Future, futuretask and completable future are often asked in interviews

1.Callable and Runnable The difference between

  - Callable and Runnable Are abstractions of tasks , Are abstractions of logical content executed by threads .
  - Callable Support return value ,Runnable Return value... Is not supported .
  - Callable Support throwing checked exceptions ,Runnable I won't support it .
  - Callable combination Future It can realize the complex functions of thread asynchronous tasks .

2.Future Interface

Future Interface is the cancellation of asynchronous tasks 、 State of the query 、 Get the abstract definition of the result . His abstract methods are ：

  - boolean cancel(boolean mayInterruptIfRunning)： Cancel the execution of the task , The parameter controls whether to interrupt the task immediately or wait until the task is completed .
  - boolean isCancelled()： Determine if the task has been canceled .
  - boolean isDone()： Judge whether the task has been completed .
  - V get()： Wait for the task to finish , Get the return value , If the mission is cancelled , Exceptions will be thrown during waiting .
  - V get(long timeout, TimeUnit unit)： Timeout waiting for the task to end , If there is no result after timeout, it will throw Timeout abnormal .

3.FutureTask class

FutureTask Realized RunnableFuture Interface , Indirectly realized Future、Runnable.FutureTask Can be regarded as Runnable To perform the task , You can also think of it as Callable To pass Future Get task results .
FutureTask task = new FutureTask<>(job); // job The class of the instance should implement Runnable perhaps Callable.
It should be noted that ,FutureTask The task state of is one-way , Once the task is completed , His state is irreversible .
FutureTask There are also shortcomings ：

  -  Cannot combine multiple asynchronous tasks .
  -  The exception handling mechanism is not enough .
  -  Cannot serialize multiple asynchronous tasks .

4.CompletableFuture

CompletableFuture It is a powerful for multiple asynchronous task choreography JUC Tools under package ,CompletableFuture It's right Future Expansion and enhancement of .CompletableFuture When submitting a task, you can specify the incoming thread pool , If it doesn't come in , It is used by default JDK Self contained ForkJoinPool.commonPool() Thread pool . Its common method ：

  - thenApply()： Hand over the execution results of the previous tasks to the following Function.
  - thenCombine： Consolidation of tasks , return .
  - runAfterBoth： After even a task is completed , Then proceed to the next task .
  - anyOf()、allOf()： Orchestrate multiple asynchronous tasks , Wait for individual completion or Multiple completion .

What I use most in the project is probably the splitting and summarizing of multiple tasks , Improve the response speed of the interface based on the overall scenario ：

CompletableFuture There are 2 An asynchronous task execution method ：runAsync、supplyAsync The difference between them is that the latter supports scenarios with return values .
CompletableFuture Provided join and get Method , All to get CompletableFuture Result . The difference is ,join Method throws an unchecked exception ,get What is thrown is the detected exception, which needs to be handled manually .