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

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

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

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

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?

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

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

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

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

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

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

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

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