Reuse Fahrschein bean if it already exists (#75).

Also document how to not use Fahrschein at all.

Author: ePaul

README.md

@@ -58,6 +58,7 @@ This library also uses:
 ### Setup
 
 If you are using maven, include the library in your `pom.xml`:
+
 ```xml
 <dependency>
     <groupId>org.zalando</groupId>
@@ -66,7 +67,10 @@ If you are using maven, include the library in your `pom.xml`:
 </dependency>
 ```
 
+The latest available version is visible in the Maven central badge at the top of this README. 
+
 Use `@EnableNakadiProducer` annotation to activate spring boot starter auto configuration:
+
 ```java
 @SpringBootApplication
 @EnableNakadiProducer
@@ -81,6 +85,23 @@ The library uses flyway migrations to set up its own database schema `nakadi_eve
 
 ### Nakadi communication configuration
 
+By default, Nakadi-producer-spring-boot starter uses the Fahrschein library to submit its events. It needs some configuration to know how to do this – we describe three ways here (basically in order of ease of use):
+
+* Using existing Fahrschein setup
+* Letting this library set things up
+* Implement Nakadi communication yourself
+
+#### Using existing Fahrschein setup
+
+If you are already using the [Fahrschein library](https://github.com/zalando-nakadi/fahrschein) directly (e.g. for event consumption) and have already a configured `org.zalando.fahrschein.NakadiClient` object, just make sure it is available as a Spring bean. Nakadi-Producer-Spring-Boot-Starter will pick it up and use it directly.
+
+The configurations in the next section are then not needed at all.
+
+#### Letting this library set things up 
+
+If you want Nakadi-Producer-Spring-Boot-Starter to configure the connection to Nakadi, you'll need to set some properties
+(and/or create beans).
+
 You must tell the library, where it can reach your Nakadi instance:
 ```yaml
 nakadi-producer:
@@ -116,6 +137,26 @@ nakadi-producer:
 
 If you do not use the STUPS Tokens library, you can implement token retrieval yourself by defining a Spring bean of type `org.zalando.nakadiproducer.AccessTokenProvider`. The starter will detect it and call it once for each request to retrieve the token. 
 
+#### Implement Nakadi communication yourself
+
+If you don't like Fahrschein, you can implement the communication with Nakadi yourself. Just provide an implementation of the `NakadiPublishingClient` interface as a Spring bean. Nakadi-producer-spring-boot-starter will just use that bean. (This is used in the [test support](#test-support) described below.)
+
+You can then also exclude the Fahrschein dependency: 
+
+```xml
+<dependency>
+    <groupId>org.zalando</groupId>
+    <artifactId>nakadi-producer-spring-boot-starter</artifactId>
+    <version>${nakadi-producer.version}</version>
+    <exclusions>
+        <exclusion>
+            <groupId>org.zalando</groupId>
+            <artifactId>fahrschein</artifactId>
+        <exclusion>
+    </exclusions>
+</dependency>
+```
+
 ### Creating events
 
 The typical use case for this library is to publish events like creating or updating of some objects.

nakadi-producer-spring-boot-starter/src/main/java/org/zalando/nakadiproducer/NakadiProducerAutoConfiguration.java

@@ -49,10 +49,10 @@
 @AutoConfigureAfter(name="org.zalando.tracer.spring.TracerAutoConfiguration")
 public class NakadiProducerAutoConfiguration {
 
-    @ConditionalOnMissingBean(NakadiPublishingClient.class)
+    @ConditionalOnMissingBean({NakadiPublishingClient.class, NakadiClient.class})
     @Configuration
-    @Import(FahrscheinNakadiClientConfiguration.StupsTokenConfiguration.class)
-    static class FahrscheinNakadiClientConfiguration {
+    @Import(FahrscheinWithTokensNakadiClientConfiguration.StupsTokenConfiguration.class)
+    static class FahrscheinWithTokensNakadiClientConfiguration {
 
         @Bean
         public NakadiPublishingClient nakadiProducerPublishingClient(AccessTokenProvider accessTokenProvider,
@@ -75,6 +75,17 @@ public StupsTokenComponent accessTokenProvider(
         }
     }
 
+    @ConditionalOnMissingBean(NakadiPublishingClient.class)
+    @ConditionalOnBean(NakadiClient.class)
+    @Configuration
+    static class ExistingFahrscheinNakadiClientConfiguration {
+
+        @Bean
+        public NakadiPublishingClient nakadiProducerPublishingClient(NakadiClient fahrscheinNakadiClient) {
+            return new FahrscheinNakadiPublishingClient(fahrscheinNakadiClient);
+        }
+    }
+
     @Bean
     @ConditionalOnMissingClass("org.zalando.tracer.Tracer")
     @ConditionalOnMissingBean(FlowIdComponent.class)