Explicitly notes version policy and corrects some pom glitches

This is a response to the following issue, where our dependency policy
was unclear: jaegertracing/jaeger-client-java#511 (comment)

README.md

@@ -31,6 +31,30 @@ You may want to put trace IDs into your log files, or change thread local
 behavior. Look at our [context libraries](context/), for integration with
 tools such as SLF4J.
 
+## Version Compatibility policy
+All Brave libraries match the minimum Java version of what's being
+traced or integrated with, and adds no 3rd party dependencies. The goal
+is to neither impact your projects' choices, nor subject your project
+to dependency decisions made by others.
+
+For example, even including a basic reporting library,
+[zipkin-sender-urlconnection](https://github.com/openzipkin/zipkin-reporter-java), Brave transitively includes no json,
+logging, protobuf or thrift dependency. This means zero concern if you
+choose to any version of any SLF4J or Log4J library, or common libraries
+like Guava.
+
+There is a floor Java version of 1.6, which allows older JREs and older
+Android runtimes, yet may limit some applications. For example, Servlet
+2.5 works with Java 1.5, but due to Brave being 1.6, you will not be
+able to trace Servlet 2.5 applications until you use at least JRE 1.6.
+
+All integrations set their associated library to "provided" scope. This
+ensures Brave doesn't interfere with the versions you choose.
+
+Some libraries update often which leads to api drift. In some cases, we
+test versions ranges to reduce the impact of this. For example, we test
+[gRPC](instrumentation/grpc) and [Kafka](instrumentation/kafka-clients) against multiple library versions.
+
 ## Artifacts
 All artifacts publish to the group ID "io.zipkin.brave". We use a common
 release version for all components.

context/rxjava2/pom.xml

@@ -12,15 +12,16 @@
 
   <properties>
     <main.basedir>${project.basedir}/../..</main.basedir>
-    <main.java.version>1.7</main.java.version>
-    <main.signature.artifact>java17</main.signature.artifact>
+    <!-- RxJava 2.x continues support for Java 1.6 and Android 2.3+ -->
+    <main.java.version>1.6</main.java.version>
+    <main.signature.artifact>java16</main.signature.artifact>
   </properties>
 
   <dependencies>
     <dependency>
       <groupId>io.reactivex.rxjava2</groupId>
       <artifactId>rxjava</artifactId>
-      <version>2.1.17</version>
+      <version>2.2.0</version>
       <scope>provided</scope>
     </dependency>
 
@@ -33,7 +34,7 @@
     <dependency>
       <groupId>com.github.akarnokd</groupId>
       <artifactId>rxjava2-extensions</artifactId>
-      <version>0.19.5</version>
+      <version>0.20.0</version>
       <scope>test</scope>
     </dependency>
     <dependency>

instrumentation/grpc/pom.xml

@@ -11,6 +11,10 @@
 
   <properties>
     <main.basedir>${project.basedir}/../..</main.basedir>
+    <!-- grpc < 1.15 supports java 6 https://github.com/grpc/grpc-java/issues/3961 -->
+    <main.java.version>1.6</main.java.version>
+    <main.signature.artifact>java16</main.signature.artifact>
+
     <protobuf.version>3.5.1</protobuf.version>
     <opencensus.version>0.15.0</opencensus.version>
     <os-maven-plugin.version>1.6.0</os-maven-plugin.version>

instrumentation/httpasyncclient/pom.xml

@@ -12,6 +12,8 @@
 
   <properties>
     <main.basedir>${project.basedir}/../..</main.basedir>
+    <main.java.version>1.6</main.java.version>
+    <main.signature.artifact>java16</main.signature.artifact>
   </properties>
 
   <dependencies>

instrumentation/jersey-server/pom.xml

@@ -12,6 +12,9 @@
 
   <properties>
     <main.basedir>${project.basedir}/../..</main.basedir>
+    <!-- Jersey 2.7 -> 2.25 are Java 7. We don't currently compile against Jersey 2.26+
+         https://jersey.github.io/documentation/latest/modules-and-dependencies.html#d0e560
+         -->
   </properties>
 
   <dependencies>

instrumentation/mysql/pom.xml

@@ -12,6 +12,7 @@
 
   <properties>
     <main.basedir>${project.basedir}/../..</main.basedir>
+    <!-- mysql-connector-java < v6 requires Java 6 -->
     <main.java.version>1.6</main.java.version>
     <main.signature.artifact>java16</main.signature.artifact>
   </properties>

instrumentation/mysql6/pom.xml

@@ -12,6 +12,9 @@
 
   <properties>
     <main.basedir>${project.basedir}/../..</main.basedir>
+    <!-- mysql-connector-java 6+ requires Java 8 -->
+    <main.java.version>1.8</main.java.version>
+    <main.signature.artifact>java18</main.signature.artifact>
   </properties>
 
   <dependencies>

instrumentation/netty-codec-http/pom.xml

@@ -12,6 +12,9 @@
 
   <properties>
     <main.basedir>${project.basedir}/../..</main.basedir>
+    <!-- Netty 4.x requires Java 6+ https://netty.io/wiki/requirements-for-4.x.html -->
+    <main.java.version>1.6</main.java.version>
+    <main.signature.artifact>java16</main.signature.artifact>
   </properties>
 
   <dependencies>

instrumentation/p6spy/pom.xml

@@ -12,6 +12,9 @@
 
   <properties>
     <main.basedir>${project.basedir}/../..</main.basedir>
+    <main.java.version>1.6</main.java.version>
+    <main.signature.artifact>java16</main.signature.artifact>
+
     <p6spy.version>3.7.0</p6spy.version>
     <derby.version>10.14.2.0</derby.version>
   </properties>

instrumentation/spring-webmvc/pom.xml

@@ -12,6 +12,8 @@
 
   <properties>
     <main.basedir>${project.basedir}/../..</main.basedir>
+    <main.java.version>1.6</main.java.version>
+    <main.signature.artifact>java16</main.signature.artifact>
   </properties>
 
   <dependencies>