Android OkHttp ETag Handling
Understanding ETags
ETags (Entity Tags) are HTTP headers that represent a unique identifier for a specific version of a resource. They are used for efficient caching and to ensure that clients receive the latest version of data.
OkHttp ETag Integration
OkHttp provides a straightforward way to handle ETags through the `CacheControl` class.
Implementing ETag Support
#### Step 1: Enabling Caching
“`java
OkHttpClient client = new OkHttpClient.Builder()
.cache(new Cache(getCacheDir(), 10 * 1024 * 1024)) // 10MB cache
.build();
“`
#### Step 2: Using CacheControl
“`java
Request request = new Request.Builder()
.url(“https://example.com/resource”)
.cacheControl(new CacheControl.Builder()
.maxAge(5, TimeUnit.SECONDS)
.maxStale(60, TimeUnit.SECONDS)
.build())
.build();
Response response = client.newCall(request).execute();
“`
* **`maxAge`:** Specifies the maximum age of a cached response (in seconds).
* **`maxStale`:** Allows the client to accept stale responses (older than `maxAge`) if the network is unavailable.
#### Step 3: Checking ETags
“`java
String etag = response.header(“ETag”);
if (etag != null) {
// Store the ETag in a local cache for future requests
// …
}
“`
#### Step 4: Conditional Requests with ETags
“`java
Request request = new Request.Builder()
.url(“https://example.com/resource”)
.header(“If-None-Match”, etag) // Send the cached ETag
.build();
Response response = client.newCall(request).execute();
if (response.code() == 304) { // Not Modified
// Use the cached data
} else {
// Retrieve and cache the updated data
}
“`
### Advantages of Using ETags
* **Reduced Network Traffic:** Avoids unnecessary data transfer when a resource hasn’t changed.
* **Faster Load Times:** Clients can access data from the cache instead of waiting for network requests.
* **Improved Performance:** Minimizes server load by serving cached content.
Example Scenario
**Scenario:** Fetching data from an API that provides ETags
“`java
// Fetch the data initially
Response response = client.newCall(request).execute();
// Store the ETag
String etag = response.header(“ETag”);
// … (store the ETag in a local cache)
// Subsequent requests
request = new Request.Builder()
.url(“https://example.com/resource”)
.header(“If-None-Match”, etag) // Send the stored ETag
.build();
response = client.newCall(request).execute();
if (response.code() == 304) { // Not Modified
// Use the cached data
} else {
// Retrieve and cache the updated data
// Update the stored ETag
}
“`
### Table: Comparing Requests with and without ETags
| Request | ETag | Response Code | Result |
|—|—|—|—|
| Initial Request | None | 200 | Data retrieved and ETag stored |
| Subsequent Request | Existing ETag | 304 | Not Modified (use cached data) |
| Subsequent Request | Invalid ETag | 200 | Data updated and new ETag stored |
Conclusion
By leveraging OkHttp’s `CacheControl` and `If-None-Match` header, you can efficiently manage ETags to improve the performance and responsiveness of your Android applications. This ensures that users receive the latest data while minimizing network overhead.