从 Mono 调用返回一个值

Question

Newbie to reactive programming in JAVA, I have a function buildRow that makes an async call to get hold of id and then use that determine a boolean value, this function always return's before async call gets completed. How do i make it return only after the async call completes and boolean value is determined?

public Row buildRow(Row row) {
    Mono<Long> id = reader.getColumn(row.getId());
    id
        .doOnNext(value-> {
            boolean isEnabled = reader.isEnabled(value);
            // This is getting returned second
            testRow(row, isEnabled);
         })
         .subscribe();
    // This is getting returned first
    return row;
}

public Row testRow(Row row, boolean isEnabled) {
  if (isEnabled) {
     return row;
  } else {
    return new Row();
  }
}

// Triggered in another function like this
map(row -> buildRow(row))

Answer 1

You can switch your logic to that one:

public Mono<Row> buildRow(Row row) {
    return reader.getColumn(row.getId())
            .map(reader::isEnabled)
            .filter(isEnabled -> isEnabled)
            .map(enabled -> row)
            .switchIfEmpty(Mono.just(new Row()));
}

After that, you can change invocation of buildRow method from:

map(row -> buildRow(row))

to:

flatMap(row -> buildRow(row))

Otherwise, there is no sense in mixing reactive with a nonreactive approach in reader logic. The more you mix, the more problems you got.

Answer 2

A Mono is a specialized Publisher that can contain only zero or one events.
In the Mono<T> Class, there is block() method:

public T block()

Subscribe to this Mono and block indefinitely until a next signal is received. Returns that value, or null if the Mono completes empty. In case the Mono errors, the original exception is thrown (wrapped in a RuntimeException if it was a checked exception).

In summary, block() method is used to get the T type value. Now, you could use block() method to return your Row Type value.

Code as followed:

public Row buildRow(Row row) {
    Mono<Long> id = reader.getColumn(row.getId());
    Row row = id.map(value-> {
            boolean isEnabled = reader.isEnabled(value);
            // use map return value
            return testRow(row, isEnabled);
         })
         .block();  // use block to compute and then get value
    // Now what you want could return
    return row;
}