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 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. The ClassLoader class uses a delegation model to search for classes and resources. Each instance of ClassLoader has an associated parent class loader. When called upon to find a class or resource, a ClassLoader instance 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 a ClassLoader instance. 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 the CLASSPATH environment 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 method defineClass converts an array of bytes into an instance of class Class. Instances of this newly defined class can be created using the newInstance method in class Class. 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 calls 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();
. . .
extends Object
A class loader is an object that is responsible for loading classes. The class ClassLoader is an abstract class. Given the 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. The ClassLoader class uses a delegation model to search for classes and resources. Each instance of ClassLoader has an associated parent class loader. When called upon to find a class or resource, a ClassLoader instance 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 a ClassLoader instance. 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 the CLASSPATH environment 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 method defineClass converts an array of bytes into an instance of class Class. Instances of this newly defined class can be created using the newInstance method in class Class. 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 calls 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();
. . .
下面分别说明：
Bootstrap ClassLoader用来加载核心类库，例如：rt.jar，jsse.jar等等！
Extension ClassLoader用来加载扩展类，即/lib/ext中的类。
AppClassLoader才是加载Classpath的。
ClassLoader加载类用的是委托模型。即先让Parent类(而不是Super，不是继承关系)寻找，Parent找不到才自己找。三者的关系为:AppClassLoader的Parent是ExtClassLoader，而ExtClassLoader的Parent为Bootstrap ClassLoader。加载一个类时，首先BootStrap先进行寻找，找不到再由ExtClassLoader寻找，最后才是AppClassLoader。
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