类加载器是怎么加载包下的类呢？

public class ClassUtil {
    
    private static final Logger LOGGER = LoggerFactory.getLogger(ClassUtil.class);
    
    /**
     * 获取类加载器
     * @return
     */
    public static ClassLoader getClassLoader() {
        //A
    }
    
    /**
     * 加载类
     * @param className
     * @param isInitialized
     * @return
     */
    public static Class<?> loadClass(String className, boolean isInitialized){
        //B
    }
    
    /**
     * 获取指定包名下的所有类
     * @param packageName
     * @return
     */
    public static Set<Class<?>> getClassSet(String packageName){
        //C
    }
}

public static ClassLoader getClassLoader() {
    return Thread.currentThread().getContextClassLoader();
}

public static Class<?> loadClass(String className, boolean isInitialized){
        Class<?> cls;
        try {
            cls = Class.forName(className, isInitialized, getClassLoader());
            ClassLoader.getSystemClassLoader().loadClass(className);
        }catch(ClassNotFoundException e) {
            LOGGER.error("load class failure", e);
            throw new RuntimeException(e);
        }
        return cls;
    }

    /**
     * Returns the {@code Class} object associated with the class or
     * interface with the given string name, using the given class loader.
     * Given the fully qualified name for a class or interface (in the same
     * format returned by {@code getName}) this method attempts to
     * locate, load, and link the class or interface.  The specified class
     * loader is used to load the class or interface.  If the parameter
     * {@code loader} is null, the class is loaded through the bootstrap
     * class loader.  The class is initialized only if the
     * {@code initialize} parameter is {@code true} and if it has
     * not been initialized earlier.*/
    @CallerSensitive
    public static Class<?> forName(String name, boolean initialize,
                                   ClassLoader loader)
        throws ClassNotFoundException
    {
        ....return forName0(name, initialize, loader, caller);
    }

    /** Called after security check for system loader access checks have been made. */
    private static native Class<?> forName0(String name, boolean initialize,
                                            ClassLoader loader,
                                            Class<?> caller)
        throws 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) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }
                ......
        }
    }

/**
* Returns a class loaded by the bootstrap class loader;
* or return null if not found.
*/
private Class<?> findBootstrapClassOrNull(String name)
{
  if (!checkName(name)) 
    return null;
  return findBootstrapClass(name);
}

/**
     * 获取指定包名下的所有类
     * @param packageName
     * @return
     */
    public static Set<Class<?>> getClassSet(String packageName){
        Set<Class<?>> classSet = new HashSet<Class<?>>();
        try {
            //将包名转化为文件路径
            Enumeration<URL> urls = getClassLoader().getResources(packageName.replace(".", "/"));
            while(urls.hasMoreElements()) {
                URL url = urls.nextElement();
                if(url != null) {
                    //获取文件的类型
                    String protocol = url.getProtocol();
                    //如果是"file"类型，就拿到这个文件里面的class文件
                    if(protocol.equals("file")) {
                        String packagePath = url.getPath().replaceAll("%20", " ");
                        addClass(classSet, packagePath, packageName);
                        //如果是'jar'类型的，先读取这个包里面的文件，如果jar中有'class'文件，则读取出来
                    }else if(protocol.equals("jar")) {
                        JarURLConnection jarURLConnection = (JarURLConnection)url.openConnection();
                        if(jarURLConnection != null) {
                            JarFile jarFile = jarURLConnection.getJarFile();
                            if(jarFile != null) {
                                Enumeration<JarEntry> jarEntries = jarFile.entries();
                                while(jarEntries.hasMoreElements()) {
                                    JarEntry jarEntry = jarEntries.nextElement();
                                    String jarEntryName = jarEntry.getName();
                                    if(jarEntryName.endsWith(".class")) {
                                        String className = jarEntryName.substring(0, jarEntryName.lastIndexOf(".")).replaceAll("/", ".");
                                        doAddClass(classSet, className);
                                    }
                                }
                            }
                        }
                    }
                }
            }
        }catch(Exception e) {
            LOGGER.error("get class set failure", e);
            throw new RuntimeException(e);
        }
        
        return classSet;
    }

    private static void doAddClass(Set<Class<?>> classSet, String className) {
        Class<?> cls = loadClass(className, false);
        classSet.add(cls);
        
    }

    private static void addClass(Set<Class<?>> classSet, String packagePath, String packageName) {
        File[] files = new File(packagePath).listFiles(new FileFilter() {

            @Override
            public boolean accept(File file) {
                return (file.isFile() && file.getName().endsWith(".class")) || file.isDirectory();
            }
            
        });
        
        for(File file: files) {
            String fileName = file.getName();
            if(file.isFile()) {
                String className = fileName.substring(0, fileName.lastIndexOf("."));
                if(StringUtil.isNotEmpty(packageName)) {
                    className = packageName + "." + className; 
                }
                doAddClass(classSet, className);
            }else {
                String subPackagePath = fileName;
                if(StringUtil.isNotEmpty(packagePath)) {
                    subPackagePath = packagePath + "/" + subPackagePath;
                }
                String subPackageName = fileName;
                if(StringUtil.isNotEmpty(packageName)) {
                    subPackageName = packageName + "." +subPackageName;
                }
            }
        }
    }