我正在做一些元编程,其中解析 JAXB bean,特别是 XmlAdapter
注释。我遇到了这样的情况:Eclipse Oxygen (4.7.2) 的编译器对某些使用类型推断的方法完全满意,但 OpenJDK 编译器 (javac 1.8.0_131) 对它们感到窒息。
我已将主要结构提取到此 MWE 中:
import java.time.LocalDate;
import javax.xml.bind.annotation.adapters.XmlAdapter;
public class Main<BoundType>
{
public void parse( Class<? extends XmlAdapter<?, BoundType>> adapterClass ) throws Exception
{
process( adapterClass );
}
private <ValueType> void process( Class<? extends XmlAdapter<ValueType, BoundType>> adapterClass ) throws Exception
{
// Do something with adapterClass ...
}
private static final class SomeAdapter extends XmlAdapter<String, LocalDate>
{
@Override
public LocalDate unmarshal( String v ) throws Exception
{
return v == null ? null : LocalDate.parse( v );
}
@Override
public String marshal( LocalDate v ) throws Exception
{
return v == null ? null : v.toString();
}
}
public static void main( String[] args ) throws Exception
{
Main<LocalDate> main = new Main<>();
main.parse( SomeAdapter.class );
}
}
OpenJDK 编译器产生此错误:
/[...]/WildcardProblem/src/Main.java:21: error: method process in class Main<BoundType> cannot be applied to given types;
process( adapterClass );
^
required: Class<? extends XmlAdapter<ValueType,BoundType>>
found: Class<CAP#1>
reason: cannot infer type-variable(s) ValueType
(argument mismatch; Class<CAP#1> cannot be converted to Class<? extends XmlAdapter<ValueType,BoundType>>)
where ValueType,BoundType are type-variables:
ValueType extends Object declared in method <ValueType>process(Class<? extends XmlAdapter<ValueType,BoundType>>)
BoundType extends Object declared in class Main
where CAP#1 is a fresh type-variable:
CAP#1 extends XmlAdapter<?,BoundType> from capture of ? extends XmlAdapter<?,BoundType>
我可以通过修改方法 process
来解决编译器错误,如下所示:
private <ValueType> void process( Class<? extends XmlAdapter<?, BoundType>> adapterClass ) throws Exception
{
@SuppressWarnings( "unchecked" )
Class<? extends XmlAdapter<ValueType, BoundType>> capturedAdapterClass = (Class<? extends XmlAdapter<ValueType, BoundType>>) adapterClass;
// Do something with adapterClass ...
}
但是,当然,未经检查的 Actor 阵容是丑陋的。我相信它应该是安全的,因为这里任意引入类型参数ValueType
只是为了捕获通配符。我只需要在处理中进一步实例化 XmlAdapter
并对其调用 unmarshal
即可。运行时不会出现异常。
所以我的问题是,首先,原始代码是否应该编译,其次,这会是 Eclipse 编译器还是 OpenJDK 编译器错误吗?
最佳答案
不是错误。您正在这里使用嵌套泛型。
Class<? extends XmlAdapter<?, BoundType>>
与 Class<? extends XmlAdapter<ValueType, BoundType>>
不同.
您必须将方法签名更改为:
public <ValueType> void parse( Class<? extends XmlAdapter<ValueType, BoundType>> adapterClass )
或者像以前一样进行转换。
<小时/>更简单的例子:
public void parse2(List<? extends Optional<?>> optionals) {
process2(optionals); // compilation error
}
public <T> void process2(List<? extends Optional<T>> optionals) {
}
关于java - Eclipse Java 编译器和 OpenJDK 编译器之间的类型推断差异 (Java 8),我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/49451943/