Concurrency Programming in Java - 07 - High-level Concurrency objects, Lock Objects, Executors, Concurrent Collections, Atomic Variables, Concurrent Random Numbers

Concurrent
Programming
07
Sachintha Gunasena MBCS
http://lk.linkedin.com/in/sachinthadtg

Recap so far…

Previous Session
• Processes & Threads
• Thread Objects
• Defining & Starting a Thread
• Pausing Execution with Sleep
• Interrupts
• Joins
• The Simple Threads Example
• Synchronisation
• Thread Interference
• Memory Consistency Errors

Previous Session
Cont.d
• Synchronization
• Synchronised Methods
• Intrinsic Locks & Synchronisation
• Atomic Access
• Liveness
• Deadlock
• Starvation & Livestock
• Guarded Blocks
• Immutable Objects
• A Synchronized Class Example
• A Strategy for Defining Immutable Objects

Today’s Session
• High Level Concurrency Objects
• Lock Objects
• Executors
• Executor Interfaces
• Thread Pools
• Fork/Join
• Concurrent Collections
• Atomic Variables
• Concurrent Random Numbers
• Questions & Exercises

High Level
Concurrency Objects

High Level Concurrency
Objects
• So far, this lesson has focused on the low-level APIs that have been part of the
Java platform from the very beginning.
• These APIs are adequate for very basic tasks, but higher-level building blocks
are needed for more advanced tasks.
• This is especially true for massively concurrent applications that fully exploit
today's multiprocessor and multi-core systems.
• In this section we'll look at some of the high-level concurrency features
introduced with version 5.0 of the Java platform.
• Most of these features are implemented in the new java.util.concurrent
packages.
• There are also new concurrent data structures in the Java Collections
Framework.

High Level Concurrency
Objects Cont.d
• Lock objects
• support locking idioms that simplify many concurrent applications.
• Executors
• define a high-level API for launching and managing threads.
• Executor implementations provided by java.util.concurrent provide thread pool management suitable for
large-scale applications.
• Concurrent collections
• make it easier to manage large collections of data, and can greatly reduce the need for synchronization.
• Atomic variables
• have features that minimize synchronization and help avoid memory consistency errors.
• ThreadLocalRandom
• (in JDK 7) provides efficient generation of pseudorandom numbers from multiple threads.

Lock Objects
• Synchronized code relies on a simple kind of reentrant lock.
• This kind of lock is easy to use, but has many limitations.
• More sophisticated locking idioms are supported by the
java.util.concurrent.locks package.
• We won't examine this package in detail, but instead will focus on its most
basic interface, Lock.
• Lock objects work very much like the implicit locks used by synchronized code.
• As with implicit locks, only one thread can own a Lock object at a time.
• Lock objects also support a wait/notify mechanism, through their associated
Condition objects.

Lock Objects Cont.d
• The biggest advantage of Lock objects over implicit locks is their ability to back out of an attempt to acquire
a lock.
• The tryLock method backs out if the lock is not available immediately or before a timeout expires (if
specified).
• The lockInterruptibly method backs out if another thread sends an interrupt before the lock is acquired.
• Let's use Lock objects to solve the deadlock problem we saw in Liveness.
• Tom and Jerry have trained themselves to notice when a friend is about to bow.
• We model this improvement by requiring that our Friend objects must acquire locks for both participants
before proceeding with the bow.
• Here is the source code for the improved model, Safelock.
• To demonstrate the versatility of this idiom,
• we assume that Tom and Jerry are so infatuated with their newfound ability to bow safely that they can't
stop bowing to each other:

Lock Objects Cont.d
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.Random;
public class Safelock {
static class Friend {
private final String name;
private final Lock lock = new ReentrantLock();
public Friend(String name) {
this.name = name;
}
public String getName() {
return this.name;
}
public boolean impendingBow(Friend bower) {
Boolean myLock = false;
Boolean yourLock = false;
try {
myLock = lock.tryLock();
yourLock = bower.lock.tryLock();
} finally {
if (! (myLock && yourLock)) {
if (myLock) {
lock.unlock();
}
if (yourLock) {
bower.lock.unlock();
}
}
}
return myLock && yourLock;
}
public void bow(Friend bower) {
if (impendingBow(bower)) {
try {
System.out.format("%s: %s has"
+ " bowed to me!%n",
this.name, bower.getName());
bower.bowBack(this);
} finally {
lock.unlock();
bower.lock.unlock();
}
} else {
System.out.format("%s: %s started"
+ " to bow to me, but saw that"
+ " I was already bowing to"
+ " him.%n",
}
}
public void bowBack(Friend bower) {
System.out.format("%s: %s has" +
" bowed back to me!%n",
}
}
static class BowLoop implements Runnable {
private Friend bower;
private Friend bowee;
public BowLoop(Friend bower, Friend bowee) {
this.bower = bower;
this.bowee = bowee;
}
public void run() {
Random random = new Random();
for (;;) {
try {
Thread.sleep(random.nextInt(10));
} catch (InterruptedException e) {}
bowee.bow(bower);
}
}
}
public static void main(String[] args) {
final Friend alphonse =
new Friend("Alphonse");
final Friend gaston =
new Friend("Gaston");
new Thread(new BowLoop(alphonse, gaston)).start();
new Thread(new BowLoop(gaston, alphonse)).start();
}
}

Executors
• In all of the previous examples,
• there's a close connection between
• the task being done by a new thread,
• as defined by its Runnable object,
• and the thread itself,
• as defined by a Thread object.
• This works well for small applications, but in large-scale applications,
• it makes sense to separate thread management and creation from the rest of the
application.
• Objects that encapsulate these functions are known as executors.

Executors Cont.d
• The following subsections describe executors in
detail.
• Executor Interfaces define the three executor
object types.
• Thread Pools are the most common kind of
executor implementation.
• Fork/Join is a framework (new in JDK 7) for
taking advantage of multiple processors.

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
• with
• 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.
(new Thread(r)).start();
e.execute(r);

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 SceduledExecutorService
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.

Thread Pools
• Most of the executor implementations in java.util.concurrent use thread pools,
• which consist of worker threads.
• This kind of thread exists separately from the Runnable and Callable tasks it
executes
• and is often used to execute multiple tasks.
• Using worker threads minimizes the overhead due to thread creation.
• Thread objects use a significant amount of memory,
• and in a large-scale application,
• allocating and deallocating many thread objects
• creates a significant memory management overhead.

Thread Pools Cont.d
• One common type of thread pool is the fixed thread pool.
• This type of pool always has a specified number of threads
running;
• if a thread is somehow terminated while it is still in use,
• it is automatically replaced with a new thread.
• Tasks are submitted to the pool via an internal queue,
• which holds extra tasks whenever there are more
active tasks than threads.

Thread Pools Cont.d
• An important advantage of the fixed thread pool is that applications using it degrade
gracefully.
• To understand this, consider a web server application
• where each HTTP request is handled by a separate thread.
• If the application simply creates a new thread for every new HTTP request,
• and the system receives more requests than it can handle immediately,
• the application will suddenly stop responding to all requests
• when the overhead of all those threads exceed the capacity of the system.
• With a limit on the number of the threads that can be created,
• the application will not be servicing HTTP requests as quickly as they come in,
• but it will be servicing them as quickly as the system can sustain.

Thread Pools Cont.d
• A simple way to create an executor that uses a
fixed thread pool is
• to invoke the newFixedThreadPool factory
method in java.util.concurrent.Executors
• This class also provides the following factory
methods:

Thread Pools Cont.d
• The newCachedThreadPool method
• creates an executor with an expandable thread pool.
• This executor is suitable for applications that launch many short-lived tasks.
• The newSingleThreadExecutor method
• creates an executor
• that executes a single task at a time.
• Several factory methods are ScheduledExecutorService versions of the above executors.
• If none of the executors provided by the above factory methods meet your needs,
• constructing instances of java.util.concurrent.ThreadPoolExecutor
• or java.util.concurrent.ScheduledThreadPoolExecutor
• will give you additional options.

Fork / Join
• The fork/join framework is an implementation of the ExecutorService interface that helps you take
advantage of multiple processors.
• It is designed for work that can be broken into smaller pieces recursively.
• The goal is to use all the available processing power to enhance the performance of your
application.
• As with any ExecutorService implementation, the fork/join framework distributes tasks to worker
threads in a thread pool.
• The fork/join framework is distinct because it uses a work-stealing algorithm.
• Worker threads that run out of things to do can steal tasks from other threads that are still busy.
• The center of the fork/join framework is the ForkJoinPool class, an extension of the
AbstractExecutorService class.
• ForkJoinPool implements the core work-stealing algorithm and can execute ForkJoinTask
processes.

Basic Use
• The first step for using the fork/join framework is to write code that performs a segment of the work.
• Your code should look similar to the following pseudocode:
• Wrap this code in a ForkJoinTask subclass,
• typically using one of its more specialized types,
• either RecursiveTask (which can return a result) or RecursiveAction.
• After your ForkJoinTask subclass is ready,
• create the object that represents all the work to be done
• and pass it to the invoke() method of a ForkJoinPool instance.
if (my portion of the work is small enough)
do the work directly
else
split my work into two pieces
invoke the two pieces and wait for the results

Blurring for Clarity
• To understand how the fork/join framework works, consider the following example.
• Suppose that you want to blur an image.
• The original source image is represented by an array of integers,
• where each integer contains the color values for a single pixel.
• The blurred destination image is also represented by an integer array with the same size as the source.
• Performing the blur is accomplished by
• working through the source array one pixel at a time.
• Each pixel is averaged with its surrounding pixels (the red, green, and blue components are averaged),
• and the result is placed in the destination array.
• Since an image is a large array, this process can take a long time.
• You can take advantage of concurrent processing on multiprocessor systems
• by implementing the algorithm using the fork/join framework.
• Here is one possible implementation:

Blurring for Clarity Cont.dpublic class ForkBlur extends RecursiveAction {
private int[] mSource;
private int mStart;
private int mLength;
private int[] mDestination;
// Processing window size; should be odd.
private int mBlurWidth = 15;
public ForkBlur(int[] src, int start, int length, int[] dst) {
mSource = src;
mStart = start;
mLength = length;
mDestination = dst;
}
protected void computeDirectly() {
int sidePixels = (mBlurWidth - 1) / 2;
for (int index = mStart; index < mStart + mLength; index++) {
// Calculate average.
float rt = 0, gt = 0, bt = 0;
for (int mi = -sidePixels; mi <= sidePixels; mi++) {
int mindex = Math.min(Math.max(mi + index, 0),
mSource.length - 1);
int pixel = mSource[mindex];
rt += (float)((pixel & 0x00ff0000) >> 16)
/ mBlurWidth;
gt += (float)((pixel & 0x0000ff00) >> 8)
/ mBlurWidth;
bt += (float)((pixel & 0x000000ff) >> 0)
/ mBlurWidth;
}
// Reassemble destination pixel.
int dpixel = (0xff000000 ) |
(((int)rt) << 16) |
(((int)gt) << 8) |
(((int)bt) << 0);
mDestination[index] = dpixel;
}
}
…

Blurring for Clarity Cont.d
• Now you implement the abstract compute() method,
• which either performs the blur directly or splits it into two smaller tasks.
• A simple array length threshold helps determine whether the work is performed or split.
protected static int sThreshold = 100000;
protected void compute() {
if (mLength < sThreshold) {
computeDirectly();
return;
}
int split = mLength / 2;
invokeAll(new ForkBlur(mSource, mStart, split, mDestination),
new ForkBlur(mSource, mStart + split, mLength - split,
mDestination));
}

Blurring for Clarity Cont.d
• If the previous methods are in a subclass of the RecursiveAction class,
• then setting up the task to run in a ForkJoinPool is straightforward,
• and involves the following steps:
• Create a task that represents all of the work to be done.
• Create the ForkJoinPool that will run the task.
• Run the task.
// source image pixels are in src
// destination image pixels are in dst
ForkBlur fb = new ForkBlur(src, 0, src.length, dst);
ForkJoinPool pool = new ForkJoinPool();
pool.invoke(fb);

Standard Implementations
• Besides using the fork/join framework to implement custom algorithms for tasks to be performed
concurrently on a multiprocessor system (such as the ForkBlur.java example in the previous section),
• there are some generally useful features in Java SE which are already implemented using the fork/join
framework.
• One such implementation, introduced in Java SE 8, is used by the java.util.Arrays class for its
parallelSort() methods.
• These methods are similar to sort(), but leverage concurrency via the fork/join framework.
• Parallel sorting of large arrays is faster than sequential sorting when run on multiprocessor systems.
• However, how exactly the fork/join framework is leveraged by these methods is outside the scope of the
Java Tutorials.
• For this information, see the Java API documentation.
• Another implementation of the fork/join framework is used by methods in the java.util.streams package,
which is part of Project Lambda scheduled for the Java SE 8 release.
• For more information, see the Lambda Expressions section in java site.

Concurrent Collections
• The java.util.concurrent package includes a number of additions to the Java Collections Framework.
• These are most easily categorized by the collection interfaces provided:
• 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.
• ConcurrentMap
• is a subinterface of java.util.Map that defines useful atomic operations.
• These operations remove or replace a key-value pair only if the key is present, or add a key-
value pair only if the key is absent.
• Making these operations atomic helps avoid synchronization.
• The standard general-purpose implementation of ConcurrentMap is ConcurrentHashMap,
which is a concurrent analog of HashMap.

Concurrent Collections
Cont.d
• ConcurrentNavigableMap
• is a subinterface of ConcurrentMap that supports approximate matches.
• The standard general-purpose implementation of ConcurrentNavigableMap
• is ConcurrentSkipListMap,
• which is a concurrent analog of TreeMap.
• All of these collections help avoid Memory Consistency Errors
• by defining a happens-before relationship between an operation
• that adds an object to the collection with subsequent operations
• that access or remove that object.

Atomic Variables
• The java.util.concurrent.atomic package defines classes that support atomic
operations on single variables.
• All classes have get and set methods that work like reads and writes on volatile
variables.
• That is, a set has a happens-before relationship with any subsequent get on the
same variable.
• The atomic compareAndSet method also has these memory consistency
features,
• as do the simple atomic arithmetic methods that apply to integer atomic
variables.
• To see how this package might be used, let's return to the Counter class we
originally used to demonstrate thread interference:

Atomic Variables Cont.d
class Counter {
private int c = 0;
public void increment() {
c++;
}
public void decrement() {
c--;
}
public int value() {
return c;
}
}

• One way to make Counter safe from thread
interference
• is to make its methods synchronized, as in
SynchronizedCounter:

class SynchronizedCounter {
private int c = 0;
public synchronized void increment() {
c++;
}
public synchronized void decrement() {
c--;
}
public synchronized int value() {
return c;
}
}

• For this simple class, synchronization is an acceptable solution.
• But for a more complicated class,
• we might want to avoid the liveness impact of unnecessary
synchronization.
• Replacing the int field with an AtomicInteger
• allows us to prevent thread interference
• without resorting to synchronization, as in AtomicCounter:

import java.util.concurrent.atomic.AtomicInteger;
class AtomicCounter {
private AtomicInteger c = new AtomicInteger(0);
public void increment() {
c.incrementAndGet();
}
public void decrement() {
c.decrementAndGet();
}
public int value() {
return c.get();
}
}

Concurrent Random
Numbers
• In JDK 7, java.util.concurrent includes a convenience class, ThreadLocalRandom,
• for applications that
• expect to use random numbers
• from multiple threads or ForkJoinTasks.
• For concurrent access, using ThreadLocalRandom
• instead of Math.random()
• results in less contention and, ultimately, better performance.
• All you need to do is
• call ThreadLocalRandom.current(),
• then call one of its methods to retrieve a random number.
• Here is one example:
int r = ThreadLocalRandom.current() .nextInt(4, 77);

Moving Forward
• http://sourceforge.net/projects/javaconcurrenta/

Questions
• Can you pass a Thread object to
Executor.execute?
• Would such an invocation make sense?

Exercises
• Compile and run BadThreads.java:
• The application should print out "Mares do eat oats."
• Is it guaranteed to always do this?
• If not, why not?
• Would it help to change the parameters of the two invocations of Sleep?
• How would you guarantee that all changes to message will be visible in
the main thread?
• Modify the producer-consumer example in Guarded Blocks to use a
standard library class instead of the Drop class.

Exercisespublic class BadThreads {
static String message;
private static class CorrectorThread
extends Thread {
public void run() {
try {
sleep(1000);
} catch (InterruptedException e) {}
// Key statement 1:
message = "Mares do eat oats.";
}
}
public static void main(String args[])
throws InterruptedException {
(new CorrectorThread()).start();
message = "Mares do not eat oats.";
Thread.sleep(2000);
// Key statement 2:
System.out.println(message);
}
}

Thank you.

Concurrency Programming in Java - 07 - High-level Concurrency objects, Lock Objects, Executors, Concurrent Collections, Atomic Variables, Concurrent Random Numbers

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