Concurrent Programmingin Java

Dr. Zoltan Papp

Motivation: event driven, responsive systems

Sequential approach:

while ( true ) { do event = getEventId() while ( event == null ); switch ( event ) { case E1: action1(); break; case E2: action2(); break; . . }}

Properties: - not a “nice” structure (e.g. extendibility)- not always responsive

Parallel approach:

Properties: - direct event – action association- independent, parallel execution

action1

action3

action2E1

E2

E3

New concepts

Process/thread/tasksingle chain of instruction execution

Interprocess communicationshare common resourcessynchronization

Concurrent programming:introduces these concepts in programming languages (i.e. new programming primitives)investigates how to build compound systems safely

Processes/threads/tasks

Main conflict:

numberOf(threads) >> numberOf(processors)

processor sharing scheduling

running

blockedrunnable

Process/thread states:

“yield”

“wait for event”

“event”

schedulerdecision

#(running) = 0 .. number of processors#(runnable) = 0 .. arbitrary

Scheduler’s main operations:

save thread contextrestore thread contextpass control to threadselect thread to run: policy!

Scheduling schemes

Aspect 1: thread selectionpriority basedround-robinFIFOor any combination of these...

Aspect 2: moments of event evaluationnon-preemptive (“cooperative scheduling”)preemptive

Java threads

java.jang package:

public class Threadextends Object implements Runnable

public abstract interface Runnable

For details check the class documentation!

Java thread scheduling:

priority basedinside priority groups: round-robinpreemptivetime slicing: platform dependent – not guaranteed!

Java thread definition 1: implementing the Runnable interface

class NewThread implements Runnable {

public void run() { try { for(int i = 5; i > 0; i--) { System.out.println("Child Thread: " + i); Thread.sleep(500); } } catch (InterruptedException e) { System.out.println("Child interrupted."); } System.out.println("Exiting child thread."); }}

public class TD1 {

public static void main(String args[]) {

Thread t = new Thread(new NewThread(), "Child Thread");

t.start(); try { for(int i = 5; i > 0; i--) { System.out.println("Main Thread: " + i); Thread.sleep(1000); } } catch (InterruptedException e) { System.out.println("Main thread interrupted."); } System.out.println("Main thread exiting."); }}

thread 1

thread 2

start newthread

just wait for a while

Java thread definition 2: extending the Thread class

class NewThread extends Thread {

NewThread(String name) { super(name); }

public void run() { try { for(int i = 5; i > 0; i--) { System.out.println("Child Thread: " + i); Thread.sleep(500); } } catch (InterruptedException e) { System.out.println("Child interrupted."); } System.out.println("Exiting child thread."); }}

public class TD2 { public static void main(String args[]) { Thread t = new NewThread("Child Thread"); // create a new thread

t.start(); try { for(int i = 5; i > 0; i--) { System.out.println("Main Thread: " + i); Thread.sleep(1000); } } catch (InterruptedException e) { System.out.println("Main thread interrupted."); } System.out.println("Main thread exiting."); }}

thread 1

thread 2

start newthread

just wait for a while

constructor for the new thread class

Interprocess communication

Common resources: the problem

thread 1: . . i11; i12; i13; i14; . .

thread 2: . . i21; i22; i23; i24; . .

R

For certain resources: access must be mutually exclusive

criticalregion

Mutual exclusion rule for protected resources:

Let C denote the union of critical regions ci for resource R. T is the set of threads tj having access to R. The mutual exclusive access to R is satisfied if at any time instance only at most one tj T executes ck C.

On programming language level: explicit notation for critical regions.On runtime system level: implementing the rule

Example 1: Not synchronized resource access

// This program is not synchronized.class Callme { void call(String msg) { System.out.print("[" + msg); try { Thread.sleep(1000); } catch(InterruptedException e) { System.out.println("Interrupted"); } System.out.println("]"); }}

class Caller implements Runnable { String msg; Callme target; Thread t;

public Caller(Callme targ, String s) { target = targ; msg = s; t = new Thread(this); t.start(); }

public void run() { target.call(msg); }}

Example 1: Not synchronized resource access (cont’d)

class Synch1 { public static void main(String args[]) { Callme target = new Callme(); Caller ob1 = new Caller(target, "Hello"); Caller ob2 = new Caller(target, "Synchronized"); Caller ob3 = new Caller(target, "World");

// wait for threads to end try { ob1.t.join(); ob2.t.join(); ob3.t.join(); } catch(InterruptedException e) { System.out.println("Interrupted"); } }}

Mutual exclusion in Java

Synchronized methods:

class Whatever { . . synchronized ReturnType method(...) { . }

}

Operation: synchronized method executions are serialized

Example 2: Synchronized resource access

// This program is not synchronized.class Callme { synchronized void call(String msg) { System.out.print("[" + msg); try { Thread.sleep(1000); } catch(InterruptedException e) { System.out.println("Interrupted"); } System.out.println("]"); }}

class Caller implements Runnable { String msg; Callme target; Thread t;

public Caller(Callme targ, String s) { target = targ; msg = s; t = new Thread(this); t.start(); }

public void run() { target.call(msg); }}

Synchronized statements:

. . synchronized ( object ) { <statements to be synchronized> }

Operation: “lock” is associated with object

Example 3: Resource access via synchronized statements

// The code is the same as that of Example 1 - except the// ‘run’ method body

class Caller implements Runnable { String msg; Callme target; Thread t;

public Caller(Callme targ, String s) { target = targ; msg = s; t = new Thread(this); t.start(); }

// synchronize calls to call() public void run() { synchronized(target) { // synchronized block target.call(msg); } }}

Important: NO MAGIC!

class Whatever {

.

synchronized ReturnType method(...) {

<BODY>

}

}

class Whatever {

.

ReturnType method(...) {

synchronized (this) { <BODY> }

}

}

THE TWO SOLUTIONS ARE EQUIVALENT!

Thread synchronization in Java

Implementation: via special operations (invoked by threads)

java.lang.Object:

public final native void wait(long timeout)throws InterruptedException;

public final void wait()throws InterruptedException;

public final native void notify();

public final native void notifyAll();

The “fine details”: direct control of thread state transitions

java.lang.Thread:

public static final int MAX_PRIORITY;public static final int MIN_PRIORITY;public static final int NORMAL_PRIORITY;

public static native void sleep(long millis)throws InterruptedException;

public static boolean interrupted();

public static native void yield();

The “fine details” (cont’d)

java.lang.Thread:

public final int getPriority();public final void setPriority();

public void interrupt();public boolean isInterrupted();

public final synchronized void join(long millis)throws InterruptedException;

public void join()throws InterruptedException;

The “fine details” (cont’d)

java.lang.Thread:

public final void suspend();public final void resume();

public final synchronized void stop(Throwable o);public final void stop();

runnable

scheduler

Thread state transitions in Java 1.1 and earlier

new

dead

suspended running blocked

blocked-susp.

new

stop

start

stop

stopstop, term

resume

suspend

suspend

resume

suspend

IO, sleep,wait, join

yield,time slice

notify, notifyAll,IO compl, sleep exp,

join compl.

IO compl.

Thread state transitions in Java 1.2

runnable

scheduler

new

dead

running blocked

new

start

term

IO, sleep,wait, join

yield,time slice

notify, notifyAll,IO compl, sleep exp,

join compl.

Reasons for the “clean-up”: stop() and suspend() are inherently unsafe!

- they encourage “messy” program structures- coherent “lock” states are difficult (or impossible) to maintain- they can result in corrupted data

Example 1: suspend/resume in Java 1

class NewThread implements Runnable { String name; // name of thread Thread t;

NewThread(String threadname) { name = threadname; t = new Thread(this, name); System.out.println("New thread: " + t); t.start(); // Start the thread }

// This is the entry point for thread. public void run() { try { for(int i = 15; i > 0; i--) { System.out.println(name + ": " + i); Thread.sleep(200); } } catch (InterruptedException e) { System.out.println(name + " interrupted."); } System.out.println(name + " exiting."); }}

Example 1: suspend/resume in Java 1 (cont’d)class SusRes1 { public static void main(String args[]) { NewThread ob1 = new NewThread("One"); NewThread ob2 = new NewThread("Two");

try { Thread.sleep(1000); ob1.suspend(); System.out.println("Suspending thread One"); Thread.sleep(1000); ob1.resume(); System.out.println("Resuming thread One"); ob2.suspend(); System.out.println("Suspending thread Two"); Thread.sleep(1000); ob2.resume(); System.out.println("Resuming thread Two"); } catch (InterruptedException e) { System.out.println("Main thread Interrupted"); }

// wait for threads to finish try { System.out.println("Waiting for threads to finish."); ob1.t.join(); ob2.t.join(); } catch (InterruptedException e) { System.out.println("Main thread Interrupted"); } System.out.println("Main thread exiting."); }}

Example 2:suspend/resumein Java 2

class NewThread implements Runnable { String name; // name of thread Thread t; boolean suspendFlag; NewThread(String threadname) { name = threadname; t = new Thread(this, name); System.out.println("New thread: " + t); suspendFlag = false; t.start(); // Start the thread }

public void run() { try { for(int i = 15; i > 0; i--) { System.out.println(name + ": " + i); Thread.sleep(200); synchronized(this) { while(suspendFlag) wait();} } } catch (InterruptedException e) { System.out.println(name + " interrupted."); } System.out.println(name + " exiting."); }

void mysuspend() { suspendFlag = true; }

synchronized void myresume() { suspendFlag = false; notify(); }}

Example 2:suspend/resumein Java 2(cont’d)

class SusRes2 { public static void main(String args[]) { NewThread ob1 = new NewThread("One"); NewThread ob2 = new NewThread("Two");

try { Thread.sleep(1000); ob1.mysuspend(); System.out.println("Suspending thread One"); Thread.sleep(1000); ob1.myresume(); System.out.println("Resuming thread One"); ob2.mysuspend(); System.out.println("Suspending thread Two"); Thread.sleep(1000); ob2.myresume(); System.out.println("Resuming thread Two"); } catch (InterruptedException e) { System.out.println("Main thread Interrupted"); }

// wait for threads to finish try { System.out.println("Waiting for threads to finish."); ob1.t.join(); ob2.t.join(); } catch (InterruptedException e) { System.out.println("Main thread Interrupted"); } System.out.println("Main thread exiting."); }}

Semaphores (locks):

resource lock

Implementation: primitive data type + operations (atomic)

E.g. semaphore + P, V operations

V(s): s := s + 1;P(s): if s == 0 then wait_until( s 0 ); s := s - 1;

Thread 1:

semaphore s1 = 1; /* to protect R1 resource initial value: free */

.

.P(s1); /* lock R1 if available */<operations on R1>V(s1); /* free R1 */..

Usage:

Example: Semaphores in Java

public class Semaphore {

long sem;

public Semaphore(long init) { sem = init; }

public synchronized void P() throws InterruptedException { while ( sem == 0 ) wait(); sem -= 1; }

public synchronized void V() { sem += 1; notifyAll(); } public boolean isDown() { return sem == 0; }

}