Question

Java：如何编写自己的Java类加载器

Answer 1

给你简单介绍一下类加载器

1.类加载器就加载字节码文件(.class)

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;        }    }

}

/*

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();        }    }}