暂停执行方法，直到回调完成

Question

I am fairly new to Java and extremely new to concurrency. However, I have worked with C# for a while. It doesn't really matter, but for the sake of example, I am trying to pull data off a table on server. I want method to wait until data is completely pulled. In C#, we have async-await pattern which can be used like this:

private async Task<List<ToDoItem>> PullItems ()
{
    var newRemoteItems = await (from p in remoteTable select p).ToListAsync();
    return newRemoteItems;
}

I am trying to have similar effect in Java. Here is the exact code I'm trying to port (Look inside SynchronizeAsync method.)! However, Java Azure SDK works with callbacks. So, I have a few options:

Use wait and notify pattern. Following code doesn't work since I don't understand what I'm doing.

final List<TEntity> newRemoteItems = new ArrayList<TEntity>();
synchronized( this ) {
    remoteTable.where().field("lastSynchronized").gt(currentTimeStamp)
    .execute(new TableQueryCallback<TEntity>() {
          public void onCompleted(List<TEntity> result, 
                                  int count, 
                                  Exception exception,
                                  ServiceFilterResponse response) {
              if (exception == null) {
                  newRemoteItems.clear();
                  for (TEntity item: result) {
                      newRemoteItems.add(item);
                      }
                  }
              } 
          });
}
this.wait();
//DO SOME OTHER STUFF

My other option is to move DO SOME OTHER STUFF right inside the callback's if(exception == null) block. However, this would result in my whole method logic chopped off into the pieces, disturbing the continuous flow. I don't really like this approach.

Now, here are questions:

1. What is recommended way of doing this? I am completing the tutorial on Java concurrency at Oracle. Still, clueless. Almost everywhere I read, it is recommended to use higher level stuff rather than wait and notify .

2. What is wrong with my wait and notify ?

3. My implementation blocks the main thread and it's considered a bad practice. But what else can I do? I must wait for the server to respond! Also, doesn't C# await block the main thread? How is that not a bad thing?

Answer 1

Either put DO SOME OTHER STUFF into callback, or declare a semaphore , and call semaphore.release in the callback and call semaphore.aquire where you want to wait. Remove synchronized(this) and this.wait.