Added SPEL evaluation against spring boot @Topic attributes (#556)

* [WIP] Added SPEL evaluation against spring boot @Topic name and pubsubname attributes

* [WIP] Updated SubscriberController to use SPEL in @Topic

* 554 - SPEL for @Topic attributes: Updated documentation and examples

* Updated documentation as discussed

* Update README.md

* Update SubscriberController.java

Co-authored-by: Artur Souza <artursouza.ms@outlook.com>

examples/src/main/java/io/dapr/examples/pubsub/http/README.md

@@ -51,13 +51,18 @@ public class Subscriber {
 ```
 `DaprApplication.start()` Method will run an Spring Boot application that registers the `SubscriberController`, which exposes the message retrieval as a POST request. The Dapr's sidecar is the one that performs the actual call to the controller, based on the pubsub features.
 
-This Spring Controller handles the message endpoint, Printing the message which is received as the POST body. The topic subscription in Dapr is handled automatically via the `@Topic` annotation. See the code snippet below:
+This Spring Controller handles the message endpoint, printing the message which is received as the POST body. 
+
+The subscription's topic in Dapr is handled automatically via the `@Topic` annotation - which also supports the same expressions in 
+[Spring's @Value annotations](https://docs.spring.io/spring-framework/docs/current/reference/html/core.html#beans-value-annotations).
+
+The code snippet below shows how to create a subscription using the `@Topic` annotation showcasing expression support. In this case, `myAppProperty` is a Java property that does not exist, so the expression resolves to the default value (`messagebus`). 
 
 ```java
 @RestController
 public class SubscriberController {
   ///...
-  @Topic(name = "testingtopic", pubsubName = "messagebus")
+  @Topic(name = "testingtopic", pubsubName = "${myAppProperty:messagebus}")
   @PostMapping(path = "/testingtopic")
   public Mono<Void> handleMessage(@RequestBody(required = false) byte[] body,
                                    @RequestHeader Map<String, String> headers) {

examples/src/main/java/io/dapr/examples/pubsub/http/SubscriberController.java

@@ -26,7 +26,7 @@ public class SubscriberController {
    * @param cloudEvent The cloud event received.
    * @return A message containing the time.
    */
-  @Topic(name = "testingtopic", pubsubName = "messagebus")
+  @Topic(name = "testingtopic", pubsubName = "${myAppProperty:messagebus}")
   @PostMapping(path = "/testingtopic")
   public Mono<Void> handleMessage(@RequestBody(required = false) CloudEvent cloudEvent) {
     return Mono.fromRunnable(() -> {

sdk-springboot/src/main/java/io/dapr/springboot/DaprBeanPostProcessor.java

@@ -11,6 +11,8 @@
 import io.dapr.Topic;
 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.config.BeanPostProcessor;
+import org.springframework.beans.factory.config.ConfigurableBeanFactory;
+import org.springframework.beans.factory.config.EmbeddedValueResolver;
 import org.springframework.stereotype.Component;
 import org.springframework.web.bind.annotation.PostMapping;
 
@@ -26,6 +28,12 @@ public class DaprBeanPostProcessor implements BeanPostProcessor {
 
   private static final ObjectMapper MAPPER = new ObjectMapper();
 
+  private final EmbeddedValueResolver embeddedValueResolver;
+
+  DaprBeanPostProcessor(ConfigurableBeanFactory beanFactory) {
+    embeddedValueResolver = new EmbeddedValueResolver(beanFactory);
+  }
+
   /**
    * {@inheritDoc}
    */
@@ -35,7 +43,7 @@ public Object postProcessBeforeInitialization(Object bean, String beanName) thro
       return null;
     }
 
-    subscribeToTopics(bean.getClass());
+    subscribeToTopics(bean.getClass(), embeddedValueResolver);
 
     return bean;
   }
@@ -52,12 +60,12 @@ public Object postProcessAfterInitialization(Object bean, String beanName) throw
    * Subscribe to topics based on {@link Topic} annotations on the given class and any of ancestor classes.
    * @param clazz Controller class where {@link Topic} is expected.
    */
-  private static void subscribeToTopics(Class clazz) {
+  private static void subscribeToTopics(Class clazz, EmbeddedValueResolver embeddedValueResolver) {
     if (clazz == null) {
       return;
     }
 
-    subscribeToTopics(clazz.getSuperclass());
+    subscribeToTopics(clazz.getSuperclass(), embeddedValueResolver);
     for (Method method : clazz.getDeclaredMethods()) {
       Topic topic = method.getAnnotation(Topic.class);
       if (topic == null) {
@@ -71,8 +79,8 @@ private static void subscribeToTopics(Class clazz) {
         route = mapping.path()[0];
       }
 
-      String topicName = topic.name();
-      String pubSubName = topic.pubsubName();
+      String topicName = embeddedValueResolver.resolveStringValue(topic.name());
+      String pubSubName = embeddedValueResolver.resolveStringValue(topic.pubsubName());
       if ((topicName != null) && (topicName.length() > 0) && pubSubName != null && pubSubName.length() > 0) {
         try {
           TypeReference<HashMap<String, String>> typeRef

sdk-tests/src/test/java/io/dapr/it/pubsub/http/SubscriberController.java

@@ -77,7 +77,7 @@ public Mono<Void> handleBinaryMessage(@RequestBody(required = false) CloudEvent
     });
   }
 
-  @Topic(name = "anothertopic", pubsubName = "messagebus")
+  @Topic(name = "#{'another'.concat('topic')}", pubsubName = "${pubsubName:messagebus}")
   @PostMapping(path = "/route3")
   public Mono<Void> handleMessageAnotherTopic(@RequestBody(required = false) CloudEvent envelope) {
     return Mono.fromRunnable(() -> {