Using Web3j to Send Ethereum JSON‑RPC Requests via WebSocket

Web3j Source Code

When first exploring Ethereum's Infura API, the author noticed a WebSocket option but could not locate comprehensive documentation, assuming HTTP was sufficient.

After deeper study of the JSON‑RPC protocol, it became clear that a request could be issued without a dedicated WebSocket API spec.

By examining the Web3j source code, the author pieced together the necessary request‑building logic. The following snippet shows the method used to obtain the network version:

public Request<?, NetVersion> netVersion() {
    return new Request("net_version", Collections.emptyList(), this.web3jService, NetVersion.class);
}

The org.web3j.protocol.core.Request constructor is defined as:

public Request(String method, List<S> params, Web3jService web3jService, Class<T> type) {
    this.method = method;
    this.params = params;
    this.id = nextId.getAndIncrement();
    this.web3jService = web3jService;
    this.responseType = type;
}

Sending a request over WebSocket simply involves transmitting a JSON‑RPC string that matches the fields of the Request object:

client.send("{\"jsonrpc\":\"2.0\",\"method\":\"net_version\",\"params\":[],\"id\":1333333}")

The author also notes the internal fields of Request, including a globally unique identifier generated with java.util.concurrent.atomic.AtomicLong:

private static AtomicLong nextId = new AtomicLong(0L);
private String jsonrpc = "2.0";
private String method;
private List<S> params;
private long id;
private Web3jService web3jService;
private Class<T> responseType;

WebSocket API Practice

Using a custom WebSocket client ( com.funtester.socket.WebSocketFunClient) connected to the Goerli test network, the author demonstrates a simple main method that sends two JSON‑RPC calls: eth_accounts and net_version.

static final String host = "wss://goerli.infura.io/ws/v3/apikey";

static void main(String[] args) {
    def client = new WebSocketFunClient(host, "infura ethereum");
    client.connect();
    client.send("{\"jsonrpc\":\"2.0\",\"method\":\"eth_accounts\",\"params\":[],\"id\":1}");
    client.send("{\"jsonrpc\":\"2.0\",\"method\":\"net_version\",\"params\":[],\"id\":1333333}");
}

The console output confirms successful connection and displays the JSON‑RPC responses:

22:03:44.299 WebSocketConnectReadThread-20 infura ethereum received: {"jsonrpc":"2.0","id":1,"result":[]}
22:03:44.544 WebSocketConnectReadThread-20 infura ethereum received: {"jsonrpc":"2.0","id":1333333,"result":"5"}

Additional links to original Chinese articles and collections are provided for further reading.