GH-676 Relax SpEL evaluation failures for input header enrichment

Add documentation

Resolves #676

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/catalog/InputEnricher.java

@@ -20,6 +20,8 @@ import java.util.Map;
 import java.util.Map.Entry;
 import java.util.function.Function;
 
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
 import org.springframework.context.expression.MapAccessor;
 import org.springframework.expression.BeanResolver;
 import org.springframework.expression.Expression;
@@ -37,7 +39,9 @@ import org.springframework.util.Assert;
  * @since 3.1.3
  *
  */
-public class InputEnricher implements Function<Object, Object> {
+class InputEnricher implements Function<Object, Object> {
+
+	protected Log logger = LogFactory.getLog(InputEnricher.class);
 
 	private final Map<String, Map<String, String>> headerExpressions;
 
@@ -62,8 +66,19 @@ public class InputEnricher implements Function<Object, Object> {
 			Map<String, String> mappings = this.headerExpressions.get("0");
 			for (Entry<String, String> keyValueExpressionEntry : mappings.entrySet()) {
 				Expression expression = this.spelParser.parseExpression(keyValueExpressionEntry.getValue());
-				Object value = expression.getValue(this.evalContext, input, Object.class);
-				messageBuilder.setHeader(keyValueExpressionEntry.getKey(), value);
+				try {
+					Object value = expression.getValue(this.evalContext, input, Object.class);
+					messageBuilder.setHeader(keyValueExpressionEntry.getKey(), value);
+				}
+				catch (Exception e) {
+					String message = "Failed while evaluating expression \"" + keyValueExpressionEntry.getValue() + "\"  on incoming message";
+					if (logger.isDebugEnabled()) {
+						logger.warn(message + ": " + input, e);
+					}
+					else {
+						logger.warn(message);
+					}
+				}
 			}
 			input = messageBuilder.build();
 		}

spring-cloud-function-context/src/test/java/org/springframework/cloud/function/context/InputHeaderMappingTests.java

@@ -34,7 +34,23 @@ import org.springframework.messaging.support.MessageBuilder;
 import static org.assertj.core.api.Assertions.assertThat;
 
 //NOTE!!! assertions for all tests are in 'echo' function since we're validating what's coming into it.
-public class FunctionPropertiesTests {
+public class InputHeaderMappingTests {
+
+	@Test
+	public void testErrorWarnAndContinue() throws Exception {
+		try (ConfigurableApplicationContext context = new SpringApplicationBuilder(
+				SampleFunctionConfiguration.class).web(WebApplicationType.NONE).run(
+						"--logging.level.org.springframework.cloud.function=DEBUG",
+						"--spring.main.lazy-initialization=true",
+						"--spring.cloud.function.configuration.echoFail.input-header-mapping-expression[0].key1=hello1",
+						"--spring.cloud.function.configuration.echoFail.input-header-mapping-expression[0].key2='hello2'",
+						"--spring.cloud.function.configuration.echoFail.input-header-mapping-expression[0].foo=headers.contentType")) {
+
+			FunctionCatalog functionCatalog = context.getBean(FunctionCatalog.class);
+			FunctionInvocationWrapper function = functionCatalog.lookup("echoFail");
+			function.apply(MessageBuilder.withPayload("helo").build());
+		}
+	}
 
 	@Test
 	public void testInputHeaderMappingPropertyWithIndex() throws Exception {
@@ -137,6 +153,16 @@ public class FunctionPropertiesTests {
 			};
 		}
 
+		@Bean
+		public Function<Message<?>, Message<?>> echoFail() {
+			return m -> {
+				assertThat(m.getHeaders().containsKey("key1")).isFalse();
+				assertThat(m.getHeaders().get("key2")).isEqualTo("hello2");
+				assertThat(m.getHeaders().containsKey("foo")).isFalse();
+				return m;
+			};
+		}
+
 		@Bean
 		public Function<Message<?>, Message<?>> split() {
 			return m -> {