[Java] Avoid RowCoder bytecode generation

[bvolpato]

RowCoderGenerator currently creates and loads one ByteBuddy class for every uncached schema. Generated encode and decode methods only load instance fields and call existing Java delegates, so class generation adds startup time, allocation, metaspace pressure, and class-loader complexity without specializing field encoding.

This replaces generated classes with one immutable CustomCoder<Row> implementation while preserving schema UUID caching, encoding-position overrides, component coders, and existing encode/decode algorithms. It also adds JMH coverage for uncached coder creation and steady-state encoding/decoding.

Benchmarks

JDK 17, same host, three isolated JVM forks. Generation includes schema construction; buildSchema is a control.

Benchmark	master	This PR	Change
Uncached coder creation	5,138.469 ± 422.881 µs/op	328.614 ± 30.207 µs/op	15.6x faster
Uncached coder allocation	554,862.4 ± 2,135.1 B/op	17,926.4 ± 33.0 B/op	31.0x lower
Schema construction control	147.588 ± 22.855 µs/op	145.541 ± 24.375 µs/op	unchanged

Command:

./gradlew :sdks:java:core:jmh:jmh --args='org.apache.beam.sdk.jmh.coders.RowCoderGenerationBenchmark -prof gc -f 3 -foe=true'

Steady-state results use identical benchmarks and three forks:

Operation	Static encoding	master ns/op	This PR ns/op
decode	false	59.354 ± 0.892	60.901 ± 1.137
decode	true	50.046 ± 1.455	50.931 ± 0.885
encode	false	61.872 ± 1.607	59.405 ± 1.554
encode	true	45.537 ± 0.958	45.286 ± 1.026

All old/new confidence intervals overlap, showing no material steady-state regression.

Historical context

RowCoder generation was introduced in 2018 to replace an implementation that inspected the schema and rebuilt component coders while processing every row. The combined change was reported as 30-40% faster. That comparison was against the former introspecting implementation, not against an ordinary coder with the same precomputed fields used here.

Over time, component-coder construction moved into regular Java and the resulting array was passed into the generated class. Its encode and decode methods remained forwarding wrappers around shared loops. Precomputing field coders is still valuable and remains unchanged in this PR; generating a class no longer provides that benefit. Current benchmarks show equivalent steady-state performance, while class generation adds substantial creation time and allocation.

Wire compatibility and testing

Encoding and decoding algorithms are unchanged. Java passes Beam's exact standard-coder vectors, including beam:coder:row:v1 vectors produced by Python.

• ./gradlew :runners:java-fn-execution:test --tests org.apache.beam.runners.fnexecution.wire.CommonCoderTest (227 passed)
• ./gradlew :sdks:java:core:test (full core suite passed; RowCoderTest 28/28 and SchemaCoderTest 19/19)
• ./gradlew :sdks:java:core:spotlessApply :sdks:java:core:jmh:spotlessApply
• Old/new RowCoderBenchmark JMH runs for dynamic and static encodings

---

• No existing issue to link.
• No CHANGES.md entry; internal performance change with unchanged API and wire format.
• Small contribution; ICLA not applicable.

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R: @Abacn for label java.

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