Concurrency in Java

1. Concurrency in Java Last Updated: Fall 2009 Paul Ammann SWE 619

2. Agenda • Some General Concurrency Mechanisms • Concurrency in Java • Synchronization in Java • Rules for Threads • Liskov Abstraction Example

3. Concepts to be covered • Threads, concurrency • Interleaving, race conditions • Atomicity of execution • Mutual exclusion • Thread states • Synchronization, locks • Inter-thread communication

4. Some General Concurrency Mechanisms • Rendevous (Ada) • Obviates need for certain low level primitives • In practice: Web Servers • Single server, scores of clients! Waiting time? • Threads • Subject of today’s talk • Database mechanisms • Concurrency entirely hidden from user • ACID properties

5. Interleaving c.inc(); // c is a counter c.inc(); System.out.println(c.get()); • Suppose 2 threads are created • Each executes the 3 statements • Order in which they are executed? • Does thread 1 get to execute all 3 before thread 2?

6. Atomicity of execution • What is the smallest unit of execution that cannot be interleaved? • Can we ensure atomicity across multiple statements? // broken because of interleaving private static int nextSerialNumber = 0; // in short nSN public static int generateSerialNumber(){ return nextSerialNumber++; } // nSN = nSN + 1

7. Atomic read and write Thread 1 reads nSN = 4 Thread 2 reads nSN = 4 //should read 5 Thread 3 reads nSN = 4 //should read 6 Thread 1 writes nSN = 5 Thread 2 writes nSN = 5 //should write 6 Thread 4 reads nSN = 5 //should read 7 Time

8. Concurrency in Java • Various mechanisms to obtain a Thread • Extend the Thread class class T extends Thread { … public void run() {…} } Thread t = new T(); t.start(); // start() calls run() • Implement the Runnable (or Callable) interface • Use an ExecutorService • Examples: Counter1.java, Counter2.java, Counter3.java, Counter4.java

9. Thread states • Initial – prior to start() • Runnable – started, but not necessarily running (may be preempted) • Blocked – waiting for some event to occur • Stop – the run() method has returned (avoid other ways of entering Stop state) • The join() method waits for a thread to stop • Or ExecutorService shutdown(), get() methods

10. Synchronization in Java • Threads use a “lock” model • Lock is associated with an object • “this” for synchronized methods • Specified object for synchronized block • Security/Reliability implications for public locks • Only one thread at a time may hold the lock • Examples • Wacky.java, Wacky1.java • BoundedQueue.java • SyncBoundedQueue.java

11. Java Synchronization • Every Java object has an associated lock • “synchronized” makes a method lockable • A thread that locked a lock is said to be holding the lock • When a thread unlocks a lock, the thread is said to release the lock • Synchronization: mutual exclusion and signal-wait

12. Communication among Threads • wait() – enter blocked state and release the lock • notify() – wake up some (other) blocked thread • notifyAll() – wake up all the (other) blocked threads. • This is harder than it looks! • Example: BoundedQueueWithGuards.java

13. What can go Wrong? • Deadlock – no progress possible • Starvation – no access to a resource • Simultaneous access to shared resources – unpredictable behavior • Various tools to analyze these issues