Transcript General Concurrency

Concurrency in Java
Last Updated: Fall 2010
Paul Ammann SWE 619
Agenda
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Some General Concurrency Mechanisms
Concurrency in Java
Synchronization in Java
Rules for Threads
Concurrency in Java
Concepts to be covered
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Threads, concurrency
Interleaving, race conditions
Atomicity of execution
Mutual exclusion
Thread states
Synchronization, locks
Inter-thread communication
Concurrency in Java
Some General Concurrency
Mechanisms
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Rendevous (Ada)
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In practice: Web Servers
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Single server, scores of clients! Waiting time?
Threads
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Obviates need for certain low level primitives
Subject of today’s talk
Database mechanisms
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Concurrency entirely hidden from user
ACID properties
Concurrency in Java
Interleaving
c.inc();
// c is a counter
c.inc();
System.out.println(c.get());
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Suppose 2 threads are created
Each executes the 3 statements
Order in which they are executed?
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Does thread 1 get to execute all 3 before thread 2?
Concurrency in Java
Atomicity of execution
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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
Concurrency in Java
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
Time
Thread 4 reads nSN = 5 //should read 7
Concurrency in Java
Concurrency in Java
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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
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Examples: Counter1.java, Counter2.java,
Counter3.java, Counter4.java
Concurrency in Java
Thread states
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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)
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The join() method waits for a thread to stop
Or ExecutorService shutdown(), get()
Concurrency in Java
methods
Synchronization in Java
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Threads use a “lock” model
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Lock is associated with an object
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“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
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Wacky.java, Wacky1.java
BoundedQueue.java
SyncBoundedQueue.java
Concurrency in Java
Java Synchronization
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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
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When a thread unlocks a lock, the thread
is said to release the lock
Synchronization: mutual exclusion and
signal-wait
Concurrency in Java
Communication among
Threads
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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
Concurrency in Java
What can go Wrong?
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Deadlock – no progress possible
Starvation – no access to a resource
Simultaneous access to shared resources
– unpredictable behavior
Various tools to analyze these issues
Concurrency in Java