How to fail gradle build on Javadoc warnings

There is a non-standard hidden javadoc option -Xwerror available on all supported Java releases. Thus you could simply do something like this:

if (JavaVersion.current().isJava8Compatible()) {
    tasks.withType(Javadoc) {
        // The '-quiet' as second argument is actually a hack,
        // since the one paramater addStringOption doesn't seem to
        // work, we extra add '-quiet', which is added anyway by
        // gradle. See https://github.com/gradle/gradle/issues/2354
        // See JDK-8200363 (https://bugs.openjdk.java.net/browse/JDK-8200363)
        // for information about the -Xwerror option.
        options.addStringOption('Xwerror', '-quiet')
    }
}

A feature request for an official '-Werror' for javadoc is tracked as JDK-8200363. This feature is now available in JDK 15+ as -Werror, -Xwerror also works as an alias.

note: i've totally replaced my original answer, because i've found a better one - which is not that ugly:

import org.gradle.logging.internal.OutputEvent
import org.gradle.logging.internal.OutputEventListener

        task("javadocCheck",type:Javadoc){
            // regular javadoc task configuration

            def outputEvents = []
            def listener=new OutputEventListener(){
                    void onOutput(OutputEvent event){
                        outputEvents << event
                    }
                };
            doFirst {
                getLogging().addOutputEventListener(listener)
            }
            doLast {
                getLogging().removeOutputEventListener(listener)
                outputEvents.each { e ->
                    if(e.toString() =~ " warning: "){
                        throw new GradleException("You have some javadoc warnings, please fix them!");
                    }
                }
            }
        }

task.getLogging() is now deprecated and LoggingManagerInternal#addOutputEventListener() got removed.

Here is a solution that should work with Gradle >2.14.

    import org.gradle.api.logging.StandardOutputListener

    task("javadocCheck",type: Javadoc) {
        // regular javadoc task configuration

        def capturedOutput = []
        def listener = { capturedOutput << it } as StandardOutputListener
        doFirst {
            logging.addStandardErrorListener(listener)
            logging.addStandardOutputListener(listener)
        }
        doLast {
            logging.removeStandardOutputListener(listener)
            logging.removeStandardErrorListener(listener)
            capturedOutput.each { e ->
                if(e.toString() =~ " warning: ") {
                    throw new GradleException("You have some javadoc warnings, please fix them!");
                }
            }
        }
    }