auto

Java 8. Auto Loading

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Task: load application classes using a custom class loader (CustomClassLoader).

Solution

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Project with three classes:

• 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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java8 -Djava.system.class.loader=ru.ispras.j8.auto.CustomClassLoader -cp . ru.ispras.j8.auto.Main

or

java8 -Djava.system.class.loader=com.habr.j8.auto.CustomClassLoader -jar auto-1.0.jar

Output:

System ClassLoader is ru.ispras.j8.auto.CustomClassLoader@75b84c92
Main Class ClassLoader is ru.ispras.j8.auto.CustomClassLoader@75b84c92
Cat Class ClassLoader is ru.ispras.j8.auto.CustomClassLoader@75b84c92
Meow

Explanation

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• When a CustomClassLoader is created, its parent becomes sun.misc.Launcher$AppClassLoader;
• Since CustomClassLoader is a system class loader, all classes to be loaded are first delegated to it, it loads application classes, and the rest is transferred to sun.misc.Launcher$AppClassLoader.

Notes

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• If you do not define a public constructor in CustomClassLoader, the JVM initialization error will occur:

Error occurred during initialization of VM
java.lang.Error: java.lang.NoSuchMethodException: ru.ispras.j8.auto.CustomClassLoader.<init>(java.lang.ClassLoader)