public static <T> T convertXmlToJavaBean(Class<T> clazz, String xmlString) throws Exception {
        try(StringReader stringReader = new StringReader(xmlString)) {
            return JAXB.unmarshal(stringReader, clazz);
        } catch (Exception e) {
            e.printStackTrace();
            throw new Exception("XML 格式错误!");
        }
    }

    public static <T> String convertJavabeanToXml(T bean) throws Exception {
        try(StringWriter stringWriter = new StringWriter()) {
            JAXB.marshal(bean, stringWriter);
            return stringWriter.toString();
        } catch (Exception e) {
            e.printStackTrace();
            throw new Exception(e.getMessage());
        }

    }

public static SAXParserFactory newInstance() {
    return FactoryFinder.find(
            /* The default property name according to the JAXP spec */
            SAXParserFactory.class,
            /* The fallback implementation class name */
            "com.sun.org.apache.xerces.internal.jaxp.SAXParserFactoryImpl");
} ``` find方法比较长，本次错误发生时候工厂实现类是由这句提供： ``` T provider = findServiceProvider(type); if (provider != null) {    return provider; } ``` 所以直接进到这个方法里面 ```
/*
 * Try to find provider using the ServiceLoader API
 *
 * @param type Base class / Service interface  of the factory to find.
 *
 * @return instance of provider class if found or null
 */
private static <T> T findServiceProvider(final Class<T> type) {
    try {
        return AccessController.doPrivileged(new PrivilegedAction<T>() {
            public T run() {
                final ServiceLoader<T> serviceLoader = ServiceLoader.load(type);
                final Iterator<T> iterator = serviceLoader.iterator();
                if (iterator.hasNext()) {
                    return iterator.next();
                } else {
                    return null;
                }
             }
        });
    } catch(ServiceConfigurationError e) {
        // It is not possible to wrap an error directly in
        // FactoryConfigurationError - so we need to wrap the
        // ServiceConfigurationError in a RuntimeException.
        // The alternative would be to modify the logic in
        // FactoryConfigurationError to allow setting a
        // Throwable as the cause, but that could cause
        // compatibility issues down the road.
        final RuntimeException x = new RuntimeException(
                "Provider for " + type + " cannot be created", e);
        final FactoryConfigurationError error =
                new FactoryConfigurationError(x, x.getMessage());
        throw error;
    }
} ``` 这个方法就是在当前线程的ContextClassLoader路径上找需要的类。