AWS Lambda Java 25 and SnapStart

The Problem#

Java on Lambda can suffer from cold starts because the JVM, framework, dependency graph, and application initialization all need time before the first request.

Why It Matters#

AWS Lambda supports Java 25 and SnapStart for Java runtimes. SnapStart snapshots an initialized execution environment and restores it later, which can dramatically reduce startup latency when used correctly.

Core Concepts#

SnapStart captures initialized state for a published function version. CRaC-style runtime hooks let you run code before checkpoint and after restore. Anything that must be unique per execution environment needs restore-time handling.

Implementation#

Register runtime hooks for resources that need refresh:

public final class ConnectionResource implements org.crac.Resource {
    public void beforeCheckpoint(org.crac.Context<? extends org.crac.Resource> ctx) {
        connectionPool.close();
    }

    public void afterRestore(org.crac.Context<? extends org.crac.Resource> ctx) {
        connectionPool.reconnect();
    }
}

Publish a version and enable SnapStart on the version, not $LATEST.

Real Project Scenario#

A Java API that validates webhook events may be called only a few times per hour, but each call needs a fast response. SnapStart can reduce initialization time by restoring a pre-initialized runtime. The handler still needs to refresh database connections, random seeds, cached credentials, and any network clients that do not survive restore cleanly.

Production Setup#

Measure three numbers separately: init duration without SnapStart, restore duration with SnapStart, and handler execution time after restore. Add a canary endpoint that exercises the restored resources so connection problems appear before customer traffic depends on the function.

Common Mistakes#

• Snapshotting stale database connections or credentials.
• Assuming random values generated before checkpoint are unique after restore.
• Testing only warm invocations.
• Forgetting that unused snapshots may need regeneration after idle periods.

Production Considerations#

Measure cold start, restore duration, error rate after restore, and downstream connection behavior. Keep function initialization deterministic and visible in logs.

Security#

Refresh secrets after restore if they may rotate. Do not bake short-lived tokens into checkpointed state.

Performance#

Reduce classpath size, avoid heavy reflection where possible, and initialize only what the handler needs. SnapStart helps most when init work is expensive and stable.

Summary#

Java 25 plus SnapStart makes Lambda more attractive for Java APIs, but restore-time resources, secrets, and uniqueness must be designed carefully.