Java &引用；在类型“上找不到属性”；在JSP EL中使用接口默认方法时

考虑以下接口：

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

以及仅重新使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}

我最初的想法Tomcat6.0对于Java1.8功能来说太旧了，但我惊讶地看到Tomcat8.0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();
    }

--但为什么默认方法对Tomcat来说是个问题呢

更新：进一步的测试显示无法找到默认属性，而默认方法可以，因此另一个解决方法（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。我在这里所做的实现扩展了

SimpleSpringBeanELResolver

。这是ELResolver的Springs实现，但是没有Spring，同样的想法应该是一样的

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 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;
    }

}

我不是100%确定，如果这是适当的地方添加我们的解析器，但它确实工作得很好。您还可以在

javax.servlet.ServletContextListener.contextInitialized

---

以下是参考资料中的

ELResolver

：

我猜内省不适用于接口默认方法？我对答案很感兴趣：）你试过添加注释@FunctionInterface吗？@rickz:没有，原因有两个：1 IRL，我的接口有不止一个方法（因此没有资格被注释），2

@FunctionInterface

有不同的范围（几乎从未与

默认方法一起使用）：通常没有默认实现，并且有很多匿名实现。我已经厌倦了IntelliJ IDEA礼貌地提醒我，每次我碰巧声明一个方法接口时，我都应该用
@functioninterface
为接口添加注释（参见相关内容）：/我还没有尝试过这个方法，但是如果覆盖了默认方法，这个解析器会返回覆盖的值还是默认值？我希望它返回被覆盖的值。@battmanz我猜它应该返回被覆盖的值。然而，我没有一个快速的方法来测试这一点。如果没有，那么您可以随时更改属性解析器的行为。
<jsp:useBean id = "c" class = "com.example.C" scope = "request"/>
<%-- ${c.property} --%>
${c.getProperty()}

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;
    }

}

@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()));
    }
}