temporalio__skill-temporal-developer/references/java/determinism.md

Java SDK Determinism

Overview

The Java SDK has no sandbox (only Python and TypeScript have sandboxing). The Java SDK relies on developer conventions to enforce determinism. The SDK provides Workflow.* APIs as safe replacements for common non-deterministic operations. A static analysis tool (temporal-workflowcheck, beta) can catch violations at build time — see references/java/determinism-protection.md.

Why Determinism Matters: History Replay

Temporal provides durable execution through History Replay. When a Worker needs to restore workflow state (after a crash, cache eviction, or to continue after a long timer), it re-executes the workflow code from the beginning, which requires the workflow code to be deterministic.

SDK Protection

Java workflow code runs in a cooperative threading model where only one workflow thread executes at a time under a global lock. The SDK does not intercept or block non-deterministic calls at runtime. If you call a forbidden operation, it will silently succeed during the initial execution but cause a NonDeterministicException when the workflow is replayed.

temporal-workflowcheck (static analysis, beta) and WorkflowReplayer (replay testing) can help uncover some violations, but they are not exhaustive — careful code review and adherence to the rules below remain essential.

Forbidden Operations in Workflows

The following are forbidden inside workflow code but are appropriate to use in activities.

• Thread.sleep() — blocks the real thread, bypasses Temporal timers
• new Thread() or thread pools — breaks the cooperative threading model
• synchronized blocks and explicit locks — can deadlock with the workflow executor
• UUID.randomUUID() — non-deterministic across replays
• Math.random() or new Random() — non-deterministic across replays
• System.currentTimeMillis() or Instant.now() — non-deterministic across replays
• Direct I/O (network, filesystem, database) — side effects must run in activities
• Mutable global/static state — shared state breaks isolation between workflow instances
• CompletableFuture — bypasses the workflow scheduler; use Promise instead

Safe Builtin Alternatives

Forbidden	Safe Alternative
Thread.sleep(millis)	Workflow.sleep(Duration.ofMillis(millis))
UUID.randomUUID()	Workflow.randomUUID()
Math.random()	Workflow.newRandom().nextInt()
System.currentTimeMillis()	Workflow.currentTimeMillis()
new Thread(runnable)	Async.function(func) / Async.procedure(proc)
CompletableFuture<T>	Promise<T> / CompletablePromise<T>
BlockingQueue<T>	WorkflowQueue<T>
Future<T>	Promise<T>

Testing Replay Compatibility

Use the WorkflowReplayer class to verify your code changes are compatible with existing histories. See the Workflow Replay Testing section of references/java/testing.md.

Best Practices

1. Use Workflow.currentTimeMillis() for all time operations
2. Use Workflow.newRandom() for random values
3. Use Workflow.randomUUID() for unique identifiers
4. Use Async.function() / Async.procedure() instead of raw threads
5. Use Promise and CompletablePromise instead of CompletableFuture
6. Test with WorkflowReplayer to catch non-determinism
7. Keep workflows focused on orchestration, delegate I/O to activities
8. Use Workflow.getLogger() for replay-safe logging