Programmation Concurrente et Asynchrone : Loom en Java 20 et 21

Programmation Concurrente et Asynchrone State of Loom in the JDK
21 José Paumard Java Developer Advocate Java Platform Group

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7/12/2023 Copyright © 2021, Oracle and/or its affiliates | 3
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Internal/Restricted/Highly Restricted 6 Loom

Adoption ?

What is wrong with this code? Concurrency Issues ExecutorService es = ...; Future<V> f1 = es.submit(SomeService::readImage); Future<V> f2 = es.submit(SomeService::readName); Page page = new Page(f1.get(), f2.get());

What is wrong with this code? Concurrency Issues Json request = buildContractRequest(id); String contractJson = contractServer.getContract(request); Contract contract = Json.unmarshal(contractJson);

What is wrong with this code? Concurrency Issues Contract contract = CompletableFuture.async( () -> buildContractRequest(id)) .thenApply( ContractServer::getContract) .exceptionnaly( exception -> doSomethingWith(exception)) .thenApply( Json::unmarshall).get();

What is wrong with this code? Concurrency Issues ExecutorService es = ...; Future<V> f1 = es.submit(SomeService::readImage); Future<V> f2 = es.submit(SomeService::readName); Page page = new Page(f1.get(), f2.get());

Processing I/O data: A Thread is idle 99.9999% of the time! How many threads do you need to keep your CPU busy? Concurrency for I/O ms ns ns

Loom fixes two things: - blocking a thread is not an issue anymore  Virtual threads - in case of exceptions, no more loose thread  Structured Concurrency Concurrency Issues

Using Virtual Threads ExecutorService service = Executors.newVirtualThreadPerTaskExecutor(); Thread virtuaThread = Thread.ofVirtual() .name("Virtual thread") .start(runnable);

A Virtual Thread runs on a Carrier Thread How Does it Work? heap Worker 1 start() Virtual thread fork join pool

When it blocks, the stack to the heap How Does it Work? heap start() blocked! Virtual thread Worker 1 fork join pool

Continuation.yield() copies the stack in the heap How Does it Work? heap start() blocked! yield() Virtual thread Worker 1 fork join pool

…until the OS signals that the data is available How Does it Work? heap start() Virtual thread Worker 1 fork join pool

… at which point Continuation.run() is called How Does it Work? heap start() Virtual thread run() Worker 1 fork join pool

Which mounts the virtual thread on a platform thread again How Does it Work? heap start() Virtual thread Worker 1 fork join pool

Which mounts the virtual thread on a platform thread again How Does it Work? heap Virtual thread ? Worker 1 fork join pool Worker 2

A virtual thread is a thread - Race conditions, visibility, locking, … are the same Loom: Virtual Threads (21) A Platform thread: - takes ~1ms to start - consumes ~2MB - context switching ~0,1ms A Virtual thread: - takes ~1ms to start - consumes ~200kB Blocking a Virtual Thread is OK!

With Loom’s virtual threads: - Launching a thread is cheap (having 1M is fine), no need to pool them anymore - Blocking a thread is cheap, no need to write non- blocking asynchronous code Loom: Virtual Threads (21)

The Travel Agency Example Quotation Weather Forecast Travel Page

Asynchronous Code var quotationCF = CompletableFuture.supplyAsync(() -> getQuotation()); var weatherCF = CompletableFuture.supplyAsync(() -> getWeather()); CompletableFuture<Page> travelPageCF = quotationCF .exceptionally(t -> { weatherCF.cancel(true); throw new RuntimeException(t); }) .thenCompose( quotation -> weatherCF // .completeOnTimeout(Weather.UNKNOWN, 100, MILLISECONDS) .exceptionally(e -> Weather.UNKNOWN) .thenApply( weather -> buildPage(quotation, weather)));

Quotation Server A 7/12/2023 Copyright © 2021, Oracle and/or its
affiliates | 26 The Travel Agency Example Quotation Server B Quotation Server C Weather Forecast Server A Weather Forecast Server B Weather Forecast Server C Travel Agency

Stuctured Concurrency Based Travel Agency try (var scope = new WeatherScope()) { scope.fork(() -> readWeatherFromA()); scope.fork(() -> readWeatherFromB()); scope.fork(() -> readWeatherFromC()); scope.join(); Weather firstWeather = scope.getFirstWeather(); return firstWeather; }

Stuctured Concurrency Based Travel Agency try (var scope = new QuotationScope()) { scope.fork(() -> readQuotationFromA()); scope.fork(() -> readQuotationFromB()); scope.fork(() -> readQuotationFromC()); scope.join(); Quotation bestQuotation = scope.getBestQuotation(); return bestQuotation; }

Stuctured Concurrency Based Travel Agency try (var scope = new TravelPageScope()) { scope.fork(() -> getFirstWeather()); scope.fork(() -> getBestQuotation()); scope.join(); TravelPage page = scope.buildTravelPage(); return page; }

Stuctured Concurrency Based Travel Agency protected void handleComplete(Subtask<Quotation> task) { switch (task.state()) { case RUNNING -> throw new IllegalStateException("Ooops"); case SUCCESS -> this.quotations.add(task.resultNow()); case FAILED -> this.exceptions.add(task.exceptionNow()); }

Stuctured Concurrency Based Travel Agency public Quotation bestQuotation() { return this.quotations.stream() .min(Comparator.comparing(Quotation::quotation)) .orElseThrow(this::exceptions); } public QuotationException exceptions() { QuotationException exception = new QuotationException(); this.exceptions.forEach(exception::addSuppressed); return exception; }

Closing PageScope closes the other scopes A Scope can Spawn more Scopes PageScope QuotationScope WeatherScope

So many things… - They are mutable - They may be kept alive forever What is Wrong with ThreadLocal?

An alternative model for ThreadLocal variables ThreadLocal are supported by virtual threads But you can do better! ScopedValue (prev 21)

ScopedValues are non-modifiable They are not bound to a particular thread They are bound to a single method call Welcome to ScopedValue! ScopedValue<String> key = new ScopedValue.newInstance(); ScopedValue.where(key, "KEY_1") .run(() -> doSomethingSmart())); ScopedValue.where(key, "KEY_2") .run(() -> doSomethingSmart()) .run(() -> soSomethingSmarter());

Welcome to ScopedValue! ScopedValues are non-modifiable They are not bound to a particular thread They are bound to a single method call void doSomethingSmart() { if (key.isBound()) { String value = key.get(); ... } else { throw new IllegalStateException("Key is not bound"); } }

Scoped Values are not transmitted to threads reason: you don’t want loose scoped values They are transmitted to StructuredTaskScope reason: you cant have loose scoped values ScopedValue (prev 21)

Virtual Threads Have Been Integrated!

Loom is a Work in Progress

Loom is a Work in Progress http://jdk.java.net/loom/

Loom is great!

Loom is great! and it’s there

Programmation Concurrente et Asynchrone : Loom en Java 20 et 21

Programmation Concurrente et Asynchrone : Loom en Java 20 et 21

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