Compile and run java9 module program: part2

In the previous post we saw baiscs about java 9 modules like, what is module, how to create module project, module descriptor file and so on. In this blog we will learn how to compile and run java9 module program.

Recommended read: Java 9 module details: part 1

Java9 Module example for hello world

Before going further let’s take an example of hello world module,

Let’s dive deeper with an example, for better understanding we will create two modules. We will put the modules under the folder ‘project’.

First module, we will create is ‘com.module.util’ module. In this module we will create module-info.java and Greeting.java files.
Second module will be ‘com.module.app’ module. In this module we will create module-info.java and Main.java files.

Let’s create two folders for modules as ‘com.module.util’ and ‘com.module.app’ in the ‘project’ folder. These modules contain files Greeting.java and Main.java respectively. Both modules will have module-info.java at top level as shown below.

Note: com.module.util and com.module.app are the folder names

Code for the ‘Greeting.java’ in ‘com.module.util’ module is as below,

Code for the ‘Main.java’ in ‘com.module.app’ module is as below,

How to compile and run module java program

Now we will see how to run and compile module program. Even if some of the commands look complex don’t worry, once you understand they will be easy and simple.

How to compile and run java program without modules

Before looking into how to run java modular program, let’s understand how to run java package program without using module-info.java file.

To separate out our source files and class files we will use ‘–d’ option to set the destination directory for class files.

Let’s assume we want to run the above Greeting class which is present in the package ‘com.module.util’. And we will put the .class files under the ‘libs’ directory.

Once we execute the above command, we can see that the .class file is created under libs\com\module\util\Greeting.class

Now if we would like to run the main class created by above command, we have to specify path where jvm can find class files. Since our classes are present under libs folder we can specify path using –cp or -classpath.

OUTPUT

Greeting class is working

Compile and run java 9 module program

Now we have seen how to run java package program and put the classes into separate folder, to avoid mixing .java and .class files. We will use -d option for modular program as well.

Most important thing to remember is that, while compiling module, we have to compile .java source files and module descriptor file(module-info.java) .

In windows path are separated by semi-colon(;) and in Linux and mac using colons(:)

Most important new argument options

There are few new parameters/argument types introduced in java 9 for running the modular program that we must know.

1. module-path: Module-path option is used to specify where the modules are located. It is used at compile and run time.
   • At compile time it is used with javac option to specify path where the dependent modules can be found.
     Syntax : javac –module-path path1;path2
   • At runtime it is used to specify dependent module and module which is to run
     Syntax : java -module-path pathslist
     Note: we can use -p path as shortcut for –module-path path.
2. module: The –module argument is used to compile list of modules or run a module.
   • Syntax : javac –module path1;path2
   • Syntax : java –module module/class
     Note: we can use -m as shortcut for –module.
3. module-source-path: the argument –module-source-path us used to specify the root directory where the source files or packages are placed.
   Sometimes the projects are organized in such a way that the code is placed in special folder.
   For example src/main/java

Note: when we are using –module or –m option it is mandatory to use –module-source-path option, even if the source files are in same folder. 
  – – module-path == >  -p 
 – – module       == >  -m 

Example
D:\project> javac -d libs –module-source-path ./ –module com.module.app 
D:\project> java –module-path libs –module com.module.app/com.module.app.Main 

OUTPUT

Have a nice day

Different ways to compile and run java modules

1) We can specify all the files separated by space in javac command:

Warning: If you are using this option don’t compile multiple modules in same folder. Otherwise it will override module-info.class files

Syntax to compile:

Syntax to run:

Note: even in windows in java ‘/’ is used as separator for module and class

Example to compile com.module.util module

After this command we should see module-info.class and Greeting.class files are created

Now we can run and check our module using following command

Greeting class is working

Now lets run the Main class from app module which is dependent on util module which is compiled in libs path.

D:\project>java –module-path app;libs –module com.module.app/com.module.app.Main

Have a nice day

2) We can specify module names instead of java files:

This is recommended option to compile the applications. If we use this option, we can compile multiple modules in single directory as separate folder for each module with module name is created.

Also if required modules are in the same folder, they are automatically compiled even if they are not specified

Syntax to compile:

Example

After executing above command, following classes are created

Syntax to run:

Example

Have a nice day

Common mistakes in running modules

Let’s assume we would like to run Main class from com.module.app module.
Com.module.app module is dependent on com.module.util module.
Com.module.util module is compiled in libs folder
Com.module.app module is compiled in app folder.

When required modules is not specified in module path

Error java.lang.module.FindException: Module com.module.util not found, required by com.module.app

When module path of the module which is to be run is not specified in module path

Error: java.lang.module.FindException Module com.module.app not found

When we use the wrong slash(\) to run –correct way is module/class

Fast track reading

• module-path: used to specify where the modules are located
• module: used to specify modules list for compile or module to run
• module-source-path: used to specify the root directroy where the source files or packages are places
• If we use –-module or –m option it is mandatory to use –-module-source-path option
• short codes -p can be used for –-module-path and -m for –module
• Syntax to compile javac –d outputDir -p requiredModulesPath –module-source-path rootOfSOurceFiles -m modulesToBeCompiles
• Run syntax: java -p requiredModulesPath -m module/package.MainClass

Frequently asked question FAQ:

What is Module resolution process?

When we run the module program, at start jvm checks for all required modules. This process of finding all modules at start is call as Module resolution process.
We can print modules scanning process log by using command –-show-module-resolution in the run command.
Note: in module resolution process first main module is searched then it keep adding required module in tree form.

What are the java restricted keyword or contextual keyword?

Newly added 10 keywords, which are considered as keyword only in case of Module descriptor file(module-info.java), are called as restricted keywords.
This keywords are open, module, requires, transitive, exports, opens, to, uses, provides, and with.
For backward compatibility in every other cases it is considered as identifiers. Unlike other keywords we can use this keywords is variables or method name.

What is module descriptor file?

From java 9, special file with name module-info.java is requied in root folder of module, which specifies the metadata of module, this file is called as module desciptor file.

Can we export multiple package in one line?

No, To export multiple packages separate exports keyword is required.

Can we add multiple modules dependancy using single requries keyword?

No for separate module new requires keyword is required.

Related topics

• Java 9 module-introduction part1
• Jigsaw Project
• Renato Athaydes guide to java9 compile jar run

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