在 JSP EL 中使用接口缺省方法时“在类型上找不到属性”

您可以通过创建处理默认方法的自定义实现来解决此问题。我在这里所做的实现扩展了。这是Spring的实现，但同样的想法应该是相同的，没有Spring。ELResolverSimpleSpringBeanELResolverELResolver

此类查找在 Bean 接口上定义的 Bean 属性签名，并尝试使用它们。如果在接口上找不到 Bean 属性签名，它将继续沿默认行为链向下发送。

import org.apache.commons.beanutils.PropertyUtils;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.access.el.SimpleSpringBeanELResolver;

import javax.el.ELContext;
import javax.el.ELException;
import java.beans.PropertyDescriptor;
import java.lang.reflect.InvocationTargetException;
import java.util.Optional;
import java.util.stream.Stream;

/**
 * Resolves bean properties defined as default interface methods for the ELResolver.
 * Retains default SimpleSpringBeanELResolver for anything which isn't a default method.
 *
 * Created by nstuart on 12/2/2016.
 */
public class DefaultMethodELResolver extends SimpleSpringBeanELResolver {
    /**
     * @param beanFactory the Spring BeanFactory to delegate to
     */
    public DefaultMethodELResolver(BeanFactory beanFactory) {
        super(beanFactory);
    }

    @Override
    public Object getValue(ELContext elContext, Object base, Object property) throws ELException {

        if(base != null && property != null) {
            String propStr = property.toString();
            if(propStr != null) {
                Optional<Object> ret = attemptDefaultMethodInvoke(base, propStr);
                if (ret != null) {
                    // notify the ELContext that our prop was resolved and return it.
                    elContext.setPropertyResolved(true);
                    return ret.get();
                }
            }
        }

        // delegate to super
        return super.getValue(elContext, base, property);
    }

    /**
     * Attempts to find the given bean property on our base object which is defined as a default method on an interface.
     * @param base base object to look on
     * @param property property name to look for (bean name)
     * @return null if no property could be located, Optional of bean value if found.
     */
    private Optional<Object> attemptDefaultMethodInvoke(Object base, String property) {
        try {
            // look through interfaces and try to find the method
            for(Class<?> intf : base.getClass().getInterfaces()) {
                // find property descriptor for interface which matches our property
                Optional<PropertyDescriptor> desc = Stream.of(PropertyUtils.getPropertyDescriptors(intf))
                        .filter(d->d.getName().equals(property))
                        .findFirst();

                // ONLY handle default methods, if its not default we dont handle it
                if(desc.isPresent() && desc.get().getReadMethod() != null && desc.get().getReadMethod().isDefault()) {
                    // found read method, invoke it on our object.
                    return Optional.ofNullable(desc.get().getReadMethod().invoke(base));
                }
            }
        } catch (InvocationTargetException | IllegalAccessException e) {
            throw new RuntimeException("Unable to access default method using reflection", e);
        }

        // no value found, return null
        return null;
    }

}

然后，您需要在应用程序中的某个位置注册您的。在我的情况下，我使用的是Spring的java配置，所以我有以下几点：ELResolver

@Configuration
...
public class SpringConfig extends WebMvcConfigurationSupport {
    ...
    @Override
    public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
        ...
        // add our default method resolver to our ELResolver list.
        JspApplicationContext jspContext = JspFactory.getDefaultFactory().getJspApplicationContext(getServletContext());
        jspContext.addELResolver(new DefaultMethodELResolver(getApplicationContext()));
    }
}

我不是100%确定这是否是添加我们的解析器的合适位置，但它确实工作得很好。您还可以在javax.servlet.ServletContextListener.contextInitialized

以下是参考：http://docs.oracle.com/javaee/7/api/javax/el/ELResolver.htmlELResolver

Spring 5删除了SimpleSpringBeanELResolver，因此上述答案不再有效，但事实证明，基类并没有真正做任何事情。下面的Spring 5版本，修复了上述评论中讨论的问题（使用XML配置）

public class DefaultMethodELResolver extends ELResolver {

    @Override
    public Object getValue(ELContext elContext, Object base, Object property) throws ELException {

        if (base != null && property != null) {
            String propStr = property.toString();
            if(propStr != null) {
                Optional<Object> ret = attemptDefaultMethodInvoke(base, propStr);
                if (ret != null) {
                    // notify the ELContext that our prop was resolved and return it.
                    elContext.setPropertyResolved(true);
                    return ret.orElse(null);
                }
            }
        }

        return null;
    }

    @Override
    public Class<?> getType(ELContext elContext, Object o, Object o1) {
        return null;
    }

    @Override
    public void setValue(ELContext elContext, Object o, Object o1, Object o2) {

    }

    @Override
    public boolean isReadOnly(ELContext elContext, Object o, Object o1) {
        return false;
    }

    @Override
    public Iterator<FeatureDescriptor> getFeatureDescriptors(ELContext elContext, Object o) {
        return null;
    }

    @Override
    public Class<?> getCommonPropertyType(ELContext elContext, Object o) {
        return Object.class;
    }

    /**
     * Attempts to find the given bean property on our base object which is defined as a default method on an interface.
     * @param base base object to look on
     * @param property property name to look for (bean name)
     * @return null if no property could be located, Optional of bean value if found.
     */
    private Optional<Object> attemptDefaultMethodInvoke(Object base, String property) {
        try {
            // look through interfaces and try to find the method
            for(Class<?> intf : ClassUtils.getAllInterfaces(base.getClass())) {
                // find property descriptor for interface which matches our property
                Optional<PropertyDescriptor> desc = Stream.of(PropertyUtils.getPropertyDescriptors(intf))
                        .filter(d->d.getName().equals(property))
                        .findFirst();

                // ONLY handle default methods, if its not default we dont handle it
                if(desc.isPresent() && desc.get().getReadMethod() != null && desc.get().getReadMethod().isDefault()) {
                    // found read method, invoke it on our object.
                    return Optional.ofNullable(desc.get().getReadMethod().invoke(base));
                }
            }
        } catch (InvocationTargetException | IllegalAccessException e) {
            throw new RuntimeException("Unable to access default method using reflection", e);
        }

        // no value found, return null
        return null;
    }

    public static class CustomResolverListener implements ServletContextListener {

        public void contextInitialized(ServletContextEvent event) {
            var jspContext =  JspFactory.getDefaultFactory().getJspApplicationContext(event.getServletContext());
            jspContext.addELResolver(new DefaultMethodELResolver());
        }

        public void contextDestroyed(ServletContextEvent event) {
        }
    }
}

然后在网络中.xml

<listener>
    <listener-class>DefaultMethodELResolver$CustomResolverListener</listener-class>
</listener>