Fix jdk10 support in travis (#4653)

For jdk10: disable errorprone and fix javadoc warnings

.travis.yml

@@ -40,8 +40,15 @@ os:
   - linux
 
 jdk:
-  - oraclejdk8
+  # net.ltgt.errorprone supports jdk8 and jdk9, but has problems with jdk10
-  - oraclejdk9
+  # For jdk10, we need to switch over to using net.ltgt.errorprone-javacplugin,
+  # and likely update to the latest com.google.errorprone:error_prone_core.
+  # We have decided not to make our build.gradle support both plugins, so when
+  # we finally move off of jdk8 and jdk9 we will need use the javac annotation
+  # processor based plugin.
+  - oraclejdk8  # if both jdk 8 and 9 are removed, migrate to net.ltgt.errorprone-javacplugin (see above comment)
+  - oraclejdk9  # if both jdk 8 and 9 are removed, migrate to net.ltgt.errorprone-javacplugin (see above comment)
+  - oraclejdk10
 
 notifications:
   email: false

build.gradle

@@ -26,7 +26,9 @@ subprojects {
     apply plugin: "com.google.osdetector"
     // The plugin only has an effect if a signature is specified
     apply plugin: "ru.vyarus.animalsniffer"
-    if (!rootProject.hasProperty('errorProne') || rootProject.errorProne.toBoolean()) {
+    // jdk10 not supported by errorprone: https://github.com/google/error-prone/issues/860
+    if (!JavaVersion.current().isJava10Compatible() &&
+        rootProject.properties.get('errorProne', true)) {
         apply plugin: "net.ltgt.errorprone"
         apply plugin: "net.ltgt.apt"
 
@@ -281,6 +283,15 @@ subprojects {
         }
     }
 
+    // For jdk10 we must explicitly choose between html4 and html5, otherwise we get a warning
+    if (JavaVersion.current().isJava10Compatible()) {
+        allprojects {
+            tasks.withType(Javadoc) {
+                options.addBooleanOption('html4', true)
+            }
+        }
+    }
+
     checkstyle {
         configFile = file("$rootDir/buildscripts/checkstyle.xml")
         toolVersion = "6.17"