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Classloaders in Java

How Java class loaders find classes, delegate loading, and organize the runtime class hierarchy.

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  • ClassLoaders in Java dynamically load .class files into the JVM at runtime.
  • They follow a hierarchical structure with parent-child relationships.
  • Bootstrap ClassLoader (native, parent null) loads core JDK classes (e.g., java.lang.*).
  • Platform ClassLoader (Java 9+) loads modular platform classes.
  • Application ClassLoader loads classes from the application's CLASSPATH.
  • ClassLoaders use the parent-first delegation model.
  • On a load request, a ClassLoader first delegates to its parent.
  • If the parent fails to load the class, the child ClassLoader attempts to load it.
  • This ensures core classes are loaded only by trusted loaders.
  • The process involves loading (find bytes), linking (verify, prepare, resolve), and initialization.
  • Custom ClassLoaders can override loadClass() to change delegation behavior.
  • Classes loaded by different ClassLoaders are treated as distinct, even if identical bytecode.
  • Namespace isolation prevents class conflicts in modular applications (e.g., web servers).

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