OCP11 – Understanding Modules

Java Platform Module System or JPMS was introduced in Java 9 as a form of encapsulation package.

A module is a group of one or more packages and a module-info.java file that contain its metadata.

In other words it consists in a ‘package of packages‘.

Benefits of using modules:

While using modules in a Java 9+ application is optional, there are a series of benefits from using them:

  • Better access control: Creates a fifth level of class access control that restricts packages to be available to outer code. Packages that are not explicitly exposed through module-info will be not available on modules external code. This is useful for encapsulation that allows to have truly internal packages.

  • Clear dependency management: Application’s dependencies will be specified in module-info.java file. This allows us to clearly identify which are the required modules/libraries.

  • Custom java builds: JPMS allow developers to specify what modules are needed. This makes it possible to create smaller runtime images discarding JRE modules that the application doesn’t need (AWT, JNI, ImageIO…).

  • Performances improvements: Having an static list of required modules and dependencies at start-up allows JVM to reduce load time and memory footprint because it allows the JVM which classes must be loaded from the beginning.

  • Unique package enforcement: A package is allowed to be supplied by only one module. JPMS prevents JAR hell scenarios such as having multiple library versions in the classpath.

The main counterpart is not all libraries have module support and, while it is possible it also makes more difficult to switch to a modular code that depends on this kind of libraries. For example, libraries that make an extensive use of reflection will need an extra configuration step because JPMS cannot identify and load classes at runtime.

Building modular applications

An application module usually is structured as it follows:

  • /mods folder: Contains the required third party modules as the /lib folder done in traditional Java applications.
  • /src folder: Contains the source code of the module as a set of one or more packages. Usually this folder is renamed with the same name as the module.
  • /src/module-info.java file: Module’s metadata file containing its definition. It’s placed in the root source code folder.

module-info.java file

The module-info.java file contains the description of current module’s metadata. This includes:

  1. the ability of mark packages as visible to outer modules
  2. list a set of modules that will be required by the current module as dependencies
  3. configure service providers
  4. enable features like reflection

The following example describes an application formed by multiple modules:

  • mod.external: A third party module that will provide some external functionality to the application.
  • mod.services: The module that will provide the service layer for the application.
  • mod.application: The top module and application’s entry point.
module mod.external
    exports com.foo.external.util;  // (1)
    exports com.foo.external.service;   // (2)
module mod.services
    requires mod.external;  // (3)
    // requires transitive mod.external;    // (3.1)

    exports com.bar.service to mod.application;     //(4)

    // (5)
    uses com.foo.external.service.IExternalService;
    provides com.foo.external.service.IExternalService with com.bar.service.impl.ExternalServiceImpl;

    // (6)
module mod.application
    requires mod.external;  // (7)
    requires mod.services;

export statement allows outer modules to use the classes of the specified package by defining them as visible.

By default, all module packages are hidden from the outside unless they are exported explicitly.

In the example, mod.external marks packages com.foo.external.util and com.foo.external.services as visible on (1) and (2). These packages can be used in mod.services requiring the mod.external module (3).

It’s also possible to export packages to specific target modules (4) so only these modules can access to them. In this example, com.bar.service will only be visible to ‘mod.application‘. Multiple modules can be specified separated by commas.

Modules can also be required in a transitive way as in (3.1). By using the transitive keyword, modules depending on mod.services also acquire the ability to use mod.external exported packages because mod.services also exports them transitively.

These modules can optionally declare its dependency on mod.external in addition to mod.services, but it’s not necessary.

In this example, if statement (3.1) was included in place of (3), statement (7) wouldn’t be necessary. Take into account that requiring multiple times the same module will produce a compilation error.

uses / provides(5) and opens (6) statements enables the use of the service provider mechanism and reflection, respectively.
Note: This features will be described in more detail in later posts.

In this example, mod.application module will be capable of consuming the following packages:

  • com.foo.external.util
  • com.foo.external.service
  • com.bar.service

Classpath vs Module path

The module path of an application is the directory or directories that contain modules. To maintain backward compatibility from Java 9, applications can still make use of the classpath that will contain all non-modularized code. Application code can access any type in classpath exposed via standard java access modifiers, such as public. Public types defined inside module path aren’t automatically available. Packages must in exported by the corresponding module.

Module types

A named module is a module containing a module-info.java file as described above and it’s in the module path. A named module that is not in module path is not considered a named module.

Along the named modules, there are other module types that must be taken into consideration:

System modules
Corresponding to Java SE and JDK modules. These are:

Modules in JRE
java.base java.naming java.smartcardio
java.compiler java.net.http java.sql
java.datatransfer java.prefs java.sql.rowset
java.desktop java.rmi java.transaction.xa
java.instrument java.scripting java.xml
java.logging java.se java.xml.crypto
java.management java.security.jgss  
java.management.rmi java.security.sasl  
Modules only in JDK
jdk.accessibility jdk.jconsole jdk.naming.dns
jdk.attach jdk.jdeps jdk.naming.rmi
jdk.charsets jdk.jdi jdk.net
jdk.compiler jdk.jdwp.agent jdk.pack
jdk.crypto.cryptoki jdk.jfr jdk.rmic
jdk.crypto.ec jdk.jlink jdk.scripting.nashorn
jdk.dynalink jdk.jshell jdk.sctp
jdk.editpad jdk.jsobject jdk.security.auth
jdk.hotspot.agent jdk.jstatd jdk.security.jgss
jdk.httpserver jdk.localdata jdk.xml.dom
jdk.jartool jdk.management jdk.zipfs
jdk.javadoc jdk.management.agent  
jdk.jcmd jdk.management.jfr  

Automatic modules

An automatic module is a normal jar file placed on the module path without a module-info.java file. In that case the list of exported packages is set to be all packages in the jar file and it will have full access to everything on the module path and the whole classpath.

The name of the module will be determined by the Automatic-Module-Name MANIFEST property or, if it’s not declared, derived from the name of the JAR following these rules:

  1. Remove file extension from jar file
  2. Remove any version info from the end of the file name, including labels like SNAPSHOT, RC, GA and similar. E.G: commons-1.0.0, commons-1.0.0-SNAPSHOT
  3. Replace characters others than letters and numbers by dots.
  4. Replace any sequence of multiple dots with a single dot.
  5. Remove dots if it’s the first or last character

For exemple, the module name for a jar named commons2-x-1.0.0-SNAPSHOT.jar will be resolved after aplying the following rules: (1) commons2-x-1.0.0-SNAPSHOT -> (2) commons2-x-> (3) commons2.x

Unnamed modules

For compatibility reasons, all code on the classpath is packaged up as a special unnamed module with no hidden packages and full access to the whole JDK.
The packages in unnamed module can only be read by the unnamed module. Unnamed module is only present if the application is executed using the --classpath option.

Build and run the application

Since its first release, the JDK includes multiple tools to compile (javac), package (jar) and run (java) applications.

Now, with the JPMS, new tools are included as well as new options for existing ones, making it possible to all of them to work with modules.

Compile modules

javac includes some new options to manage module compilation. The common way to compile a module is executing the following command:

javac --module-path <path> --d <path> <classes to compile>


  • --module-path, -p: Location of module dependencies JARs.
  • -d: Folder where *.class files will be generated.
  • classes to compile: Sources to compile. Multiple space separated sources can be specified and they can include wildcards. For example: javac -p mods -d build src/com/bar/service/*.java src/com/bar/service/impl/*.java src/module-info.java

Package modules

Modules can be packages using jar command as it follows:

jar --create --file=<jar file name> [--main-class <main class fully qualified name>] -C <path to build dir> .


  • --create, -c: Indicates that a jar will be created.
  • --file, f: The jar file name. Any directories you want the output JAR file to be under must already exist!.
  • -C: Tells the jar command to change directory to compiled module root directory and then include everything found in that directorydue to the following . argument (which signals "current directory").
  • --main-class: Specifies the fully qualified name of the entry point class of the jar file.

Run modules

To run packaged modules with the java command line tool, the following commands can be used:

# Run from build folder
java --module-path <build folder> --module <module name>/<main class fully qualified name>
# Run from jar (without Main class specified in Manifest)
java --module-path <modules folder> --module <module name>/<main class fully qualified name>
# Run from jar (with Main class specified in Manifest)
java --module-path <modules folder> --module <module name>


  • --module-path, -p: Location of modules. Multiple ; (Windows) or : (*nix) separated sources can be specified.
  • --module, -m: Module to execute. If the module’s jar Manifest doesn’t spcified a main class it must be appended to the module name separated by ‘/’

Inspecting modules

Several mechanisms were added to JDK tools in order to inspect and analyse module dependencies graph and resolution, including new tools and new options to the existing ones.

java command options
  • --show-module-resolution: Using this option when running a program with the java command shows used modules in the execution.
  • --list-modules: Lists application’s observable modules without actually running the application.
  • --describe-module, -d: Lists module’s qualified exports to an specific target module (export .. to), packages not exported (appearring as contains in the output), etc.
jar command options
  • --describe-module, -d: similar to javac‘s.
jdeps command options

Gives information about dependencies within a module.

It scans code in addition to module-info declarations so the listed results will include implicit dependencies (e.g. requires transient).
The available options for jdeps includes:

  • --module-path: Location of module dependencies JARs.
  • -summary, -s: Output summary info. If not included, the output will contain the full information.
  • --jdk-internals, -jdkinternals: List classes that use any internal unsupported API (sun.*). In the resulting list may also appear a suggestion to replace each use of the unsupported API.
jlink command options

Packages a executable module into an standalone application, including only the needed dependency modules.

The main use case of this tool is to generate a JVM distribution specially for an application that will only contain the needed JVM modules.
Its syntax is:

    jlink --module-path <modules folder> --add-modules <local modules to include> --output <output folder>


  • --module-path, -p: Location of modules. Multiple ; (Windows) or : (*nix) separated sources can be specified. This parameter must contain the %JAVA_HOME%\jmods location which is where JDK core modules are located. e.g. --module-path "mods;%JAVA_HOME%\jmods".
  • --add-modules: Comma separated list of local modules to include.
  • --output: Output folder where the resulting distribution will be generated.
jmods command options

Tool to create JMOD files and list the content of existing JMOD files.

While modularized applications are still packaged as JAR files, some resources cannot be included in this JAR files, such as native libraries or resources for example.

JMOD files are recommended only for package modules that contains this kind of stuff.

Its syntax is:

    jmod (create|extract|list|describe|hash) [--class-path <path>][--cmds <path>][--config <path>] <file name>


  • create: Creates a new JMOD archive file.
  • extract: Extracts all the files from the JMOD archive file.
  • list: Prints the names of all the entries.
  • describe: Prints the module details.
  • hash: Determines leaf modules and records the hashes of the dependencies that directly and indirectly require them.
  • --class-path: On jmod creation, specifies the location of application JAR files or a directory containing classes to copy into the resulting JMOD file.
  • --cmds: On jmod creation, specifies the location of native commands to copy into the resulting JMOD file.
  • --config: On jmod creation, specifies the location of user-editable configuration files to copy into the resulting JMOD file.
  • file name: Specifies the name of the JMOD file to create or from which to retrieve information.