GH-676 Relax SpEL evaluation failures for input header enrichment

Add documentation

Resolves #676

docs/src/main/asciidoc/spring-cloud-function.adoc

@@ -258,6 +258,78 @@ due to the nature of the reactive functions which are invoked only once to pass
 is handled by the reactor, hence we can not access and/or rely on the routing instructions communicated via individual
 values (e.g., Message).
 
+=== Input Enrichment
+
+There are often times when you need to modify or refine an incoming Message and to keep your code clean of non-functional concerns, and you don’t want to 
+do it inside of your business logic.
+
+You can always accomplish it via <<Function Composition>>. Such approach provides several benefits:
+
+- It allows you to isolate this non-functional concern into a separate function which you can compose with the business function as function definition.
+- It provides you with complete freedom (and danger) as to what you can modify before incoming message reaches the actual business function.
+
+[source, java]
+----
+@Bean
+public Function<Message<?>, Message<?>> enrich() {
+    return message -> MessageBuilder.fromMessage(message).setHeader("foo", "bar").build();
+}
+
+@Bean
+public Function<Message<?>, Message<?>> myBusinessFunction() {
+    // do whatever
+}
+----
+
+And then compose your function by providing the following function definition `enrich|myBusinessFunction`.
+
+While the described approach is the most flexible, it is also the most involved as it requires you to write some code, make it a bean or 
+manually register it as a function before you can compose it with the business function as you can see from the preceding example. 
+
+But what if modifications (enrichments) you are trying to make are trivial as they are in the preceding example? Is there a simpler and more dynamic and configurable
+ mechanism to accomplish the same?
+
+Since version 3.1.3, the framework allows you to provide SpEL expression to enrich individual message headers. Let’s look at one of the tests as the example.
+
+[source, java]
+----
+@Test
+public void testInputHeaderMappingPropertyWithoutIndex() throws Exception {
+	try (ConfigurableApplicationContext context = new SpringApplicationBuilder(
+			SampleFunctionConfiguration.class).web(WebApplicationType.NONE).run(
+				"--spring.cloud.function.configuration.echo.input-header-mapping-expression.key1='hello1'",
+				"--spring.cloud.function.configuration.echo.input-header-mapping-expression.key2='hello2'",
+				"--spring.cloud.function.configuration.echo.input-header-mapping-expression.foo=headers.contentType")) {
+
+	FunctionCatalog functionCatalog = context.getBean(FunctionCatalog.class);
+	FunctionInvocationWrapper function = functionCatalog.lookup("echo");
+	function.apply(MessageBuilder.withPayload("helo")
+					.setHeader(MessageHeaders.CONTENT_TYPE, "application/json").build());
+  }
+}
+----
+
+Here you see a property called `input-header-mapping-expression` preceded by the name of the function (i.e., `echo`) and followed by the name of the 
+message header key you want to set and the value as SpEL expression. The first two expressions (for 'key1' and 'key2') are literal SpEL expressions enclosed in 
+single quotes, effectively setting 'key1' to value `hello1` and 'key2' to value `hello2`. The third  one will map Message header ‘foo’ to the value of the 
+current ‘contentType’ header.
+
+NOTE: if for whatever reason the provided expression evaluation fails, the execution of the function will proceed as if nothing ever happen. 
+However you will see the WARN message in your logs informing you about it 
+
+[source, text]
+----
+o.s.c.f.context.catalog.InputEnricher    : Failed while evaluating expression "hello1"  on incoming message. . .
+----
+
+In the event you are dealing with functions that have multiple inputs (next section), you can use index immediately after `input-header-mapping-expression`
+
+[source, text]
+----
+--spring.cloud.function.configuration.echo.input-header-mapping-expression[0].key1=‘hello1'
+--spring.cloud.function.configuration.echo.input-header-mapping-expression[1].key2='hello2'
+----
+
 === Function Arity
 
 There are times when a stream of data needs to be categorized and organized. For example,
@@ -470,16 +542,6 @@ same rules for signature transformation outlined in "Java 8 function support" se
 To enable Kotlin support all you need is to add Kotlin SDK libraries on the classpath which will trigger appropriate
 autoconfiguration and supporting classes.
 
-=== Function Component Scan
-
-Spring Cloud Function will scan for implementations of `Function`,
-`Consumer` and `Supplier` in a package called `functions` if it
-exists. Using this feature you can write functions that have no
-dependencies on Spring - not even the `@Component` annotation is
-needed. If you want to use a different package, you can set
-`spring.cloud.function.scan.packages`. You can also use
-`spring.cloud.function.scan.enabled=false` to switch off the scan
-completely.
 
 == Standalone Web Applications
 

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 -> {