为什么我不能在这里调用Exact（），即使MethodType是可以的？

对于我的一个项目，我必须对构造函数进行动态调用。但是由于这是Java 7，而不是“经典”反射API，我使用java.lang.invoke。

法典：

@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，这有效。

我遇到的问题是这行：

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

如果我用 替换 ，我会得到这个堆栈跟踪：invokeinvokeExact

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。两者都使用带有 a 作为参数的单个构造函数，因此 for 两者都是 在 中定义的。如果不是这样，我无论如何都不可能“抓住”s。GlobPathMatcherRegexPathMatcherStringMethodTypeCONSTRUCTOR_TYPEMethodHandle

然而，我得到了一个.为什么？WrongMethodTypeException

编辑：这是我阅读答案后的代码;现在我不需要中间映射：我只需要有一个映射，将 a 映射到 ：StringMethodHandle

@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, 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 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", throwable);
        }
    }

    protected final void registerPathMatcher(@Nonnull final String name,
        @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);
        }
    }
}