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CSCI 6900: Design, Implementation, and Verification of Concurrent Software

Eileen KraemerAugust 24th, 2010

The University of Georgia

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Java Threads & Concurrency, continued 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

ExecutorsExecutor InterfacesThread Pools

Concurrent CollectionsAtomic Variables

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Executors

Separate thread management & control from the rest of the appSeparation of concernsReusability

Executor Interfaces Three kinds of object types

Thread poolsMost common implementation of executor

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Executor Interfaces

Executor ExecutorService ScheduledExecutorService

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The Executor Interface

See: java.util.concurrent Interface Executor

An “executor” is an object that executes submitted Runnable tasks. Separates task submission from details of how to schedule & run

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Contains one method:

executevoid execute(Runnable command) Executes the given command at some time in the future. The command

may execute in a new thread, in a pooled thread, or in the calling thread, at the discretion of the Executor implementation.

Parameters:command - the runnable task

Throws: RejectedExecutionException - if this task cannot be accepted for

execution. NullPointerException - if command is null

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Instead of writing

new Thread(new(RunnableTask1())).start(); new Thread(new(RunnableTask2())).start();

Each of which creates a new thread and launches it immediately

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Write:

Executor executor = anExecutor(); executor.execute(new RunnableTask1()); executor.execute(new RunnableTask2());

Depending on the implementation of the Executor interface, the thread may use an existing thread, place r in a queue to wait for a worker thread, etc.

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This executor runs the submitted task in the caller’s thread …

class DirectExecutor implements Executor { public void execute(Runnable r) {

r.run(); }

}

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This one spawns a new thread to execute the task …

class ThreadPerTaskExecutor implements Executor { public void execute(Runnable r) {

new Thread(r).start(); }

}

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Others may impose some limitation on how and when tasks are scheduled. See example SerialExecutor

serializes submission of tasks to second executor

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The ExecutorService Interface

Extends the Executor Interface Adds methods for:

submitaccepts Runnable and Callable (can return a value) objectsreturns a Future objectallows the submission of collections of Callable objects

invokeAll, invokeAnyshutdownawaitTermination isShutdown, isTerminated

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The ScheduledExecutorService Interface

Offers a schedule methodExecutes a Runnable or Callable task after a specified delay

AlsoscheduleAtFixedRatescheduleWithFixedDelay

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Thread Pools

Worker threads Used to execute multiple tasks

Minimizes overhead of thread creation

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java.util.concurrent.Executors

Contains factory and utility methods for Executor, ExecutorService, ScheduledExecutorService, ThreadFactory, and Callable classesMethods that create and return an ExecutorService set up with

commonly useful configuration settings. Methods that create and return a ScheduledExecutorService set

up with commonly useful configuration settings. Methods that create and return a "wrapped" ExecutorService, that

disables reconfiguration by making implementation-specific methods inaccessible.

Methods that create and return a ThreadFactory that sets newly created threads to a known state.

Methods that create and return a Callable out of other closure-like forms, so they can be used in execution methods requiring Callable.

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Fixed thread pool

Maintains a fixed number of worker threads An internal queue holds submitted tasks

Holds tasks when #tasks > #threads

AdvantagesApplications degrade gracefully

By limiting resources consumed by threads to fixed number

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newFixedThreadPool factory method

Creates a thread pool that reuses a fixed number of threads operating off a shared unbounded queue.

At any point, at most n Threads threads will be actively processing tasks.

If additional tasks are submitted when all threads are active, they will wait in the queue until a thread is available.

If any thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks.

The threads in the pool will exist until it is explicitly shutdown.

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newCachedThreadPool

creates an executor with an expandable thread pool. suitable for applications that launch many short-lived tasks.

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newSingleThreadExecutor

creates an executor that executes a single task at a time.

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Concurrent Collections

Help avoid memory consistency errorsDefine a happens-before relationship between an operation that

adds an object to the collection and and subsequent operations that remove the object

BlockingQueue ConcurrentMap ConcurrentNavigableMap

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BlockingQueue

defines a first-in-first-out data structure that blocks or times out when you attempt to

add to a full queue, or retrieve from an empty queue.

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ConcurrentMap

a subinterface of java.util.Map defines useful atomic operations.

remove or replace a key-value pair only if the key is present, add a key-value pair only if the key is absent.

The standard general-purpose implementation of ConcurrentMap is ConcurrentHashMap, which is a concurrent analog of HashMap.

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ConcurrentNavigableMap

a subinterface of ConcurrentMap supports approximate matches.

The standard general-purpose implementation of ConcurrentNavigableMap is ConcurrentSkipListMap, which is a concurrent analog of TreeMap.

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Atomic Variables – java.util.concurrent.atomic

defines classes that support atomic operations on single variables. have get and set methods that work like reads and writes on volatile

variables.a set has a happens-before relationship with any subsequent get

on the same variable. compareAndSet method simple atomic arithmetic methods for integer atomic variables.

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Next steps…

Read Ch. 2 of Magee and Kramer, Processes and Threads