如何使用Java FFI访问MemoryLayout中的C_指针

使用JDK 16中的FFI预览，我有以下内存布局：

class FfiTest {
    static GroupLayout layout = MemoryLayout.ofStruct(
        C_INT.withName("someInt"),
        MemoryLayout.ofPaddingBits(32), // So the following pointer is aligned at 64 bits
        C_POINTER.withName("somePtr")
    );
}

然后，我在本机代码的回调中接收到指向此类结构的指针：

public static void someCallback(MemoryAddress address) {
    try (MemorySegment seg = address.asSegmentRestricted(FfiTest.layout.byteSize())) {                

        // Works: fetching int from native structure, correct value is returned
        VarHandle intHandle = FfiTest.layout.varHandle(int.class, MemoryLayout.PathElement.groupElement("someInt"));
        int intResult = (int) vh.get(seg);

        // Does not work: get the pointer as a MemoryAddress, fatal JVM crash with Hotspot log
        VarHandle badPtrHandle = FfiTest.layout.varHandle(MemoryAddress.class, MemoryLayout.PathElement.groupElement("somePtr"));

        // Works: get the pointer as a long, correct value is returned
        VarHandle goodPtrHandle = FfiTest.layout.varHandle(long.class, MemoryLayout.PathElement.groupElement("somePtr"));
        long longResult = (long) goodPtrHandle.get(seg);
    }
}

在

JDK.internal.foreign.Utils中的JDK代码中引发异常：

    public static void checkPrimitiveCarrierCompat(Class<?> carrier, MemoryLayout layout) {
        checkLayoutType(layout, ValueLayout.class);
        if (!isValidPrimitiveCarrier(carrier))
            throw new IllegalArgumentException("Unsupported carrier: " + carrier); // Throws this exception, carrier has the value MemoryAddress
        if (Wrapper.forPrimitiveType(carrier).bitWidth() != layout.bitSize())
            throw new IllegalArgumentException("Carrier size mismatch: " + carrier + " != " + layout);
    }

publicstaticvoidcheckprimitivecarriercompat（类载体，MemoryLayout布局）{
checkLayoutType（布局，ValueLayout.class）；
如果（！isValidPrimitiveCarrier（载波））
抛出新的IllegalArgumentException（“不支持的承运人：+carrier”）；//抛出此异常，承运人的值为MemoryAddress
if（Wrapper.forPrimitiveType（carrier）.bitWidth（）！=layout.bitSize（））
抛出新的IllegalArgumentException（“载波大小不匹配：“+Carrier+”！=“+layout”）；
}

根据巴拿马的文档，C_指针的Java载体应该是
MemoryAddress
，但这在这里不起作用

那么使用long来访问这样的指针是否正确呢？还是别的什么？
正如Johannes在评论中指出的，可以用来调整
长
句柄，您必须接受并返回
内存地址
：

VarHandle goodPtrHandle = FfiTest.layout.varHandle(long.class, MemoryLayout.PathElement.groupElement("somePtr"));
long longResult = (long) goodPtrHandle.get(seg);
VarHandle addrHandle = MemoryHandles.asAddressVarHandle(goodPtrHandle);
MemoryAddress addrResult = (MemoryAddress) addrHandle.get(seg);

简而言之，您必须使用
long
，然后调整生成的
VarHandle
。