java – 为什么我不能.invokeExact()在这里,即使MethodType是OK

码：

@ParametersAreNonnullByDefault
public class PathMatcherProvider
{
    private static final MethodHandles.Lookup LOOKUP
        = MethodHandles.publicLookup();
    private static final MethodType CONSTRUCTOR_TYPE
        = MethodType.methodType(void.class,String.class);

    private final Map<String,Class<? extends PathMatcher>> classMap
        = new HashMap<>();
    private final Map<Class<? extends PathMatcher>,MethodHandle> handleMap
        = new HashMap<>();

    public PathMatcherProvider()
    {
        registerPathMatcher("glob",GlobPathMatcher.class);
        registerPathMatcher("regex",RegexPathMatcher.class);
    }

    public final PathMatcher getPathMatcher(final String name,final String arg)
    {
        Objects.requireNonNull(name);
        Objects.requireNonNull(arg);

        final Class<? extends PathMatcher> c = classMap.get(name);
        if (c == null)
            throw new UnsupportedOperationException();

        try {
            return c.cast(handleMap.get(c).invoke(arg));
        } catch (Throwable throwable) {
            throw new RuntimeException("Unhandled exception",throwable);
        }
    }

    protected final void registerPathMatcher(@Nonnull final String name,@Nonnull final Class<? extends PathMatcher> matcherClass)
    {
        Objects.requireNonNull(name);
        Objects.requireNonNull(matcherClass);
        try {
            classMap.put(name,matcherClass);
            handleMap.put(matcherClass,findConstructor(matcherClass));
        } catch (NoSuchMethodException | IllegalAccessException e) {
            throw new RuntimeException("cannot find constructor",e);
        }
    }

    private static <T extends PathMatcher> MethodHandle findConstructor(
        final Class<T> matcherClass)
        throws NoSuchMethodException,IllegalAccessException
    {
        Objects.requireNonNull(matcherClass);
        return LOOKUP.findConstructor(matcherClass,CONSTRUCTOR_TYPE);
    }

    public static void main(final String... args)
    {
        new PathMatcherProvider().getPathMatcher("regex","^a");
    }
}

好的,这个工作.

我遇到的问题是这一行：

return c.cast(handleMap.get(c).invoke(arg));

如果我用invokeExact替换invoke,我得到这个堆栈跟踪：

Exception in thread "main" java.lang.RuntimeException: Unhandled exception
    at com.github.fge.filesystem.path.matchers.PathMatcherProvider.getPathMatcher(PathMatcherProvider.java:62)
    at com.github.fge.filesystem.path.matchers.PathMatcherProvider.main(PathMatcherProvider.java:89)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)
Caused by: java.lang.invoke.WrongMethodTypeException: expected (String)RegexPathMatcher but found (String)Object
    at java.lang.invoke.Invokers.newWrongMethodTypeException(Invokers.java:350)
    at java.lang.invoke.Invokers.checkExactType(Invokers.java:361)
    at com.github.fge.filesystem.path.matchers.PathMatcherProvider.getPathMatcher(PathMatcherProvider.java:60)

我不太明白GlobPathMatcher和RegexPathMatcher都使用一个构造函数,其中一个String作为参数,因此,两者的MethodType都是在CONSTRUCTOR_TYPE中定义的.如果不是,我无法“抓住”MethodHandles.

但我得到一个WrongMethodTypeException.为什么？

编辑：这是我看完答案后的代码现在我不需要中间地图：我只需要有一个地图,将一个String映射到一个MethodHandle：

@ParametersAreNonnullByDefault
public class PathMatcherProvider
{
    private static final MethodHandles.Lookup LOOKUP
        = MethodHandles.publicLookup();
    private static final MethodType CONSTRUCTOR_TYPE
        = MethodType.methodType(void.class,final String arg)
    {
        Objects.requireNonNull(name);
        Objects.requireNonNull(arg);

        final MethodHandle handle = handleMap.get(name);
        if (handle == null)
            throw new UnsupportedOperationException();

        try {
            return (PathMatcher) handle.invokeExact(arg);
        } catch (Throwable throwable) {
            throw new RuntimeException("Unhandled exception",@Nonnull final Class<? extends PathMatcher> matcherClass)
    {
        Objects.requireNonNull(name);
        Objects.requireNonNull(matcherClass);

        final MethodHandle handle;
        final MethodType type;

        try {
            handle = LOOKUP.findConstructor(matcherClass,CONSTRUCTOR_TYPE);
            type = handle.type().changeReturnType(PathMatcher.class);
            handleMap.put(name,handle.asType(type));
        } catch (NoSuchMethodException | IllegalAccessException e) {
            throw new RuntimeException("cannot find constructor",e);
        }
    }
}