A module is a named set of packages, resources and native libraries. Its a new Java entity that is similiar to an executable jar in its production state, but includes a name and a descriptor class that provides benefits that did not exist prior to Java 9.
The benefits of using modules are:
- Strong encapsulation at a higher level which improves security
- Modules can hide unwanted and internal details very safely, better security
- Ease of testing and maintainability
- Reliable dependencies which improves stability
- Supports better performance
- Its easy to support less coupling between components
- Its easy to support Single Responsibility Princple (SRP)
The smaller sets of code nicely packaged into modular jar files comprise the Modular JDK
The modular JDK kit is split into two sections SE and JDK
- SE - define the core java platform, the modules start with a 'java.' or 'javax.' prefixes
- JDK - are specifc to the JDK, the modules start with the 'jdk.' prefix, these are non-standard modules
Before Java 9 the JDK was a monolithic set of packages, packaged in one file (rt.jar), which any Java application would need to deploy along with its own code. Even if your application only ever used one class in a package, you still needed the rt.jar file which included the neccessary frameworks. Project Jigsaw was the name of the project to modularize the JDK.
The goals of project Jigsaw are:
- More stability, by allowing he JDK to more easily scale down to smaller platforms
- Improved security by protecting the internals of the JDK
- Improved maintainability by publishing dependencies and reducing the amount of interdependecies
- Improved application performance by allowing configuration flexibility
- Easier construction of and maintenance of libraries and large applications
A module graph is a diagram which attempts to visualize the module dependencies of a particular module, it only includes dependencies and not packages.
The java.base module is described as the foundational API of the Java SE Platform, which includes the core packages (java.lang, java.util, java.io, etc)
| Commandline tools to view Java modules | java --list-modules # view SE and JDK modules java --describe-module java.logging # get more details of a module (version, state, etc) jdeps --print-module-deps -m java.sql # view summary of dependencies of a specific module jdeps --list-reduced-deps --module java.sql # same as above but different formatting |
Declare modules and enable access between modules
Below is a simple module using intellij IDEA, the first screenshot is a normal Java not using modules
If you notice that when intellij runs the code you can see that it still uses the classpath parameter
By simply adding a module-info.java file in the src root folder and then rerunning the code you can see that now is uses the -p (path) and -m (module) parameters and thus is running as a module, there are a number of commandline tools that you can use to create jars, run Jars list dependencies, etc
| Execute the module | java -p out\production\ -m MyFirstModule/modular.HelloModularWorld java --module-path . --module MyFirstModule java -p . -m MyFirstModule # shorthand of above Note: there are a number of different parameters that you can use to execute the module |
| Create a Jar file | jar --create --file MyFirstModule.jar --main-class modular.HelloModularWorld -C out\production\MyFirstModule\ .
Note: the dot at the end means all files |
| List contents of a jar file | jar -f MyFirstModule.jar --list |
| List dependencies | jar -f MyFirstModule.jar -d java [--module-path|-p] . [--describe-module|-d] MyFirstModule Note: shortcut parameters can be used for example -p is the same as --module-path |
| List class dependencies | jdeps MyFirstModule.jar jdeps [-v|-s] --module-path .;out MyFirstModule # -v verbose and -s summary (good for module graph) jdeps --module-path .;out -m MyFirstModule jdeps --module-path .;out --check MyFirstModule Note: -p is not the same as --module-path in other commands, -p is package when using jdeps |
| List all modules that the code uses | java --list-modules -p MyFirstModule |
| Dry run checking module dependencies are satisfied | java --dry-run -p . -m MyFirstModule java --dry-run -p .;out -m MyFirstModule # if using multiple modules you can also add other directories as well Note: The code does not execute but checks all the dependencies |
| Compiling modules | javac -d out\production --module-source-path . -m MyFirstModule |
Describe how a modular project is compiled and run
The module-info.java file has a number of directives that can be used within the file as shown in the table below, I will only cover the basics whicxh is part of the OCA exam which are requires and exports
| Module directive | Example | Description |
| requires | requires org.modules.a; requires java.logging; requires transitive org.modules.d; requires static org.module.e; |
A module specified in the requires directive is one which exports packages that the current module may or does have a dependency on. The current module is said to read the module specified in the required directive. A requires directive with the transitive modifier allows any module which requires the current module to have an implicit requires directive on the specified module. A static directive requires the specified module at compile time but its optional at run time. |
| exports | exports org.pkg.base; exports org.pkg.util to org.module.a, org.module.b |
A package specified in the exports directive will expose its public and protected types, to modules that read the current module. The 'to' keyword make an exports directive qualified and will be followed by a comma delimited list of modules that are termed friends of the current module. Friends of a module have access to public and protected types of the exported package. No other modules will have access. You limit the exposure of the exported package types to its friends. |
So creating a 2 module example you can see below that one modules exports its package and the other requires a module, notice that we export the package and require the module
