I have multiple JDKs installed.

In my build.gradle, I set `sourceCompatibility = 1.8` to ensure the right one is used. This works fine.

However, this seems to be ignored by the Javadoc task (`./gradlew javadoc`), which fails with an error (`error: package sun.net.www.protocol.http is not visible`) -- from [this question](https://stackoverflow.com/questions/36910746/package-java-net-http-does-not-exist-error-on-jdk9) I learned that this is an issue with a new feature in Java 9.

As of now, the project is aimed at Java 8 only. One day it will be upgraded to Java 9, but not today, so I just want to use the Java 8 javadoc generator instead of the Java 9 version.

I checked the [task documentation](https://docs.gradle.org/current/dsl/org.gradle.api.tasks.javadoc.Javadoc.html) but it doesn't look like there's any option to specify a JDK version. What can I do?

I'm expecting the solution to be a Gradle configuration, so it can be easily shared with the other devs on different machines.

Gradle version in which the behavior was seen (installed through IntelliJ):

```
------------------------------------------------------------
Gradle 4.4
------------------------------------------------------------

Build time:   2017-12-06 09:05:06 UTC
Revision:     cf7821a6f79f8e2a598df21780e3ff7ce8db2b82

Groovy:       2.4.12
Ant:          Apache Ant(TM) version 1.9.9 compiled on February 2 2017
JVM:          10.0.1 (Oracle Corporation 10.0.1+10-Debian-4)
OS:           Linux 4.16.0-2-amd64 amd64

```

Gradle version in which there was a warning instead of a failure of the Javadoc task (installed through Debian's repos):

```
------------------------------------------------------------
Gradle 3.4.1
------------------------------------------------------------

Build time:   2012-12-21 00:00:00 UTC
Revision:     none

Groovy:       2.4.15
Ant:          Apache Ant(TM) version 1.10.3 compiled on June 13 2018
JVM:          10.0.1 (Oracle Corporation 10.0.1+10-Debian-4)
OS:           Linux 4.16.0-2-amd64 amd64

```

---

Edit -- I have found [this page](https://docs.oracle.com/javase/9/javadoc/javadoc-command.htm#JSJAV-GUID-C27CE557-E5C6-4688-9FA5-9E9DE886A569) that specifies that a "-source *release*" parameter that is probably the solution to this problem, however I cannot find the way it should be called:

```
javadoc {
    options.addStringOption('-source', '8')
}

```

This compiles & runs with no warnings (in `build.gradle`), but doesn't change anything and doesn't appear in `/build/tmp/javadoc/javadoc.options`.