Which system modules are on the module path by default?

Question

When I run an application via java -cp (without --add-modules or --limit-modules), some Java system modules are observable while the others are not.

For example, all java.se modules are observable. All java.se.ee modules are not observable. I know that javafx.* modules are observable. jdk.unsupported and jdk.shell are observable too.

So, is my assumption correct: if no --add-modules and --limit-modules are specified, the set of observable system modules consists of all system modules except java.se.ee?

Is there a reliable way to know the exact list of default observable system modules? I know there is a --list-modules option, but it lists all modules including java.se.ee.

Answer 1

So, is my assumption correct: if no --add-modules and --limit-modules are specified, the set of observable system modules consists of all system modules except java.se.ee?

In short, yes that is correct.

The default set of modules enabled in Java 9 are known as the root modules. Per JEP 261, the default set of root modules are defined as:

• The java.se module is a root, if it exists. If it does not exist then every java.* module on the upgrade module path or among the system modules that exports at least one package, without qualification, is a root.

• Every non-java.* module on the upgrade module path or among the system modules that exports at least one package, without qualification, is also a root.

Here is a nice graphic of what is included in the java.se module: (Source: Java 9 javadoc)

Like the java.se aggregate module, the java.se.ee module itself does not provide any classes, it is an aggregate module that includes the following modules:

java.se
java.activation
java.annotations.common
java.corba
java.transaction
java.xml.bind
java.xml.ws

---

Is there a reliable way to know the exact list of default observable system modules? I know there is a --list-modules option, but it lists all modules including java.se.ee.

Your terminology is slightly off here. In Java 9 a module is observable if both of the following are true:

• the module is a system module (i.e. comes from the JDK) OR it is added to the module path
• the module is not excluded via --limit-modules

This means java.se.ee is observable by default.

I think instead you are wondering what modules are the default set of root modules? In which case, see the above definition of root modules.