java17style

Java 17. Auto Loading (Java 17 style)

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Task: load application classes using a custom class loader (CustomClassLoader) in Java 8 style (with module-info.java).

Solution

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Modules (declared via module-info.java):

• auto.onemodule.j17style:
  • Main - a class containing a main method creating an instance of the Cat class and calling the Cat::talk method;
  • Cat - loadable class with the talk method, which prints the string "Meow" to stdout;
  • CustomClassLoader - a class that is an implementation of a custom class loader.

To change the system loader to a custom CustomClassLoader in documentation of the ClassLoader.getSystemClassLoader() method it is written that for this it is necessary for the JVM to pass the name of the new system class loader through the argument java.system.class.loader, and also to define a public constructor with a parameter of type ClassLoader, which will be passed as AppClassLoader when created.

Run

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Using classpath the results are the same as in [[Java 17. Auto Loading (Java 8 style)]].

Using modulepath:

java17 -Djava.system.class.loader=ru.ispras.j17.auto.onemodule.j17style.CustomClassLoader \
-p . -m auto.onemodule.j17style

Output:

Error occurred during initialization of VM
java.lang.Error: class java.lang.ClassLoader (in module java.base) cannot access class ru.ispras.j17.auto.onemodule.j17style.CustomClassLoader (in module auto.onemodule.j17style) because module auto.onemodule.j17style does not export ru.ispras.j17.auto.onemodule.j17style to module java.base

We get an error because the java.base module cannot access the CustomClassLoader class. Solved by simply exporting the package to module-info.java:

// module-info.java
module auto.onemodule.j17style {
    exports ru.ispras.j17.auto.onemodule.j17style to java.base;
}

New output:

Main Class Module is module auto.onemodule.j17style
Cat Class Module is module auto.onemodule.j17style
System ClassLoader is ru.ispras.j17.auto.onemodule.j17style.CustomClassLoader@76ed5528
Main Class ClassLoader is jdk.internal.loader.ClassLoaders$AppClassLoader@4554617c
Cat Class ClassLoader is jdk.internal.loader.ClassLoaders$AppClassLoader@4554617c
Meow

Explanation

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• Regarding Java 17. Auto Loading (Java 8 style) is different with the module name manual.onemodule.j17style in the jar file, which is anchored with module-info .java, so this module is considered an Application Module, and not an Automatic Module.

Notes

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Same as in Java 17. Auto Loading (Java 8 style).