考虑以下接口:
public interface I { default String getProperty() { return "..."; } }
和只重用默认实现的实现类:
public final class C implements I { // empty }
每当C在JSP EL脚本上下文中使用的实例时:
C
<jsp:useBean id = "c" class = "com.example.C" scope = "request"/> ${c.property}
-我收到PropertyNotFoundException:
PropertyNotFoundException
javax.el.PropertyNotFoundException: Property 'property' not found on type com.example.C javax.el.BeanELResolver$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功能而言太旧了,但是我很惊讶地看到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此问题:
@Override public String getProperty() { return I.super.getProperty(); }
-但是为什么对Tomcat来说默认方法可能是个问题?
更新 :进一步的测试表明找不到默认属性,而可以找到默认方法,因此另一个解决方法(Tomcat 7+)是:
<jsp:useBean id = "c" class = "com.example.C" scope = "request"/> <%-- ${c.property} --%> ${c.getProperty()}
您可以通过创建ELResolver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可以扩展SimpleSpringBeanELResolver。这是Spring的实现,ELResolver但没有Spring,相同的想法应该是相同的。
ELResolver
SimpleSpringBeanELResolver
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。如果在接口上未找到bean prop签名,它将继续按照默认行为链发送它。
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ELResolver在您的应用程序中的某个地方注册。就我而言,我使用的是Spring的Java配置,因此我需要以下内容:
@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())); } }
我不能100%地确定这是否是添加我们的解析器的适当位置,但是它确实可以正常工作。您也可以在以下期间加载ELResolverjavax.servlet.ServletContextListener.contextInitialized
javax.servlet.ServletContextListener.contextInitialized
这是ELResolver参考资料:http : //docs.oracle.com/javaee/7/api/javax/el/ELResolver.html