并发和并行实际上是相同的原理，两者都与同时执行的任务有关，尽管我想说并行任务应该是真正的多任务，“同时”执行，而并发可能意味着任务共享执行线程，同时仍然看起来是并行执行。

异步方法与前两个概念没有直接关系，异步被用来呈现并发或并行任务的印象，但实际上异步方法调用通常用于需要在当前应用程序之外执行工作的进程，我们不希望等待和阻塞应用程序等待响应。

例如，从数据库获取数据可能需要时间，但我们不想阻塞UI等待数据。异步调用接受回调引用，并在请求被放置到远程系统后立即将执行返回给您的代码。当远程系统执行所需的任何处理时，UI可以继续响应用户，一旦它将数据返回给回调方法，那么该方法就可以适当地更新UI(或移交更新)。

从用户的角度来看，它看起来像多任务处理，但它可能不是。

EDIT

可能值得补充的是，在许多实现中，异步方法调用会导致线程启动，但这不是必要的，这实际上取决于正在执行的操作以及如何将响应通知回系统。

每个人都很难将异步与并行或并发相关联，因为异步既不是并行的反义词，也不是并发的反义词。它是同步的反义词。它只是表示某些东西，在这个例子中是线程，是否会与其他东西同步，在这个例子中是另一个线程。

并发与并行: 并发在一个时间点上只能完成一个任务。例如:单CPU处理器 并行性在一个点上我们可以做多个任务。例如:双核或多核处理器

Concurrency Concurrency means that an application is making progress on more than one task at the same time (concurrently). Well, if the computer only has one CPU the application may not make progress on more than one task at exactly the same time, but more than one task is being processed at a time inside the application. It does not completely finish one task before it begins the next. Parallelism Parallelism means that an application splits its tasks up into smaller subtasks which can be processed in parallel, for instance on multiple CPUs at the exact same time. Concurrency vs. Parallelism In Detail As you can see, concurrency is related to how an application handles multiple tasks it works on. An application may process one task at at time (sequentially) or work on multiple tasks at the same time (concurrently). Parallelism on the other hand, is related to how an application handles each individual task. An application may process the task serially from start to end, or split the task up into subtasks which can be completed in parallel. As you can see, an application can be concurrent, but not parallel. This means that it processes more than one task at the same time, but the tasks are not broken down into subtasks. An application can also be parallel but not concurrent. This means that the application only works on one task at a time, and this task is broken down into subtasks which can be processed in parallel. Additionally, an application can be neither concurrent nor parallel. This means that it works on only one task at a time, and the task is never broken down into subtasks for parallel execution. Finally, an application can also be both concurrent and parallel, in that it both works on multiple tasks at the same time, and also breaks each task down into subtasks for parallel execution. However, some of the benefits of concurrency and parallelism may be lost in this scenario, as the CPUs in the computer are already kept reasonably busy with either concurrency or parallelism alone. Combining it may lead to only a small performance gain or even performance loss. Make sure you analyze and measure before you adopt a concurrent parallel model blindly.

从http://tutorials.jenkov.com/java-concurrency/concurrency-vs-parallelism.html

并发是指多个任务的执行是交错的，而不是每个任务依次执行。

并行性是指这些任务实际上是并行执行的。

异步是一个单独的概念(尽管在某些上下文中是相关的)。它指的是一个事件可能发生在与另一个事件不同的时间(不同步)。下面的图表说明了同步执行和异步执行之间的区别，其中参与者可以对应于不同的线程、进程甚至服务器。

Parallel : It's a broad term that means that two pieces of code execute that "at the same time". It doesn't matter if it's "real" parallelism or if it's faked through some clever design pattern. The point is that you can start the "tasks" at the same time and then control them separately (with mutex and all the appropriate tricks). But usually you prefer to use the word "parallel" only for "true" parallelism, as in : you make it happen through non-cooperative multitasking (whether be throuch CPU/GPU cores, or only at software level by letting the OS managing it at a very low level). People are reluctant to say "parallel" just for complicated sequential code that fakes parallelism, like you would find in a browser window's javascript for example. Hence the reason why people in this thread say "asynchronous has nothing to do with parallelism". Well it does, but just don't confuse them.

并发:没有并行性就不可能有并发性(无论是模拟的还是真实的，正如我上面解释的那样)，但是这个术语特别关注的是两个系统将试图在某个时间点同时访问同一资源的事实。它强调了一个事实，那就是你必须要处理这个问题。

异步:每个人都说异步与并行无关，这是对的，但它为并行铺平了道路(让事情并行或不并行的负担在你身上——继续阅读)。

“异步”指的是并行性的一种表示形式，它形式化了并行性中通常涉及的三个基本内容:1)定义任务的初始化(比如它何时开始以及它获得哪些参数)，2)任务完成后必须做什么，3)代码在此期间应该继续做什么。

但它仍然只是语法(通常表示为回调方法)。在后台，底层系统可能简单地认为这些所谓的“任务”只是堆积起来的代码片段，直到它完成当前正在执行的代码。然后它一个接一个地把它们拆开，然后依次执行。与否。它还可以为每个任务创建一个线程，并并行运行它们。谁在乎呢?这部分不包括在概念中;)

并发性可能发生在以下几种场景中:

异步——这意味着你的程序执行非阻塞操作。例如，它可以通过HTTP发起对远程资源的请求，然后在等待接收响应的同时继续执行一些其他任务。这有点像你发了一封电子邮件，然后继续你的生活，没有等待回复。

并行性——这意味着您的程序利用多核机器的硬件，通过将工作分解为任务，在同一时间执行任务，每个任务都在单独的核心上执行。这有点像洗澡时唱歌:你实际上是在同时做两件事。

多线程——这是一种允许不同线程并发执行的软件实现。多线程程序似乎同时在做几件事，即使它运行在单核机器上。这有点像通过各种IM窗口与不同的人聊天;虽然你实际上是在来回切换，但最终结果是你同时进行了多个对话。

这里有一些语义需要澄清:

并发或并行是资源争用的问题，而异步是关于控制流的问题。

不同的过程(或它们的组成操作)被称为异步的，当它们的处理顺序没有确定的实现时;换句话说，它们中的任何一个都有可能在任何给定的时间t被处理。根据定义，多个处理器(例如cpu或Persons)使多个处理器同时被处理成为可能;在单个处理器上，它们的处理是交错的(例如线程)。

当异步过程或操作共享资源时，它们被称为Concurrent;当没有资源共享(例如不同的处理器和存储)时，并行性是微不足道的保证;否则必须解决并发控制问题。

因此，异步过程或操作可以与其他过程或操作并行或并发地处理。

“同步和异步是编程模型。并发和并行是任务执行的方式……” 来源:https://medium.com/better - programming/sync vs -异步并发- vs - vs - 5754 cdb60f66平行

换句话说，同步和异步描述了在进行函数调用时程序如何执行(它是等待还是继续执行?)，而并发和并行描述了函数(任务)将如何执行(并发=可能同时执行，并行=有效地同时执行)。

并发性意味着同时执行多个任务，但不一定是同时执行。当你必须执行多个任务，但你只有一个资源时，我们就会追求并发性。在单核环境中，并发是通过上下文切换实现的。

并行性就像同时执行多个任务，就像你可以一边唱歌一边洗澡。现在你在并行地做这些任务。

术语异步与线程执行有关。在异步模型中，当执行一个任务时，您可以切换到另一个任务，而无需等待前一个任务完成。

异步编程帮助我们实现并发性。多线程环境中的异步编程是实现并行的一种方法。

我会让它简短而有趣，让你们能够理解这些概念。

并发与并行——任务执行的方式。

Take an example in real life: There’s a challenge that requires you to both eat a whole huge cake and sing a whole song. You’ll win if you’re the fastest who sings the whole song and finishes the cake. So the rule is that you sing and eat concurrently. How you do that does not belong to the rule. You can eat the whole cake, then sing the whole song, or you can eat half a cake, then sing half a song, then do that again, etc. Parallelism is a specific kind of concurrency where tasks are really executed simultaneously. In computer science, parallelism can only be achieved in multicore environments.

同步与异步——编程模型。

在同步中，您将代码编写为按顺序从上到下执行的步骤 底部。在异步编程模型中，你将代码作为任务编写， 然后并发执行。并发执行意味着 所有的任务都可能同时执行。

我用真实的场景来解释3个话题 假设你想去艾哈迈达巴德到孟买旅行，但你不知道怎么走，所以你决定使用地图应用程序(谷歌Maps)。

很正常但效率很低的一种方法是，你可以在开车前看完整的路径，然后你开始开车并到达目的地。

平行-你可以不断地驾驶和观察路径。 异步-你的朋友和你在车里，你给他你的手机打开地图应用程序，告诉他看地图和指导你。 同时行驶——你开了几公里，把车停在一边，看地图，找方向，然后继续开车。

用类比来解释这些术语。

你的房子需要洗碗和洗衣服。

并发性:你不需要等完成一个任务后再启动另一个。例如，你可以先洗碗，也可以同时洗碗。他们可以按照任何顺序完成，也就是说，即使你先洗碗，也可能会先洗衣服。

平行性:你的房子里有不止一个人在做这件事;例如，你可以洗碗，而另一个人可以洗衣服。

异步:你让别人去洗衣服，你让别人去洗碗。他们实际上可以是同一个人(也就是说，你告诉他们去洗衣服，同时也告诉他们去洗碗)。他们处理完每个人后会向你报告。

同步:你叫别人去洗碗。你等着他们。当他们完成后，你可以做其他的事情(你可以告诉他们接下来洗衣服，或者你自己洗，或者你可以完全做其他的事情;重点是你在完成第一个任务时被阻塞，你与他们是同步的)。

总结

并发(Concurrent):多件事情似乎同时发生(具有在并发任务之间快速切换的能力;它们是并发的，因为每个任务都想要一块资源，CPU等等。)

并行是指同时发生多件事情(执行线程的数量与执行内核的数量密切相关)

异步是简单的非阻塞，当我们必须等待的事情不会让我们忙于等待(需要某种通知机制从我们离开的地方继续)

当一个经理有几个员工，并且可以给他们每个人一个单独的任务时，并行性就会发生。工人们做他们的工作，并向经理提供结果。如果任务不能完全分离，例如任务之间的结果有一定的依赖性，或者需要在没有其他推理的情况下使用相同的资源，那么并行度就受到这些约束，不能完全实现。

Concurrency happens when a manager has several tasks but only less workers, hence some workers are given more than one task. Any worker given multiple tasks, divides each original given task to several steps and does the steps interleaved, each task result will be given back to manager as soon as every steps of it finished. Manager receive a task result while other tasks started and progressed several steps but have not finished yet. If any worker with multiple task decides not to start a single step of a given task before finishing every steps of an already started task, this is called sequentiality.

Asynchrony is any of the two above mixed or separated, seen from the manager's point of view. When the manager assigns the tasks to either few or enough workers he shall not be awaited stalled until any results are given back. He can do his personal jobs or whatever, while jobs are progressing. Usually workers do not decide how tasks should be divided into steps. An inversion of control means manager decides over steps and gives single steps to workers. So when he receives a step result from a worker, give him another step, maybe from another task. The whom under control is responsible for composing back step results into task results as well. So Asynchronicity comes with responsibility for control and probably coordination. If any worker is urged to work sequentially, from manager's point of view he is a synchronous worker.

Summary As it's simple to guess, full parallelism is an unrealisable idea unless otherwise in rare mostly trivial cases. Since reality comes with interdependent tasks and shared resources and lack of workers. So concurrency is the reality. From manager's point of view this concurrency is best if it does not hinder him from fine controlling the tasks, and if positive it is called asynchronous. Also computer software engineering best practices, augmented by S in SOLID principle, historically made servers single step runners called micro-services, this returned back control to the clients. So current situation is concurrency from server point of view and asynchronicity from client point of view.

并发+并行都意味着同时运行多个任务。我看到一些人认为这是有区别的，但这取决于你咨询的参考文献，没有真正的正确或错误的答案。

Asynchronous: In some communities this means non-blocking code, which can mean two things: It almost always means it will not block an OS thread. However, non-blocking can optionally mean that the next line of source code in a function will continue to run without delay. In Python await asyncio.sleep(5) blocks execution of the function, but not the OS thread, and that is considered async. In Golang, you have "goroutines" that similarly to Python's await, they block execution of code, but not OS threads, however, this is not referred to as async in the Golang community. It's just concurrent programming.