java - 在 JSP EL 中使用接口(interface)默认方法时出现 "Property not found on type"

Question

考虑以下接口(interface):

public interface I {
    default String getProperty() {
        return "...";
    }
}

和只是重用默认实现的实现类:

public final class C implements I {
    // empty
}

每当在 JSP EL 脚本上下文中使用 C 的实例时:

<jsp:useBean id = "c" class = "com.example.C" scope = "request"/>
${c.property}

-- 我收到一个 PropertyNotFoundException:

javax.el.PropertyNotFoundException: Property 'property' not found on type com.example.C
    javax.el.BeanELResolver$BeanProperties.get(BeanELResolver.java:268)
    javax.el.BeanELResolver$BeanProperties.access$300(BeanELResolver.java:221)
    javax.el.BeanELResolver.property(BeanELResolver.java:355)
    javax.el.BeanELResolver.getValue(BeanELResolver.java:95)
    org.apache.jasper.el.JasperELResolver.getValue(JasperELResolver.java:110)
    org.apache.el.parser.AstValue.getValue(AstValue.java:169)
    org.apache.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:184)
    org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate(PageContextImpl.java:943)
    org.apache.jsp.index_jsp._jspService(index_jsp.java:225)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:438)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

我最初的想法是 Tomcat 6.0 对于 Java 1.8 的功能来说太旧了，但我惊讶地发现 Tomcat 8.0 也受到了影响。当然，我可以通过显式调用默认实现来解决这个问题:

    @Override
    public String getProperty() {
        return I.super.getProperty();
    }

-- 但到底为什么默认方法可能成为 Tomcat 的问题？

更新:进一步测试显示无法找到默认属性，而可以找到默认方法，因此另一种解决方法 (Tomcat 7+) 是:

<jsp:useBean id = "c" class = "com.example.C" scope = "request"/>
<%-- ${c.property} --%>
${c.getProperty()}

Answer 1

您可以通过创建处理默认方法的自定义 ELResolver 实现来解决此问题。我在这里所做的实现扩展了 SimpleSpringBeanELResolver。这是 ELResolver 的 Springs 实现，但如果没有 Spring，相同的想法应该是相同的。

此类查找定义在 bean 接口(interface)上的 bean 属性签名并尝试使用它们。如果在接口(interface)上没有找到 bean prop 签名，它将继续沿着默认行为链发送它。

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;
    }

}

然后您需要在您的应用程序中的某个地方注册您的ELResolver。在我的例子中，我使用的是 Spring 的 java 配置，所以我有以下内容:

@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% 确定那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

期间加载 ELResolver

这是引用中的 ELResolver:http://docs.oracle.com/javaee/7/api/javax/el/ELResolver.html