How to get FD on datagram socket?

Question

Before java 17, we could get the FD on a datagram socket like below trick with reflection, in order to set a socket option for SO_PORTREUSE using a kernel API. But the implementation changed in latest java and the field is now called delegate, and the setAccessible API will not work with it due to a java.lang.reflect.InaccessibleObjectException.

Has anyone found an alternative way to get this from the datagram socket?

    private int getFD(DatagramSocket ds) throws Exception {
        Field dsImpl = DatagramSocket.class.getDeclaredField("impl");
        dsImpl.setAccessible(true);

        Field fd = DatagramSocketImpl.class.getDeclaredField("fd");
        fd.setAccessible(true);

        FileDescriptor fdi = (FileDescriptor) fd.get(dsImpl.get(ds));

        Field fdVal = FileDescriptor.class.getDeclaredField("fd");
        fdVal.setAccessible(true);

        return fdVal.getInt(fdi);
    }

Eh? You can already set SO_REUSEPORT with DatagramSocket.setOption() and SO_REUSEPORT. You don't need Reflection at all. — user207421
– user207421, Commented Nov 14, 2023 at 2:57

diesel10 · Accepted Answer · 2023-09-21 15:34:50Z

-2

It appears that using the opens JVM flags we can get this work with java 17 again.

--add-opens=java.base/java.io=ALL-UNNAMED 
--add-opens=java.base/java.net=ALL-UNNAMED 
--add-opens=java.base/sun.nio.ch=ALL-UNNAMED 
--add-opens java.base/java.lang=ALL-UNNAMED 
--add-opens java.base/java.util=ALL-UNNAMED

answered Sep 21, 2023 at 15:34

diesel10

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1 Comment

VGR Over a year ago

Private fields are private for a reason. They are implementation details. Any future release of Java is free to change or remove them. Java has warned people for many years not to rely on such internals.

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