[jdbc-v2] Convert Clickhouse Array(T) to Java T[] instead of Object[]

[dolfinus]

Summary

I've faced an issue when reading Array(Decimal) with clickhouse-jdbc 0.9.2 and Spark 3.5.7 - instead of converting values to Spark's DecimalType, it failed with exception:

Exception java.lang.ClassCastException: class [Ljava.lang.Object; cannot be cast to class [Ljava.math.BigDecimal; ([Ljava.lang.Object; and [Ljava.math.BigDecimal; are in module java.base of loader 'bootstrap')

This is because JdbcUtils.convert(value, java.math.BigDecimal.class, column) returns Object[], and Java doesn't allow casts like those:

(java.math.BigDecimal[]) new Object[]{};

Even if each item of array is actually a BigDecimal.

Now this method returns BigDecimal[] for these arguments. This requires some reflection usage to construct T[] with T known only in runtime.

I haven't run integration tests on this yet.

Checklist

Delete items not relevant to your PR:

• Closes JDBC-V2: ResultSet inferred type for arrays is always Object[] #2457
• Unit and integration tests covering the common scenarios were added

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[windsurf-bot]

Looks good to me 🤙

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[chernser]

Good day, @dolfinus !
Thank you for contributing!

However getObject() by default should return Array object for all arrays.
Defined in 8.9.3 JDBC Types Mapped to Java Object Types

There is a java.sql.ResultSet#getObject(int, java.util.Map<java.lang.String,java.lang.Class<?>>) that should be used in case user wants stripped array.

[dolfinus]

master branch already contains clause:

else if (type != java.sql.Array.class && value instanceof List<?>

The only thing changed there is that generic Object[] replaced with T[].
If PR title is misleading, I can update it.

[chernser]

@dolfinus
I'll review this week.

[dolfinus]

Thanks!

[chernser]

@dolfinus
Please take a look #2667
This is related to the PR - tests for nested arrays were failing.