Java 8接口默认方法似乎没有声明属性 [英] Java 8 interface default method doesn't seem to declare property
问题描述
在我的应用程序中,我遇到一个问题,当类中的getter仅在接口中默认(Java 8特性)时,结果没有Java Beans属性。即对于普通方法调用,它只是作为标准方法工作,但是对于通过属性访问它突然表现不同......
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...
这是一个测试用例:
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
{ }
}
结果:
special foo
default foo
class, foo
class
special foo
Exception in thread "main" java.lang.NoSuchMethodException: Unknown property 'foo' on class 'class test$Bean2'
at org.apache.commons.beanutils.PropertyUtilsBean.getSimpleProperty(PropertyUtilsBean.java:1257)
at org.apache.commons.beanutils.PropertyUtilsBean.getNestedProperty(PropertyUtilsBean.java:808)
at org.apache.commons.beanutils.PropertyUtilsBean.getProperty(PropertyUtilsBean.java:884)
at org.apache.commons.beanutils.PropertyUtils.getProperty(PropertyUtils.java:464)
at test.main(test.java:21)
问题:我做错了什么或是不是一个bug在Java?是否有一种解决方法,除了从不使用默认方法(对于getter / setter),以防你以后需要在某个时候将它们作为属性访问?
Questions: do I do something wrong or is it a bug in Java? Is there a workaround other than never using defaulted methods (for getters/setters) in case you might need to access them as a "property" at some point later?
我总是讨厌Java按惯例的属性,因为你打喷嚏的方式很容易打破。
推荐答案
这似乎确实是Beans Introspector
中的错误遗漏。除了不使用默认
方法之外,还有一个解决方法:
This seems to be indeed an erroneous omission in the Beans Introspector
. Here is a workaround other than not using default
methods:
public static void main (String[] arguments) throws Exception {
testBean(new Bean1());
System.out.println();
testBean(new Bean2());
}
static void testBean(Object bean) throws Exception {
PropertyDescriptor[] pd
= Introspector.getBeanInfo(bean.getClass()).getPropertyDescriptors();
System.out.println(Arrays.stream(pd)
.map(PropertyDescriptor::getName).collect(Collectors.joining(", ")));
for(PropertyDescriptor p: pd)
System.out.println(p.getDisplayName()+": "+p.getReadMethod().invoke(bean));
}
public interface Foo {
default String getFoo() {
return "default foo";
}
}
public static class Bean1 implements Foo {
@Override
public String getFoo() {
return "special foo";
}
}
public static class Bean2BeanInfo extends SimpleBeanInfo {
private final BeanInfo ifBeanInfo;
public Bean2BeanInfo() throws IntrospectionException {
ifBeanInfo=Introspector.getBeanInfo(Foo.class);
}
@Override
public BeanInfo[] getAdditionalBeanInfo() {
return new BeanInfo[]{ifBeanInfo};
}
}
public static class Bean2 implements Foo { }
class, foo
class: class helper.PropTest$Bean1
foo: special foo
class, foo
class: class helper.PropTest$Bean2
foo: default foo
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