Java

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The Java Classloader is a part of the Java Runtime Environment that dynamically loads Java classes into the Java Virtual Machine.[1] Usually classes are only loaded on demand. The Java run time system does not need to know about files and file systems because of classloaders. Delegation is an important concept to understand when learning about classloaders.

When the JVM is started, three class loaders are used:[3][4]

Bootstrap class loader

Extensions class loader

System class loader

The bootstrap class loader loads the core Java libraries[5] located in the /jre/lib directory. This class loader, which is part of the core JVM, is written in native code.

The extensions class loader loads the code in the extensions directories (/jre/lib/ext,[6] or any other directory specified by the java.ext.dirs system property). It is implemented by the sun.misc.Launcher$ExtClassLoaderclass.

The system class loader loads code found on java.class.path, which maps to the CLASSPATH environment variable. This is implemented by the sun.misc.Launcher$AppClassLoader class.

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