Class Loading Followed by Class Initialization

This section provides a tutorial example showing that class initialization happens immediately after class loading.

When a class is loaded into the JVM, the class will be initialized immediately. The initialization process includes allocating static variables and creating objects used by those static variables.

In order to create objects for those static variables, other classes will be loaded too.

To verify this behavior, I wrote 3 classes:

Client.java - The primary class:

class Client {
   String name;
   static Address home = new Address();
   static Account checking = null;
}

Address.java - A supporting class for Client.java:

class Address {
   String street;
   String city;
   String country;
}

Account.java: Another supporting class for Client.java:

class Account {
   String status;
   double balance;
}

Let's compile all classes, then load the "Client" class. I see no problem.

C:\herong>javac Address.java

C:\herong>javac Account.java

C:\herong>javac Client.java

C:\herong>java -classpath . ClassCheck Client

Class: Client
Loader: jdk.internal.loader.ClassLoaders$AppClassLoader@1ff8b8f
Loaded from: file:/C:/herong/Client.class
All locations:
   file:/C:/herong/Client.class

Delete "Account.class", then load the "Client" class. I see no problem, because Account.class is not really needed to initialize the "Client" class.

C:\herong>del Account.class

C:\herong>java -classpath . ClassCheck Client

Class: Client
Loader: jdk.internal.loader.ClassLoaders$AppClassLoader@1ff8b8f
Loaded from: file:/C:/herong/Client.class
All locations:
   file:/C:/herong/Client.class

Delete "Address.class", then load the "Client" class. The class loader fails during "Client" class initialization:

C:\herong>del Address.class

C:\herong>java -classpath . ClassCheck Client

Exception in thread "main" java.lang.NoClassDefFoundError: Address
   at Client.<clinit>(Client.java:3)
   at java.base/java.lang.Class.forName0(Native Method)
   at java.base/java.lang.Class.forName(Unknown Source)
   at ClassChecker.main(ClassChecker.java:15)
Caused by: java.lang.ClassNotFoundException: Address
   at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(...)
   at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader
      .loadClass(...)
   at java.base/java.lang.ClassLoader.loadClass(...)   

This proves that class initialization happens immediately after class loading.

Last update: 2018.

Table of Contents

 About This Book

 JVM (Java Virtual Machine) Specification

 Java HotSpot VM - JVM by Oracle/Sun

 java.lang.Runtime Class - The JVM Instance

 java.lang.System Class - The Operating System

ClassLoader Class - Class Loaders

 What Is Class Loader

 What Is java.lang.ClassLoader Class

 Accessing the ClassLoader of a Class

 JVM "-verbose:class" Option

 loadClass() Method - Loading Classes Explicitly

 getSystemResource() Method - Finding Files

 Class Loading Problem - JAR Hell

 ClassChecker.java - Reports Class Loader

 ClassChecker.java - Reports Class Locations

 "superclass access check failed" Class Load Error

Class Loading Followed by Class Initialization

 Class Class - Class Reflections

 JVM Runtime Data Areas

 JVM Stack, Frame and Stack Overflow

 Thread Testing Program and Result

 CPU Impact of Multi-Thread Applications

 I/O Impact of Multi-Thread Applications

 CDS (Class Data Sharing)

 Micro Benchmark Runner and JVM Options

 Micro Benchmark Tests on "int" Operations

 Micro Benchmark Tests on "long" Operations

 Micro Benchmark Tests in JIT Compilation Mode

 Micro Benchmark Tests on "float" and "double" Operations

 JRockit JVM 28.2.7 by Oracle Corporation

 Outdated Tutorials

 References

 Full Version in PDF/EPUB