Checkout.com Java SDK

Getting started

Version 6.0.0 is here!

We improved the initialization of SDK making it easier to understand the available options.
Now NAS accounts are the default instance for the SDK and ABC structure was moved to a previous prefixes.
If you have been using this SDK before, you may find the following important changes:

Marketplace module was moved to Accounts module, same for classes and references.

In most cases, IDE can help you determine from where to import, but if you’re still having issues don't hesitate to open a ticket.

Gradle

dependencies {
    implementation 'com.checkout:checkout-sdk-java:<version>'
}

Building from source

Once you check out the code from GitHub, the project can be built using Gradle:

gradlew build

# skip tests
gradlew build -x test

The execution of integration tests require the following environment variables set in your system:

For default account systems (NAS): CHECKOUT_DEFAULT_PUBLIC_KEY & CHECKOUT_DEFAULT_SECRET_KEY
For default account systems (OAuth): CHECKOUT_DEFAULT_OAUTH_CLIENT_ID & CHECKOUT_DEFAULT_OAUTH_CLIENT_SECRET
For Previous account systems (ABC): CHECKOUT_PREVIOUS_PUBLIC_KEY & CHECKOUT_PREVIOUS_SECRET_KEY

Maven

<dependency>
    <groupId>com.checkout</groupId>
    <artifactId>checkout-sdk-java</artifactId>
    <version>version</version>
</dependency>

🚀 Please check in GitHub releases for all the versions available.

📖 Checkout our official documentation.

📚 Check out our official API documentation guide, where you can also find more usage examples.

How to use the SDK

This SDK can be used with two different pair of API keys provided by Checkout. However, using different API keys implies using specific API features.
Please find in the table below the types of keys that can be used within this SDK.

Account System	Public Key (example)	Secret Key (example)
Default	pk_abcdef123456ghijkl789mnopqr	sk_123456ghijklm7890abcdefxyz
Previous	pk_12345678-abcd-efgh-ijkl-mnopqrstuvwx	sk_abcdef12-3456-ghij-klmn-opqrstuvwxyz

Note: sandbox keys have a sbox_ or test_ identifier, for Default and Previous accounts respectively.

If you don't have your own API keys, you can sign up for a test account here.

PLEASE NEVER SHARE OR PUBLISH YOUR CHECKOUT CREDENTIALS.

Default

Default keys client instantiation can be done as follows:

import com.checkout.CheckoutApi;

public static void main(String[] args) {
    
    final CheckoutApi checkoutApi = CheckoutSdk.builder()
            .staticKeys()
            .publicKey("public_key")  // optional, only required for operations related with tokens
            .secretKey("secret_key")
            .environment(Environment.PRODUCTION)  // required
            .environmentSubdomain("subdomain") // optional, Merchant-specific DNS name
            .executor() // optional for a custom Executor Service
            .build();
    
    final PaymentsClient client = checkoutApi.paymentsClient();
    
    final CompletableFuture<RefundResponse> refundPayment = client.refundPayment("payment_id");
}

Default OAuth

The SDK supports client credentials OAuth, when initialized as follows:

import com.checkout.CheckoutApi;

public static void main(String[] args) {

final CheckoutApi checkoutApi = CheckoutSdk.builder()
        .oAuth()
        .clientCredentials("client_id", "client_secret")
        // for a specific authorization endpoint
        //.clientCredentials(new URI("https://access.sandbox.checkout.com/connect/token"), "client_id", "client_secret")
        .scopes(OAuthScope.GATEWAY, OAuthScope.VAULT, OAuthScope.FX)
        .environment(Environment.PRODUCTION)  // required
        .environmentSubdomain("subdomain") // optional, Merchant-specific DNS name
        .executor() // optional for a custom Executor Service
        .build();

final PaymentsClient client = checkoutApi.paymentsClient();

final CompletableFuture<RefundResponse> refundPayment = client.refundPayment("payment_id");
}

import com.checkout.previous.CheckoutApi;

public static void main(String[] args) {
    final CheckoutApi checkoutApi = CheckoutSdk.builder()
            .previous()
            .staticKeys()
            .publicKey("public_key")  // optional, only required for operations related with tokens
            .secretKey("secret_key")
            .environment(Environment.PRODUCTION)  // required
            .environmentSubdomain("subdomain") // optional, Merchant-specific DNS name
            .executor() // optional for a custom Executor Service
            .build();
    
    final PaymentsClient client = checkoutApi.paymentsClient();
    
    final CompletableFuture<RefundResponse> refundPayment = client.refundPayment("payment_id");
        }

Logging

The SDK supports SLF4J as logger provider, you need to provide your configuration file through resources folder.

Exception handling

All the API responses that do not fall in the 2** status codes will cause a CheckoutApiException. The exception encapsulates the responseHeaders, httpStatusCode and a map of errorDetails, if available.

Telemetry

Request telemetry is enabled by default in the Java SDK. Request latency is included in the telemetry data. Recording the request latency allows Checkout.com to continuously monitor and improve the merchant experience.

Request telemetry can be disabled by opting out during CheckoutSdk builder step:

final CheckoutApi checkoutApi = CheckoutSdk.builder()
        .staticKeys()
        .secretKey("secret_key")
        .environment(Environment.PRODUCTION)
        .recordTelemetry(false)
        .build();

Asynchronous and Synchronous Operations

The SDK provides flexibility in how you handle API operations by supporting both asynchronous and synchronous execution modes. You can freely use the async and sync functions of each client, and they will return CompletableFuture or ResponseObject respectively, both functions are existing in all client, maintaining a consistent API interface. For example, the flow client looks like:

        public interface FlowClient {

        CompletableFuture<PaymentSessionResponse> requestPaymentSession(PaymentSessionRequest paymentSessionRequest);

        CompletableFuture<SubmitPaymentSessionResponse> submitPaymentSessions(
                String paymentId,
                SubmitPaymentSessionRequest submitPaymentSessionRequest
        );

        CompletableFuture<PaymentSessionWithPaymentResponse> requestPaymentSessionWithPayment(
                PaymentSessionWithPaymentRequest paymentSessionRequest
        );

        // Synchronous methods
        PaymentSessionResponse requestPaymentSessionSync(PaymentSessionRequest paymentSessionRequest);

        SubmitPaymentSessionResponse submitPaymentSessionsSync(
                String paymentId,
                SubmitPaymentSessionRequest submitPaymentSessionRequest
        );

        PaymentSessionWithPaymentResponse requestPaymentSessionWithPaymentSync(
                PaymentSessionWithPaymentRequest paymentSessionRequest
        );
        }

The default operation mode of the SDK is async, so you can use async and sync functions freely, or combine them.

Aync/Sync Operation Mode

The SDK can be configured to operate in two different modes:

Asynchronous Mode (Default)

Configuration: synchronous(false) or omit the setting (default)
Behavior: HTTP requests are executed asynchronously using the configured executor (except the specific sync methods)
Transport: Uses Transport.invoke() and Transport.submitFile() methods
Resilience: Resilience4j patterns are NOT supported in async mode
Performance: Non-blocking execution
Return: CompletableFuture

Synchronous Mode

Configuration: synchronous(true)
Behavior: HTTP requests are executed synchronously, even async ops will be executed synchronous is this mode is activated!!!
Transport: Uses Transport.invokeSync() and Transport.submitFileSync() methods
Resilience: Full Resilience4j support (retry, rate limiter, circuit breaker)
Performance: Blocking execution with enhanced reliability patterns
Return: ResponseObject

How the Transport Layer Works

Asynchronous Transport Behavior

In asynchronous mode, the transport layer:

Immediate Return: Methods return CompletableFuture immediately
Async HTTP Execution: HTTP calls are made asynchronously using Apache HttpClient
Thread Pool: Uses the configured executor for CompletableFuture operations
Non-blocking: The calling thread is NOT blocked during HTTP execution

// Asynchronous mode configuration
final CheckoutApi asyncApi = CheckoutSdk.builder()
        .staticKeys()
        .secretKey("secret_key")
        .environment(Environment.PRODUCTION)
        .synchronous(false) // or omit (default)
        .executor(Executors.newFixedThreadPool(20)) // Custom thread pool for async operations
        .build();

// Usage - returns immediately, HTTP call happens asynchronously
CompletableFuture<PaymentResponse> future = asyncApi.paymentsClient().requestPayment(request);

// Process result when available
future.thenAccept(response -> {
    System.out.println("Payment ID: " + response.getId());
});

Synchronous Transport Behavior

In synchronous mode, the transport layer:

Blocking HTTP Call: HTTP request is made synchronously with resilience patterns applied
Wrapped in CompletableFuture: Result are returned directly as response objects when the call completes
Resilience4j Applied: Retry, rate limiting and circuit breaker are applied during the synchronous HTTP call
Blocking: The calling thread is blocked during HTTP execution until the call ends

// Synchronous mode configuration with resilience
final CheckoutApi syncApi = CheckoutSdk.builder()
        .staticKeys()
        .secretKey("secret_key")
        .environment(Environment.PRODUCTION)
        .synchronous(true) // Enable synchronous mode
        .resilience4jConfiguration(Resilience4jConfiguration.defaultConfiguration())
        .executor(Executors.newFixedThreadPool(10)) // Thread pool for wrapping sync calls
        .build();

// Usage - HTTP call executes synchronously with resilience patterns
PaymentResponse response = syncApi.paymentsClient().requestPaymentSync(request);
System.out.println("Payment ID: " + response.getId());

Performance Characteristics

Asynchronous Mode

Pros:
- Non-blocking execution
- Better resource utilization for I/O bound operations
- Lower memory footprint per request
Cons:
- No built-in resilience patterns
- Must implement retry logic manually
- More complex error handling

Synchronous Mode

Pros:
- Built-in resilience patterns (retry, rate limiting, circuit breaker)
- Simplified error handling with automatic retries
- Easier debugging and testing
- Transient error handling out-of-the-box
Cons:
- Blocking HTTP execution (but mitigated by executor wrapping)
- Higher resource usage due to resilience overhead

Choosing the Right Mode

Use Asynchronous Mode When:

Implementing custom retry/resilience logic
Working with reactive programming patterns
Resource efficiency is critical
You need fine-grained control over error handling

Use Synchronous Mode When:

You need robust error handling and retries
Reliability is more important than raw performance
You want built-in transient error handling
Implementing payment processing where consistency is critical

Example Comparison

// Both modes have identical parameters for the interfaces
PaymentRequest request = PaymentRequest.builder()
        .source(cardSource)
        .amount(1000L)
        .currency(Currency.USD)
        .build();

// Asynchronous - fast return, manual error handling
CompletableFuture<PaymentResponse> asyncResult = asyncApi.paymentsClient().requestPayment(request)
        .exceptionally(throwable -> {
            // Manual retry logic if needed
            if (shouldRetry(throwable)) {
                return retryPayment(request);
            }
            throw new CompletionException(throwable);
        });

// Synchronous - built-in resilience, automatic retries
PaymentResponse syncResult = syncApi.paymentsClient().requestPaymentSync(request);
        // Resilience4j automatically handles retries for transient errors

// Both return CompletableFuture - same API!

Thread Pool Considerations

Async Mode: Executor primarily used for CompletableFuture operations, HTTP I/O is non-blocking
Sync Mode: Executor used to wrap synchronous operations, size should account for concurrent blocking calls
Recommendation: Use larger thread pools for sync mode to handle blocking operations

Custom Environment

In case that you want to use an integrator or mock server, you can specify your own configuration as follows:

final CustomEnvironment environment = CustomEnvironment.builder()
        .checkoutApi(create("https://the.base.uri/")) // the uri for all Checkout operations
        .oAuthAuthorizationApi(create("https://the.oauth.uri/connect/token")) // the uri used for OAUTH authorization, only required for OAuth operations
        .filesApi(create("https://the.files.uri/")) // the uri used for Files operations, only required for Accounts module
        .transfersApi(create("https://the.transfers.uri/")) // the uri used for Transfer operations, only required for Transfers module
        .balancesApi(create("https://the.balances.uri/")) // the uri used for Balances operations, only required for Balances module
        .sandbox(false) // flag to determine if Sanbox or Production configured, default false
        .build();

final CheckoutApi checkoutApi = CheckoutSdk.builder()
        .staticKeys()
        .secretKey("secret_key")
        .environment(environment)  // required
        .executor(customHttpClient) // optional for a custom Executor Service
        .build();

You don't need to specify all the previous URI's, only the ones for the modules that you're going to use.

Custom HttpClient

The SDK allows you to customize the underlying HTTP client and executor configurations to meet your specific needs, such as custom connection pooling, proxy settings, timeout configurations, or multithreaded operations.

Basic HttpClient Configuration

You can provide your own HttpClientBuilder to customize HTTP connection properties:

final HttpClientBuilder customHttpClient = HttpClientBuilder.create()
        .setProxy(HttpHost.create("https://proxy"))
        .setConnectionTimeToLive(3, TimeUnit.SECONDS);

final CheckoutApi checkoutApi = CheckoutSdk.builder()
        .staticKeys()
        .secretKey("secret_key")
        .environment(Environment.PRODUCTION)  // required
        .environmentSubdomain("subdomain") // optional, Merchant-specific DNS name
        .httpClientBuilder(customHttpClient) // optional for a custom HttpClient
        .build();

Custom Executor

You can provide a custom Executor for handling asynchronous operations. By default, the SDK uses ForkJoinPool.commonPool().

final Executor customExecutor = Executors.newSingleThreadExecutor();

final CheckoutApi checkoutApi = CheckoutSdk.builder()
        .staticKeys()
        .secretKey("secret_key")
        .environment(Environment.PRODUCTION)  // required
        .executor(customExecutor) // optional for a custom Executor Service
        .build();

Multithreaded Configuration for High-Performance Applications

For applications requiring high concurrency and performance, you can configure both a custom executor with a larger thread pool and an HTTP client optimized for connection pooling:

// Configure HttpClient for high concurrency
final HttpClientBuilder multithreadedHttpClient = HttpClientBuilder.create()
        .setMaxConnTotal(200)                             // Maximum total connections
        .setMaxConnPerRoute(100)                          // Maximum connections per route/destination
        .setConnectionTimeToLive(60, TimeUnit.SECONDS)    // Connection time-to-live
        .evictIdleConnections(30, TimeUnit.SECONDS);      // Evict idle connections after 30 seconds

// Configure a fixed thread pool executor
final Executor multithreadedExecutor = Executors.newFixedThreadPool(100);

final CheckoutApi checkoutApi = CheckoutSdk.builder()
        .staticKeys()
        .secretKey("secret_key")
        .environment(Environment.PRODUCTION)  // required
        .executor(multithreadedExecutor)      // custom thread pool for async operations
        .httpClientBuilder(multithreadedHttpClient) // custom HTTP client for connection pooling
        .build();

Configuration Options Explained:

executor(Executor): Configures the executor service for handling asynchronous operations. Default is ForkJoinPool.commonPool().
httpClientBuilder(HttpClientBuilder): Configures the Apache HttpClient settings. Default is HttpClientBuilder.create().

Common HttpClient Settings:

setMaxConnTotal(int): Maximum number of total connections across all routes
setMaxConnPerRoute(int): Maximum number of connections per specific route/destination
setConnectionTimeToLive(long, TimeUnit): Connection lifetime before being evicted from the pool
evictIdleConnections(long, TimeUnit): Time after which idle connections are evicted
setProxy(HttpHost): Configure HTTP proxy server
setDefaultConnectionConfig(ConnectionConfig): Set default connection configuration
setDefaultSocketConfig(SocketConfig): Set socket configuration including timeouts

Advanced Customization for Different Throughput Requirements

This section provides specific configuration patterns for different application requirements, focusing on rate limiting and HTTP connection pool optimization.

HTTP Connection Pool Optimization by Throughput

Configure your HTTP connection pool based on your application's expected throughput and concurrency requirements.

Performance Tuning Guidelines

Connection Pool Sizing Rules of Thumb

MaxConnTotal: Should be at least 2-3x your expected concurrent requests
MaxConnPerRoute: Usually 50-80% of MaxConnTotal for single-endpoint applications
Thread Pool Size: Should match or slightly exceed MaxConnPerRoute
Connection TTL: Shorter for high-load (10-30s), longer for low-load (60-300s)

Monitoring and Observability

When using custom configurations, monitor these metrics:

Connection Pool Utilization: Ensure pools aren't exhausted
Request Rate: Verify rate limiting effectiveness
Response Times: Monitor impact of connection pooling
Error Rates: Track timeouts and connection failures
Memory Usage: Watch for connection pool memory impact

Retry Strategy and Resilience Configuration

The SDK includes built-in resilience patterns using the Resilience4j library to handle transient errors and improve reliability.

Supported Resilience Patterns

The SDK supports three main resilience patterns:

Retry: Automatically retries failed requests
Rate Limiter: Controls the rate of requests to prevent overloading
Circuit Breaker: Prevents cascading failures by temporarily stopping requests to failing services

Libraries Used

Resilience4j Retry: io.github.resilience4j:resilience4j-retry:1.7.1
Resilience4j Rate Limiter: io.github.resilience4j:resilience4j-ratelimiter:1.7.1
Resilience4j Circuit Breaker: io.github.resilience4j:resilience4j-circuitbreaker:1.7.1

How It Works

The resilience patterns are applied as decorators around your HTTP requests in the following order:

Rate Limiter (if configured) - Controls request rate
Retry (if configured) - Retries failed requests
Circuit Breaker (if configured) - Wraps everything to prevent cascading failures (by default deactivated)

Transient Errors Handled

The retry mechanism can handle various types of transient errors, including:

Network connectivity issues (timeouts, connection refused)
HTTP 5xx server errors (500, 502, 503, 504)
Temporary service unavailability
Rate limiting responses (429)

Individual Component Configuration

final Resilience4jConfiguration resilience4jConfig = Resilience4jConfiguration.builder()
        .withDefaultRetry()
        // Deactivated by default
        //.withDefaultRateLimiter()
        // Deactivated by default
        //.withDefaultCircuitBreaker()
        .build();

final CheckoutApi checkoutApi = CheckoutSdk.builder()
        .staticKeys()
        .secretKey("secret_key")
        .environment(Environment.PRODUCTION)
        .synchronous(true)
        .resilience4jConfiguration(resilience4jConfig)
        .build();

Custom Resilience Configuration

Default behavior: Asynchronous mode Resilience4j: NOT applied by default Transport: Uses async methods (transport.invoke(), transport.submitFile()) Performance: Faster, non-blocking execution but no automatic retries

To enable synchronous mode with Resilience4j patterns, you need to explicitly set:

    CheckoutSdk.builder()    .synchronous(true)

So the SDK is designed to be async-first by default, which makes sense for most modern applications that prefer non-blocking operations.

// SYNC transport (with Resilience4j)
@Override
public Response invokeSync(...) {
    // ... build request ...
    final Supplier<Response> callSupplier = () -> performCall(...);
    return executeWithResilience4j(callSupplier);  // ← Resilience4j applied here
}

// ASYNC transport (no Resilience4j)
@Override
public CompletableFuture<Response> invoke(...) {
    return CompletableFuture.supplyAsync(() -> {
        // ... build request ...
        return performCall(...);  // ← Direct call, no Resilience4j
    }, executor);
}

Custom Retry Configuration

final RetryConfig retryConfig = RetryConfig.custom()
        .maxAttempts(5)                                    // Maximum retry attempts
        .waitDuration(Duration.ofMillis(1000))             // Wait time between retries
        .retryOnException(throwable -> 
            throwable instanceof CheckoutApiException &&   // Retry on API exceptions
            ((CheckoutApiException) throwable).getHttpStatusCode() >= 500) // Only 5xx errors
        .build();

final Resilience4jConfiguration resilience4jConfig = Resilience4jConfiguration.builder()
        .withRetry(retryConfig)
        .build();

Custom Rate Limiter Configuration

(deactivated by default)

final RateLimiterConfig rateLimiterConfig = RateLimiterConfig.custom()
        .limitForPeriod(50)                                // 50 requests
        .limitRefreshPeriod(Duration.ofSeconds(1))         // Per second
        .timeoutDuration(Duration.ofSeconds(2))            // Wait up to 2 seconds for permission
        .build();

final Resilience4jConfiguration resilience4jConfig = Resilience4jConfiguration.builder()
        .withRateLimiter(rateLimiterConfig)
        .build();

Custom Circuit Breaker Configuration

(deactivated by default)

final CircuitBreakerConfig circuitBreakerConfig = CircuitBreakerConfig.custom()
        .failureRateThreshold(60)                          // Open circuit at 60% failure rate
        .waitDurationInOpenState(Duration.ofSeconds(60))   // Wait 60 seconds before trying again
        .slidingWindowSize(20)                             // Consider last 20 requests
        .minimumNumberOfCalls(10)                          // Minimum calls before evaluation
        .permittedNumberOfCallsInHalfOpenState(5)          // Calls allowed in half-open state
        .build();

final Resilience4jConfiguration resilience4jConfig = Resilience4jConfiguration.builder()
        .withCircuitBreaker(circuitBreakerConfig)
        .build();

Complete Custom Configuration

final Resilience4jConfiguration resilience4jConfig = Resilience4jConfiguration.builder()
        .withRetry(RetryConfig.custom()
                .maxAttempts(3)
                .waitDuration(Duration.ofMillis(500))
                .retryOnException(throwable -> 
                    throwable instanceof CheckoutApiException &&
                    ((CheckoutApiException) throwable).getHttpStatusCode() >= 500)
                .build())
        // Deactivated by default
        //.withRateLimiter(RateLimiterConfig.custom()           
        //        .limitForPeriod(100)
        //        .limitRefreshPeriod(Duration.ofSeconds(1))
        //        .timeoutDuration(Duration.ofSeconds(5))
        //        .build())
        // Deactivated by default
        //.withCircuitBreaker(CircuitBreakerConfig.custom()
        //        .failureRateThreshold(50)
        //        .waitDurationInOpenState(Duration.ofSeconds(30))
        //        .slidingWindowSize(10)
        //        .build())
        .build();

final CheckoutApi checkoutApi = CheckoutSdk.builder()
        .staticKeys()
        .secretKey("secret_key")
        .environment(Environment.PRODUCTION)
        .synchronous(true)
        .resilience4jConfiguration(resilience4jConfig)
        .build();

Important Notes:

Resilience4j configuration only works with synchronous mode (synchronous(true))
For asynchronous operations, implement your own retry logic using CompletableFuture patterns
All resilience patterns are optional - configure only what you need
Rate limiter helps respect API rate limits and prevent overwhelming the service

Code of Conduct

Please refer to Code of Conduct

Licensing

MIT

Name		Name	Last commit message	Last commit date
Latest commit History 629 Commits
.github		.github
gradle/wrapper		gradle/wrapper
src		src
.gitignore		.gitignore
CODE_OF_CONDUCT.md		CODE_OF_CONDUCT.md
LICENSE.md		LICENSE.md
README.md		README.md
build.gradle		build.gradle
gradle.properties		gradle.properties
gradlew		gradlew
gradlew.bat		gradlew.bat
settings.gradle		settings.gradle

License

checkout/checkout-sdk-java

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Checkout.com Java SDK

Getting started

Gradle

Building from source

Maven

🚀 Please check in GitHub releases for all the versions available.

📖 Checkout our official documentation.

📚 Check out our official API documentation guide, where you can also find more usage examples.

How to use the SDK

Default

Default OAuth

Previous

Logging

Exception handling

Telemetry

Asynchronous and Synchronous Operations

Aync/Sync Operation Mode

Asynchronous Mode (Default)

Synchronous Mode

How the Transport Layer Works

Asynchronous Transport Behavior

Synchronous Transport Behavior

Performance Characteristics

Asynchronous Mode

Synchronous Mode

Choosing the Right Mode

Use Asynchronous Mode When:

Use Synchronous Mode When:

Example Comparison

Thread Pool Considerations

Custom Environment

Custom HttpClient

Basic HttpClient Configuration

Custom Executor

Multithreaded Configuration for High-Performance Applications

Advanced Customization for Different Throughput Requirements

HTTP Connection Pool Optimization by Throughput

Performance Tuning Guidelines

Connection Pool Sizing Rules of Thumb

Monitoring and Observability

Retry Strategy and Resilience Configuration

Supported Resilience Patterns

Libraries Used

How It Works

Transient Errors Handled

Individual Component Configuration

Custom Resilience Configuration

Custom Retry Configuration

Custom Rate Limiter Configuration

Custom Circuit Breaker Configuration

Complete Custom Configuration

Code of Conduct

Licensing

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