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Chapter 8 of Inside the Java Virtual Machine
The Linking Model
by Bill Venners

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Class Loaders and the Parent-Delegation Model

As mentioned in Chapter 3, "Security," version 1.2 introduced a formal parent-delegation model for class loaders. Although legacy class loaders written prior to 1.2 that don't take advantage of the parent- delegation model will still work in 1.2, the recommended way to create class loaders from 1.2 on is to use the parent-delegation model. Each user-defined class loader created in 1.2 is assigned a "parent" class loader when it is created. If the parent class loader is not passed explicitly to the constructor of the user-defined class loader, the system class loader is assigned to be the parent by default. Alternatively, a parent loader can be explicitly passed to the constructor of a new user-defined class loader. If a reference to an existing user- defined class loader is passed to the constructor, that user-defined class loader is assigned to be the parent. If null is passed to the constructor, the bootstrap class loader is assigned to be the parent.

To better visualize the parent-delegation model, imagine a Java application creates a user-defined class loader named "Grandma." Because the application passes null to Grandma's constructor, Grandma's parent is set to the bootstrap class loader. Time passes. Sometime later, the application creates another class loader named "Mom." Because the application passes to Mom's constructor a reference to Grandma, Mom's parent is set to the user-defined class loader referred to affectionately as Grandma. More time passes. At some later time, the application creates a class loader named, "Cindy." Because the application passes to Cindy's constructor a reference to Mom, Cindy's parent is set to the user- defined class loader referred to as Mom.

Now imagine the application asks Cindy to load a type named java.io.FileReader. When a class that follows the parent delegation model loads a type, it first delegates to its parent -- it asks its parent to try and load the type. Its parent, in turn, asks its parent, which first asks its parent, and so on. The delegation continues all the way up to the end-point of the parent-delegation chain, which is usually the bootstrap class loader. Thus, the first thing Cindy does is ask Mom to load the type. The first thing Mom does is ask Grandma to load the type. And the first thing Grandma does is ask the bootstrap class loader to load the type. In this case, the bootstrap class loader is able to load (or already has loaded) the type, and returns the Class instance representing java.io.FileReader to Grandma. Grandma passes this Class reference back to Mom, who passes it back to Cindy, who returns it to the application.

Note that given delegation between class loaders, the class loader that initiates loading is not necessarily the class loader that actually defines the type. In the previous example, the application initially asked Cindy to load the type, but ultimately, the bootstrap class loader defined the type. In Java terminology, a class loader that is asked to load a type, but returns a type loaded by some other class loader, is called an initiating class loader of that type. The class loader that actually defines the type is called the defining class loader for the type. In the previous example, therefore, the defining class loader for java.io.FileReader is the bootstrap class loader. Class Cindy is an initiating class loader, but so are Mom, Grandma, and even the bootstrap class loader. Any class loader that is asked to load a type and is able to return a reference to the Class instance representing the type is an initiating loader of that type.

For another example, imagine the application asks Cindy to load a type named com.artima.knitting.QuiltPattern. Cindy delegates to Mom, who delegates to Grandma, who delegates to the bootstrap class loader. In this case, however, the bootstrap class loader is unable to load the type. So control returns back to Grandma, who attempts to load the type in her custom way. Because Grandma is responsible for loading standard extensions, and the com.artima.knitting package is wisely installed in a JAR file in the standard extensions directory, Grandma is able to load the type. Grandma defines the type and returns the Class instance representing com.artima.knitting.QuiltPattern to Mom. Mom passes this Class reference back to Cindy, who returns it to the application. In this example, Grandma is the defining loader of the com.artima.knitting.QuiltPattern type. Cindy, Mom, and Grandma -- but not the bootstrap class loader -- are initiating class loaders for the type.

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