如何在kotlin中从线程中提取返回值

Question

Assume I have a function in which I send a http request and get the results. Obviously, I have some wait inside it:

fun getDocuments(): JsonObject {
        Thread {
            return@Thread functionToGetHttpResultAsJsonObject()
        }.start()
    }

What I need is to extract return value of functionToGetHttpResultAsJsonObject() into getDocuments() return value. But I get Unit instead my desired JsonObject. Any help will be appreciated

Answer 1

Let's start that your request is a bit strange. You normally don't spin a thread to block the main thread waiting for it to finish. The main point of threads is to not do that, that said.

There is no simple way to pass information between threads.

If you want to wait for a thread to finish inside another thread you should call .join() instead of .start()

But that will not return anything. If you want to synchronize two threads. So one waits for the other to compute some data and get that data, one way to do it is to use a BlockingQueue

val queue = LinkedBlockingQueue<Int>()

Thread {
    println("concurrency ftw")
    Thread.sleep(1_000)
    println("finish sleeping!")
    queue.add(1)
}.start()

println("first")
println(queue.take())

Notice I'm using .start() here because I don't want to block the second Thread to do it's processing.

The main thread will block at the .take until someone adds some data. At the same time, if the second thread arrived first to the .add it will also block until someone consumes that data.

At this point I also should mention that Kotlin has the concept of coroutines. They are very similar in concept to Go "channels" and I would advice to look into those as well .