How do you write tests for async Apex (@future, Queueable, Batch)?

Async testing requires careful use of Test.startTest() and Test.stopTest() to force async work to run synchronously inside the test.

Pattern:

`apex @isTest static void testAsync() { Account acc = new Account(Name='Test'); insert acc;

Test.startTest(); MyClass.doFutureWork(acc.Id); // async work queued Test.stopTest(); // <-- async work runs HERE, synchronously

Account refreshed = [SELECT Phone FROM Account WHERE Id=:acc.Id]; System.assertEquals('updated', refreshed.Phone); } `

Key rules:

`Test.startTest()` resets governor limits for the test code that follows. Useful for isolating limit usage.
`Test.stopTest()` flushes async work — @future, Queueable, Batch jobs queued during the test all execute synchronously when stopTest is called.
Without stopTest, async work doesn't run during the test; you can't assert on its effects.

Specifics by async type:

`@future`:

Test calls the future method between startTest/stopTest. The work runs at stopTest.
Cannot test future call from within another future (Salesforce restriction).

Queueable:

Same pattern. The Queueable's execute() runs synchronously at stopTest.
For chained Queueables: only the FIRST chain link runs in the test. Subsequent links require additional test-method invocations or the chain is mocked.

Batch Apex:

Same pattern, but the test must process records you've inserted.
start() returns the QueryLocator (or iterable); test setup must populate the source records.
execute() runs once per chunk. In tests, the entire query result becomes one chunk regardless of batch size — keep test data small.
finish() runs at the end.

Common pitfalls:

Forgetting Test.stopTest() — async never runs, assertions fail.
Trying to chain Queueables in test — can't easily test deep chains.
Querying for the result before stopTest — async hasn't run yet, results are stale.
Not setting up enough test data to make assertions meaningful.

Mocking external callouts in async tests:

The async test pattern is one of the most common interview questions and most common production-code mistakes.

Why this answer works