Java Multithreading and Concurrency

JAVA MULTITHREADING &
CONCURRENCY

Rajesh

Ananda Kumar

(rajesh_1290k@yahoo.com)

1. Introduction
 What is a Process?






What is a Thread?
Benefits of Threads
Risks of Threads
Real-time usage of threads
Where are Threads used in Java?

WHAT IS A PROCESS?




Process is a isolated, independently executing
programs to which OS allocate resources such as
memory, file handlers, security credentials, etc.
Processes communicate with one another through
mechanisms like sockets, signal handlers, shared
memory, semaphores and files.

WHAT IS A THREAD?







Threads are called lightweight processes
Threads are spawned from Processes. So all the
threads created by one particular process can share
the process's resources (memory, file handlers, etc).
Threads allow multiple program flows to coexist
within the process.
Even though threads share process-wide resources
(memory, file handlers), but each thread has its own
Program Counter, Stack and local variables.

What are the benefits of Threads?

BENEFITS OF THREADS




Exploiting multiple processors
Handling asynchronous events
More responsive user interfaces

What are the risks of Threads?

RISKS OF THREADS
Threads in Java are like double-ended swords.
Following are few risks;

Safety Hazards

Liveness Hazards

Performance Hazards

RISKS OF THREADS – SAFETY HAZARDS


Unsafe codes may result in race conditions.
public class SequenceGenerator {
private int currentSequence = 0;

}


public int getNextSequence() {
return currentSequence++;
}

Proper synchronization should be done.

RISKS OF THREADS – LIVENESS

HAZARDS

A liveness failure occurs when an activity gets into a
state such that it permanently unable to make
forward progress. (Eg. Infinite loop)

A liveness hazard scenario;
Thread A waits for a lock to be released by Thread B
and vice versa. In this scenario, these programs will
wait for ever.


RISKS OF THREADS – PERFORMANCE
HAZARDS



Context Switches
When threads share data, they must use
synchronization which prevents compiler
optimizations, flush or invalidate memory caches and
create synchronization traffic on shared memory bus.

Where are Threads used in Java? Or how
frequently are we using threads in our day-today
work?

WHERE ARE THREADS USED IN JAVA?






Threads are everywhere. Every Java application uses
thread. Even a simple CUI based application that
runs in a single class uses threads.
When JVM starts, it creates threads for JVM
housekeeping tasks (garbage collection, finalization)
and a main thread for running “main” method.
Examples; Timers, Servlets & JSPs, RMI, Swing &
AWT

2. Java Threading Basics







Defining a Thread
Instantiate a Thread
Start a Threads
Thread Life-cycle
Thread Priorities
Important methods from java.lang.Thread & java.lang.Object class;

sleep()
yield()
join()
wait()
notify() & notifyAll()
 Synchronization
Locks
When & how to Synchronize?
 Deadlocks
 Thread Interactions

DEFINING A THREAD


Extending java.lang.Thread class
public class SampleThread extends Thread {
public void run() {
system.out.println(“Running....”);
}
}



Implementing java.lang.Runnable interface
public class SampleRunnable implements Runnable {
public void run() {
system.out.println(“Running....”);
}
}

INSTANTIATE A THREAD




We can use any of the above mentioned ways to
define a thread. But can instantiate a thread only by
using java.lang.Thread class.
If we create a thread by extending Thread class;
SampleThread thread1 = new SampleThread();



If we create a thread by implementing Runnable
interface;
SampleRunnable runnable1 = new SampleRunnable();
Thread thread2 = new Thread(runnable1);

START A THREAD


Use the java.lang.Thread class's start()
method to start a thread.
public class TestThread {
public static void main(String... args) {
SampleThread t1 = new SampleThread();
t1.start();
}
}



Once if we start a thread, it can never be
started again.

THREAD PRIORITIES








In java Threads always run with some priorities,
usually a number between 1 to 10.
If a thread enters the runnable state, and it has
higher priority than the current running thread, the
lower-priority thread will be bumped back to
runnable and highest-priority thread will be chosen
to run.
All priority threads being equal, a JVM
implementation if the scheduler is free to do just
about anything it likes.
It is recommended not to depend on that behavior for
correctness

THREAD PRIORITIES (CONT.)


Use the following syntax to set priorities;
thread1.setPriority(8);
thread1.start();

Default priority is 5.

We can use the pre-defined constants like;
Thread.MIN_PRIORITY, Thread.NORM_PRIORITY
and Thread.MAX_PRIORITY


IMPORTANT METHODS FROM
JAVA.LANG.THREAD &
JAVA.LANG.OBJECT - SLEEP()








Since sleep is a static method in java.lang.Thread
class, the call affects only the current thread in
execution.
Used for pausing the thread execution for a specific
milliseconds.
Throws InterruptedException when another thread
is trying to interrupt a sleeping thread.
Syntax;
try {
Thread.sleep(60 * 1000); // sleep for 1 second
} catch(InterruptedException iex) { }

JAVA.LANG.THREAD &
JAVA.LANG.OBJECT - YIELD()






Since yield is a static method in java.lang.Thread
class, the call affects only the current thread in
execution.
This method is supposed to make the current thread
head back to runnable to allow other threads of the
same priority to get their turn.
There is no guarantee to do what it claims.

JAVA.LANG.THREAD &
JAVA.LANG.OBJECT - JOIN()



This is a non-static method.
Syntax;
public class Test {
public static void main(String... args) {
thread1.start();
thread1.join();
// Some more code here
}
}



The above code takes the main thread and attaches it
to the end of 'thread1'. Main thread will be paused.
'thread1' will be finished and main thread will
continue its execution.

JAVA.LANG.THREAD & JAVA.LANG.OBJECT –
WAIT(), NOTIFY() & NOTIFYALL()

Use non-static wait method to prevent or pause a
thread from execution. Three overloaded versions
of wait are available. Preferably use the method
which takes a long as argument. This will make
the thread wake-up after a particular time.

'notify' and 'notifyAll' are methods used to inform
the waiting threads to stop waiting and move
them from blocked to runnable state.

All these 3 methods should be called only with in
a synchronization context. A thread can't invoke
these methods on an object unless it owns that
object's lock.

JAVA.LANG.THREAD & JAVA.LANG.OBJECT –
WAIT(), NOTIFY() & NOTIFYALL() (CONT.)




When the wait method is called, the locks acquired
by the objects are temporarily released.
An InterruptedException is thrown when a waiting
thread is interrupted by another thread.

SYNCHRONIZATION







Synchronization can be done only for methods and
blocks of code. Not for classes.
Use synchronize keyword in places where we
refer/use the mutable instance variables, as these
mutable instance variables can be accessed by
multiple threads simultaneously.
Another use of synchronization is memory visibility.
Synchronization ensures atomicity and visibility

SYNCHRONIZATION - LOCKS






Every object in java has exactly one built-in lock by
default. This is called intrinsic lock.
When we synchronize a block of code with 'this' or
when we synchronize a method, actually we
synchronize it using intrinsic locks.
We can use our own locks in place of intrinsic locks.
We can declare an object as instance variable in a
class and we can use block synchronization where the
block is synchronized using this object.

SYNCHRONIZATION – WHEN & HOW?









Synchronize all parts of code that access the mutable
instance variables declared inside a class.
Follow the same synchronization policy when
modifying the code.
Make granular synchronizations.
Two ways to use synchronization.
1) Add synchronized keyword to method
2) Wrap a block of code with-in synchronized
keyword

DEADLOCK
public class DeadlockDemo {
private Object lockA = new Object();
private Object lockB = new Object();
public void read() {
synchronized(lockA) {
synchronized(lockB) {
// Code for read operation
}
}
}

}

public void write() {
synchronized(lockB) {
synchronized(lockA) {
// Code for write operation
}
}
}

THREAD INTERACTIONS
public class Operator extends Thread {
public void run() {
while(true) {
// Get data
synchronized(this) {
// Calculate input required for Machine
notify();
}
}
}
}
public class Machine extends Thread {
Operator operator; // Assume we have a constructor
public void run() {
while(true) {
synchronized(operator) {
try { operator.wait(); }
catch (InterruptedException ex) { }
// Get the generated input and process it
}
}
}
}

3. Advanced Java thread concepts
Immutable objects
Thread-safety
What is Thread-safety?
Tops for creating thread-safe classes?
Volatile variables
Synchronized collections
Synchronized collections - Problems

IMMUTABLE OBJECTS


An object is immutable if;
−
−
−




Its state cannot be modified after construction
All its fields are final
It is properly constructed

Example is String.
Create a custom immutable class.

THREAD-SAFETY – WHAT IS THE
MEANING?


A class is tread-safe when it continues to behave
correctly when accessed from multiple threads,
regardless of the scheduling or interleaving of the
execution of those threads by the runtime
environment, and with no additional synchronization
or other coordination on the part of the calling code.

THREAD-SAFETY – TIPS







Stateless objects are thread-safe.
State-full objects with ALL immutable instance
variable are thread-safe.
Access to immutable instance variables don't have to
be synchronized.
Synchronize atomic operations (on mutable instance
variables). If not, there will be race condition. For
example;

THREAD-SAFETY – TIPS (CONT.)
public class CounterServlet implements Servlet {
private long count = 0;
public void service(ServletRequest request,
ServletResponse response) {
// Some code here
++count;
// Some code here
}
}




To fix the above scenario use AutomicLong instead of
‘long’.
For mutable state variables that may be accessed by
more than one thread, all accesses to that variable
must be performed with the same lock held. In this
case, we say that the variable is guarded by the same
lock.

VOLATILE VARIABLES






Volatile variables are not cached in registers or in
caches where they are hidden from other processors,
so read of a volatile variable always returns the most
recent write by any thread.
Accessing volatile variables perform no locking and
so cannot cause the executing thread to block.
Volatile variables are called light-weight
synchronization.
Locking can guarantee both visibility and atomicity;
but volatile variables can only guarantee visibility.

VOLATILE VARIABLE (CONT.)
Use volatile variables only when;

Writes to the variable do not depend on its current
value.

Variable does not participate in invariants with other
state variables.

Locking not required for any other reason while the
variable is being accessed.

SYNCHRONIZED COLLECTIONS




Vector and Hashtable are synchronized classed from
JDK 1.2
Collections.synchronizedXxx helps to create
synchronized wrapper classes. These classes achieve
thread-safety by encapsulating their state and
synchronizing all public methods so that only one
thread can access the collection at a time.

SYNCHRONIZED COLLECTIONS PROBLEMS


Compound actions are still technically thread-safe
even without client-side locking, but they may not
behave as you might expect when other threads can
concurrently modify the collection.

public Object getLast(Vector list) {
int lastIndex = list.size() - 1;
return list.get(lastIndex);
}
public void deleteLast(Vector list) {
list.remove(lastIndex);
}

SYNCHRONIZED COLLECTION –
PROBLEMS (CONT.)

SYNCHRONIZED COLLECTION –
PROBLEMS (CONT.)


To fix this issue the code inside the methods has to be
synchronized using the input argument 'list'.
public Object getLast(Vector list) {
synchronized(list) {
return list.get(lastIndex);
}
}
public void deleteLast(Vector list) {
synchronized(list) {
list.remove(lastIndex);
}
}

SYNCHRONIZED COLLECTIONS –
PROBLEMS (CONT.)






The iterators returned by the synchronized
collections are fail-fast iterators.
While iteration, if another thread modifies the list,
then the iteration would be stopped by throwing the
exception ConcurrentModificationException.
Ways to avoid; clone the collection and send it for
iteration or wrap the collection with
UnmodifiableXxx class or use concurrent collections.

4. Java Concurrency
Use of java.util.concurrent.lock.Lock
Use of java.util.concurrent.atomic package
Concurrent Collections
Synchronizers
Latches
FutureTask
Semaphores & Barriers
Executor Framework
Thread Pools
Executor Lifecycle
Callable & Future
Task Cancellation
Cancellation via Future
ExecutorService shutdown

USE OF
JAVA.UTIL.CONCURRENT.LOCK.LOCK








An alternative synchronization mechanism
introduced in JDK 1.5.
This was developed to solve the problems that we had
with traditional synchronization.
A commonly used Lock implementation is
ReenterantLock.
In general a reentrant lock mean, there is an
acquisition count associated with the lock, and if a
thread that holds the lock acquires it again, the
acquisition count is incremented and the lock then
needs to be released twice to truly release the lock.

USE OF
(CONT.)
Lock lock = new ReentrantLock();
lock.lock();
try {
// update object state
}
finally {
lock.unlock();
}


The above is the way to use it.
Many benchmarks have proven that, Reenterant
locks perform much better than synchronization.

USE OF
(CONT.)






This might tempt us to stop using synchronization.
But experts say that the new concurrency Lock is
only for advanced users for 2 reasons;

1) It is easy to forget to use a finally block to
release the lock, to the great detriment of your
program.
2) When the JVM manages lock acquisition and
release using synchronization, the JVM is able to
include locking information when generating
thread dumps. The Lock classes are just ordinary
classes, and the JVM does not (yet) have any
knowledge of which Lock objects are owned by
specific threads.

USE OF
JAVA.UTIL.CONCURRENT.ATOMIC
PACKAGE






A small toolkit of classes that support lock-free
thread-safe programming on single variables.
Few classes under this package are AtomicInteger,
AtomicLong, AtomicReference, etc.
Classes like AtomicInteger can be used in situations
like atomically incremented counter.

USE OF
PACKAGE (CONT.)
To understand the atomic package correctly, we
should understand 2 concepts;
 CAS: Compare And Swap. This idea is used in
processors also. A CAS operation includes three
operands -- a memory location (V), the expected
old value (A), and a new value (B). The processor
will atomically update the location to the new
value if the value that is there matches the
expected old value, otherwise it will do nothing. In
either case, it returns the value that was at that
location prior to the CAS instruction. CAS
effectively says "I think location V should have the
value A; if it does, put B in it, otherwise, don't
change it but tell me what value is there now."


USE OF
JAVA.UTIL.CONCURRENCY.ATOMIC
PACKAGE (CONT.)
Lock-Free: Concurrent algorithms based on CAS are
called lock-free, because threads do not ever have to
wait for a lock (sometimes called a mutex or critical
section, depending on the terminology of your
threading platform). Either the CAS operation
succeeds or it doesn't, but in either case, it completes
in a predictable amount of time. If the CAS fails, the
caller can retry the CAS operation or take other
action as it sees fit.

USE OF
PACKAGE (CONT.)
package java.util.concurrent.atomic;
public class AtomicInteger {
private int value;
public final int get() {
Return value;
}

}

public final int incrementAndGet() {
for (;;) {
int current = get();
int next = current + 1;
if (compareAndSet(current, next))
return next;
}
}

CONCURRENT COLLECTIONS










These collections are designed for concurrent access
from multiple threads.
Interface ConcurrentMap is introduced for adding
support for compound actions like put-if-absent,
replace and conditional remove.
Interface BlockingQueue is useful for producerconsumer design scenarios.
ConcurrentHashMap is replacement for synchronized
hash-based map.
CopyOnWriteArrayList is replacement for
synchronized list.

SYNCHRONIZERS





A Synchronizer is any object that co-ordinates the
control flow of threads based on its state.
Blocking queues can act as synchronizers.
Other synchronizers are Latches, Semaphores &
Barriers.

SYNCHRONIZER - LATCHES






A latch is a synchronizer that can delay the process
of threads until it reaches its terminal state.
Once the latch reaches the terminal state, it cannot
change the state again. So it remains open forever.
Implementation call is CountDownLatch. Uses
await() and countDown() methods for controlling the
thread flow.

SYNCHRONIZERS – LATCHES (CONT.)
Class TaskTimeCalculator {
public long timeTaks(int noOfThreads, final Runnable task) {
CountDownLatch startGate = new CountDownLatch(1);
CountDownLatch endGate = new CountDownLatch(noOfThreads);
for(int i = 0; i < noOfThreads; i++) {
Thread t = new Thread() {
public void run() {
startGate.await();
try { task.run(); }
finally { endGate.countDown(); }
}
};
t.start();
} // End of for

}

}

long start = System.nanoTime();
startGate.countDown();
endGate.await();
long end = System.nanoTime();
return end – start;

SYNCHRONIZERS - FUTURETASK






FutureTask acts like a latch.
It implements Future.
Computation represented by FutureTask is
implemented with a Callable interface.
Behavior of Future.get() depends on the state of the
task. If task is not completed, get() method waits or
blocks till the task is completed. Once completed, it
returns the result or throws an ExecutionException.

SYNCHRONIZERS – FUTURETASK
(CONT.)
class PreLoader {
private final FutureTask<ProductInfo> future =
new FutureTask<ProductInfo>(new Callable<ProductInfo>() {
public ProductInfo call() throws DataLoadException {
return loadProductInfo();
}
});
private final Thread thread = new Thread(future);
public void start() { thread.start(); }

}

public ProductInfo get()
throws DataLoadException, InterruptedException {
try {
return future.get();
} catch(ExecutionException eex) {
Throwable cause = e.getCause();
// other exception handling code here
}
}

SYNCHRONIZERS – SEMAPHORES &
BARRIERS




Counting Semaphores are used to control the number
of activities that can access a certain resource or
perform a given action at the same time. This can be
used for implementing resource pooling. Methods
used; acquire() & release().
While Latches are for waiting fir events, Barriers are
for waiting for other threads. All the threads must
come together at a barrier point at the same time in
order to proceed.

EXECUTOR FRAMEWORK






Tasks are logical units of work, and threads are a
mechanism by which tasks can run asynchronously.
Java.util.concurrent provides a flexible thread pool
implementation as part of the Executor framework.
The primary abstraction for task execution in the
java class libraries is not Thread, but Executor.

EXECUTOR FRAMEWORK (CONT.)
public interface Executor {
void execute(Runnable command);
}




This simple interface forms the basic for flexible &
powerful framework for asynchronous task execution
that support wide variety of task execution policy.
Executors help us decoupling task submission from
task execution.

THREAD POOLS
Thread pool manages a homogenous pool of worker
threads.

Different thread pool implementation are;
1) FixedThreadPool
2) CachedThreadPool
3) SingleThreadExecutor
4) ScheduledThreadPool

These pools are created using Executors class.


EXECUTOR LIFECYCLE


Since Executors provide a service to applications,
they should be able to shut down properly.
public interface ExecutorService
extends Executor {
void shutdown();
List<Runnable> shutdownNow();
boolean isShutdown();
boolean isTerminated();
boolean awaitTermination(
long timeout, TimeUnit unit)
throws InterruptedException;
}

// Other methods

EXECUTOR LIFECYCLE - EXAMPLE
class LifeCycleWebServer {
private final ExecutorSerive exec =
Executors.newFixedThreadPool(100);
public void start() throws IOException {
ServerSocket socket = new ServerSocket(80);

}

while(!exec.isShutdown()) {
final Socket conn = socket.accept();
exec.execute(new Runnable() {
public void run() { handleRequest(conn); }
});
}

public void stop() { exec.shutdown(); }

}

void handleRequest(Socket connection) {
Request req = readRequest(connection);
if(isShutdownRequest(req)
stop();
else
dispatchRequest(req);
}

CALLABLE & FUTURE






In Runnable interface, run cannot return a value or
throw checked exception. Callable interface solves
this problem.
Callable expects a main entry point called 'call'
method and can return a value. Also it can throw an
Exception.
Future represents the lifecycle of a task and provides
methods to test wherher a task is completed or been
canceled, retrieve its result and cancel the task.

CALLABLE & FUTURE (CONT.)
public interface Callable<V> {
V call() throws Exception;
}
public interface Future<V> {
boolean cancel(boolean mayInterruptIfRunning);
boolean isCancelled();
boolean isDone();
V get() throws …;
V get(long timeout, TimeUnti unit) throws …;
}

TASK CANCELLATION




An activity is cancellable if external code can move it
to completion before its normal completion.
There are different ways to achieve this.

TASK CANCELLATION – MANUAL
public class PrimeGenerator implements Runnable {
private final List<BigInteger> primes =
new ArrayList<BigInteger>();
private volatile boolean cancelled = false;
public void run() {
BigInteger p = BigInteger.ONE;
while(!cancelled) {
p = p.nextProbablePrime();
primes.add(p);
}
}
}
public void cancel() { cancelled = true; }

}

public synchronized List<BigInteger> get() {
return new ArrayList<BigInteger>(primes);
}

TASK CANCELLATION – THREAD
INTERRUPTION




The problem with previous approch is, it cancels all
the executing threads. We cannot cancel a specific
instance.
We can use the following methods in Thread class
instead.
public class Thread {
public void interrupt() { .. }
public boolean isInterrupted() { .. }
public static boolean interrupted() { .. }
}

TASK CANCELLATION – THREAD
INTERRUPTION (CONT.)
public class PrimeGenerator implements Runnable {
private final List<BigInteger> primes =
new ArrayList<BigInteger>();
private volatile boolean cancelled = false;
public void run() {
BigInteger p = BigInteger.ONE;
While(!Thread.currentThread().isInterrupted()) {
p = p.nextProbablePrime();
primes.add(p);
}
}
}
public void cancel() { interrupt(); }

}

public synchronized List<BigInteger> get() {
return new ArrayList<BigInteger>(primes);
}

CANCELLATION VIA FUTURE
public void timedRun(Runnable r,
long timeout,
TimeUnit unit) throws InterruptedException {
Future<?> task = taskExec.submit(r);

}

try {
task.get(timeout, unit);
} catch(TimeoutException tex) {
// task will be cancelled below
} catch(ExecutionException eex) {
// Process eex and re-throw if needed
} finally {
if(!task.isCancelled() && !task.isDone()) {
task.cancel(true);
}
}

EXECUTORSERVICE SHUTDOWN
If Executors are not shutdown properly, it won't
allow the JVM process to close.

Two ways to shutdown;
1) Call shutdown()
2) Call shutdownNow()


That’s all flocks! 

Thank you!! Please send me your valuable feedbacks.

Java Multithreading and Concurrency

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