Enable compiling against Java 9+ (tests disabled)

This change only enables compilation to ensure code compiles against
recent Java versions going forward. Tests are still disabled in this
profile until test failures are addressed.

.travis.yml

@@ -31,6 +31,9 @@ cache:
 
 matrix:
   include:
+      # Java 11 build
+    - jdk: openjdk11
+
       # license checks
     - env:
        - NAME="license checks"

pom.xml

@@ -73,6 +73,7 @@
     <properties>
         <maven.compiler.source>1.8</maven.compiler.source>
         <maven.compiler.target>1.8</maven.compiler.target>
+        <java.version>8</java.version>
         <project.build.resourceEncoding>UTF-8</project.build.resourceEncoding>
         <apache.curator.version>4.1.0</apache.curator.version>
         <apache.curator.test.version>2.12.0</apache.curator.test.version>
@@ -1348,6 +1349,7 @@
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>
                     <artifactId>maven-compiler-plugin</artifactId>
+                    <version>3.8.1</version>
                     <configuration>
                         <source>${maven.compiler.source}</source>
                         <target>${maven.compiler.target}</target>
@@ -1367,6 +1369,33 @@
     </build>
 
     <profiles>
+        <profile>
+            <id>java-9+</id>
+            <activation>
+                <jdk>[9,)</jdk>
+            </activation>
+            <build>
+                <plugins>
+                    <plugin>
+                        <groupId>org.apache.maven.plugins</groupId>
+                        <artifactId>maven-compiler-plugin</artifactId>
+                        <inherited>true</inherited>
+                        <!-- prefer release instead of source/target in JDK 9 and above -->
+                        <configuration>
+                            <release>${java.version}</release>
+                        </configuration>
+                    </plugin>
+                    <plugin>
+                        <groupId>org.apache.maven.plugins</groupId>
+                        <artifactId>maven-surefire-plugin</artifactId>
+                        <configuration combine.self="override">
+                            <!-- disable tests until we fully support JDK 9 and above -->
+                            <skipTests>true</skipTests>
+                        </configuration>
+                    </plugin>
+                </plugins>
+            </build>
+        </profile>
         <profile>
             <id>strict</id>
             <build>