variables);
+
+		assertThat(result).isEqualTo("Hello Spring AI!");
+	}
+
+	/**
+	 * Tests validation behavior with property access syntax. Should only require the
+	 * top-level variable, not the property names.
+	 */
+	@Test
+	void shouldValidatePropertyAccessCorrectly() {
+		StTemplateRenderer renderer = StTemplateRenderer.builder().build();
+		Map<String, Object> variables = new HashMap<>();
+		// Only provide the top-level variable, not the properties
+		variables.put("user", Map.of("profile", Map.of("name", "John")));
+
+		// This should work fine since we provide the required top-level variable
+		String result = renderer.apply("Hello {user.profile.name}!", variables);
+		assertThat(result).isEqualTo("Hello John!");
+
+		// Test with missing top-level variable - should throw exception
+		Map<String, Object> missingVariables = new HashMap<>();
+		// Wrong: providing nested variable instead of top-level
+		missingVariables.put("profile", Map.of("name", "John"));
+
+		assertThatThrownBy(() -> renderer.apply("Hello {user.profile.name}!", missingVariables))
+			.isInstanceOf(IllegalStateException.class)
+			.hasMessageContaining(
+					"Not all variables were replaced in the template. Missing variable names are: [user]");
+	}
+
 }