JavaSE_坚持读源码_ClassLoader对象_Java1.7

ClassLoader

 java.lang
 public abstract class ClassLoader
 extends Object

 //类加载器的责任就是加载类，说了跟没说一样
 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.
 //通过Class.getClassLoader()获取数组的类加载器和获取数组的元素的类加载器返回的是同样的加载器
 The class loader for an array class, as returned by Class.getClassLoader() is the same as the class loader for its element type;
 //如果数组的元素是基础数据类型的话，那这个数组没有类加载器(调用数组对象的Class对象的getClassLoader()方法返回值为null)
 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.
 //每个类加载器，都有一个关联的父-类加载器
 Each instance of ClassLoader has an associated parent class loader. When requested to find a class or resource,
 //当请求加载一个类时，类加载器会将请求交给父-类加载器，而不是自己去尝试加载类
 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.
 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).
 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 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"

 Since:
 1.0
 See Also:
 resolveClass(Class)