Class loaders are hierarchical. Classes are introduced into the JVM as they are referenced by name in a class that is already running in the JVM. So how is the very first class loaded? The very first class is specially loaded with the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the classes, which are already loaded and running. A class loader creates a namespace. All JVMs include at least one class loader that is embedded within the JVM called the primordial (or bootstrap) class loader. Now let’s look at non-primordial class loaders. The JVM has hooks in it to allow user defined class loaders to be used in place of primordial class loader. Let us look at the class loaders created by the JVM.

 

CLASS LOADER

Reloadable?

Explanation

Bootstrap
(primordial)
 
No
Loads JDK internal classes, java.* packages. (as defined in the sun.boot.class.path
system property, typically loads rt.jar and i18n.jar)

Extensions

 

No
Loads jar files from JDK extensions directory (as defined in the java.ext.dirs system
property – usually lib/ext directory of the JRE)

System

No
Loads classes from system classpath (as defined by the java.class.path property, which
is set by the CLASSPATH environment variable or –classpath or –cp command line
options)

 

 

 

 

 

 

 

 

 

Class loaders are hierarchical and use a delegation model when loading a class. Class loaders request their parent to load the class first before attempting to load it themselves. When a class loader loads a class, the child class loaders in the hierarchy will never reload the class again. Hence uniqueness is maintained. Classes loaded by a child class loader have visibility into classes loaded by its parents up the hierarchy but the reverse is not true.

Important: Two objects loaded by different class loaders are never equal even if they carry the same values, which mean a class is uniquely identified in the context of the associated class loader. This applies to singletons too, where each class loader will have its own singleton.


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Sandeep Joshi
Mathematics, Technology and Programming are my passion. I am a part of Java Ecosystem and through this blog, I contribute to it. I am here to blog about my interests, views and experiences.
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I feel proud to be listed as a "National Memory Record Holder" in the Limca Book of Records, 2009 and have attempted for an International Memory record in the Guiness Book of Records. I can remember the value of PI upto 10,000 digits after the decimal (3.1415.....). You can contact me on javagenious.com(At)gmal.com ; I would like to hear from you :)