Understand ClassLoader in Java

A class loader is an object that is responsible for loading classes. The classClassLoaderis an abstract class. Given thebinary nameof 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.

EveryClassobject contains areferenceto theClassLoaderthat defined it.

Classobjects 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 byClass.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 ofClassLoaderin 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.

TheClassLoaderclass uses a delegation model to search for classes and resources. Each instance ofClassLoaderhas an associated parent class loader. When requested to find a class or resource, aClassLoaderinstance will delegate the search for the class or resource to its parent class loader before attempting to find the class or resource itself. The virtual machine's built-in class loader, called the "bootstrap class loader", does not itself have a parent but may serve as the parent of aClassLoaderinstance.

Class loaders that support concurrent loading of classes are known asparallel capableclass loaders and are required to register themselves at their class initialization time by invoking theClassLoader.registerAsParallelCapablemethod. Note that theClassLoaderclass 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 (seeloadClassmethods).

Normally, the Java virtual machine loads classes from the local file system in a platform-dependent manner. For example, on UNIX systems, the virtual machine loads classes from the directory defined by theCLASSPATHenvironment variable.

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 methoddefineClassconverts an array of bytes into an instance of classClass. Instances of this newly defined class can be created usingClass.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 theloadClassmethod 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 methodsfindClassandloadClassDatato load a class from the network. Once it has downloaded the bytes that make up the class, it should use the methoddefineClassto 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
             ...
         }
     }