在JSP EL中使用接口默认方法时,java – “在类型上找不到属性”

考虑以下界面：

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

和刚刚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Tomcat 6.0对于Java 1.8功能来说太旧了,但是我很惊讶,看到Tomcat 8.0也受到影响.当然,我可以通过明确地调用默认实现来解决这个问题：

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

– 但是为什么地球上的默认方法可能是Tomcat的问题？

更新：进一步测试显示默认属性无法找到,而默认方法可以,所以另一个解决方法(Tomcat 7)是：

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

您可以通过创建一个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Springs实现的ELResolver,但同样的想法应该是一样的没有Spring.

该类查找在bean的接口上定义的Bean属性签名,并尝试使用它们.如果在接口上没有发现bean prop签名,它会继续将其发送给默认行为链.

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注册您的ELResolver.在我的情况下,我使用的是Spring的java配置,所以我有以下几点：

@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％肯定如果这是适当的地方添加我们的解析器,但它的工作正常.您也可以在javax.servlet.ServletContextListener.contextInitialized期间加载ELResolver

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