MongoDB异步Java驱动程序find（）

Question

I have a webapp in which I have to return the results from a mongodb find() to the front-end from my java back-end. I am using the Async Java driver, and the only way I think I have to return the results from mongo is something like this:

public String getDocuments(){
  ...
  collection.find(query).map(Document::toJson)
        .into(new HashSet<String>(), new SingleResultCallback<HashSet<String>>() {
            @Override
            public void onResult(HashSet<String> strings, Throwable throwable) {
              // here I have to get all the Json Documents in the set,
              // make a whole json string and wake the main thread
            }
        });
  // here I have to put the main thread to wait until I get the data in
  // the onResult() method so I can return the string back to the front-end
  ...
  return jsonString;
}

Is this assumption right or there´s another way to do it?

Answer 1

Asynchronous APIs (any API based on callbacks, not necessarily MongoDB) can be a true blessing for multithreaded applications. But to really benefit from them, you need to design your whole application architecture in an asynchronous fashion. This is not always feasible, especially when it is supposed to fit into a given framework which isn't built on callbacks.

So sometimes (like in your case) you just want to use an asynchronous API in a synchronous fashion. In that case, you can use the class CompletableFuture .

This class provides (among others) two methods <T> get() and complete(<T> value) . The method get will block until complete is called to provide the return value (should complete get called before get , get returns immediately with the provided value).

public String getDocuments(){
  ...
  CompletableFuture<String> result = new CompletableFuture<>(); // <-- create an empty, uncompleted Future

  collection.find(query).map(Document::toJson)
        .into(new HashSet<String>(), new SingleResultCallback<HashSet<String>>() {
            @Override
            public void onResult(HashSet<String> strings, Throwable throwable) {
              // here I have to get all the Json Documents in the set and
              // make a whole json string

              result.complete(wholeJsonString); // <--resolves the future
            }
        });

  return result.get(); // <-- blocks until result.complete is called
}

The the get() -method of CompletableFuture also has an alternative overload with a timeout parameter . I recommend using this to prevent your program from accumulating hanging threads when the callback is not called for whatever reason. It will also be a good idea to implement your whole callback in a try { block and do the result.complete in the finally { block to make sure the result always gets resolved, even when there is an unexpected error during your callback.

Answer 2

Yes, you're right.

That's the correct behaviour of Mongo async driver (see MongoIterable.into ).

However, Why don't you use sync driver in this situation? Is there any reason to use async method?