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External libraries example

In this example we include external libraries, which haven't been modularized so far. These jar files can be translated into so called [automatic modules](The State of the Module System) by javac. To get an idea of how those modules will look and which dependencies they will require you can use the jar tool:

$ jar -d --file=lib/junit-4.12.jar
No module descriptor found. Derived automatic module.

module [email protected] (automatic)
  requires mandated java.base
   contains junit.extensions
   contains junit.framework
   contains junit.runner
   contains junit.textui
   contains org.junit
   ...

This output tells us, that the automatic module will be called junit and will have the version number 4.12. It sadly doesn't tell us, that JUnit depends on Hamcrest. We can get a hint towards that using jdeps however:

$ jdeps -s lib/junit-4.12.jar 
junit-4.12.jar -> java.base
junit-4.12.jar -> java.management
junit-4.12.jar -> not found

That last not found entry tells us that there is some unknown dependency. The -s option requests that only the summary is printed. So to take a closer look, let's omit that:

$ jdeps lib/junit-4.12.jar | grep "not found"
junit-4.12.jar -> not found
   org.junit                                          -> org.hamcrest                                       not found
   org.junit.experimental.results                     -> org.hamcrest                                       not found
   org.junit.internal                                 -> org.hamcrest                                       not found
   org.junit.internal.matchers                        -> org.hamcrest                                       not found
   org.junit.matchers                                 -> org.hamcrest                                       not found
   org.junit.matchers                                 -> org.hamcrest.core                                  not found
   org.junit.rules                                    -> org.hamcrest                                       not found

Aha! Hamcrest it is. And we also see, that there are no other unknown dependencies from JUnit. Let's just check Hamcrest to be sure:

$ jar -d --file=lib/hamcrest-core-1.3.jar
o module descriptor found. Derived automatic module.

module [email protected] (automatic)
  requires mandated java.base
  contains org.hamcrest
  contains org.hamcrest.core
  contains org.hamcrest.internal

And

$ jdeps -s lib/hamcrest-core-1.3.jar
hamcrest-core-1.3.jar -> java.base

So, Hamcrest has no further dependencies.

Perform the below commands to see the contents of the respective sources contained in the src folder:

$ cat src/com.greetings/module-info.java
$ cat src/com.greetings/main/java/com/greetings/Main.java
$ cat src/com.greetings/main/java/com/greetings/Greet.java
$ cat src/com.greetings/test/java/com/greetings/GreetTest.java

As you can see in the module-info.java, we have added a dependency to JUnit using the module name retrieved with the above commands. Hamcrest is a transitive dependency and as such does not have to be added separately. Also, since JUnit will have to access our classes to run tests, we export com.greetings to the JUnit module.

Note: in case one of the below .sh script fails due to the tree command, please take a look at Download and install the tree and wget command section in the README.md file and apply the appropriate solution.

You can see the dependency information by running the command:

$ ./deps.sh

Try to compile the modules using the below command:

$ ./compile.sh

And we run the example with the following command:

$ ./run.sh

We should get the following output:

     *** Running from within the mods folder without arguments. *** 
    
    Hello World!
    
     *** Running from within the mods folder with arguments. *** 
    
    Hello Alice!
    Hello Bob!
    Hello Charlie!
    
     *** Running tests *** 
    
    JUnit version 4.12
    .....
    Time: 0.009
    
    OK (5 tests)

Check the contents of this script file (use the cat command or a text editor) to see what they are doing and why - interesting instructions and information in there.

See ../01_Greetings/README.md to learn more about package and module naming conventions and how to avoid confusions between them.