Skip to content

Instantly share code, notes, and snippets.

@benjchristensen
Last active December 12, 2023 09:36
Show Gist options
  • Save benjchristensen/4671081 to your computer and use it in GitHub Desktop.
Save benjchristensen/4671081 to your computer and use it in GitHub Desktop.
FuturesB.java Example of using Futures for nested calls showing how it blocks inefficiently.
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
public class FuturesB {
/**
* Demonstrate how futures can easily become blocking and prevent other work
* from being performed asynchronously.
*/
public static void run() throws Exception {
ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
try {
// get f3 with dependent result from f1
Future<String> f1 = executor.submit(new CallToRemoteServiceA());
Future<String> f3 = executor.submit(new CallToRemoteServiceC(f1.get()));
/* The work below can not proceed until f1.get() completes even though there is no dependency */
// also get f4/f5 after dependency f2 completes
Future<Integer> f2 = executor.submit(new CallToRemoteServiceB());
Future<Integer> f4 = executor.submit(new CallToRemoteServiceD(f2.get()));
Future<Integer> f5 = executor.submit(new CallToRemoteServiceE(f2.get()));
System.out.println(f3.get() + " => " + (f4.get() * f5.get()));
} finally {
executor.shutdownNow();
}
}
/**
* Demonstrates how reordering of Future.get() can improve the situation
* but that it still doesn't address differing response latencies of f1 and f2.
*/
public static void run2() throws Exception {
ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
try {
// kick of f1/f2 in parallel
Future<String> f1 = executor.submit(new CallToRemoteServiceA());
Future<Integer> f2 = executor.submit(new CallToRemoteServiceB());
// get f3 with dependent result from f1 (blocks)
Future<String> f3 = executor.submit(new CallToRemoteServiceC(f1.get()));
/* The work below can not proceed until f1.get() completes even if f2.get() is done. */
// get f4/f5 after dependency f2 completes (blocks)
Future<Integer> f4 = executor.submit(new CallToRemoteServiceD(f2.get()));
Future<Integer> f5 = executor.submit(new CallToRemoteServiceE(f2.get()));
System.out.println(f3.get() + " => " + (f4.get() * f5.get()));
} finally {
executor.shutdownNow();
}
}
/**
* Demonstrate how changing where threads are injected can solve the issue of run2()
* at the cost of incidental complexity being added to the code.
* <p>
* This same example could be accomplished by refactoring CallToRemoteServiceC
* to accept a Future<String> instead of String but the principle is the same.
*/
public static void run3() throws Exception {
ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
try {
// kick of f1/f2 in parallel
final Future<String> f1 = executor.submit(new CallToRemoteServiceA());
Future<Integer> f2 = executor.submit(new CallToRemoteServiceB());
// spawn in another thread so waiting on f1 for f3 doesn't block f4/f5
Future<String> f3 = executor.submit(new Callable<String>() {
@Override
public String call() throws Exception {
// get f3 with dependent result from f1 (blocks)
return new CallToRemoteServiceC(f1.get()).call();
}
});
/* The following can now proceed as soon as f2.get() completes even if f1.get() isn't done yet */
// get f4/f5 after dependency f2 completes (blocks)
Future<Integer> f4 = executor.submit(new CallToRemoteServiceD(f2.get()));
Future<Integer> f5 = executor.submit(new CallToRemoteServiceE(f2.get()));
System.out.println(f3.get() + " => " + (f4.get() * f5.get()));
} finally {
executor.shutdownNow();
}
}
/**
* Demonstrate typical handling of responding to Futures as they complete.
* <p>
* This successfully executes multiple calls in parallel but then synchronously handles
* each response in the order they were put in the list rather than the order they complete.
*/
public static void run4() throws Exception {
ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
try {
List<Future<?>> futures = new ArrayList<Future<?>>();
// kick off several async tasks
futures.add(executor.submit(new CallToRemoteServiceA()));
futures.add(executor.submit(new CallToRemoteServiceB()));
futures.add(executor.submit(new CallToRemoteServiceC("A")));
futures.add(executor.submit(new CallToRemoteServiceC("B")));
futures.add(executor.submit(new CallToRemoteServiceC("C")));
futures.add(executor.submit(new CallToRemoteServiceD(1)));
futures.add(executor.submit(new CallToRemoteServiceE(2)));
futures.add(executor.submit(new CallToRemoteServiceE(3)));
futures.add(executor.submit(new CallToRemoteServiceE(4)));
futures.add(executor.submit(new CallToRemoteServiceE(5)));
// as each completes do further work
for (Future<?> f : futures) {
/* this blocks so even if other futures in the list complete earlier they will wait until this one is done */
doMoreWork(f.get());
}
} finally {
executor.shutdownNow();
}
}
private static void doMoreWork(Object s) {
// do work
System.out.println("do more work => " + s);
}
/**
* Demonstrate polling approach to handling Futures as they complete.
* <p>
* This becomes unwieldy and error prone quickly.
*/
public static void run5() throws Exception {
ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
try {
List<Future<?>> futures = new ArrayList<Future<?>>();
// kick off several async tasks
futures.add(executor.submit(new CallToRemoteServiceA()));
futures.add(executor.submit(new CallToRemoteServiceB()));
futures.add(executor.submit(new CallToRemoteServiceC("A")));
futures.add(executor.submit(new CallToRemoteServiceC("B")));
futures.add(executor.submit(new CallToRemoteServiceC("C")));
futures.add(executor.submit(new CallToRemoteServiceD(1)));
futures.add(executor.submit(new CallToRemoteServiceE(2)));
futures.add(executor.submit(new CallToRemoteServiceE(3)));
futures.add(executor.submit(new CallToRemoteServiceE(4)));
futures.add(executor.submit(new CallToRemoteServiceE(5)));
// as each completes do further work
// keep polling until all work is done
while (futures.size() > 0) {
// use an iterator so we can remove from it
Iterator<Future<?>> i = futures.iterator();
while (i.hasNext()) {
Future<?> f = i.next();
if (f.isDone()) {
// only do work if the Future is done
doMoreWork(f.get());
i.remove();
}
// otherwise we continue to the next Future
}
}
} finally {
executor.shutdownNow();
}
}
public static void main(String args[]) {
try {
long start = System.currentTimeMillis();
run();
System.out.println("Finished in: " + (System.currentTimeMillis() - start) + "ms");
run2();
run3();
run4();
run5();
} catch (Exception e) {
e.printStackTrace();
}
}
private static final class CallToRemoteServiceA implements Callable<String> {
@Override
public String call() throws Exception {
// simulate fetching data from remote service
Thread.sleep(100);
return "responseA";
}
}
private static final class CallToRemoteServiceB implements Callable<Integer> {
@Override
public Integer call() throws Exception {
// simulate fetching data from remote service
Thread.sleep(40);
return 100;
}
}
private static final class CallToRemoteServiceC implements Callable<String> {
private final String dependencyFromA;
public CallToRemoteServiceC(String dependencyFromA) {
this.dependencyFromA = dependencyFromA;
}
@Override
public String call() throws Exception {
// simulate fetching data from remote service
Thread.sleep(60);
return "responseB_" + dependencyFromA;
}
}
private static final class CallToRemoteServiceD implements Callable<Integer> {
private final Integer dependencyFromB;
public CallToRemoteServiceD(Integer dependencyFromB) {
this.dependencyFromB = dependencyFromB;
}
@Override
public Integer call() throws Exception {
// simulate fetching data from remote service
Thread.sleep(140);
return 40 + dependencyFromB;
}
}
private static final class CallToRemoteServiceE implements Callable<Integer> {
private final Integer dependencyFromB;
public CallToRemoteServiceE(Integer dependencyFromB) {
this.dependencyFromB = dependencyFromB;
}
@Override
public Integer call() throws Exception {
// simulate fetching data from remote service
Thread.sleep(55);
return 5000 + dependencyFromB;
}
}
}
@simon-zhou
Copy link

Very good example demonstrating how Java Future becomes complex to use in fully asynchronous use scenario. Thanks!

@orirawlings
Copy link

It would also be good to see how run5() changes if we introduce usage of java.util.concurrent.ExecutorCompletionService<V>

@UncleBarney
Copy link

Thanks Ben! This is an excellent example on Java Future and how things can go pretty ugly.

@Turbo87
Copy link

Turbo87 commented Jul 21, 2015

@benjchristensen have you considered using the thenCompose() of the CompletableFuture class instead?

    public static void run2() throws Exception {
        ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
        try {
            // kick of f1/f2 in parallel
            Future<String> f1 = executor.submit(new CallToRemoteServiceA());
            Future<Integer> f2 = executor.submit(new CallToRemoteServiceB());

            // get f3 with dependent result from f1
            Future<String> f3 = f1.thenCompose(f1Result -> executor.submit(new CallToRemoteServiceC(f1Result)));

            // get f4/f5 after dependency f2 completes
            Future<Integer> f4 = f2.thenCompose(f2Result -> executor.submit(new CallToRemoteServiceD(f2Result)));
            Future<Integer> f5 = f2.thenCompose(f2Result -> executor.submit(new CallToRemoteServiceE(f2Result)));

            // first blocking command
            System.out.println(f3.get() + " => " + (f4.get() * f5.get()));
        } finally {
            executor.shutdownNow();
        }
    }

@vadlamak
Copy link

@Turbo87 true that...completable future is available with java 8, rxjava started earlier

@zinking
Copy link

zinking commented Dec 7, 2015

I am a bit confused, what about scala's Future and chaining using onSuccess etc... will that help?

@huangyg11
Copy link

don't understand why in the run(), f2,f4,f5 can not proceed until f1.get() completes. Doesn't the ThreadPoolExecutor can hold 4 threads at most?

@huangyg11
Copy link

I got it, because f1.get() blocks.

@Sergey80
Copy link

Sergey80 commented Jul 8, 2017

Is that only because it is Java future? Scala Future works better with nesting until you do Awaits.result()

@crazyrunsnail
Copy link

very handsome!

@SauravDas90
Copy link

Concise example with point to point explanation!
In line 55 does the call to f5 is blocked, until f4 is finished? both of them depend on f2, so once f2 is available, can both f4, f5 proceed simultaneously?

@wwzskyrain
Copy link

@Turbo87 true that...completable future is available with java 8, rxjava started earlier

that’s all?

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment