Demystifying Java Class Loading: From Load to Initialization

Overview

Any program must load bytecode into memory before the CPU can execute it. The JVM's ClassLoader loads .class files using the parent‑delegation model.

Class Loading Process

Loading consists of three steps: Load, Link, Initialize. The article breaks them down as follows.

Load

The ClassLoader reads the binary byte stream of a class (usually from the classpath) and creates a java.lang.Class object. It performs basic checks such as the magic number, constant‑pool size, and superclass existence.

Link

Linking includes Verification, Preparation, and Resolution.

Verification : ensures the bytecode conforms to the class‑file format and JVM constraints. It can be disabled with -Xverify:none for performance.

Preparation : allocates memory for static fields, assigns default values, and resolves symbolic references to direct references.

Resolution : replaces symbolic references in the constant pool with actual memory addresses.

Initialize

Executes static initializers and <clinit> code, recursively initializing super‑classes. If a class is loaded on demand, initialization occurs when it is first actively used.

Class vs. Array Loading

Arrays have a special type (e.g., Ljava.lang.String) and are created directly by the JVM; their element type is then loaded by the ClassLoader. Ordinary classes can be loaded by the bootstrap loader or custom loaders that override loadClass().

String[] str = new String[10];

Common Pitfalls and Tips

In JDK 9, Class.newInstance() is deprecated; use getDeclaredConstructor().newInstance() instead.

Access to private members can be forced with setAccessible(true); otherwise IllegalAccessException is thrown.

Verification is costly but optional for already‑trusted code; it can be skipped with -Xverify:none.

References

《码到成功》

《深入理解Java虚拟机》第三版