collectList

As explained in Spring in Action, Sixth Edition

collectList is a Project Reactor operator that collects all source elements into a List emitted by a Mono.

collectList() gathers every element emitted by a Flux and returns a Mono containing those elements in a List. section 11.3.3

Collecting fruit

Flux.just("apple", "orange", "banana", "kiwi", "strawberry") produces a Mono<List<String>> through collectList(). StepVerifier checks that the Mono emits the complete fruit list and then completes. section 11.3.3

Use collectList() when the entire stream should become one collection:

• It emits one List from the source Flux through a Mono, unlike buffer(), which produces a Flux of lists. section 11.3.3
• Use collectMap() instead when elements should be gathered into a Map using calculated keys. section 11.3.3

Figure 11.19 The collectList() operation results in a Mono containing a list of all messages emitted by the incoming Flux.

Shows the resulting Mono containing a List of all messages emitted by the incoming Flux. section 11.3.3

How Spring in Action, Sixth Edition covers it

How deep it goes

Defines the Flux-to-Mono conversion and demonstrates it with a complete Java test, without detailing operator internals.

Examples

Uses a fruit Flux containing apple, orange, banana, kiwi, and strawberry, then verifies the resulting List.

What you can do after

Apply it via collectList() and verify the resulting Mono<List<String>> with StepVerifier.

How it connects

Connects Flux, Mono, buffer(), collectMap(), and StepVerifier through reactive stream collection and verification.

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