// loadClass
protected Class<?> loadClass(String name, boolean resolve)
  throws ClassNotFoundException
{
  synchronized (getClassLoadingLock(name)) {
    // 首先检查当前类是否被加载
    Class<?> c = findLoadedClass(name);
    if (c == null) {
      long t0 = System.nanoTime();
      try {
        if (parent != null) {
          // 如果父类类加载器不为空，先尝试父类加载来加载
          c = parent.loadClass(name, false);
        } else {
          // 引导类加载器尝试加载
          c = findBootstrapClassOrNull(name);
        }
      } catch (ClassNotFoundException e) {
        // ClassNotFoundException thrown if class not found
        // from the non-null parent class loader
      }
      if (c == null) {
        // If still not found, then invoke findClass in order
        // to find the class.
        long t1 = System.nanoTime();
        // 尝试自己加载 
        c = findClass(name);
        // this is the defining class loader; record the stats
        sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
        sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
        sun.misc.PerfCounter.getFindClasses().increment();
      }
    }
    if (resolve) {
      resolveClass(c);
    }
    return c;
  }
}
// 类加载器的包含关系
public abstract class ClassLoader {
    private static native void registerNatives();
    static {
        registerNatives();
    }
    // 当前 ClassLoader 和 parent ClassLoader 的包含关系
    private final ClassLoader parent;
}

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
public class ClassLoaderTest extends ClassLoader {
    private static String rxRootPath;
    static {
        rxRootPath = "/temp/class/";
    }
    @Override
    public Class findClass(String name) {
        byte[] b = loadClassData(name);
        return defineClass(name, b, 0, b.length);
    }
    /**
     * 读取 .class 文件为字节数组
     *
     * @param name 全路径类名
     * @return
     */
    private byte[] loadClassData(String name) {
        try {
            String filePath = fullClassName2FilePath(name);
            InputStream is = new FileInputStream(new File(filePath));
            ByteArrayOutputStream bos = new ByteArrayOutputStream();
            byte[] buf = new byte[2048];
            int r;
            while ((r = is.read(buf)) != -1) {
                bos.write(buf, 0, r);
            }
            return bos.toByteArray();
        } catch (Throwable e) {
            e.printStackTrace();
        }
        return null;
    }
    /**
     * 全限定名转换为文件路径
     *
     * @param name
     * @return
     */
    private String fullClassName2FilePath(String name) {
        return rxRootPath + name.replace(".", "//") + ".class";
    }
    public static void main(String[] args) throws ClassNotFoundException {
        ClassLoaderTest classLoader = new ClassLoaderTest();
        String className = "com.test.TestAA";
        Class clazz = classLoader.loadClass(className);
        System.out.println(clazz.getClassLoader());
        // 输出结果 
        //cn.xxx.xxx.loader.ClassLoaderTest@3764951d
    }
}