Java Equivalent of C async await

Question

I am a normal C  developer but occasionally I develop application in Java  I m wondering if there is any Java equivalent of C  async await   In simple words what is the java equivalent of   async Task lt int gt  AccessTheWebAsync          HttpClient client   new HttpClient        var urlContents   await client GetStringAsync  http   msdn microsoft com        return urlContents Length

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As it was mentioned  there is no direct equivalent  but very close approximation could be created with Java bytecode modifications  for both async await-like instructions and underlying continuations implementation    I m working right now on a project that implements async await on top of JavaFlow continuation library  please check https   github com vsilaev java-async-await  No Maven mojo is created yet  but you may run examples with supplied Java agent  Here is how async await code looks like   public class AsyncAwaitNioFileChannelDemo    public static void main final String   argv  throws Exception                final AsyncAwaitNioFileChannelDemo demo   new AsyncAwaitNioFileChannelDemo        final CompletionStage lt String gt  result   demo processFile     project        System out println  Returned to caller     LocalTime now                 public  async CompletionStage lt String gt  processFile final String fileName  throws IOException       final Path path   Paths get new File fileName  toURI         try               final AsyncFileChannel file   new AsyncFileChannel                  path  Collections singleton StandardOpenOption READ   null                                          final FileLock lock   await file lockAll true                        System out println  In process  shared lock      lock           final ByteBuffer buffer   ByteBuffer allocateDirect  int file size              await  file read buffer  0L             System out println  In process  bytes read      buffer           buffer rewind             final String result   processBytes buffer            return asyncResult result          catch  final IOException ex            ex printStackTrace System out           throw ex             async is the annotation that flags a method as asynchronously executable  await   is a function that waits on CompletableFuture using continuations and a call to  return asyncResult someValue   is what finalizes associated CompletableFuture Continuation  As with C   control flow is preserved and exception handling may be done in regular manner  try catch like in sequentially executed code

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First  understand what async await is  It is a way for a single-threaded GUI application or an efficient server to run multiple  fibers  or  co-routines  or  lightweight threads  on a single thread   If you are ok with using ordinary threads  then the Java equivalent is ExecutorService submit and Future get  This will block until the task completes  and return the result  Meanwhile  other threads can do work   If you want the benefit of something like fibers  you need support in the container  I mean in the GUI event loop or in the server HTTP request handler   or by writing your own   For example  Servlet 3 0 offers asynchronous processing  JavaFX offers javafx concurrent Task  These don t have the elegance of language features  though  They work through ordinary callbacks

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Java doesn t have direct equivalent of C  language feature called async await  however there s a different approach to the problem that async await tries to solve  It s called project Loom  which will provide virtual threads for high-throughput concurrency  It will be available in some future version of OpenJDK   This approach also solves  colored function problem  that async await has   Similar feature can be also found in Golang  goroutines

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async and await are syntactic sugars  The essence of async and await is state machine  The compiler will transform your async await code into a state machine   At the same time  in order for async await to be really practicable in real projects  we need to have lots of Async I O library functions already in place  For C   most original synchronized I O functions has an alternative Async version  The reason we need these Async functions is because in most cases  your own async await code will boil down to some library Async method    The Async version library functions in C  is kind of like the AsynchronousChannel concept in Java  For example  we have AsynchronousFileChannel read which can either return a Future or execute a callback after the read operation is done  But it   s not exactly the same  All C  Async functions return Tasks  similar to Future but more powerful than Future    So let   s say Java do support async await  and we write some code like this   public static async Future lt Byte gt  readFirstByteAsync String filePath        Path path   Paths get filePath       AsynchronousFileChannel channel   AsynchronousFileChannel open path        ByteBuffer buffer   ByteBuffer allocate 100 000       await channel read buffer  0  buffer  this       return buffer get 0       Then I would imagine the compiler will transform the original async await code into something like this   public static Future lt Byte gt  readFirstByteAsync String filePath         CompletableFuture lt Byte gt  result   new CompletableFuture lt Byte gt          AsyncHandler ah   new AsyncHandler result  filePath        ah completed null  null        return result      And here is the implementation for AsyncHandler   class AsyncHandler implements CompletionHandler lt Integer  ByteBuffer gt        CompletableFuture lt Byte gt  future      int state      String filePath       public AsyncHandler CompletableFuture lt Byte gt  future  String filePath                this future   future          this state   0          this filePath   filePath              Override     public void completed Integer arg0  ByteBuffer arg1            try               if  state    0                    state   1                  Path path   Paths get filePath                   AsynchronousFileChannel channel   AsynchronousFileChannel open path                    ByteBuffer buffer   ByteBuffer allocate 100 000                   channel read buffer  0  buffer  this                   return                else                   Byte ret   arg1 get 0                   future complete ret                            catch  Exception e                future completeExceptionally e                         Override     public void failed Throwable arg0  ByteBuffer arg1            future completeExceptionally arg0

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AsynHelper Java library includes a set of utility classes methods for such asynchronous calls  and wait     If it is desired to run a set of method calls or code blocks asynchronously  the It includes an useful helper method AsyncTask submitTasks as in below snippet   AsyncTask submitTasks         - gt  getMethodParam1 arg1  arg2          - gt  getMethodParam2 arg2  arg3         - gt  getMethodParam3 arg3  arg4          - gt                   Some other code to run asynchronously                      If it is desired to wait till all asynchronous codes are completed running  the AsyncTask submitTasksAndWait varient can be used   Also if it is desired to obtain a return value from each of the asynchronous method call or code block  the AsyncSupplier submitSuppliers can be used so that the result can be then obtained by from the result suppliers array returned by the method  Below is the sample snippet   Supplier lt Object gt    resultSuppliers       AsyncSupplier submitSuppliers          - gt  getMethodParam1 arg1  arg2           - gt  getMethodParam2 arg3  arg4           - gt  getMethodParam3 arg5  arg6         Object a   resultSuppliers 0  get    Object b   resultSuppliers 1  get    Object c   resultSuppliers 2  get     myBigMethod a b c     If the return type of each method differ  use the below kind of snippet   Supplier lt String gt  aResultSupplier   AsyncSupplier submitSupplier    - gt  getMethodParam1 arg1  arg2    Supplier lt Integer gt  bResultSupplier   AsyncSupplier submitSupplier    - gt  getMethodParam2 arg3  arg4    Supplier lt Object gt  cResultSupplier   AsyncSupplier submitSupplier    - gt  getMethodParam3 arg5  arg6     myBigMethod aResultSupplier get    bResultSupplier get    cResultSupplier get       The result of the asynchronous method calls code blocks can also be obtained at a different point of code in the same thread or a different thread as in the below snippet   AsyncSupplier submitSupplierForSingleAccess    - gt  getMethodParam1 arg1  arg2    a    AsyncSupplier submitSupplierForSingleAccess    - gt  getMethodParam2 arg3  arg4    b    AsyncSupplier submitSupplierForSingleAccess    - gt  getMethodParam3 arg5  arg6    c        Following can be in the same thread or a different thread Optional lt String gt  aResult   AsyncSupplier waitAndGetFromSupplier String class   a    Optional lt Integer gt  bResult   AsyncSupplier waitAndGetFromSupplier Integer class   b    Optional lt Object gt  cResult   AsyncSupplier waitAndGetFromSupplier Object class   c      myBigMethod aResult get   bResult get   cResult get

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No  there isn t any equivalent of async await in Java - or even in C  before v5   It s a fairly complex language feature to build a state machine behind the scenes   There s relatively little language support for asynchrony concurrency in Java  but the java util concurrent package contains a lot of useful classes around this   Not quite equivalent to the Task Parallel Library  but the closest approximation to it

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Check out ea-async which does Java bytecode rewriting to simulate async await pretty nicely  Per their readme   It is heavily inspired by Async-Await on the  NET CLR

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The await uses a continuation to execute additional code when the asynchronous operation completes  client GetStringAsync          So  as the most close approximation I would use a CompletableFuture lt T gt   the Java 8 equivalent to  net Task lt TResult gt   based solution to process the Http request asynchronously    UPDATED on 25-05-2016 to AsyncHttpClient v 2 released on Abril 13th of 2016   So the Java 8 equivalent to the OP example of AccessTheWebAsync   is the following   CompletableFuture lt Integer gt  AccessTheWebAsync         AsyncHttpClient asyncHttpClient   new DefaultAsyncHttpClient        return asyncHttpClient         prepareGet  http   msdn microsoft com           execute           toCompletableFuture           thenApply Response  getResponseBody          thenApply String  length       This usage was taken from the answer to How do I get a CompletableFuture from an Async Http Client request  and which is according to the new API provided in version 2 of AsyncHttpClient released on Abril 13th of 2016  that has already intrinsic support for CompletableFuture lt T gt    Original answer using version 1 of AsyncHttpClient   To that end we have two possible approaches     the first one uses non-blocking IO and I call it AccessTheWebAsyncNio  Yet  because the AsyncCompletionHandler is an abstract class  instead of a functional interface  we cannot pass a lambda as argument  So it incurs in inevitable verbosity due to the syntax of anonymous classes  However  this solution is the most close to the  execution flow of the given C  example  the second one is slightly less verbose however it will submit a new Task that ultimately will block a thread on f get   until the response is complete    First approach  more verbose but non-blocking   static CompletableFuture lt Integer gt  AccessTheWebAsyncNio        final AsyncHttpClient asyncHttpClient   new AsyncHttpClient        final CompletableFuture lt Integer gt  promise   new CompletableFuture lt  gt         asyncHttpClient          prepareGet  https   msdn microsoft com            execute new AsyncCompletionHandler lt Response gt                  Override             public Response onCompleted Response resp  throws Exception                   promise complete resp getResponseBody   length                     return resp                                return promise      Second approach less verbose but blocking a thread   static CompletableFuture lt Integer gt  AccessTheWebAsync        try AsyncHttpClient asyncHttpClient   new AsyncHttpClient             Future lt Response gt  f   asyncHttpClient              prepareGet  https   msdn microsoft com                execute            return CompletableFuture supplyAsync                 - gt  return f join   getResponseBody   length

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I make and released Java async await library  https   github com stofu1234 kamaitachi  This library don t need compiler extension  and realize stackless IO processing in Java       async Task lt int gt  AccessTheWebAsync             HttpClient client  new HttpClient            var urlContents  await client GetStringAsync  http   msdn microsoft com           return urlContents Length                      LikeWebApplicationTester java     BlockingQueue lt Integer gt  AccessTheWebAsync            HttpClient client   new HttpClient           return awaiter await                 - gt  client GetStringAsync  http   msdn microsoft com                urlContents - gt                    return urlContents length                              public void doget            BlockingQueue lt Integer gt  lengthQueue AccessTheWebAsync            awaiter awaitVoid   - gt lengthQueue take                length- gt                   System out println  Length   length

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If you re just after clean code which simulates the same effect as async await in java and don t mind blocking the thread it is called on until it is finished  such as in a test  you could use something like this code   interface Async       void run Runnable handler      static void await Async async  throws InterruptedException        final CountDownLatch countDownLatch   new CountDownLatch 1       async run new Runnable               Override         public void run                 countDownLatch countDown                          countDownLatch await YOUR TIMEOUT VALUE IN SECONDS  TimeUnit SECONDS          await new Async              Override         public void run final Runnable handler                yourAsyncMethod new CompletionHandler                       Override                 public void completion                         handler run

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There is no equivalent of C  async await in Java at the language level  A concept known as Fibers aka cooperative threads aka lightweight threads could be an interesting alternative  You can find Java libraries providing support for fibers   Java libraries implementing Fibers   JetLang Kilim Quasar   You can read this article  from Quasar  for a nice introduction to fibers  It covers what threads are  how fibers can be implemented on the JVM and has some Quasar specific code

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There isn t anything native to java that lets you do this like async await keywords  but what you can do if you really want to is use a CountDownLatch  You could then imitate async await by passing this around  at least in Java7   This is a common practice in Android unit testing where we have to make an async call  usually a runnable posted by a handler   and then await for the result  count down     Using this however inside your application as opposed to your test is NOT what I am recommending  That would be extremely shoddy as CountDownLatch depends on you effectively counting down the right number of times and in the right places

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Java itself has no equivalent features  but third-party libraries exist which offer similar functionality  e g Kilim

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Java has unfortunately no equivalent of async await  The closest you can get is probably with ListenableFuture from Guava and listener chaining  but it would be still very cumbersome to write for cases involving multiple asynchronous calls  as the nesting level would very quickly grow   If you re ok with using a different language on top of JVM  fortunately there is async await in Scala which is a direct C  async await equivalent with an almost identical syntax and semantics  https   github com scala async     Note that although this functionality needed a pretty advanced compiler support in C   in Scala it could be added as a library thanks to a very powerful  macro system in Scala and therefore can be added even to older versions of Scala like 2 10  Additionally Scala is class-compatible with Java  so you can write the async code in Scala and then call it from Java   There is also another similar project called Akka Dataflow http   doc akka io docs akka 2 3-M1 scala dataflow html which uses different wording but conceptually is very similar  however implemented using delimited continuations  not macros  so it works with even older Scala versions like 2 9

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