How to do it...

1. Compile the code and place the compiled classes in a directory, say, mods:

      javac -d mods --module-source-path . $(find . -name *.java)

2. Build a modular JAR for the math.util module:

      jar --create --file=mlib/math.util@1.0.jar --module-version 1.0
      -C mods/math.util .

3. Build a modular JAR for the calculator module, specifying the main class to make the JAR executable:

      jar --create --file=mlib/calculator@1.0.jar --module-version 1.0 
      --main-class com.packt.calculator.Calculator -C mods/calculator .

The critical piece in the preceding command is the --main-class option. This enables us to execute the JAR without providing the main class information during execution.

4. Now, we have two JARs in the mlib directory: math.util@1.0.jar and calculator@1.0.jar. These JARs are called modular JARs. If you want to run the example, you can use the following command:

      java -p mlib -m calculator

5. A new command-line option for the JAR command has been introduced in Java 9, called -d , or  --describe-module. This prints the information about the module that the modular JAR contains:

jar -d --file=mlib/calculator@1.0.jar

The output of jar -d for calculator@1.0.jar is as follows: 

calculator@1.0
  requires mandated java.base
  requires math.util
  conceals com.packt.calculator
  main-class com.packt.calculator.Calculator

jar -d --file=mlib/math.util@1.0.jar

The output of jar -d for math.util@1.0.jar is as follows:

math.util@1.0
  requires mandated java.base
  exports com.packt.math

We have provided the following scripts to try out the recipe code on Windows:

• compile-math.bat
• compile-calculator.bat
• jar-math.bat
• jar-calculator.bat
• run.bat

We have provided the following scripts to try out the recipe code on Linux:

• compile.sh
• jar-math.sh
• jar-calculator.sh
• run.sh

You have to run the scripts in the order they have been listed.