co2101 threads in java - tom ridge · 15/10/2019 · java thread class, and runnable interface...

CO2101 — Threads in Java

Tom Ridge (tr61)

15th October 2019

tr61 Java Threads 15th October 2019 1 / 14

Recap/overview

Recap and Lecture Overview

The nature of concurrent programming is multiple processes/threadsrunning in parallelProcesses may share resourcesSolutions need to guarantee mutual exclusion, progress, bounded waiting

Overview of this lecture

How can Java support concurrent programming?

Java Thread class, and Runnable interfaceJava Executors and ThreadPoolsSimple examples

Recap/overview

Recap and Lecture Overview

The nature of concurrent programming is multiple processes/threadsrunning in parallelProcesses may share resourcesSolutions need to guarantee mutual exclusion, progress, bounded waiting

Overview of this lecture

How can Java support concurrent programming?

Java Thread class, and Runnable interfaceJava Executors and ThreadPoolsSimple examples

Concurrency in Java

Java Virtual Machine (JVM) allows an application to have multiplethreads running concurrently

the thread concept is supported by

the classes java.lang.Thread and java.util.concurrentthe interface java.lang.Runnable

Two ways to create a new thread

Extending the Thread classImplementing the Runnable interface

Concurrency in Java

Two ways to create a thread

Extend Thread class

Implement Runnable interface

MyRunnable

+run(): void

Thread

+run(): void

ExampleThread

+run(): void

Runnable

+run(): voidThread

Concurrency in Java Extend Thread class

Extending Java Thread Class

Declare a subclass of Thread, overriding its run() method

public class ExampleThread extends Thread {int parameter;ExampleThread(int para) { parameter = para; }

public void run() {... // what the thread should do

Create an instance of the subclass and start executing it

ExampleThread t = new ExampleThread(10);t.start();

Concurrency in Java Extend Thread class

Extending Java Thread Class

Declare a subclass of Thread, overriding its run() method

public class ExampleThread extends Thread {int parameter;ExampleThread(int para) { parameter = para; }

public void run() {... // what the thread should do

Create an instance of the subclass and start executing it

ExampleThread t = new ExampleThread(10);t.start();

Concurrency in Java Implement Runnable interface

Implementing the Interface Runnable

Java does not permit multiple inheritance

So usually more convenient to implement run() from Java interfaceRunnable instead of deriving from Thread

Runnable declares (just) the run() method

public interface Runnable {public abstract void run();

We can extend other classes while implementing Runnable

In fact, the Thread class just implements Runnable

public class Threadextends Objectimplements Runnable

Implementing the Interface Runnable (Cont’d)

First declare a class implementing Runnable

public class MyRunnable extends SomeClassimplements Runnable {

int parameter;MyRunnable (int para) { parameter = para; }

public void run() {... // what should the thread do?

Threads can then be allocated and started by

MyRunnable r = new MyRunnable(10);Thread t = new Thread(r);t.start();

Implementing the Interface Runnable (Cont’d)

First declare a class implementing Runnable

public class MyRunnable extends SomeClassimplements Runnable {

int parameter;MyRunnable (int para) { parameter = para; }

public void run() {... // what should the thread do?

Threads can then be allocated and started by

MyRunnable r = new MyRunnable(10);Thread t = new Thread(r);t.start();

Concurrency in Java Thread methods

Methods of Thread Class

Some principal methods of the Thread class

t.start(): Causes thread t to begin execution; (JVM will call run()concurrently)

t.run(): Call the run method of thread t directly (with no concurrency;typically you don’t do this)Thread.sleep(ms): Cease the execution of current thread for msmillisecondst.join(): Causes the calling thread to wait (block) for the thread t tofinish

Refer to the Java documents about Thread class and Runnableinterface.

t.start(): Causes thread t to begin execution; (JVM will call run()concurrently)t.run(): Call the run method of thread t directly (with no concurrency;typically you don’t do this)

Thread.sleep(ms): Cease the execution of current thread for msmillisecondst.join(): Causes the calling thread to wait (block) for the thread t tofinish

t.start(): Causes thread t to begin execution; (JVM will call run()concurrently)t.run(): Call the run method of thread t directly (with no concurrency;typically you don’t do this)Thread.sleep(ms): Cease the execution of current thread for msmilliseconds

t.join(): Causes the calling thread to wait (block) for the thread t tofinish

t.start(): Causes thread t to begin execution; (JVM will call run()concurrently)t.run(): Call the run method of thread t directly (with no concurrency;typically you don’t do this)Thread.sleep(ms): Cease the execution of current thread for msmillisecondst.join(): Causes the calling thread to wait (block) for the thread t tofinish

Concurrency in Java Using threads

Outline code using Threads

public class T1 extends Thread {// attributes and constructorpublic void run() { /* thread functionality */ }

}......public class Tn extends Thread {// attributes and constructorpublic void run() { /* thread functionality */ }

}public class MainProgram {public static void main(String[] args) {

B;T1 t1 = new T1(); ...; Tn tn = new Tn();t1.start(); ...; tn.start();I;t1.join(); ...; tn.join();F; }

The Semantics of start() and join()

The execution of B finishes before t1,. . . ,tn as they have not beenstarted yet;

The effect of t1.start(),. . . ,tn.start() is equivalent to interleavingthe atomic actions of the started threads;

Execution of statement I can interleave with t1,. . . ,tn

Execution of statement F does not begin until all of t1,. . . ,tn havecompleted

Execution of main() terminates only if all of the started threads (andI and F) terminate

If we replace t1.start(),. . . ,tn.start() with t1.run(),. . . ,tn.run(),the program will become a sequential program

Concurrency in Java Examples

Thread Example

class ExampleThread extends Thread {int _id;ExampleThread(int id) { _id = id; }public void run() {

System.out.println("This is thread: " + _id);}public static void main(String[] args) {

ExampleThread t1 = new ExampleThread(42);ExampleThread t2 = new ExampleThread(43);ExampleThread t3 = new ExampleThread(44);t1.start();t2.start();t3.start();System.out.println("All threads have terminated.");

Thread Example

Execution Result

A possible result is as follows:

This is thread 42This is thread 43All threads have terminated.This is thread 44

The statements in main() after the start() statements can interleavewith the statements of the started threads.

The result is non-deterministic, and can differ with each run of theprogram.

Runnable example

class ExampleRunnable implements Runnable {int _id;

ExampleRunnable(int id){ _id = id; }public void run() {

System.out.println("This is runnable thread: " + _id);}public static void main(String[] args) {

Thread t1 = new Thread(new ExampleRunnable(42));Thread t2 = new Thread(new ExampleRunnable(43));Thread t3 = new Thread(new ExampleRunnable(44));t1.start();t2.start();t3.start();System.out.println("All threads have terminated."); }

Runnable example

Summary

A thread in Java can be created via:

Extending Thread classImplementing Runnable interface

The result of running programs with multiple threads may benon-deterministic

We have not yet really considered critical sections

co2101 threads in java - tom ridge · 15/10/2019 · java thread class, and runnable interface...

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