1. Concurrency in Java
Last Updated: Fall 2010
Paul Ammann SWE 619

2. Agenda Some General Concurrency Mechanisms Concurrency in Java
Synchronization in Java
Rules for Threads

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 Suppose 2 threads are created
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