Extract historic Ethereum transactions using web3j

EthereumTransactionExtractor.java

public class EthereumTransactionExtractor implements AutoCloseable {


    private final WebSocketService webSocketService;
    private final Web3j web3j;
    private final ObjectMapper mapper = new ObjectMapper();
    private final JsonGenerator jsonGenerator;

    public EthereumTransactionExtractor(String url, String fileName) throws IOException {
        webSocketService = new WebSocketService(url, false);
        webSocketService.connect();
        web3j = Web3j.build(webSocketService);
        jsonGenerator = new JsonFactory()
                .createGenerator(new File(fileName), JsonEncoding.UTF8)
                .setCodec(new ObjectMapper());
    }

    public void extractFromTo(int startBlock, int endBlock) throws IOException {
        jsonGenerator.writeStartArray();
        web3j.replayPastTransactionsFlowable(
                new DefaultBlockParameterNumber(startBlock),
                new DefaultBlockParameterNumber(endBlock)
        ).doOnError(Throwable::printStackTrace)
                .blockingSubscribe(pojo -> {
                            System.out.println("foo");
                            jsonGenerator.writeObject(pojo);
                        },
                        100000);
        jsonGenerator.writeEndArray();
        jsonGenerator.close();
    }

    @Override
    public void close() {
        webSocketService.close();
    }
}