How to Add Address Autocomplete in Java / Spring Boot

What the API returns

The API wraps every response in a standard envelope. The address suggestions live in the Result field, which for autocomplete is a plain array of strings:

{
  "Id": "3f2504e0-4f89-11d3-9a0c-0305e82c3301",
  "Result": [
    "123 Main St APT 1, Andover, MA 01810-3816",
    "123 Main St APT 1, Delhi, NY 13753-1257",
    "123 Main St STE 1, Caldwell, ID 83605-5476",
    "123 Main St STE 1, Corinth, NY 12822-1010",
    "123 Main St STE 1, Delhi, NY 13753-1258"
  ],
  "ClientSessionId": null,
  "StatusCode": 200,
  "IsError": false,
  "Errors": []
}

Each suggestion includes the full street, the unit designation (APT, STE, UNIT), city, state code, and ZIP+4. The API handles abbreviations (St, Ave, Blvd) and directional prefixes (N, S, E, W) on the way in, and returns clean, standardized output. In Java you map the envelope to a record and read the result() list.

Get your API key

1. Sign up at sthan.io and subscribe to the free Address Autocomplete tier
2. Open your dashboard and create an API key
3. Copy the key - it looks like sthan_live_xxxxxxxxxxxxxxxx

You get the key immediately, with no approval queue. An API key is the simplest way to authenticate: you send it as a Bearer token on every request and there is no separate login step. (If you prefer a short-lived token, there is a JWT flow covered later.)

Configure the project

Bind the key to a property that reads from an environment variable, so the secret never lives in source you commit:

# src/main/resources/application.properties
sthan.base=https://api.sthan.io
sthan.api-key=${STHAN_API_KEY}

Build the RestClient service

First, map the response envelope to a record. The JSON keys are upper camel case, so annotate each component with @JsonProperty:

import com.fasterxml.jackson.annotation.JsonProperty;
import java.util.List;

public record SthanResponse<T>(
    @JsonProperty("Id") String id,
    @JsonProperty("Result") T result,
    @JsonProperty("StatusCode") int statusCode,
    @JsonProperty("IsError") boolean isError,
    @JsonProperty("Errors") List<String> errors
) {}

Now the service. A RestClient with a default Authorization header sends the API key on every call, and the {text} URI template variable is URL-encoded for you:

import org.springframework.beans.factory.annotation.Value;
import org.springframework.core.ParameterizedTypeReference;
import org.springframework.http.HttpHeaders;
import org.springframework.stereotype.Service;
import org.springframework.web.client.RestClient;
import java.util.List;

@Service
public class AddressService {

    private final RestClient client;

    public AddressService(
        @Value("${sthan.base}") String base,
        @Value("${sthan.api-key}") String apiKey
    ) {
        this.client = RestClient.builder()
            .baseUrl(base)
            .defaultHeader(HttpHeaders.AUTHORIZATION, "Bearer " + apiKey)
            .build();
    }

    public List<String> autocomplete(String query) {
        SthanResponse<List<String>> body = client.get()
            .uri("/AutoComplete/USA/Address/{text}", query)
            .retrieve()
            .body(new ParameterizedTypeReference<>() {});

        // The envelope wraps the data — suggestions are in result()
        return body != null && body.result() != null
            ? body.result()
            : List.of();
    }
}

The whole integration is this one request. Everything else is plumbing to keep the key on the server and to debounce the front end.

Add the controller

The browser should call your server, and your server calls sthan.io. There are two reasons for this. First, the API does not enable CORS for browser requests, so a direct call from the page would be blocked. Second, and more important, putting your API key in client-side JavaScript would expose it to anyone who opens the network tab. The controller keeps the key on the server.

import org.springframework.web.bind.annotation.*;
import java.util.List;

@RestController
@RequestMapping("/api/address")
public class AddressController {

    private final AddressService service;

    public AddressController(AddressService service) {
        this.service = service;
    }

    @GetMapping("/autocomplete")
    public List<String> autocomplete(@RequestParam String query) {
        String text = query == null ? "" : query.strip();
        if (text.length() < 3) {
            return List.of();
        }
        return service.autocomplete(text);
    }
}

Your front end now has a clean URL to call: /api/address/autocomplete?query=123 main st returns a JSON array of addresses, and the API key never leaves the server.

Wire up the front-end input

The last piece is a debounced input that calls your controller. Debouncing matters: without it, "123 main st" fires eleven requests, one per keystroke. With a 250ms debounce, it fires one request after the user pauses. Add this to a template or static page:

<input type="text" id="address" autocomplete="off"
       placeholder="Start typing your address..." />
<ul id="suggestions"></ul>

<script>
const input = document.getElementById("address");
const list = document.getElementById("suggestions");
let timer;

input.addEventListener("input", () => {
  clearTimeout(timer);
  const query = input.value.trim();

  if (query.length < 3) {
    list.innerHTML = "";
    return;
  }

  // Wait 250ms after the last keystroke before calling the server
  timer = setTimeout(async () => {
    const res = await fetch(
      `/api/address/autocomplete?query=${encodeURIComponent(query)}`);
    const items = await res.json();
    list.innerHTML = items.map((a) => `<li>${a}</li>`).join("");
  }, 250);
});
</script>

The browser only ever talks to /api/address/autocomplete on your own domain. No key, no CORS, no third-party script. From here you can style the list, add keyboard navigation, and fill the form fields when a user clicks a suggestion.

Alternative: JWT authentication

An API key is the simplest option and is all most apps need. If your security policy prefers short-lived credentials, the platform also supports a 2-step JWT flow. You call GET /Auth/Token once with your profileName and profilePassword headers, receive a token valid for up to 60 minutes, then send that token as the Bearer value on subsequent calls:

public record TokenResult(
    @JsonProperty("access_token") String accessToken,
    @JsonProperty("expiration") String expiration
) {}

public String fetchToken(String profileName, String profilePassword) {
    SthanResponse<TokenResult> body = client.get()
        .uri("/Auth/Token")
        .header("profileName", profileName)
        .header("profilePassword", profilePassword)
        .retrieve()
        .body(new ParameterizedTypeReference<>() {});

    return body.result().accessToken();
}

You would cache that token until it nears expiry and set the Authorization header per request instead of a static key. Everything else - the endpoint, the envelope, the parsing - stays the same.

Handle errors

Two status codes are worth handling explicitly so a hiccup never crashes your form:

• 401 - The key or token was rejected. Check the value and, on the JWT flow, refresh and retry once.
• 429 - Rate limit reached. Back off and return what the user has typed so far rather than throwing.

public List<String> safeAutocomplete(String query) {
    try {
        return autocomplete(query);
    } catch (RestClientResponseException ex) {
        // 401, 429, or any non-2xx — degrade gracefully
        return List.of();
    }
}

Returning an empty list on failure means a momentary hiccup shows no suggestions rather than a broken page. The user can still type the address by hand.

What's next: confirm the address is deliverable

Autocomplete gets the user to a clean, well-formed address fast. It does not, on its own, confirm that mail or a package will actually arrive there - a suggestion can be correctly formatted yet point at a unit that no longer accepts delivery.

The natural next step is to run the chosen address through the Address Verification API at the moment the user submits the form. It returns a Delivery Point Validation (DPV) result and a deliverable status, standardizes the address to standard postal format, and appends ZIP+4 and county. The call is the same pattern you already built - one GET, the same envelope:

var body = client.get()
    .uri("/v2/address-verification/usa/speculative/{address}", selected)
    .retrieve()
    .body(new ParameterizedTypeReference<SthanResponse<VerificationResult>>() {});
// body.result().deliverableStatus(), body.result().dpvConfirmation()

Address Verification has its own free tier of 100 requests/month, with paid plans from $12/month. Pairing autocomplete (volume, real-time, as the user types) with verification (one confirming call at submit) keeps your costs low and your delivery data clean. For the broader Java walkthrough across verification, parsing, and geocoding, see Integrate Address APIs in Java.