README.md

# Apache DataFusion Java

Java bindings for [Apache DataFusion]. Queries run in native Rust and results
return to the JVM as [Apache Arrow] batches via the Arrow C Data Interface.

[Apache DataFusion]: https://datafusion.apache.org/
[Apache Arrow]: https://arrow.apache.org/

> Early development: no releases yet, API will change. Bug reports and
> contributions welcome.

## Quickstart

```java
import org.apache.arrow.memory.RootAllocator;
import org.apache.arrow.vector.ipc.ArrowReader;
import org.apache.datafusion.DataFrame;
import org.apache.datafusion.SessionContext;

try (var allocator = new RootAllocator();
     var ctx = new SessionContext()) {

    ctx.registerParquet("orders", "/path/to/orders.parquet");

    try (DataFrame df = ctx.sql(
            "SELECT o_orderpriority, COUNT(*) AS n " +
            "FROM orders GROUP BY o_orderpriority");
         ArrowReader reader = df.collect(allocator)) {
        while (reader.loadNextBatch()) {
            var batch = reader.getVectorSchemaRoot();
            // ...
        }
    }
}
```

`SessionContext` and `DataFrame` are `AutoCloseable` and not thread-safe.

## Project status

Query interfaces:

- [x] SQL: `SessionContext.sql(String)`
- [x] DataFrame: `select`, `filter` (other transformations TBD)
- [x] DataFusion-Proto `LogicalPlanNode`: `SessionContext.fromProto(byte[])`.
      The `datafusion-proto` Java classes are generated by the build.

Data sources:

- [x] Parquet via `registerParquet` / `readParquet`, with `ParquetReadOptions`
- [ ] CSV, JSON, Avro
- [ ] Custom catalog and table providers

Results:

- [x] `DataFrame.collect(allocator)` — Arrow C Data Interface stream
- [x] `DataFrame.count()`, `show()`, `show(int)`
- [x] `SessionContext.tableSchema(String)`

Not yet:

- [ ] `SessionConfig` / `RuntimeEnv` knobs
- [ ] Java UDFs
- [ ] `write_*` outputs

## Requirements

JDK 17+. Building from source: see [CONTRIBUTING.md](CONTRIBUTING.md).

## Layout

- `src/` — Java sources and tests
- `native/` — Rust crate (JNI + Arrow C Data Interface)

## Contributing

Open an issue to discuss non-trivial changes before sending a PR.
See [CONTRIBUTING.md](CONTRIBUTING.md).

## License

Apache License 2.0. See [LICENSE.txt](LICENSE.txt) and [NOTICE.txt](NOTICE.txt).