Threads

Doing Several Things at Once

Threads

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

What are Threads?

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

Thread Context

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

Two Ways to Obtain a New Thread

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

Extension of java.lang.Thread

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.

Extending the Thread Class

Class AThread extends Thread

{

…

public void run()

{

// your code here

}

}

Running a Thread 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

The Runnable Interface

public interface Runnable

{ // Must write run()

public abstract void run();

}

Implement Runnable 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 constructorThread t = new Thread( new ARunnable() );

t.start(); // start the thread

See Java Doc on Runnable See NameUsingThread.java

Notes on Runnable

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

Remarks

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.

Terminating Threads

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

Interrupt Coding

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

}

Parameters to Threads

The run method cannot have parameters

The constructor can have parameters The Thread constructor can have

certain parameters- see API for java.lang.Thread

Thread Lifecycle

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

Priorities 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)

Four Kinds of Thread Programming

1. Unrelated Threads

2. Related Unsynchronized Threads

3. Mutually-Exclusive Threads

4. Communicating Mutually-Exclusive Threads

Thread Diagram

A ClassA Class

An ObjectAn Object

An ObjectAn Object

An Object has MethodsAn Object has Methods

An Object has MethodsAn Object has Methods

Thread runs in a methodThread runs in a method

Unrelated Threads

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

Related, Unsynchronized Threads

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

Mutally-Exclusive Threads

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.

The Pressure Gauge

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

BadPressure output

>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

Mutual Exclusion 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

The keyword synchronized

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.

Some considerations

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

Mutual Exclusion over an Entire Class 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

Class Mutual Exclusion

From SetPressure.java

// Only one thread may be executing in herestatic synchronized void raisePressure() { if(BadPressure.pressureGauge < BadPressure.SafetyLimit - 15) { ....

>java SetPressureGauge reads 15, safe limit is 20

Mutual Exclusion over a Block

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

Block Mutual Exclusion

class pressure extends Thread{ static Object lock = new Object();  void raisePressure() { synchronized(lock); Mutex ‘til end { if(SetPressure.pressureGauge < SetPressure.SafetyLimit - 15) { ... } ...}

Mutual Exclusion over a Method 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

Method Synchronization

synchronized void raisePressure() { if(p.pressureGauge < p.SafetyLimit - 15) { ...same as void raisePressure() { synchronized(this) // on this object { if(p.pressureGauge < p.SafetyLimit - 15) { ... } }

Communicating Mutually Exclusive Threads

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 / Notify

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 Pseudo-code

// producer thread

enter synchronized code // get lock

while(buffer_full)

wait();

produce_items()

notify() // tell waiting consumers

leave synchronized code // release lock

consumerPseudo-code

// consumer thread

enter synchronized code // get lock

while(no_items)

wait();

consume_items()

leave synchronized code

Notes: 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

Examples, etc;

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.