13multithreaded Programming

1 © 2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com
core
programming
Multithreaded
Programming

Multithreaded Programming2 www.corewebprogramming.com
Agenda
• Why threads?
• Approaches for starting threads
– Separate class approach
– Callback approach
• Solving common thread problems
• Synchronizing access to shared resources
• Thread life cycle
• Stopping threads

Concurrent Programming
Using Java Threads
• Motivation
– Efficiency
• Downloading network data files
– Convenience
• A clock icon
– Multi-client applications
• HTTP Server, SMTP Server
• Caution
– Significantly harder to debug and maintain
• Two Main Approaches:
– Make a self-contained subclass of Thread with the
behavior you want
– Implement the Runnable interface and put behavior in
the run method of that object

Thread Mechanism One:
Making a Thread Subclass
• Create a separate subclass of Thread
– No import statements needed: Thread is in java.lang
• Put the actions to be performed in the run
method of the subclass
– public void run() { … }
• Create an instance of your Thread subclass
– Or lots of instances if you want lots of threads
• Call that instance’s start method
– You put the code in run, but you call start!

Making a Thread Subclass
public class DriverClass extends SomeClass {
...
public void startAThread() {
// Create a Thread object
ThreadClass thread = new ThreadClass();
// Start it in a separate process
thread.start();
}
}
public class ThreadClass extends Thread {
public void run() {
// Thread behavior here
}
}

Example
public class Counter extends Thread {
private static int totalNum = 0;
private int currentNum, loopLimit;
public Counter(int loopLimit) {
this.loopLimit = loopLimit;
currentNum = totalNum++;
}
private void pause(double seconds) {
try { Thread.sleep(Math.round(1000.0*seconds)); }
catch(InterruptedException ie) {}
}
...

Example (Continued)
...
/** When run finishes, the thread exits. */
public void run() {
for(int i=0; i<loopLimit; i++) {
System.out.println("Counter " + currentNum
+ ": " + i);
pause(Math.random()); // Sleep for up to 1 second
}
}
}

Example (Continued)
public class CounterTest {
public static void main(String[] args) {
Counter c1 = new Counter(5);
c1.start();
c2.start();
c3.start();
}
}

Thread Mechanism One: Result
Counter 0: 0
Counter 1: 0
Counter 2: 0
Counter 1: 1
Counter 2: 1
Counter 1: 2
Counter 0: 1
Counter 0: 2
Counter 1: 3
Counter 2: 2
Counter 0: 3
Counter 1: 4
Counter 0: 4
Counter 2: 3
Counter 2: 4

Thread Mechanism Two:
Implementing Runnable
• Put the actions to be performed in the run
method of your existing class
• Have class implement Runnable interface
– If your class already extends some other class (e.g.,
Applet), why can't it still extend Thread? Because Java
does not support multiple inheritance.
• Construct an instance of Thread passing in
the existing object (i.e., the Runnable)
– Thread t = new Thread(theRunnableObject);
• Call that Thread’s start method
– t.start();

Implementing Runnable (Cont.)
public class ThreadedClass extends AnyClass
implements Runnable {
public void run() {
// Thread behavior here
// If you want to access thread instance
// (e.g. to get private per-thread data), use
// Thread.currentThread().
}
public void startThread() {
Thread t = new Thread(this);
t.start(); // Calls back to run method in this
}
...
}

Example
public class Counter2 implements Runnable {
private static int totalNum = 0;
private int currentNum, loopLimit;
public Counter2(int loopLimit) {
this.loopLimit = loopLimit;
currentNum = totalNum++;
Thread t = new Thread(this);
t.start();
}
...

Example (Continued)
...
}
public void run() {
System.out.println("Counter " + currentNum
+ ": " + i);
pause(Math.random()); // Sleep for up to 1 second
}
}
}

Example (Continued)
public class Counter2Test {
public static void main(String[] args) {
Counter2 c1 = new Counter2(5);
}
}

Thread Mechanism Two: Result
Counter 0: 0
Counter 1: 0
Counter 2: 0
Counter 1: 1
Counter 1: 2
Counter 0: 1
Counter 1: 3
Counter 2: 1
Counter 0: 2
Counter 0: 3
Counter 1: 4
Counter 2: 2
Counter 2: 3
Counter 0: 4
Counter 2: 4

Race Conditions: Example
public class BuggyCounterApplet extends Applet
implements Runnable{
private int totalNum = 0;
private int loopLimit = 5;
public void start() {
Thread t;
for(int i=0; i<3; i++) {
t = new Thread(this);
t.start();
}
}
}
...

Race Conditions: Example
(Continued)
...
public void run() {
int currentNum = totalNum;
System.out.println("Setting currentNum to "
+ currentNum);
totalNum = totalNum + 1;
System.out.println("Counter "
+ currentNum + ": " + i);
pause(Math.random());
}
}
}
• What's wrong with this code?

Race Conditions: Result
• Occasional Output
Setting currentNum to 0
Counter 0: 0
Counter 0: 1
Counter 1: 0
Counter 1: 0
Counter 0: 2
Counter 0: 3
Counter 1: 1
Counter 0: 4
Counter 1: 1
Counter 1: 2
Counter 1: 3
Counter 1: 2
Counter 1: 3
Counter 1: 4
Counter 1: 4
• Usual Output
Counter 0: 0
Counter 1: 0
Counter 2: 0
Counter 2: 1
Counter 1: 1
Counter 0: 1
Counter 2: 2
Counter 0: 2
Counter 1: 2
Counter 1: 3
Counter 0: 3
Counter 2: 3
Counter 1: 4
Counter 2: 4
Counter 0: 4

Race Conditions: Solution?
• Do things in a single step
public void run() {
int currentNum = totalNum++;
System.out.println("Setting currentNum to "
+ currentNum);
System.out.println("Counter "
+ currentNum + ": " + i);
pause(Math.random());
}
}

Arbitrating Contention for
Shared Resources
• Synchronizing a Section of Code
synchronized(someObject) {
code
}
• Normal interpretation
– Once a thread enters the code, no other thread can enter
until the first thread exits.
• Stronger interpretation
– Once a thread enters the code, no other thread can enter
any section of code that is synchronized using the same
“lock” tag

Arbitrating Contention for
Shared Resources
• Synchronizing an Entire Method
public synchronized void someMethod() {
body
}
• Note that this is equivalent to
public void someMethod() {
synchronized(this) {
body
}
}

Common Synchronization Bug
• What’s wrong with this class?
public class SomeThreadedClass extends Thread {
private static RandomClass someSharedObject;
...
public synchronized void doSomeOperation() {
accessSomeSharedObject();
}
...
public void run() {
while(someCondition) {
doSomeOperation(); // Accesses shared data
doSomeOtherOperation();// No shared data
}
}
}

Synchronization Solution
• Solution 1: synchronize on the shared data
public void doSomeOperation() {
synchronized(someSharedObject) {
}
}
• Solution 2: synchronize on the class object
synchronized(SomeThreadedClass.class) {
}
}
– Note that if you synchronize a static method, the lock is the
corresponding Class object, not this

Synchronization Solution
(Continued)
• Solution 3: synchronize on arbitrary object
public class SomeThreadedClass extends Thread {
private static Object lockObject
= new Object();
...
synchronized(lockObject) {
}
}
...
}
– Why doesn't this problem usually occur with Runnable?

Thread Lifecycle
new
wait()
yield()
notify()
dispatch
ready
start()
running
waiting
blocked
sleeping
sleep()
Block on I/O
times expires
or interrupted
I/O completed
deadrun completes

Useful Thread Constructors
• Thread()
– Default version you get when you call constructor of your
custom Thread subclass.
• Thread(Runnable target)
– Creates a thread, that, once started, will execute the run
method of the target
• Thread(ThreadGroup group,
Runnable target)
– Creates a thread and places it in the specified thread group
– A ThreadGroup is a collection of threads that can be
operated on as a set
• Thread(String name)
– Creates a thread with the given name
– Useful for debugging

Thread Priorities
• A thread’s default priority is the same as the
creating thread
• Thread API defines three thread priorities
•Thread.MAX_PRIORITY (typically 10)
•Thread.NORM_PRIORITY (typically 5)
•Thread.MIN_PRIORITY (typically 1)
• Problems
– A Java thread priority may map differently to the thread
priorities of the underlying OS
• Solaris has 232–1 priority level; Windows NT has 7
user priority levels
– Starvation can occur for lower-priority threads if the
higher-priority threads never terminate, sleep, or wait for
I/O

Useful Thread Methods
• currentThread
– Returns a reference to the currently executing thread
– This is a static method that can be called by arbitrary methods,
not just from within a Thread object
• I.e., anyone can call Thread.currentThread
• interrupt
– One of two outcomes:
• If the thread is executing join, sleep, or wait, an
InterruptedException is thrown
• Sets a flag, from which the interrupted thread can check
(isInterrupted)
• interrupted
– Checks whether the currently executing thread has a request for
interruption (checks flag) and clears the flag

(Continued)
• isInterrupted
– Simply checks whether the thread’s interrupt flag has
been set (does not modify the flag)
• Reset the flag by calling interrupted from within
the run method of the flagged thread
• join
– Joins to another thread by simply waiting (sleeps) until
the other thread has completed execution
• isDaemon/setDaemon
– Determines or set the thread to be a daemon
– A Java program will exit when the only active threads
remaining are daemon threads

(Continued)
• start
– Initializes the thread and then calls run
– If the thread was constructed by providing a Runnable,
then start calls the run method of that Runnable
• run
– The method in which a created thread will execute
– Do not call run directly; call start on the thread object
– When run completes the thread enters a dead state and
cannot be restarted

(Continued)
• sleep
– Causes the currently executing thread to do a
nonbusy wait for at least the amount of time
(milliseconds), unless interrupted
– As a static method, may be called for nonthreaded
applications as well
• I.e., anyone can call Thread.sleep
• Note that sleep throws InterruptedException. Need
try/catch
• yield
– Allows any other threads of the same or higher
priority to execute (moves itself to the end of the
priority queue)
– If all waiting threads have a lower priority, then the
yielding thread remains on the CPU

(Continued)
• wait/waitForAll
– Releases the lock for other threads and suspends itself
(placed in a wait queue associated with the lock)
– Thread can be restarted through notify or notifyAll
– These methods must be synchronized on the lock object of
importance
• notify/notifyAll
– Wakes up all threads waiting for the lock
– A notified doesn’t begin immediate execution, but is placed in
the runnable thread queue

Stopping a Thread
public class ThreadExample implements Runnable {
private boolean running;
public ThreadExample()
Thread thread = new Thread(this);
thread.start();
}
public void run(){
running = true;
while (running) {
...
}
doCleanup();
}
public void setRunning(boolean running) {
this.running = running;
}
}

Signaling with wait and notify
public class ConnectionPool implements Runnable {
...
public synchronized Connection getConnection() {
if (availableConnections.isEmpty()) {
try {
wait();
} catch(InterruptedException ie) {}
// Someone freed up a connection, so try again.
return(getConnection());
} else {
// Get available connection
...
return(connection)
}
}

Signaling with wait and notify
(Continued)
public synchronized void free(Connection connection) {
busyConnections.removeElement(connection);
availableConnections.addElement(connection);
// Wake up threads that are waiting
// for a connection
notifyAll();
}
...
}

Summary
• Achieve multithreaded behavior by
– Inheriting directly from Thread
(separate class approach)
– Implementing the Runnable interface
(callback approach)
• In either case, put your code in the run
method. Call start on the Thread object.
• Avoid race conditions by placing the shared
resource in a synchronized block
• You can’t restart a dead thread
• Stop threads by setting a flag that the
thread's run method checks

37 © 2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com
core
programming
Questions?

13multithreaded Programming

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to 13multithreaded Programming (20)

More from Adil Jafri (20)

Recently uploaded (20)

13multithreaded Programming