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Trail: Essential Classes
Lesson: Concurrency
Section: High Level Concurrency Objects
Subsection: Executors
Concurrency
Processes and Threads
Thread Objects
Defining and Starting a Thread
Pausing Execution with Sleep
Interrupts
Joins
The SimpleThreads Example
Synchronization
Thread Interference
Memory Consistency Errors
Synchronized Methods
Intrinsic Locks and Synchronization
Atomic Access
Liveness
Deadlock
Starvation and Livelock
Guarded Blocks
Immutable Objects
A Synchronized Class Example
A Strategy for Defining Immutable Objects
High Level Concurrency Objects
Lock Objects
Executors
Executor Interfaces
Thread Pools
Concurrent Collections
Atomic Variables
For Further Reading
Questions and Exercises
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Executor Interfaces

> The `java.util.concurrent` package defines three executor interfaces:
> - `Executor`, a simple interface that supports launching new tasks.
> - `ExecutorService`, a subinterface of `Executor`, which adds features that help manage the lifecycle, both of the individual tasks and of the executor itself.
> - `ScheduledExecutorService`, a subinterface of `ExecutorService`, supports future and/or periodic execution of tasks.
> Typically, variables that refer to executor objects are declared as one of these three interface types, not with an executor class type.

> ### The `Executor` Interface

> The `Executor` interface provides a single method, `execute`, designed to be a drop-in replacement for a common thread-creation idiom. If `r` is a `Runnable` object, and `e` is an `Executor` object you can replace

> >     (new Thread(r)).start();

> with

> >     e.execute(r);

> However, the definition of `execute` is less specific. The low-level idiom creates a new thread and launches it immediately. Depending on the `Executor` implementation, `execute` may do the same thing, but is more likely to use an existing worker thread to run `r`, or to place `r` in a queue to wait for a worker thread to become available. (We'll describe worker threads in the section on Thread Pools.)

> The executor implementations in `java.util.concurrent` are designed to make full use of the more advanced `ExecutorService` and `ScheduledExecutorService` interfaces, although they also work with the base `Executor` interface.

> ### The `ExecutorService` Interface

> The `ExecutorService` interface supplements `execute` with a similar, but more versatile `submit` method. Like `execute`, `submit` accepts `Runnable` objects, but also accepts `Callable` objects, which allow the task to return a value. The `submit` method returns a `Future` object, which is used to retrieve the `Callable` return value and to manage the status of both `Callable` and `Runnable` tasks.

> `ExecutorService` also provides methods for submitting large collections of `Callable` objects. Finally, `ExecutorService` provides a number of methods for managing the shutdown of the executor. To support immediate shutdown, tasks should handle interrupts correctly.

> ### The `ScheduledExecutorService` Interface

> The `ScheduledExecutorService` interface supplements the methods of its parent `ExecutorService` with `schedule`, which executes a `Runnable` or `Callable` task after a specified delay. In addition, the interface defines `scheduleAtFixedRate` and `scheduleWithFixedDelay`, which executes specified tasks repeatedly, at defined intervals.

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