Modules

Syntax#

requires java.xml;
requires public java.xml; # exposes module to dependents for use
exports com.example.foo; # dependents can use public types in this package
exports com.example.foo.impl to com.example.bar; # restrict usage to a module

Remarks#

The use of modules is encouraged but not required, this allows existing code to continue working in Java 9. It also allows for a gradual transition to modular code.

Any non-modular code is put in an unnamed module when it is compiled. This is a special module that is able to use types from all other modules but only from packages which have an exports declaration.

All packages in the unnamed module are exported automatically.

Keywords, e.g. module etc…, are restricted in use within the module declaration but can be continue to be used as identifiers elsewhere.

Defining a basic module

Modules are defined in a file named module-info.java, named a module descriptor. It should be placed in the source-code root:

 |-- module-info.java
 |-- com
     |-- example
         |-- foo
             |-- Foo.java
         |-- bar
             |-- Bar.java

Here is a simple module descriptor:

module com.example {
    requires java.httpclient;
    exports com.example.foo;
}

The module name should be unique and it is recommended that you use the same Reverse-DNS naming notation as used by packages to help ensure this.

The module java.base, which contains Java’s basic classes, is implicitly visible to any module and does not need to be included.

The requires declaration allows us to use other modules, in the example the module java.httpclient is imported.

A module can also specify which packages it exports and therefore makes it visible to other modules.

The package com.example.foo declared in the exports clause will be visible to other modules. Any sub-packages of com.example.foo will not be exported, they need their own export declarations.

Conversely, com.example.bar which is not listed in exports clauses will not be visible to other modules.