在JSP EL中使用接口默认方法时,“在类型上找不到属性”
请考虑以下界面:
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:
${c.property}
– 我收到一个PropertyNotFoundException
:
javax.el.PropertyNotFoundException: Property 'property' not found on type com.example.C javax.el.BeanELResolver$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 1.8function来说太旧了,但我很惊讶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(); }
– 但是为什么默认方法可能是Tomcat的问题?
更新 :进一步测试显示无法找到默认属性,而默认方法可以,因此另一种解决方法(Tomcat 7+)是:
${c.getProperty()}
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。 我在这里做的实现扩展了SimpleSpringBeanELResolver
。 这是ELResolver
Springs实现,但没有Spring,同样的想法应该是相同的。
此类查找在bean的接口上定义的bean属性签名,并尝试使用它们。 如果在接口上找不到bean prop签名,它将继续向下发送默认行为链。
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
然后,您ELResolver
在应用程序中的某个位置注册ELResolver
。 在我的情况下,我使用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%肯定,如果这是添加我们的解析器的适当的地方,但它确实工作得很好。 您还可以在javax.servlet.ServletContextListener.contextInitialized
期间加载ELResolver。
以下是ELResolver
的参考: http : ELResolver