I am trying to understand how JPMS works.
From here
The classpath is not completely gone yet. All JARs (modular or not) and classes on the classpath will be contained in the Unnamed Module. Similar to automatic modules, it exports all packages and reads all other modules. But it does not have a name, obviously. For that reason, it cannot be required and read by named application modules. The unnamed module in turn can access all other modules.
Please, note ...on the classpath will be contained in the Unnamed Module. Module is singular.
From here
For compatibility, all code on the classpath is packaged up as a special unnamed module, with no hidden packages and full access to the whole JDK.
Again unnamed module. Module is singular.
Do I understand right that there is always only one unnamed module in JPMS? Does it mean that applications that were developed before Java9 and not updated for Java9 will be loaded as one unnamed module?
Do I understand right that there is always only one unnamed module in JPMS?
In short
Generally speaking, no. But let's put it this way: If you place some or even all JARs on the class path and your application does not create class loaders to load any additional content, then there is only one unnamed module you need to care about.
In more detail
Every ClassLoader has its own unnamed module that it uses to represent classes that it loaded from the class path. This is necessary because the module system requires everything to be in a module.
As nullpointer's answer explains in detail, an application will by default use three separate class loaders. It is possible that it might spin up its own class loaders, for example to load plugins. If it doesn't do that, though, all application code will end up in the system/application class loader and hence in the same unnamed module. That's why there is typically only one you need to care about.
Does it mean that applications that were developed before Java9 and not updated for Java9 will be loaded as one unnamed module?
This has nothing to do with whether code (application, frameworks, libraries) targets Java 9 - it only depends on which path you place a JAR, on the class path or the module path.
If it's on the class path, it ends up in the unnamed module together with other class path content. This is true for plain JARs without module descriptor but also for modular JARs that contain one.
If it's on the module path, it gets its own module. If it's a modular JAR, it gets an explicit module as those described throughout the State of the Module System - plain JARs get turned into automatic modules (note the plural: one automatic module per JAR).