Issue
As the title says, I'm looking for a simple example in github that contains a code example that shows how to use the [the maven-shade-plugin] to rename classes when two versions of a library must be included in an application.
I found this answer on SO: href="https://stackoverflow.com/a/13620420/3281336">https://stackoverflow.com/a/13620420/3281336 which shows exactly what I'm trying to do and I'm looking for a github repo that has that code example in it.
In the mean time, I'm starting to create my on version of this example and will post what I've created if no one points one out to me.
I have found several web pages that describe the plugin but nothing that has included a pointer to a repo with sample code.
Some useful writes up on the maven-shade-plugin
The plugin documentation: https://maven.apache.org/plugins/maven-shade-plugin/examples/class-relocation.html
What is the maven-shade-plugin used for, and why would you want to relocate Java packages? - the answer that included the diagram that shows how maven-shade-plugin allows two versions of a library to be included in an application.
In the same question this answer was great because it gave several real-life examples of how and why the maven-shade-plugin is used: https://stackoverflow.com/a/19386712/3281336
What is the maven-shade-plugin used for, and why would you want to relocate Java packages?
Difference between maven plugins ( assembly-plugins , jar-plugins , shaded-plugins) - differences between the different plugins to create a JAR executable.
Searching on google there were several articles I found that explained how to use the maven-shade-plugin. Some were:
https://howtodoinjava.com/maven/maven-shade-plugin-create-uberfat-jar-example/
https://mkyong.com/maven/create-a-fat-jar-file-maven-shade-plugin/
Solution
I created a github repo so others can learn/benefit from this question and answer.
The github repo is public at: https://github.com/steranka/maven-shade-example
The Readme.md explains the solution. Some if it is documented here in the event that goes away (some day).
Overview of the application
This is an example that illustrates the problem that occurs in most large projects and how the maven-shade-plugin works to fix the problem.
This code example was developed in order to allow me to understand how the maven-shade-plugin works with a real code but very simplified.
The idea of the simplicity was inspired by the StackOverflow answer: What is the maven-shade-plugin used for, and why would you want to relocate Java packages?
which explains why the maven-shade-plugin is needed and the problem it solves.
In this code the following JAR files exist
helloworld.jar - A Java executable jar that is the main program.
loglib.jar - There are two versions of this jar file (in different directories).
goodlib.jar - This is another JAR file that uses version 1 of loglib.jar (dir loglibv1).
Overview of the Dependency Tree
The following diagram shows the dependency tree for the helloworld application.
Each of the pom files create a JAR file so there are 4 (four) jar files created by this code example.
Summary of the Problem
The next diagram shows the loglib.jar file being used in both the main application (helloworld)
It shows the class LogIt contains a method sayHello which takes one argument String name in version 1.0.0 of the package (liblog.jar)
and sayHello takes two arguments in the 2nd version of the package.
While this example is contrived to demonstrate the general problem that faces Java developers on more complex code bases, this simplified version of the problem is intended to make the problem easy to understand and to fix.
So by default, when a FAT jar is created for the HelloWorld application, the Fat jar only contains one copy of the class file com.steranka.play.LogIt.
The result is that either the HelloWorld class will crash or the GoodFeature class will crash. Where crash means throw an exception
Below is an example of the exception I saw when I first ran the application.
Hello World!
What's up, Sam
Exception in thread "main" java.lang.NoSuchMethodError: 'java.lang.String com.steranka.play.LogIt.sayHello(java.lang.String)'
at com.steranka.play.GoodFeature.sayGoodbye(GoodFeature.java:6)
at com.steranka.play.HelloWorldApp.main(HelloWorldApp.java:15)
This problem occurs because version 2.0.0 of the loglib.jar was used which contained the signature:
sayHello(String name, String greeting)
and the code in GoodFeature.sayGoodbye had bytecode which called the version 1.0.0 of the signature which was:
sayHello(String name)
Since that signature did not exist, the exception occured.
The Solution
The solution is to include both versions of the loglib jar file as shown in the next diagram.
Next Steps
If you want to follow along with what I did, and learn how I solved this problem continue here with the doc/01-Starting.md file.
Answered By - PatS
Answer Checked By - David Marino (JavaFixing Volunteer)
