13/04/2015Client-server Programming1 Block 6: Threads 1 Jin Sa.

18/04/23 Client-server Programming 1

Block 6: Threads 1

Jin Sa


Outline of block 6

• Why multi-threads and typical applications

• Defining and creating threads

• An example with two threads

• Life cycle of a thread

• An example with user interface

• Problem with threads

• Synchronisation among threads


Why multithreading?

• Most of the java programs you have seen so far can only do one thing at a time. In the real world many actions are happening concurrently.

• Typical applications: – User interfacing, e.g. One deals with the drawing,

the other deals with user interface– Many instances of similar behaviour, e.g. a

concurrent server serves many clients – Many different tasks, e.g. open day simulation


Two ways to define threads in Java

• Extending the Thread class

• Implementing the Runnable interface


Creating threads by extending the Thread class

• Define a class, e.g. NewThread, extending the Thread class

• Override the run() method to tell the system how the thread will be executed when it runs.

• Create an instance of NewThread, • Invoke the start() method to tell the system to

start the thread and to execute the run() method.


Key elements for extending Thread 1

class NewThread extends Thread {

…

//override the run method

public void run() {

//define how the thread runs

…

}

…

}


Key element for extending Thread 2

class MyApplication {

…

public void someMethod() {

NewThread mythread=new NewThread();

…

mythread.start();

}

…

}


An example: a thread printing its id repeatedly

class MyThread extends Thread {

private String threadID;

MyThread(String s) { threadID=s; }

public void run() {

for (int i=0;i<200;i++){

System.out.println("Print thread "

+ threadID);

}//for

}//run

} //MyThread


An example: two threads printing their ids

public class SeveralThreads {

public static void main(String [] args) {MyThread t1,t2;

//create threadst1 = new MyThread("t1");t2 = new MyThread("t2");

//start threads t1.start();

t2.start();}//main

}//SeveralThreads


An example: two threads printing their ids

Print thread t1Print thread t2Print thread t2Print thread t1Print thread t1Print thread t2Print thread t1Print thread t2Print thread t1Print thread t2…

Run the Severalthread program in E:\Teaching\Client_server\Run2006-7\Code\ClientServer. Package threads


Life cycle of a thread

• A thread can be in one of these states once it is created: ready, running, blocked, finish

• A ready thread is runnable, but not running, it needs allocation of CPU time.

• Common ways for a running thread to become blocked include: waiting for I/O, sleep

• A common way for a thread to finish is when it completes its execution of the run() method


Creating threads by implementing the Runnable interface

• Define a class, e.g. NewTask, implementing the Runnable interface

• implement the run() method to tell the system how the task will be executed when it runs.

• Create an instance of NewTask, e.g. t1• The task needs to be executed in a thread. Create

an instance of Thread with t1 as the parameter• Invoke the start() method of the thread to tell

the system to start the thread.


Key elements for implementing Runnable 1

class NewTask extends AnotherClass implements Runnable {…//implement the run methodpublic void run() {

//define how the thread runs…

}…

}


Key elements for implementing Runnable 2

class MyApplication {…public void someMethod() {

NewTask mytask=new NewTask();Thread mythread=new Thread(mytask);

…mythread.start();

}…

}


Implementing Runnable or Extending Thread

• Use Runnable if need to extend another class

• Even if NewTask does not extend another class, this is the preferred method for creating a thread if we only want to define the run() method, and not using the other methods in Thread.


An example: thread printing its id using implementing Runnable

class MyTask implements Runnable { private String threadID; MyTask(String s) { threadID=s; } public void run() { for (int i=0;i<200;i++){ System.out.println("Print thread " + threadID); }//for }//run } //MyTask


An example: two threads printing their ids using implementing Runnable

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

//create instances of MyTaskMyTask t1 = new MyTask("t1");MyTask t2 = new MyTask("t2");

// create Thread instances Thread tt1=new Thread(t1); Thread tt2=new Thread(t2);

//start threads tt1.start();

tt2.start();}//main

}//SeveralThreads

Thread synchronisation

Problem with accessing shared resources– an example

• The Account class is a bank account, contains methods such as deposit and withdraw.

• Cardholder is a thread allows the card holder to deposit £10 repeatedly for 10 times.

• FamilyAccount has a main methods that creates one instance of an Account, two instances of Cardholders, both putting money into the same account.

Balance=50

An instance of Account

c1 c2

deposit(10)

deposit(10)

deposit(10)

deposit(10)

deposit(10)

deposit(10)

deposit(10)

deposit(10)

Problem with accessing shared resources– an example 1

class Account { private int balance=0; public void deposit(int amount) { //balance=balance+amount;

int tmp = balance; Thread.sleep(200); balance = tmp + amount;

System.out.println("Current balance is "+balance); }}


class CardHolder extends Thread{ private Account acc; private String accName; CardHolder(Account a,String nm) { acc=a; accName=nm; } public void run() { for (int i=0;i<10;i++) { System.out.println(accName+" is depositing

£10 ..."); acc.deposit(10); } }//run}//CardHolder


class FamilyAccount { public static void main(String [] args) { Account ourAccount=new Account(); CardHolder c1 = new CardHolder(ourAccount,"john"); CardHolder c2 = new CardHolder(ourAccount,"Kate"); c1.start(); c2.start(); }}//FamilyAccount


• Run FamilyAccount in the E:clientserver/threads (account,cardholder,familyAccount)

• The result should be:The balance is : £10

The balance is : £20


…


• But a shared resource may be corrupted if it is accessed simultaneously by multiple threads

Problem• For example, the current balance in ourAccount is £50,• The c1 thread: tries to deposit £10; loading the current

value, i.e. 50, to the temporary storage place before doing the arithmetic. The thread is suspended; switches to the c2 thread.

• The c2 thread: tries to deposit £10; loading the current balance, which is still £50. The thread is then suspended; resumes the c1 thread,

• Back with the c1 thread: add £10 to the value, i.e. £50, in the temporary storage; £60 is stored in the attribute balance; suspend the c1 thread

• Back with the c2 thread: the c2 thread adds £10 to the current value in the temporary storage that is 50. The result 60 is now stored in the balance attribute.

• Should be 70, not 60!

Race condition

• The problem is that the two threads (c1 and c2) are accessing the same resource (account) in a way that causes a conflict.

• This is a common problem, known as a race condition, in multithreaded programs.

• A class is said to be thread-safe if an object of the class does not cause a race condition in the presence of multiple threads.

• The Account class is not thread-safe at the moment.

Synchronized method

• There is a need to protect the shared data.• Use synchronized methods so that there can be

only one thread executing this method at a time.• Java guarantees once a thread has gained access to

a synchronized method, it will finish the method before any other thread gains access to that or any other synchronized method in that object.

• A synchronised method (implicitly) acquires a lock on the instance.

Deposit as a Synchronized method

class Account { int balance=0;

public synchronized void deposit(int amount) { int tmp = balance; Thread.sleep(200); balance = tmp + amount;

System.out.println("Current balance is "+balance);

}}//Account

• Run the program familyAccount, change the first line of deposit


Multithreading for user interface in GUI applications


Example: Flashing label

• Press start, program displays a flashing rectangle until user presses the stop button.

• One thread is tied to display the flashing rectangle.

• In order to be able to accept user’s input, need more than one thread.

• Run the FlashStop program. (E:\Teaching\Client_server\Run2006-7\Code\ClientServer\src\threads)


Implementations• Two classes: FlashStop and FlashRec• FlashStop deals with the graphical interface and the

interaction with the user– Presents gui elements (behaves as a Frame)– Implements the actonPerformed method (behaves as an

ActionListener)• To start a thread for displaying the “flashing”• To stop the thread

• FlasRec deals with displaying the “flashing” effect.– A thread repeatedly sets the background colour to be red/white


Flash rectangle 1class FlashRec extends JPanel implements

Runnable{ private boolean keepgoing=false; public void run() { while (keepgoing) { try { setBackground(Color.red); Thread.sleep(800); setBackground(Color.white); Thread.sleep(800); } catch (Exception e) {…} }//while }//runTo continue…


Flash rectangle 2

public void pleaseStop() {

keepgoing=false;

}

public void pleaseStart(){

keepgoing=true;

}

}//FlashRec


FlashStop class 1

public class FlashStop extends JFrame

implements ActionListener {

private JButton flash=

new JButton(“flash");

private JButton stop=new JButton("stop");

private FlashRec fr;

private Thread recThread;

…


FlashStop class 2

FlashStop() { setSize(400,200); fr=new FlashRec(); Container c = getContentPane(); c.add(flash, "North"); start.addActionListener(this); c.add(stop,"South"); stop.addActionListener(this); c.add(fr, "Center");// }//FlashStop


FlashStop class 3

public void actionPerformed(ActionEvent e) { if (e.getSource() == start ) { fr.pleaseStart(); recThread=new Thread(fr); recThread.start();} if (e.getSource() ==stop) { fr.pleaseStop();}}//actionPerformed

The actual code is slightly more complex.


Review

• Understand how to define and create threads in Java: extends Thread and implement Runnable

• Be able to write simple Java thread applications• Problem and solution with concurrent thread • Understand the GUI example and how to deal with

events.

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13/04/2015Client-server Programming1 Block 6: Threads 1 Jin Sa.

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