Fix ReflectionCache to be serializable

[k163377]

Maybe fixes #295

I honestly don't understand the required specifications, but is this correct?
The following test confirms that the LRUMap set to reflectionCacheSize is retained, but not its contents.

import com.fasterxml.jackson.module.kotlin.ReflectionCache
import org.junit.Test
import java.io.FileInputStream
import java.io.FileNotFoundException
import java.io.FileOutputStream
import java.io.IOException
import java.io.ObjectInputStream
import java.io.ObjectOutputStream

class ReflectionCacheTest {
    private val serialFile = "./serial.ser"

    private fun mkSerialFile(obj: Any) {
        try {
            FileOutputStream(serialFile).use { fileOutStream ->
                ObjectOutputStream(fileOutStream).use { outStream ->
                    outStream.writeObject(obj)
                    outStream.flush()
                    outStream.reset()
                }
            }
        } catch (e: FileNotFoundException) {
            e.printStackTrace()
        } catch (e: IOException) {
            e.printStackTrace()
        }
    }

    private fun readSerialFile(): ReflectionCache {
        var reflectionCache: ReflectionCache? = null
        try {
            FileInputStream(serialFile).use { fileInStream ->
                ObjectInputStream(fileInStream).use { inStream ->
                    reflectionCache = inStream.readObject() as ReflectionCache
                }
            }
        } catch (e: ClassNotFoundException) {
            e.printStackTrace()
        } catch (e: IOException) {
            e.printStackTrace()
        } catch (e: Exception) {
            e.printStackTrace()
        }
        return reflectionCache ?: throw RuntimeException()
    }

    @Test
    fun test() {
        val cache = ReflectionCache(100)
        cache.kotlinFromJava(this::class.java.methods.first())
        mkSerialFile(cache)
        val tmp = readSerialFile()

        println()
    }
}

[cowtowncoder]

@k163377 There are existing tests that can help. json-core f.ex has:

https://github.com/FasterXML/jackson-core/blob/2.15/src/test/java/com/fasterxml/jackson/core/TestJDKSerializability.java

so you need not write a file, but just write as byte[]/read from byte[]. Helper methods like:

    protected byte[] jdkSerialize(Object o) throws IOException
    {
        ByteArrayOutputStream bytes = new ByteArrayOutputStream(1000);
        ObjectOutputStream obOut = new ObjectOutputStream(bytes);
        obOut.writeObject(o);
        obOut.close();
        return bytes.toByteArray();
    }

    protected <T> T jdkDeserialize(byte[] raw) throws IOException
    {
        ObjectInputStream objIn = new ObjectInputStream(new ByteArrayInputStream(raw));
        try {
            return (T) objIn.readObject();
        } catch (ClassNotFoundException e) {
            fail("Missing class: "+e.getMessage());
            return null;
        } finally {
            objIn.close();
        }
    }

and with that I think 2 things need to be tested, if not already:

1. ObjectMapper with Kotlin module registered (so that everything needed is java.io.Serializabe and
2. ReflectionCache itself

Of these (1) might fail, and figuring out which things it refers to are not serializable can be some work. So here, just adding test for (2) would make sense, I think.

... although there is also difference between empty cache with no entries (not referencing things that might not be Serializable) and cache with entries. This is why test (1) is often done only after serializing once, deserializing once, to let referenced components be initialized.

But on verifying that a class is JDK serializable/deserializable, all that is needed is for write-then-read to succeed without exception.

I hope this helps.

[cowtowncoder]

Oh, also; when testing a non-empty cache, I am pretty sure you will find out that actual cache LRUMaps need to be changed to transient (not serialized); and that readResolve() is needed to construct an empty cache instance. Otherwise LRUMap members were left as nulls as they are not (and should not be) serialized.

[k163377]

@cowtowncoder
Thank you, that was very helpful.
The test I added has been pushed.

[cowtowncoder]

LGTM