Java 8 interface default method doesn't seem to declare property

问题

In my application I run into a problem that when a getter in a class is defaulted in an interface only (Java 8 feature), there is no Java Beans property as a result. I.e. for normal method invocation it works just as a standard method, but for access through "properties" it suddenly behaves differently...

Here is a test case:

import java.beans.Introspector;
import java.util.Arrays;
import java.util.stream.Collectors;
import org.apache.commons.beanutils.PropertyUtils;

public class test
{
    public static void main (String[] arguments) throws Exception
    {
        // Normal language-level invocation, works fine.
        System.out.println (new Bean1 ().getFoo ());
        System.out.println (new Bean2 ().getFoo ());

        // Printing Java Beans properties; Bean2 doesn't have 'foo' property...
        System.out.println (Arrays.stream (Introspector.getBeanInfo (Bean1.class).getPropertyDescriptors ())
                            .map ((property) -> property.getName ())
                            .collect (Collectors.joining (", ")));
        System.out.println (Arrays.stream (Introspector.getBeanInfo (Bean2.class).getPropertyDescriptors ())
                            .map ((property) -> property.getName ())
                            .collect (Collectors.joining (", ")));

        // First call behaves as expected, second dies with exception.
        System.out.println (PropertyUtils.getProperty (new Bean1 (), "foo"));
        System.out.println (PropertyUtils.getProperty (new Bean2 (), "foo"));
    }

    public interface Foo
    {
        default String getFoo ()
        {
            return "default foo";
        }
    }

    public static class Bean1 implements Foo
    {
        @Override
        public String getFoo ()
        {
            return "special foo";
        }
    }

    public static class Bean2 implements Foo
    { }
}

Result:

special foo
default foo
class, foo
class
special foo
Exception in thread "main" java.lang.NoSuchMethodException: Unknown property 'foo' on class 'class test$Bean2'
        at org.apache.commons.beanutils.PropertyUtilsBean.getSimpleProperty(PropertyUtilsBean.java:1257)
        at org.apache.commons.beanutils.PropertyUtilsBean.getNestedProperty(PropertyUtilsBean.java:808)
        at org.apache.commons.beanutils.PropertyUtilsBean.getProperty(PropertyUtilsBean.java:884)
        at org.apache.commons.beanutils.PropertyUtils.getProperty(PropertyUtils.java:464)
        at test.main(test.java:21)

Questions: do I do something wrong or is it a bug in Java? Is there a workaround other than never using defaulted methods (for getters/setters) in case you might need to access them as a "property" at some point later?

I always hated Java "properties by convention" that tend to break because you sneeze the wrong way.

回答1:

This seems to be indeed an erroneous omission in the Beans Introspector. Here is a workaround other than not using default methods:

public static void main (String[] arguments) throws Exception {
    testBean(new Bean1());
    System.out.println();
    testBean(new Bean2());
}
static void testBean(Object bean) throws Exception {
    PropertyDescriptor[] pd
        = Introspector.getBeanInfo(bean.getClass()).getPropertyDescriptors();
    System.out.println(Arrays.stream(pd)
        .map(PropertyDescriptor::getName).collect(Collectors.joining(", ")));
    for(PropertyDescriptor p: pd)
        System.out.println(p.getDisplayName()+": "+p.getReadMethod().invoke(bean));
}
public interface Foo {
    default String getFoo() {
        return "default foo";
    }
}
public static class Bean1 implements Foo {
    @Override
    public String getFoo() {
        return "special foo";
    }
}
public static class Bean2BeanInfo extends SimpleBeanInfo {
    private final BeanInfo ifBeanInfo;
    public Bean2BeanInfo() throws IntrospectionException {
        ifBeanInfo=Introspector.getBeanInfo(Foo.class);
    }
    @Override
    public BeanInfo[] getAdditionalBeanInfo() {
        return new BeanInfo[]{ifBeanInfo};
    }
}
public static class Bean2 implements Foo { }

class, foo
class: class helper.PropTest$Bean1
foo: special foo
class, foo
class: class helper.PropTest$Bean2
foo: default foo

回答2:

A quick work-around:

try {
    return PropertyUtils.getProperty(bean, property);
}
catch (NoSuchMethodException e) {
    String getMethod = "get" + property.substring(0, 1).toUpperCase() + property.substring(1);
    return MethodUtils.invokeMethod(bean, getMethod, new Object[]{});
}

回答3:

I don't know if my answer will be helpful, but I solved similar problem by using BeanUtils.getPropertyDescriptors(clazz) from Spring. It understands default methods.

来源：https://stackoverflow.com/questions/31703563/java-8-interface-default-method-doesnt-seem-to-declare-property

标签

java

java-8

javabeans

default-method