Java语言中的自定义类加载器实例解析

package com.lg.test;
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
/**
 * Created by 鏉庢灉 on 2016/8/6.
 */
public class FileClassLoader extends ClassLoader {
	String rootDir=null;
	public FileClassLoader(String rootDir) {
		this.rootDir = rootDir;
	}
	@Override
	  protected Class<?> findClass(String className) throws ClassNotFoundException {
		//首先检查是否已经被加载了。
		Class<?> c = findLoadedClass(className);
		String path = rootDir + "/" + className.replace('.', '/') + ".class";
		if (c != null) {
			return c;
		} else {
			/*双亲委托模式*/
			ClassLoader loaderParent = this.getParent();
			c = loaderParent.loadClass(className);
			if (c != null) {
				return c;
			} else {
				/*如果再不行的话，就再进行加载。因为字节码的本质就是一个字节数组*/
				InputStream is = null;
				ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
				try {
					is = new FileInputStream(path);
					byte[] buffer = new byte[1024];
					int len = 0;
					while ((len = is.read(buffer)) != -1) {
						outputStream.write(buffer, 0, len);
					}
					c = defineClass(className, buffer, 0, buffer.length);
				}
				catch (Exception e) {
					e.printStackTrace();
				}
				finally {
					if (is != null) {
						try {
							is.close();
						}
						catch (IOException e) {
							e.printStackTrace();
						}
					}
				}
			}
			return c;
		}
	}
}

package com.lg.test;
/**
 * Created by 鏉庢灉 on 2016/8/6.
 */
/*相同的类加载器对同一个类进行加载，得到的hascode是相同的
 * 不同的类加载器对同一类进行加载，得到的hascode是不一样的*/
public class Demo {
	public static void main(String[] args) {
		FileClassLoader loader = new FileClassLoader("c://myjava");
		FileClassLoader loader2=new FileClassLoader("c://myjava");
		try {
			Class<?> c = loader.findClass("com.lg.test.HelloWorld");
			Class<?> c0=loader.findClass("com.lg.test.HelloWorld");
			Class<?> c1=loader2.findClass("com.lg.test.HelloWorld");
			Class<?> c2=loader.findClass("com.lg.test.Demo01");
			Class<?> c3=loader.findClass("java.lang.String");
			System.out.println(c.hashCode());
			System.out.println(c.getClassLoader());
			System.out.println(c0.hashCode());
			System.out.println(c0.getClassLoader());
			System.out.println(c1.hashCode());
			System.out.println(c1.getClassLoader());
			System.out.println(c2.hashCode());
			System.out.println(c2.getClassLoader());
			System.out.println(c3.hashCode());
			System.out.println(c3.getClassLoader());
		}
		catch (ClassNotFoundException e) {
			e.printStackTrace();
		}
	}
}