ClassLoader (Java SE 9 & JDK 9 ) (original) (raw)

public abstract class ClassLoader
extends Object
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

The Java run-time has the following built-in class loaders: 

    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 inClassLoader must be a binary name as defined byThe 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 inClassLoader must be either the empty string (denoting an unnamed package) or a fully qualified name as defined byThe Java™ Language Specification.
Since:
1.0
See Also:
resolveClass(Class)
See The Java™ Language Specification:
6.7 Fully Qualified Names, 13.1 The Form of a Binary