Threads - PowerPoint PPT Presentation

Threads

Doing Several Things at Once

• What are Threads?
• Two Ways to Obtain a New Thread
• The Lifecycle of a Thread
• Four Kinds of Thread Programming

• A Thread is a unit of program execution that runs independently from other threads.
• lightweight process
• Threads behave as if they were running on different CPUs
• Extend the concept of time-sharing.
• Garbage collectors and listeners run in separate threads

• Context switched when JVM switched to a different thread
• Not as costly as switching processes
• Threads can make a program run faster
• Threads allow a program to do more that one thing at a time
• one processor?
• the system handles the sharing

• Extend the java.lang.Thread Class
• Implement the “Runnable” interface
• Applets extend the class Applet by definition.

 an applet must always use the second technique; extends is taken by (J)Applet

• Create a class extending Thread Class
• Put the code for the task into the run method of the new class
• Create an instance of that class
• Call the start method for that instance
• The start method will call the run method
• The thread executes until the run method terminates.

Class AThread extends Thread

{

…

public void run()

{

// your code here

}

}

• In another thread, create the thread

AThread t1 = new AThread();

• Start the thread by calling its start method

t1.start();

• The call to start creates and schedules the thread to execute. The run method is called by the JVM when it is the thread’s turn to execute
• See Java Documentation on Thread
• See:
• NameThread.java
• ThreadTest.java

public interface Runnable

{

// Must write run()

public abstract void run();

}

• Create a class ARunnable which implements the “runnable” interface.

public class ARunnable implements Runnable

{

public void run() { … }

}

• Use that class in a call to the Thread constructor

Thread t = new Thread( new ARunnable() );

t.start(); // start the thread

• See Java Doc on Runnable
• See NameUsingThread.java

• Statements within the Runnable Interface implementation of run() can’t invoke Thread methods like “sleep()”, “getName()”, etc. because no “this” object is available in Runnable.
• the object isn’t the thread

• A call to currentThread() can appear anywhere in Java code. Any of the methods of that thread can be called via the thread returned in currentThread().
• Runnable vs Thread: Use Runnable when only the “run()” method will be overridden. Classes should not be extended unless there is a fundamental enhancement of behavior.

• A thread terminates when its run method returns – the normal way
• A thread can be interrupted (never stopped) by using the thread.interrupt() method.
• A thread’s run method should occasionally check for the interrupt signal

public void run()

{

try // DO NOT PUT try in a loop, but loop in try

{ // do some work

}

catch (InterruptedException e)

{ // do whatever needs to be done

}

// clean up

}

• The run method cannot have parameters
• The constructor can have parameters
• The Thread constructor can have certain parameters
• see API for java.lang.Thread

• A thread is created, then “started”
• The thread “dies” when
• The run method terminates:
• normally
• through an exception or return
• A thrown exception will cause the thread to terminate but not the parent

• A thread runs at the priority of its parent unless its priority is changed.
• A higher priority executes first if they are both ready to run
• Java threads
• can be preempted.
• may or may not be time-sliced
• Computationally intensive threads should “yield” periodically
• Raising a thread’s priority does not affect other (heavyweight) processes, only parent, sibling, children threads (competing lightweight processes)

• Unrelated Threads
• Related Unsynchronized Threads
• Mutually-Exclusive Threads
• Communicating Mutually-Exclusive Threads

A Class

An Object has Methods

An Object

An Object

An Object has Methods

Thread runs in a method

• The simplest thread programs involves threads that do different things
• Don’t interact with one another
• Coffee_Tea Example
• Change sleep time; run on PC and Suns to see difference

• The problem is partitioned into subproblems
• A thread solves each subproblem
• The threads don’t interact
• They don’t work on shared data
• Example: testPrime.java
• A server connection for each socket connection is a good example
• A “Work-to-order” thread is called a daemon
• always on

Threads that access common data concurrently need to be controlled so that the correct state of the data is preserved at all times. Such a situation occurs frequently in the real world.

Consider reading and setting a pressure gauge. The gauge should not be set above 20 psi. But the pressure is set by pressure setting objects, independent of one another. Each must check the gauge and if it is safe to do so, increase the pressure by a fixed amount.

• Example:
• BadPressure.java

>java BadPressure

Gauge reads 150, safe limit is 20

What happened?

• code appeared to restrict gauge value, but didn’t
• Setting the gauge is a critical section
• Mutual exclusion required

• Simultaneous reading and setting of the pressure gauge by different threads
• set followed check; all did check, THEN did change
• check-change must be atomic
• one object needs to lock others out until it is finished
• can be done at the class level, method level, or on a block of code

This keyword obtains a mutex (mutual exclusion) lock for the executing thread on the named object. The code is then executed, and the lock is released. If another object already holds the lock, the thread is suspended until the lock is released. Lock-competing threads are queued.

• Java programmers don’t need to do the low level details of creating, acquiring, and releasing locks
• Unix semaphores, locks
• Specify the portion of code (critical section) for mutual exclusion, and the object that must be exclusively locked
• The region of code must be as small as possible

• apply the keyword synchronized to a class method.
• e.g. static synchronized void RaisePressure()
• Only one static synchronized method for a given class can be running at any given time in the JVM, regardless of how many objects there are of that class
• the class object is used to synchronize the threads
• There is one lock for static synchronized methods, and a different lock for synchronized methods. One thread could have the static lock while another thread has a method lock, while other threads could be running other unsynchronized methods
• Example: SetPressure.java

• From SetPressure.java

// Only one thread may be executing in here

static synchronized void raisePressure()

{ if(BadPressure.pressureGauge <

BadPressure.SafetyLimit - 15)

{

....

>java SetPressure

Gauge reads 15, safe limit is 20

• use the keyword synchronized before a block of code
• Use a static object for the lock
• any available, convenient object will do
• cannot be a local object or instance variable

class pressure extends Thread

{ static Object lock = new Object();

  void raisePressure()

{

synchronized(lock); Mutex ‘til end

{

if(SetPressure.pressureGauge <

SetPressure.SafetyLimit - 15)

{

...

}

...

}

• use synchronized keyword on the instance method.
• guarantees that only one of the perhaps many synchronized instance methods will be executing at any one time on a given instance of that class
• equivalent to synchronized(this) over a block
• Examples:
• raisePressure
• Monitor in MessagePass.java

synchronized void raisePressure()

{

if(p.pressureGauge <

p.SafetyLimit - 15)

{

...

same as

void raisePressure()

{

synchronized(this) // on this object

{

if(p.pressureGauge < p.SafetyLimit - 15)

{ ...

}

}

• Threads that need to access common data, but also communicate with one another
• the hardest kind of thread programming
• Producer/Consumer type of problem
• I produce, you consume. Don’t try to produce until there is something to consume. Don’t produce too much, until something is consumed
• Wait/Notify is the solution

• Wait says “I have the lock, but there is nothing to consume. I give up the lock and wait
• Notify says “I just produced something, I will place it in common area, release the lock and wait”

// producer thread

enter synchronized code // get lock

while(buffer_full)

wait();

produce_items()

notify() // tell waiting consumers

leave synchronized code // release lock

// consumer thread

enter synchronized code // get lock

while(no_items)

wait();

consume_items()

leave synchronized code

• Examples
• plum.java
• inelegant dependant producer-consumer
• Operator Examples
• MessagePass.java
• Operator with Monitor
• Operator.java
• Operator w/o monitor (inelegant, too)
• Tally Examples
• TallyWrong.java
• TallyRight.java
• Note how inner classes compile
• PutGet.java
• vary relative wait times and see what happens
• Deadlock.java
• oh, oh; somewhat inelegant – works with zero buffer

• CommunicatingThreads.java
• applet with piped communication between two threads
• Exercise
• Try the PutGet example with multiple producers/consumers. Change the constructors so that you know who is producing what and who is consuming what.