Java 9 Modules

Concepts

• modules are conceptually one level above a package
• they tend to be at the same level as an individual .jar file that does not include other .jar files
• modules specify dependency relationships among one another
• modules specify which packages have their public types available to other modules that depend on them
• modules have nothing at all to do with a versioning system, and they do not specify versions

Types of paths

• class path
• module path (defined by the host system)

Types of modules

• unnamed module (legacy)
  • code compiled by Java 9 that does not have a module descriptor and is not on the module path
  • reads all modules
  • readable by all automatic modules
  • not readable by modules
• module (encapsulated)
  • code that has a module descriptor and is on the module path
  • reads modules it specifies with a "requires" declaration
  • readable by modules, if they declare it with their own "requires" declaration
  • readable by the unnamed module
  • may not read the unnamed module
  • legacy code can be incrementally converted to modules from the bottom up
  • represented as a regular .jar file with the addition of a module-info.class file at the root
• automatic module (anarchy)
  • code that does not have a module descriptor, but is on the module path
  • reads all modules
  • reads the unnamed module
  • readable by modules, if they declare it with their own "requires" declaration
  • readable by all automatic modules
  • readable by the unnamed module
  • can be used as a bridge between modules and legacy code when bottom up conversion to modules is not practical
  • represented as a regular .jar file placed on a module path
  • the module name is derived from the name of the .jar file

Packages

• Modules use "exports" declarations to indicate which packages are readable by other modules
• Only public types are exported
• It is a compile time error for two modules to export the same package
• When a package can be loaded from either the module path or the class path, the module path is searched first
• Apparent relationships between module names and package names have no significance with regards to behavior, it is convention only

Examples

• MainEntryPoint is in the unnamed module, so it can access MainA in the module learn.module.a
• MainA is in module learn.module.a, which requires module learn.module.b, so it can access MainB
• MainLegacyCode is in the automatic module learn.module.legacycode, so it can see MainThirdPartyCode in the unnamed module

Other details

• jar has this option :-d, --describe-module Print the module descriptor, or automatic module name
• Oracle's Java9 uses the non-standardized "jmod" format for modules, which includes support for native code