jetbrick-commons/src/main/java/jetbrick/beans/ClassUtils.java
public static boolean available(String qualifiedClassName, ClassLoader loader) {
        return ClassLoaderUtils.loadClass(qualifiedClassName, loader) != null;
    }
public static boolean available(String qualifiedClassName, ClassLoader loader) {
        return ClassLoaderUtils.loadClass(qualifiedClassName, loader) != null;
    }
判断一个 Class 是否存在并且可用.