Fixes #3179 – Restore _class metadata for collections in MappingRedisConverter

[xyraclius]

Overview

This PR resolves a regression in Redis Stream serialization introduced in spring-data-redis 4.0.0-SNAPSHOT, where the _class metadata is no longer written when serializing collections. As a result, deserialization fails with:

java.lang.IllegalArgumentException: Value must not be null

The issue is tracked in GitHub issue #3179.

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The Problem

In Spring Data Redis 3.5.1, when using ReactiveRedisTemplate to write and read a collection into a Redis Stream, the MappingRedisConverter correctly writes the _class metadata using the configured typeMapper. This metadata allows the deserializer to reconstruct the correct types during a read operation.

In 4.0.0-SNAPSHOT, an optimization for Collection types bypasses the call to typeMapper.writeType(...), resulting in the _class field being omitted. This omission breaks deserialization compatibility with existing data and behavior.

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The Solution

To restore the previous behavior while preserving the new optimization, this PR adds the following line:

typeMapper.writeType(ClassUtils.getUserClass(source), sink.getBucket().getPath());

Before:

if (source instanceof Collection collection) {
    writeCollection(sink.getKeyspace(), "", collection, TypeInformation.of(Object.class), sink);
    return;
} 

After:

if (source instanceof Collection collection) {
    typeMapper.writeType(ClassUtils.getUserClass(source), sink.getBucket().getPath());
    writeCollection(sink.getKeyspace(), "", collection, TypeInformation.of(Object.class), sink);
    return;
}

This ensures _class metadata is written for collection values in Redis Streams, restoring compatibility with existing deserialization logic.

Test Coverage

This fix is already verified by the existing test case:

@Test // GH-2168, GH-3179
void writePlainList() { ... }

This test verifies that _class metadata is written when serializing collections.
It fails without the fix and passes once the metadata is correctly applied.

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Related Issue

• Fixes: #3179

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