自定义类加载器

自定义加载器只需要继承 java.lang.ClassLoader 有两种核心方法，一种是loadClasss(String, boolean)，另一种实现父母分配机制的方法是findClass，默认实现是空的，所以我们自定义的加载器主要是重写findClass方法。

public class MyClassLoaderTest {    static class TestClassLoader extends ClassLoader {        private String classPath;        public TestClassLoader(String classPath) {            this.classPath = classPath;        }        private byte[] loadByte(String name) throws Exception {            name = name.replaceAll("\\.", "/");            FileInputStream fis = new FileInputStream(classPath + "/" + name + ".class");            int len = fis.available();            byte[] data = new byte[len];            fis.read(data);            fis.close();            return data;        }        protected Class<?> findClass(String name) throws ClassNotFoundException {            try {                byte[] data = loadByte(name);                return defineClass(name, data, 0, data.length);            } catch (Exception e) {                e.printStackTrace();                throw new ClassNotFoundException();            }        }        protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {            synchronized (getClassLoadingLock(name)) {                // First, check if the class has already been loaded                Class<?> c = findLoadedClass(name);                if (c == null) {                    // If still not found, then invoke findClass in order                    // to find the class.                    long t1 = System.nanoTime();                    if (!name.startsWith("org.example")) {                        c = getParent().loadClass(name);                    } else {                        c = findClass(name);                    }                    // this is the defining class loader; record the stats                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);                    sun.misc.PerfCounter.getFindClasses().increment();                }                if (resolve) {                    resolveClass(c);                }                return c;            }        }    }    public static void main(String args[]) throws Exception {        TestClassLoader classLoader = new TestClassLoader("F:/test");        Class clazz = classLoader.loadClass("org.example.Test");        Object obj = clazz.newInstance();        Method method = clazz.getDeclaredMethod("test", null);        Object invoke = method.invoke(obj, null);        System.out.println("=====" + invoke);        System.out.println(clazz.getClassLoader().getClass().getName());    }}