Java Concurrency

J2SE 5.0 Concurrency, Carol McDonald Java Technology Architect Sun Microsystems, Inc.

Speaker’s Qualifications Carol cDonald: Java Architect at Sun Microsystems Before Sun, worked on software development of: Application to manage car Loans for Toyota (>10 million loans) Pharmaceutical Intranet ( Roche Switzerland) Telecom Network Mgmt ( Digital France) X.400 Email Server ( IBM Germany)

Motivation for Concurrency Utilities Developing concurrent classes was too hard Java has concurrency primitives: wait() , notify() , sleep() , interrupt() , synchronized These are too primitive, Too low level Easy to use incorrectly Incorrect use can produce poor performance

Concurrency Utilities Goals Provide a good set of concurrency building blocks library for concurrency like Collections for data structures Enhance scalability, performance, readability and thread safety of Java applications Beat C performance in high-end server applications

Puzzle: “Ping Pong” 01 class PingPong { 02 public static synchronized void main (String[] a) { 03 Thread t = new Thread() { 04 public void run() { 05 pong(); 06 } 07 }; 08 09 t.run(); 10 System.out.print("Ping"); 11 } 12 13 static synchronized void pong() { 14 System.out.print("Pong"); 15 } 16 }

What Does It Print? (a) PingPong (b) PongPing (c) It varies

What Does It Print? (a) PingPong (b) PongPing (c) It varies Not a multithreaded program!

Example How to start a thread public class HelloRunnable implements Runnable { public void run() { System.out.println("Hello from a thread!"); } public static void main(String args[]) { (new Thread (new HelloRunnable())) .start(); } }

Another Look 01 class PingPong { 02 public static synchronized void main(String[] a) { 03 Thread t = new Thread() { 04 public void run () { 05 pong(); 06 } 07 }; 08 09 t. run (); // Common typo! 10 System.out.print("Ping"); 11 } 12 13 static synchronized void pong() { 14 System.out.print("Pong"); 15 } 16 }

How Do You Fix It? 01 class PingPong { 02 public static synchronized void main(String[] a) { 03 Thread t = new Thread() { 04 public void run() { 05 pong(); 06 } 07 }; 08 09 t. start (); 10 System.out.print("Ping"); 11 } 12 13 static synchronized void pong() { 14 System.out.print("Pong"); 15 } 16 }

The Moral Invoke Thread.start , not Thread.run Common error Can be very difficult to diagnose

Concurrency Utilities: JSR-166 Task Scheduling Framework: Executor interface replaces direct use of Thread Callable and Future Synchronisers Semaphore, CyclicBarrier, CountDownLatch Concurrent collections : BlockingQueue Lock Atomic

Concurrency Utilities: JSR-166 Task Scheduling Framework: Executor interface replaces direct use of Thread No more direct Thread invocation Use myExecutor.execute(aRunnable); Not new Thread(aRunnable).start(); public interface Executor { void execute (Runnable command); }

Executor Framework for asynchronous execution public interface Executor { void execute (Runnable command); } public interface ExecutorService extends Executor { .. } public class Executors { //Factory methods static ExecutorService newFixedThreadPool(int poolSize); ... } Executor pool = Executors.newFixedThreadPool(5); pool.execute ( runnable ) ;

Creating Executors Factory methods in the Executors class public class Executors { static ExecutorService newSingleThreadedExecutor (); static ExecutorService newFixedThreadPool (int poolSize); static ExecutorService newCachedThreadPool (); static ScheduledExecutorService newScheduledThreadPool (); // Other methods not listed }

Thread Pool Example class WebService { public static void main(String[] args) { Executor pool = Executors.newFixedThreadPool(5); ServerSocket socket = new ServerSocket(999); for (;;) { final Socket connection = socket.accept(); Runnable task = new Runnable() { public void run() { new Handler().process ( connection ); } } pool.execute ( task ) ; } } } class Handler { void process (Socket s); }

ExecutorService for Lifecycle Support ExecutorService supports graceful and immediate shutdown public interface ExecutorService extends Executor { void shutdown (); List<Runnable> shutdownNow (); boolean isShutdown (); boolean isTerminated (); boolean awaitTermination (long timeout, TimeUnit unit); // additional methods not listed }

ScheduledExecutorService Deferred and recurring tasks Schedule execution of Callable or Runnable to run once after a fixed delay schedule () Schedule a Runnable to run periodically at a fixed rate scheduleAtFixedRate () Schedule a Runnable to run periodically with a fixed delay between executions scheduleWithFixedDelay () Submission returns a ScheduledFuture Can be used to cancel task

ScheduledExecutorService Example ScheduledExecutorService sched = Executors. newSingleThreadScheduledExecutor (); public void runTwiceAnHour(long howLong) { final Runnable rTask = new Runnable() { public void run() { /* Work to do */ } }; final ScheduledFuture<?> rTaskFuture = sched. scheduleAtFixedRate (rTask, 0, 1800, SECONDS); sched. schedule (new Runnable { public void run { rTaskFuture.cancel(true); } }, howLong, SECONDS); }

Synchronize Critical Section E.g., shared resource is an customer account. Certain methods called by multiple threads. Hold monitor lock for as short a time as possible . synchronized double getBalance() { Account acct = verify(name, password); return acct.balance; } Lock held for long time double getBalance() { synchronized (this) { Account acct = verify(name, password); return acct.balance; } } Current object is locked Equivalent to above double getBalance() { Account acct = verify(name, password); synchronized (acct) { return acct.balance}; } Better Only acct object is locked – for shorter time

Locks Java provides basic locking via synchronized Good for many situations, but some issues Single monitor per object Not possible to interrupt thread waiting for lock Not possible to time-out when waiting for a lock Block structured approach Aquiring multiple locks is complex Advanced techniques like hand-over-hand locking are not possible New Lock interface addresses these issues

Lock Interface No automatic unlocking Interface Lock { void lock (); void lockInterruptibly () throws IE ; boolean tryLock (); boolean tryLock (long t, TimeUnit u) throws IE ; //returns true if lock is aquired void unlock (); Condition newCondition () throws UnsupportedOperationException; } IE = InterruptedException

RentrantLock Simplest concrete implementation of Lock Same semantics as synchronized, but with more features Generally better performance under contention than synchronized Remember Lock is not automatically released Must use a finally block to release Multiple wait-sets supported Using Condition interface

Lock Example Lock lock = new RentrantLock(); public void accessProtectedResource() throws IllegalMonitorStateException { lock.lock(); try { // Access lock protected resource } finally { // Ensure lock is always released lock.unlock(); } }

ReadWriteLock Interface Has two locks controlling read and write access Multiple threads can aquire the read lock if no threads have a write lock Only one thread can aquire the write lock Methods to access locks rwl.readLock().lock(); rwl.writeLock().lock(); Better performance for read-mostly data access

ReadWriteLock Example ReentrantReadWriteLock rwl = new ReentrantReadWriteLock (); Lock rLock = rwl.readLock (); Lock wLock = rwl.writeLock (); ArrayList<String> data = new ArrayList<String>(); public String getData(int pos) { r.lock () ; try { return data.get (pos); } finally { r.unlock (); } } public void addData(int pos, String value) { w.lock (); try { data.add (pos, value); } finally { w.unlock (); } }

Synchronizers Co-ordinate access and control Semaphore Manages a fixed sized pool of resources CountDownLatch One or more threads wait for a set of threads to complete an action CyclicBarrier Set of threads wait until they all reach a specified point Exchanger Two threads reach a fixed point and exchange data

BlockingQueue Interface Provides thread safe way for multiple threads to manipulate collection Interface BlockingQueue<E> { void put (E o) throws IE; boolean offer (E o) throws IE; boolean offer (E o, long t, TimeUnit u) throws IE; E take () throws IE; E poll () throws IE; E poll (long t, TimeUnit u) throws IE; int drainTo (Collection<? super E> c); int drainTo (Collection<? super E> c, int max); // Other methods not listed }

BlockingQueue Implementations ArrayBlockingQueue Bounded queue, backed by an array, FIFO LinkedBlockingQueue Optionally bounded queue, backed by linked nodes, FIFO PriorityBlockingQueue Unbounded queue Uses comparator or natural ordering to determine the order of the queue

Blocking Queue Example: 1 private BlockingQueue<String> msgQueue ; public Logger ( BlockingQueue<String> mq) { msgQueue = mq; } public void run() { try { while (true) { String message = msgQueue.take (); /* Log message */ } } catch (InterruptedException ie) { } }

Blocking Queue Example: 2 private ArrayBlockingQueue messageQueue = new ArrayBlockingQueue<String>(10); Logger logger = new Logger( messageQueue ); public void run() { String someMessage; try { while (true) { /* Do some processing */ /* Blocks if no space available */ messageQueue.put(someMessage) ; } } catch (InterruptedException ie) { } }

Concurrent Collections ConcurrentMap (interface) Extends Map interface with atomic operations ConcurrentHashMap Fully concurrent retrieval Tunable concurrency for updates Constructor takes number of expected concurrent threads ConcurrentLinkedQueue Unbounded, thread safe queue, FIFO CopyOnWriteArrayList Optimised for frequent iteration, infrequent modifications

Summary New concurrency features are very powerful Lots of great features Take time to learn how to use them correctly Use them!

For More Information (1/2) Memory management white paper http://java.sun.com/j2se/reference/whitepapers/ Destructors, Finalizers, and Synchronization http://portal.acm.org/citation.cfm?id=604153 Finalization, Threads, and the Java Technology Memory Model http://developers.sun.com/learning/javaoneonline/2005/coreplatform/TS-3281.html Memory-retention due to finalization article http://www.devx.com/Java/Article/30192

For More Information (2/2) FindBugs http://findbugs.sourceforge.net Heap analysis tools Monitoring and Management in 6.0 http://java.sun.com/developer/technicalArticles/J2SE/monitoring/ Troubleshooting guide http://java.sun.com/javase/6/webnotes/trouble/ JConsole http://java.sun.com/developer/technicalArticles/J2SE/jconsole.html

Resources http://java.sun.com/javase Java.net http://jdk.dev.java.net

Stay in Touch with Java SE http://java.sun.com/javase JDK Software Community planetjdk.org community.java.net/jdk JDK 6 http://jdk6.dev.java.net/ http://jcp.org/en/jsr/detail?id=270 JDK 7 http://jdk7.dev.java.net/ http://jcp.org/en/jsr/detail?id=277

Thank You! Carol McDonald Java Technology Architect Sun Microsystems, Inc.

Java Concurrency

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