The first request to an AWS service can take noticeably longer than subsequent requests. A Java application may need to initialize SDK components, load request serialization code, establish a network connection, perform DNS resolution, and complete a TLS handshake before it can receive a response.

For latency-sensitive applications, this startup overhead can affect the first user request, a newly started container, or an AWS Lambda function recovering from a cold start.

The AWS SDK for Java 2.x provides SdkWarmUp, an API designed to move much of this initialization work into application startup. Instead of waiting for the first business operation to trigger initialization, an application can warm up selected SDK clients before it begins processing traffic.

This article explains how to configure SDK client warm-up, integrate it into a Java service, choose which clients to warm, and measure whether the change improves first-request latency.

Why the First AWS SDK Request Can Be Slow

Consider a Spring Boot service that reads customer records from Amazon DynamoDB. The service starts successfully, but the first request to retrieve a customer takes significantly longer than later requests.

Several operations may contribute to that delay:

Not every request performs every operation. For example, a reused HTTP connection can avoid another connection handshake, and credential providers may cache or refresh credentials according to their implementation.

The key point is that client construction alone does not guarantee that the complete request path has been exercised.

Creating a service client during startup is still a good practice. Explicit warm-up goes further by exercising SDK initialization paths before real application traffic arrives.

Add the AWS SDK for Java Dependency

The warm-up API is available through the AWS SDK for Java 2.x sdk-core module. You also need the service module for each AWS service your application uses.

For a Maven application using Amazon S3, add the following dependencies. Keep the SDK modules on the same version, preferably through the AWS SDK BOM.

<properties>
    <aws.sdk.version>2.54.0</aws.sdk.version>
</properties>

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>software.amazon.awssdk</groupId>
            <artifactId>bom</artifactId>
            <version>${aws.sdk.version}</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>

<dependencies>
    <dependency>
        <groupId>software.amazon.awssdk</groupId>
        <artifactId>s3</artifactId>
    </dependency>
</dependencies>

The version shown is an example baseline for the warm-up API. In an existing application, use a compatible SDK version approved for your project rather than downgrading dependencies solely to match this example.

Using the BOM helps keep service modules and core SDK components aligned.

Warm Up All SDK Clients

The simplest option is to invoke SdkWarmUp.warmUp() during application initialization.

import software.amazon.awssdk.core.warmup.SdkWarmUp;

public final class AwsSdkInitializer {

    private AwsSdkInitializer() {
    }

    public static void initialize() {
        SdkWarmUp.warmUp();
    }
}

Call the initializer before the application starts accepting requests:

public static void main(String[] args) {
    AwsSdkInitializer.initialize();

    startApplication(args);
}

Here, startApplication represents your application's startup logic. Replace it with the actual framework bootstrap in your project.

The no-argument warm-up method targets service clients available on the classpath. This can be convenient for a small application, but it may perform unnecessary work if your dependency tree contains service modules that the application does not use.

Warm-up also adds work to startup. That trade-off is worthwhile when reducing first-request latency matters more than minimizing initialization time.

Warm Up Only the Clients Your Application Uses

For a service that calls only a few AWS services, select the relevant client classes explicitly.

For example, an application that uses Amazon S3 and Amazon DynamoDB can warm their synchronous client paths:

import software.amazon.awssdk.core.warmup.SdkWarmUp;
import software.amazon.awssdk.services.dynamodb.DynamoDbClient;
import software.amazon.awssdk.services.s3.S3Client;

public final class AwsSdkInitializer {

    private AwsSdkInitializer() {
    }

    public static void initialize() {
        SdkWarmUp.warmUp(
            S3Client.class,
            DynamoDbClient.class
        );
    }
}

The class-based overload lets you avoid warming service clients that your application does not need.

The choice of client type matters. Warming S3Client.class exercises the synchronous path; if your application uses S3AsyncClient, warm that asynchronous client type instead. A synchronous and asynchronous client can use different HTTP implementations and initialization paths.

For example:

import software.amazon.awssdk.core.warmup.SdkWarmUp;
import software.amazon.awssdk.services.s3.S3AsyncClient;

public final class AsyncAwsSdkInitializer {

    private AsyncAwsSdkInitializer() {
    }

    public static void initialize() {
        SdkWarmUp.warmUp(S3AsyncClient.class);
    }
}

Warm the client types your application actually uses, and avoid treating every service module on the classpath as a requirement.

Integrate Warm-Up into a Spring Boot Application

In a Spring Boot application, initialize the SDK before the application reports readiness to receive traffic.

One option is an ApplicationRunner:

import org.springframework.boot.ApplicationArguments;
import org.springframework.boot.ApplicationRunner;
import org.springframework.stereotype.Component;
import software.amazon.awssdk.core.warmup.SdkWarmUp;
import software.amazon.awssdk.services.s3.S3Client;

@Component
public class AwsSdkWarmUpRunner implements ApplicationRunner {

    @Override
    public void run(ApplicationArguments args) {
        SdkWarmUp.warmUp(S3Client.class);
    }
}

This example invokes warm-up during Spring Boot startup. However, an ApplicationRunner runs after the application context has been refreshed. Depending on your readiness configuration, the application might already be considered ready at that point.

For a strict readiness requirement, coordinate warm-up with your application's readiness lifecycle so that the instance does not receive production traffic until initialization has completed successfully.

Also note that this runner warms the SDK request path, not necessarily every application-specific operation. If the application needs to validate access to a particular bucket, table, or other resource, perform that validation separately using an appropriate service operation and error-handling policy.

Understand What Warm-Up Does and Does Not Do

The SDK warm-up feature exercises service-client and HTTP-client initialization paths.

At a high level, the service-client warm-up uses a local-only operation with a predefined response to exercise request marshalling and response unmarshalling. HTTP-client warm-up makes a network request to an AWS endpoint to exercise connection setup.

The HTTP warm-up request is unsigned and does not invoke an AWS service operation. Consequently, it does not require AWS service credentials or permissions for the warm-up request itself, and it does not incur AWS service-operation charges.

However, this does not mean warm-up validates your application's IAM permissions. A successful warm-up does not prove that the application can read a particular S3 object, query a DynamoDB table, or invoke another protected API.

Keep these checks separate:

Separating these checks prevents a successful warm-up from creating a false sense of operational readiness.

Combine Warm-Up With Long-Lived Service Clients

Warm-up is not a replacement for reusing AWS SDK clients.

AWS SDK for Java 2.x service clients are thread-safe and are generally intended to be long-lived. Each client maintains resources such as its HTTP connection pool, so creating a new client for every request adds avoidable overhead.

For example, create an S3 client once and inject it into the application service:

import software.amazon.awssdk.services.s3.S3Client;

public final class ObjectStorageService {

    private final S3Client s3Client;

    public ObjectStorageService(S3Client s3Client) {
        this.s3Client = s3Client;
    }

    // Add application-specific S3 operations here.
}

Manage the client's lifecycle through dependency injection or an application-level component. Close the client when the application shuts down or when it is no longer needed.

Avoid constructing a new client inside every controller method or business operation. Otherwise, you may repeatedly pay initialization costs and lose the benefits of connection pooling, even after performing warm-up.

Use Warm-Up With AWS Lambda SnapStart

Lambda workloads can experience cold-start latency when a new execution environment initializes the Java runtime and application dependencies.

SDK client warm-up can be useful with AWS Lambda, including workflows that use Lambda SnapStart. With SnapStart, initialization work can be included in the snapshot, so restored execution environments may benefit from the initialized SDK request path.

For example:

import software.amazon.awssdk.core.warmup.SdkWarmUp;
import software.amazon.awssdk.services.s3.S3Client;

public final class LambdaInitialization {

    private LambdaInitialization() {
    }

    public static void initialize() {
        SdkWarmUp.warmUp(S3Client.class);
    }
}

Invoke this initialization during the appropriate initialization phase for your Lambda application.

Snapshot-based execution introduces additional lifecycle considerations. Do not assume that every resource or network connection established before snapshotting remains valid after restoration. Validate behavior with the actual Lambda runtime and deployment configuration, particularly where resources depend on network state or external credentials.

Warm-up can reduce initialization overhead, but it does not eliminate every cold-start cost. Runtime startup, dependency loading, application initialization, and other external dependencies can still contribute to latency.

Measure the Improvement

Warm-up adds work to startup in exchange for potentially reducing latency on the first business request. Whether that trade-off is beneficial depends on your workload.

Measure at least these metrics:

Metric

What it tells you

Application startup duration

How much additional time warm-up adds

First business-request latency

Whether the initial request becomes faster

Subsequent request latency

Whether steady-state performance changes

Warm-up failures

Whether initialization encounters connectivity or runtime problems

Readiness time

When the instance actually becomes eligible for traffic

Compare the same application build and deployment environment with warm-up enabled and disabled.

Use a repeatable test procedure:

  1. Start a fresh application instance.

  2. Measure startup and readiness duration.

  3. Send the first real business request and record its latency.

  4. Send additional requests and record their latency.

  5. Repeat across multiple fresh instances.

  6. Compare the distributions rather than relying on a single measurement.

Avoid reporting a fixed latency reduction without measurements from your own workload. Results depend on the JVM, SDK version, HTTP client, network, deployment platform, and application behavior.

If first-request latency remains high after warm-up, investigate credential resolution, DNS, downstream service latency, retries, connection reuse, and application-level initialization separately.

Common Mistakes to Avoid

Warming every client unnecessarily. A large dependency tree can increase startup time if unused service clients are warmed. Prefer the class-based overload when the required client types are known.

Creating clients after warm-up without considering lifecycle. Initialize and reuse the actual service clients according to the application's dependency-injection and lifecycle design. Avoid repeatedly constructing clients during request processing.

Treating warm-up as an IAM test. The warm-up request does not validate permissions for your business operations. Test required service access separately.

Ignoring startup latency. Warm-up shifts work earlier in the lifecycle. Measure whether the reduction in first-request latency justifies the added initialization time.

Assuming asynchronous and synchronous paths are identical. Warm the client type that matches the application's actual request path.

Assuming all cold-start costs disappear. Warm-up targets SDK request-path initialization, not every source of startup or service latency.

Summary

SdkWarmUp in AWS SDK for Java 2.x provides a way to initialize service-client and HTTP-client request paths before the first business operation. Use SdkWarmUp.warmUp() when you want to warm the clients available on the classpath, or pass specific client classes when you want more control over startup work.

Combine warm-up with long-lived, reusable service clients and integrate it with the application's readiness lifecycle. For Lambda applications, evaluate its behavior alongside cold-start optimization features such as SnapStart.

Finally, measure startup duration and first-request latency under realistic conditions. Warm-up is most effective when it addresses a measured initialization bottleneck rather than being added without evidence.