FULL PRODUCT VERSION :
java version "9-ea"
Java(TM) SE Runtime Environment (build 9-ea+118)
Java HotSpot(TM) Client VM (build 9-ea+118, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Windows 10

A DESCRIPTION OF THE PROBLEM :
Since refresh of module system in build 118, some modules are now not by default on javac classpath.
as described in http://download.java.net/java/jdk9/changes/jdk-9+118.html

Problem occurs when we are cross compiling to java 8 as target and where it is expected that this classes are available.
Further on, if we use new -release 8 flag java.annotations.common module is present at compile of the classes but it is still missing from annotation processors that are used in same compile cycle.

the only way to compile code that worked fine in Java 8 or even earlier builds of JDK9. is to compile it for -source 9 -target 9 and adding also -addmods java.annotations.common -J-addmods -Jjava.annotations.common second one is the one that passes it to annotation processor.

Trying to use -J-addmods or -addmods in combination with -target 8 or -release 8, doesn't work as well.

This is preventing us from compiling WildFly and related projects.

REGRESSION. Last worked in version 8u92

ADDITIONAL REGRESSION INFORMATION:
Create simple annotation processor that tries to use javax.annotation.Generated

annotation it will process

public @interface TestAnnotation {
}
annotation processor

@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class TestAnnotationProcessor extends AbstractProcessor {

    @Override
    public Set<String> getSupportedAnnotationTypes() {
        return Collections.singleton(TestAnnotation.class.getName());
    }

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        try {
            Class generated = Class.forName(Generated.class.getName()); //try to load annotation from java.annotations.common module
        } catch (ClassNotFoundException e) {
            processingEnv.getMessager().printMessage(Diagnostic.Kind.ERROR, "Could not load javax.annotation.Generated class");
            throw new RuntimeException(e);
        }
        return true;
    }
}
and register it in META-INF/services/javax.annotation.processing.Processor
with content of "TestAnnotationProcessor"

create simple class that will use annotation that is processed with processor

@TestAnnotation //to trigger annotation processor
public class ClassWithAnnotation {

     public void someMethod() throws Exception {
    //try to load class from java.annotations.common, to see if "main" javac classpath has classes available
        Class generated = Class.forName(Generated.class.getName());
    }
}

than first compile the processor and put it into jar file.
once you have processor.jar follow the step to reproduce


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
//try to compile for java 8 target:
javac -d out -classpath processor.jar; -source 8 -target 8 code\src\main\java\ClassWithAnnotation.java
//new way to compile for java 8, by using new -release 8 flag
javac -d out -classpath processor.jar; -release 8 code\src\main\java\ClassWithAnnotation.java
//use java 9 as target and add module java.annotations.common, to javac compile
javac -d out -classpath processor.jar; -addmods java.annotations.common code\src\main\java\ClassWithAnnotation.java
//use java 9 as target and add module java.annotations.common, to javac and pass it to annotation processor by using -J<params>
javac -d out -classpath processor.jar; -addmods java.annotations.common -J-addmods -Jjava.annotations.common code\src\main\java\ClassWithAnnotation.java


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
all 4 should succeed, but only 4th command results in successful compile

even 3 is somewhat expected to work, as we do add module to javac, but it is not passed over to annotation processor.

ACTUAL -
1 & 2 & 3 fail with
An annotation processor threw an uncaught exception.
Consult the following stack trace for details.
java.lang.NoClassDefFoundError: javax/annotation/Generated
        at processor.TestAnnotationProcessor.process(TestAnnotationProcessor.java:28)
        at com.sun.tools.javac.processing.JavacProcessingEnvironment.callProcessor(jdk.compiler@9-ea/JavacProcessingEnvironment.java:960)
        at com.sun.tools.javac.processing.JavacProcessingEnvironment.discoverAndRunProcs(jdk.compiler@9-ea/JavacProcessingEnvironment.java:876)
        at com.sun.tools.javac.processing.JavacProcessingEnvironment.access$2100(jdk.compiler@9-ea/JavacProcessingEnvironment.java:106)
        at com.sun.tools.javac.processing.JavacProcessingEnvironment$Round.run(jdk.compiler@9-ea/JavacProcessingEnvironment.java:1183)
        at com.sun.tools.javac.processing.JavacProcessingEnvironment.doProcessing(jdk.compiler@9-ea/JavacProcessingEnvironment.java:1291)
        at com.sun.tools.javac.main.JavaCompiler.processAnnotations(jdk.compiler@9-ea/JavaCompiler.java:1229)
        at com.sun.tools.javac.main.JavaCompiler.compile(jdk.compiler@9-ea/JavaCompiler.java:903)
        at com.sun.tools.javac.main.Main.compile(jdk.compiler@9-ea/Main.java:269)
        at com.sun.tools.javac.main.Main.compile(jdk.compiler@9-ea/Main.java:144)
        at com.sun.tools.javac.Main.compile(jdk.compiler@9-ea/Main.java:55)
        at com.sun.tools.javac.Main.main(jdk.compiler@9-ea/Main.java:41)
Caused by: java.lang.ClassNotFoundException: javax.annotation.Generated
        at java.net.URLClassLoader.findClass(java.base@9-ea/URLClassLoader.java:384)
        at java.lang.ClassLoader.loadClass(java.base@9-ea/ClassLoader.java:486)
        at java.lang.ClassLoader.loadClass(java.base@9-ea/ClassLoader.java:419)
        ... 12 more


4 works.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
public @interface TestAnnotation {
}
annotation processor

@SupportedSourceVersion(SourceVersion.RELEASE_8)
public class TestAnnotationProcessor extends AbstractProcessor {

    @Override
    public Set<String> getSupportedAnnotationTypes() {
        return Collections.singleton(TestAnnotation.class.getName());
    }

    @Override
    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        try {
            Class generated = Class.forName(Generated.class.getName()); //try to load annotation from java.annotations.common module
        } catch (ClassNotFoundException e) {
            processingEnv.getMessager().printMessage(Diagnostic.Kind.ERROR, "Could not load javax.annotation.Generated class");
            throw new RuntimeException(e);
        }
        return true;
    }
}
and register it in META-INF/services/javax.annotation.processing.Processor
with content of "TestAnnotationProcessor"

create simple class that will use annotation that is processed with processor

@TestAnnotation //to trigger annotation processor
public class ClassWithAnnotation {

     public void someMethod() throws Exception {
    //try to load class from java.annotations.common, to see if "main" javac classpath has classes available
        Class generated = Class.forName(Generated.class.getName());
    }
}

for compilable example see https://github.com/ctomc/jdk9-compiler
---------- END SOURCE ----------