Can't build javadoc anymore after upgrading from JDK 1.6, "path may not be null or empty string"

Question

I used to use the following Gradle task in build.gradle to build Javadocs for my Android app:

task javadoc(type: Javadoc) {
    title = "My Docs v${android.defaultConfig.versionName}"
    failOnError false
    options.memberLevel = JavadocMemberLevel.PUBLIC
    source = android.sourceSets.main.java.srcDirs
    destinationDir = file("${project.rootDir}/docs/")
    classpath += android.sourceSets.main.compileClasspath + android.sourceSets.main.output
    // classpath += project.files(android.getBootClasspath().join(File.pathSeparator))

    // packages to exclude from the docs
    exclude "com/example/README*"

    // workaround for https://github.com/gradle/gradle/issues/19726 when using "exclude"/"include"
    options.addStringOption("-source-path", android.sourceSets.main.java.srcDirs.join(":"))
}

However, this no longer works after upgrading from Gradle 7 and JDK 1.6 to Gradle 8 and JDK 17. I get this error:

Execution failed for task ':example:javadoc'.
> path may not be null or empty string. path=''

When I uncomment the line with the android.getBootClasspath(), I get other errors like:

…/Example.java:21: error: package android.util does not exist
import android.util.Log;

Looking at related questions these all don't apply, or the fixes there do not work:

• Cause: path may not be null or empty string. path=''
• A problem occurred evaluating project ':app'. > path may not be null or empty string. path='null'
• Cannot build my android studio project on another PC, gradle says "Cause: path may not be null or empty string. path='null'"
• ERROR: path may not be null or empty string. path='null'
• Android proguard issue: path may not be null or empty string. path='null'
• https://github.com/expo/expo/issues/22584

What can I do?

Answer 1

The solution seems to be to remove these lines:

classpath += android.sourceSets.main.compileClasspath + android.sourceSets.main.output
classpath += project.files(android.getBootClasspath().join(File.pathSeparator))

And instead add this:

classpath += files("${android.sdkDirectory}/platforms/${android.compileSdkVersion}/android.jar")

This seems to fix the issue with JDK 17.

Hat tip to https://github.com/emanuele-f for providing this solution.