Class Loading Followed by Class Initialization

JVM Tutorials - Herong's Tutorial Examples

∟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.

herong> javac Address.java

herong> javac Account.java

herong> javac Client.java

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.

herong> del Account.class

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:

herong> del Address.class

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(...)