Java class loading option

Class ClassLoader

A class loader is an object that is responsible for loading classes. The class ClassLoader is an abstract class. Given the binary name of a class, a class loader should attempt to locate or generate data that constitutes a definition for the class. A typical strategy is to transform the name into a file name and then read a «class file» of that name from a file system.

Every Class object contains a reference to the ClassLoader that defined it.

Class objects for array classes are not created by class loaders, but are created automatically as required by the Java runtime. The class loader for an array class, as returned by Class.getClassLoader() is the same as the class loader for its element type; if the element type is a primitive type, then the array class has no class loader.

Applications implement subclasses of ClassLoader in order to extend the manner in which the Java virtual machine dynamically loads classes.

Class loaders may typically be used by security managers to indicate security domains.

In addition to loading classes, a class loader is also responsible for locating resources. A resource is some data (a » .class » file, configuration data, or an image for example) that is identified with an abstract ‘/’-separated path name. Resources are typically packaged with an application or library so that they can be located by code in the application or library. In some cases, the resources are included so that they can be located by other libraries.

The ClassLoader class uses a delegation model to search for classes and resources. Each instance of ClassLoader has an associated parent class loader. When requested to find a class or resource, a ClassLoader instance will usually delegate the search for the class or resource to its parent class loader before attempting to find the class or resource itself.

Class loaders that support concurrent loading of classes are known as parallel capable class loaders and are required to register themselves at their class initialization time by invoking the ClassLoader.registerAsParallelCapable method. Note that the ClassLoader class is registered as parallel capable by default. However, its subclasses still need to register themselves if they are parallel capable. In environments in which the delegation model is not strictly hierarchical, class loaders need to be parallel capable, otherwise class loading can lead to deadlocks because the loader lock is held for the duration of the class loading process (see loadClass methods).

Run-time Built-in Class Loaders

• Bootstrap class loader. It is the virtual machine’s built-in class loader, typically represented as null , and does not have a parent.
• Platform class loader. The platform class loader is responsible for loading the platform classes. Platform classes include Java SE platform APIs, their implementation classes and JDK-specific run-time classes that are defined by the platform class loader or its ancestors. The platform class loader can be used as the parent of a ClassLoader instance. To allow for upgrading/overriding of modules defined to the platform class loader, and where upgraded modules read modules defined to class loaders other than the platform class loader and its ancestors, then the platform class loader may have to delegate to other class loaders, the application class loader for example. In other words, classes in named modules defined to class loaders other than the platform class loader and its ancestors may be visible to the platform class loader.
• System class loader. It is also known as application class loader and is distinct from the platform class loader. The system class loader is typically used to define classes on the application class path, module path, and JDK-specific tools. The platform class loader is the parent or an ancestor of the system class loader, so the system class loader can load platform classes by delegating to its parent.

Normally, the Java virtual machine loads classes from the local file system in a platform-dependent manner. However, some classes may not originate from a file; they may originate from other sources, such as the network, or they could be constructed by an application. The method defineClass converts an array of bytes into an instance of class Class . Instances of this newly defined class can be created using Class.newInstance .

The methods and constructors of objects created by a class loader may reference other classes. To determine the class(es) referred to, the Java virtual machine invokes the loadClass method of the class loader that originally created the class.

For example, an application could create a network class loader to download class files from a server. Sample code might look like:

ClassLoader loader = new NetworkClassLoader(host, port); Object main = loader.loadClass("Main", true).newInstance(); . . .

The network class loader subclass must define the methods findClass and loadClassData to load a class from the network. Once it has downloaded the bytes that make up the class, it should use the method defineClass to create a class instance. A sample implementation is:

class NetworkClassLoader extends ClassLoader < String host; int port; public Class findClass(String name) < byte[] b = loadClassData(name); return defineClass(name, b, 0, b.length); >private byte[] loadClassData(String name) < // load the class data from the connection . . . >>

Binary names

Any class name provided as a String parameter to methods in ClassLoader must be a binary name as defined by The Java Language Specification .

Examples of valid class names include:

"java.lang.String" "javax.swing.JSpinner$DefaultEditor" "java.security.KeyStore$Builder$FileBuilder$1" "java.net.URLClassLoader$3$1"

Any package name provided as a String parameter to methods in ClassLoader must be either the empty string (denoting an unnamed package) or a fully qualified name as defined by The Java Language Specification .

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Understanding Extension Class Loading

The extension framework makes use of the class-loading delegation mechanism. When the runtime environment needs to load a new class for an application, it looks for the class in the following locations, in order:

1. Bootstrap classes: the runtime classes in rt.jar, internationalization classes in i18n.jar, and others.
2. Installed extensions: classes in JAR files in the lib/ext directory of the JRE, and in the system-wide, platform-specific extension directory (such as /usr/jdk/packages/lib/ext on the Solaris™ Operating System, but note that use of this directory applies only to Java™ 6 and later).
3. The class path: classes, including classes in JAR files, on paths specified by the system property java.class.path. If a JAR file on the class path has a manifest with the Class-Path attribute, JAR files specified by the Class-Path attribute will be searched also. By default, the java.class.path property’s value is . , the current directory. You can change the value by using the -classpath or -cp command-line options, or setting the CLASSPATH environment variable. The command-line options override the setting of the CLASSPATH environment variable.

The precedence list tells you, for example, that the class path is searched only if a class to be loaded hasn’t been found among the classes in rt.jar, i18n.jar or the installed extensions.

Unless your software instantiates its own class loaders for special purposes, you don’t really need to know much more than to keep this precedence list in mind. In particular, you should be aware of any class name conflicts that might be present. For example, if you list a class on the class path, you’ll get unexpected results if the runtime environment instead loads another class of the same name that it found in an installed extension.

The Java Class Loading Mechanism

The Java platform uses a delegation model for loading classes. The basic idea is that every class loader has a «parent» class loader. When loading a class, a class loader first «delegates» the search for the class to its parent class loader before attempting to find the class itself.

Here are some highlights of the class-loading API:

• Constructors in java.lang.ClassLoader and its subclasses allow you to specify a parent when you instantiate a new class loader. If you don’t explicitly specify a parent, the virtual machine’s system class loader will be assigned as the default parent.
• The loadClass method in ClassLoader performs these tasks, in order, when called to load a class:
  1. If a class has already been loaded, it returns it.
  2. Otherwise, it delegates the search for the new class to the parent class loader.
  3. If the parent class loader does not find the class, loadClass calls the method findClass to find and load the class.
• The findClass method of ClassLoader searches for the class in the current class loader if the class wasn’t found by the parent class loader. You will probably want to override this method when you instantiate a class loader subclass in your application.
• The class java.net.URLClassLoader serves as the basic class loader for extensions and other JAR files, overriding the findClass method of java.lang.ClassLoader to search one or more specified URLs for classes and resources.

To see a sample application that uses some of the API as it relates to JAR files, see the Using JAR-related APIs lesson in this tutorial.

Class Loading and the java Command

The Java platform’s class-loading mechanism is reflected in the java command.

• In the java tool, the -classpath option is a shorthand way to set the java.class.path property.
• The -cp and -classpath options are equivalent.
• The -jar option runs applications that are packaged in JAR files. For a description and examples of this option, see the Running JAR-Packaged Software lesson in this tutorial.

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