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CLASS LOADING
  • In Java, a class loader is responsible for loading classes into the JVM
  • When you run a Java program, the JVM doesn't load all classes at once
  • Instead, classes are loaded dynamically as they are needed during runtime
  • This process is handled by one or more class loaders.

Types of Class Loaders​

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Java uses a hierarchical class loading mechanism with the following types of class loaders

Bootstrap Class Loader​

  • The root class loader.
  • Loads core Java classes (e.g., classes in java.lang, java.util, etc.) from the rt.jar file in the JRE.
  • Written in native code (not Java).

Extension Class Loader​

  • Loads classes from the lib/ext directory of the JRE or other specified directories.
  • Handles extensions to the standard Java libraries.

Application Class Loader (System Class Loader)​

  • Loads classes from the application's classpath (e.g., your JAR files, -cp or -classpath arguments).
  • This is the default class loader for user-defined classes.

Custom Class Loaders​

  • Developers can create their own class loaders by extending the ClassLoader class.
  • Used in frameworks like flink_, Tomcat, or OSGi to implement custom class loading strategies (e.g., isolating user code from framework code)

How Class Loading Works​

Delegation Model​

  • Class loaders follow a parent-delegation model. When a class loader is asked to load a class, it first delegates the request to its parent class loader.
  • The parent class loader repeats the process until the request reaches the Bootstrap Class Loader.
  • If the parent class loader cannot find the class, the child class loader attempts to load it

Class Loading Steps​

  1. Loading: The class loader reads the .class file (bytecode) from the file system, network, or other sources.
  2. Linking:
    • Verification: Ensures the bytecode is valid and adheres to JVM specifications.
    • Preparation: Allocates memory for static fields and initializes them to default values.
    • Resolution: Resolves symbolic references to other classes, methods, and fields.
  • Initialization: Executes static initializers and assigns correct values to static fields.
  1. Class Loader Isolation:
    • Each class loader has its own namespace. Two classes loaded by different class loaders are considered different, even if they have the same fully qualified name.
    • This can lead to issues like ClassCastException or LinkageError if the same class is loaded by multiple class loaders.

Why Class Loading Matters​

  1. Modularity:
    • Class loaders allow applications to load classes dynamically, enabling features like plugins, hot deployment, and modular architectures.
  2. Isolation:
    • Frameworks like Apache Flink use custom class loaders to isolate user code from framework code. This prevents conflicts between user dependencies and framework dependencies.
  3. Security:
    • Class loaders can enforce security policies by controlling which classes are loaded and from where.

References​

  • Deepseek