LAB-23: Java Threads

A thread is a thread of execution in a program. The Java Virtual Machine allows an application to have multiple threads of execution running concurrently. This hands-on lab takes you through the basics of using Java threading.

Exercise 1: Extending Thread class

(1.1) The start() method is NOT in the constructor of the subclass

0. Start NetBeans IDE if you have not done so yet.
1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in ExtendThreadClassTest0 as project name.
For Create Main Class field, type in ExtendThreadClassTest0. (Figure-1.10 below)
Click Finish.

Figure-1.10: Create a new project

Observe that ExtendThreadClassTest0 project appears and IDE generated ExtendThreadClassTest0.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated ExtendThreadClassTest0.java as shown in Code-1.11 below. Study the code by paying special attention to the bold fonted parts. Note that the start() is invoked after the object instance of PrintNameThread class is created.

Code-1.11: ExtendThreadClassTest0.java

3. Write PrintNameThread.java as shown in Code-1.12 below.

// Subclass extends Thread class
public class PrintNameThread extends Thread {

    PrintNameThread(String name) {
        super(name);
    }

    // Override the run() method of the Thread class.
    // This method gets executed when start() method
    // is invoked.
    public void run() {
        System.out.println("run() method of the " + this.getName() + " thread is called" );

        for (int i = 0; i < 10; i++) {
            System.out.print(this.getName());
        }
    }
}

Code-1.12: PrintNameThread.java

4. Build and run the project

Right click ExtendThreadClassTest0 project and select Run.
Observe the result in the Output window. (Figure-1.13 below)

Creating PrintNameThread object instance..
Calling start() method of A thread
run() method of the A thread is called
AAAAAAAAAA

Figure-1.13: Result of running ExtendThreadClassTest0 application

5. Modify the ExtendThreadClassTest0.java as shown in Code-1.15 below. The code fragments that need to be added are highlighted in bold and blue-colored font.

public class ExtendThreadClassTest0 {

    public static void main(String args[]) {

         // Create object instance of a class that is subclass of Thread class
        System.out.println("Creating PrintNameThread object instance..");
        PrintNameThread pnt1 =
                new PrintNameThread("A");

        // Start the thread by invoking start() method
        System.out.println("Calling start() method of " + pnt1.getName() + " thread");
        pnt1.start();

        System.out.println("Creating PrintNameThread object instance..");
        PrintNameThread pnt2 =
                new PrintNameThread("B");
        System.out.println("Calling start() method of " + pnt2.getName() + " thread");
        pnt2.start();

        System.out.println("Creating PrintNameThread object instance..");
        PrintNameThread pnt3 =
                new PrintNameThread("C");
        System.out.println("Calling start() method of " + pnt3.getName() + " thread");
        pnt3.start();
    }
}

Code-1.15: Modified ExtendThreadClassTest0.java

6. Build and run the project

Right click ExtendThreadClassTest0 project and select Run.
Observe the result in the Output window. (Figure-1.16 below)

Creating PrintNameThread object instance..
Calling start() method of A thread
Creating PrintNameThread object instance..
Calling start() method of B thread
AAAAAAAAAACreating PrintNameThread object instance..
BCalling start() method of C thread
BBBBBBBBBCCCCCCCCCC

Figure-1.16: Result

7. For your own exercise, modify ExtendThreadClassTest0.java as following. Build and run the application.

Create and start another thread.
Set the name of the thread as "MyOwn"

(1.2) The start() method is in the constructor of the subclass

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in ExtendThreadClassTest2 as project name.
For Create Main Class field, type in ExtendThreadClassTest2.
Click Finish.

Observe that ExtendThreadClassTest2 project appears and IDE generated ExtendThreadClassTest2.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated ExtendThreadClassTest2.java as shown in Code-1.21 below.

public class ExtendThreadClassTest2 {

    public static void main(String args[]) {


        PrintNameThread pnt1 =
                new PrintNameThread("A");


        PrintNameThread pnt2 =
                new PrintNameThread("B");


        PrintNameThread pnt3 =
                new PrintNameThread("C");

    }
}

Code-1.21: ExtendThreadClassTest2.java

3. Write PrintNameThread.java as shown in Code-1.22 below. Note that the start() method is invoked as part of the constructor method of the PrintNameThread class.

public class PrintNameThread extends Thread {

    PrintNameThread(String name) {
        super(name);
        // start() method is inside the constructor of the subclass
        start();
    }

    public void run() {
        String name = getName();
        for (int i = 0; i < 10; i++) {
            System.out.print(name);
        }
    }
}

Code-1.22: PrintNameThread.java

4. Build and run the project

Right click ExtendThreadClassTest2 project and select Run.
Observe the result in the Output window. (Figure-1.23 below)

AAAAAAAAAABBBBBBBBBBCCCCCCCCCC

Figure-1.23: Result of running ExtendThreadClassTest2 application

5. For your own exercise, modify ExtendThreadClassTest2.java as following. Build and run the application.

Create and start another thread.
Set the name of the thread as "MyOwn"

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Summary

Exercise 2: Implement Runnable interface

In this exercise, you are going to create and start a thread by writing a class that implements Runnable interface.

(2.1) Create and start a thread by implementing Runnable interface - start() method is not in the constructor

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in RunnableThreadTest1 as project name.
For Create Main Class field, type in RunnableThreadTest1.
Click Finish.

Observe that RunnableThreadTest1 project appears and IDE generated RunnableThreadTest1.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated RunnableThreadTest1.java as shown in Code-2.11 below. Study the code by paying special attention to the bold fonted parts. Note that the start() method needs to be invoked explicitly after an object instance of the PrintNameRunnable class is created.

public class RunnableThreadTest1 {

    public static void main(String args[]) {

        PrintNameRunnable pnt1 = new PrintNameRunnable("A");
        Thread t1 = new Thread(pnt1);
        t1.start();

        PrintNameRunnable pnt2 = new PrintNameRunnable("B");
        Thread t2 = new Thread(pnt2);
        t2.start();

        PrintNameRunnable pnt3 = new PrintNameRunnable("C");
        Thread t3 = new Thread(pnt3);
        t3.start();

    }
}

Code-2.11: RunnableThreadTest1.java

3. Write PrintNameRunnable.java as shown in Code-2.12 below.

// The class implements Runnable interface
class PrintNameRunnable implements Runnable {

    String name;

    PrintNameRunnable(String name) {
        this.name = name;
    }

    // Implementation of the run() defined in the
    // Runnable interface.
    public void run() {
        for (int i = 0; i < 10; i++) {
            System.out.print(name);
        }
    }
}

Code-2.12: PrintNameRunnable.java

4. Build and run the project

Right click RunnableThreadTest1 project and select Run.
Observe the result in the Output window. (Figure-2.13 below)

ACBACBACBACBACABCABCABCABCABCB

Figure-2.13: Result of running RunnableThreadTest1 application

5. For your own exercise, do the following. Build and run the application.

Create another class called MyOwnRunnableClass that implements Runnable interface
MyOwnRunnableClass displays values 1 to 10 inside its run() method
Modify RunnableThreadTest1.java to start 2 thread instances of MyOwnRunnableClass.

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(2.2) Create and start a thread by implementing Runnable interface - start() method is in the constructor

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in RunnableThreadTest2 as project name.
For Create Main Class field, type in RunnableThreadTest2.
Click Finish.

Observe that RunnableThreadTest2 project appears and IDE generated RunnableThreadTest2.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated RunnableThreadTest2.java as shown in Code-2.21 below. Study the code by paying special attention to the bold fonted parts.

public class RunnableThreadTest2 {

    public static void main(String args[]) {

        // Since the constructor of the PrintNameRunnable
        // object creates a Thread object and starts it,
        // there is no need to do it here.
        new PrintNameRunnable("A");

        new PrintNameRunnable("B");
        new PrintNameRunnable("C");
    }
}

Code-2.21: RunnableThreadTest2.java

3. Write PrintNameRunnable.java as shown in Code-2.22 below. Study the code by paying special attention to the bold fonted parts. Note that the start() method is in the constructor of the PrintNameRunnable class.

// The class implements Runnable interface
class PrintNameRunnable implements Runnable {

    Thread thread;

    PrintNameRunnable(String name) {
        thread = new Thread(this, name);
        thread.start();
    }

    // Implementation of the run() defined in the
    // Runnable interface.
    public void run() {
        String name = thread.getName();
        for (int i = 0; i < 10; i++) {
            System.out.print(name);
        }
    }
}

Code-2.22: PrintNameRunnable.java

4. Build and run the project

Right click RunnableThreadTest2 project and select Run.
Observe the result in the Output window. (Figure-1.23 below)

ABCABCABCABCABCABCABCBACBACBAC

Figure-2.23: Result of running RunnableThreadTest2 application

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Summary

In this exercise, you have learned how to create a class that implements Runnable interface and starts a thread.

Exercise 3: ThreadsGroup, View all threads, ThreadPriority

(3.1) Display threads of a ThreadGroup

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in ThreadGroupTest as project name.
For Create Main Class field, type in ThreadGroupTest.
Click Finish.

Observe that ThreadGroupTest project appears and IDE generated ThreadGroupTest.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated ThreadGroupTest.java as shown in Code-3.11 below. Study the code by paying special attention to the bold fonted parts.

public class ThreadGroupTest {

    public static void main (String[] args) {

        // Start three threads first. They should belong
        // to a same ThreadsGroup.
        new SimpleThread("Boston").start();
        new SimpleThread("New York").start();
        new SimpleThread("Seoul").start();

        // Get ThreadGroup of the current thread and display
        // the number of active threads that belong to the
        // ThreadGroup.
        ThreadGroup group = Thread.currentThread().getThreadGroup();
        System.out.println("Number of active threads in this thread group = "
                            + group.activeCount());

        // Display the names of the threads in the current
        // ThreadGroup.
        Thread[] tarray = new Thread[10];
        int actualSize = group.enumerate(tarray);
        for (int i=0; i<actualSize;i++){
            System.out.println("Thread " + tarray[i].getName()
                               + " in thread group " + group.getName());
        }

    }
}

Code-3.11: ThreadGroupTest.java

3. Write SimpleThread.java as shown in Code-3.12 below.

public class SimpleThread extends Thread {

    public SimpleThread(String str) {
        super(str);
    }

    public void run() {
        for (int i = 0; i < 5; i++) {
            // System.out.format("%d %s%n", i, getName());
            try {
                sleep((long)(Math.random() * 1000));
            } catch (InterruptedException e) {}
        }
        System.out.format("DONE! %s%n", getName());
    }
}

Code-3.12: SimpleThread.java

4. Build and run the project

Right click ThreadGroupTest project and select Run.
Observe the result in the Output window. (Figure-3.13 below)

Number of active threads in this thread group = 4
Thread main in thread group main
Thread Boston in thread group main
Thread New York in thread group main
Thread Seoul in thread group main
DONE! Seoul
DONE! New York
DONE! Boston

Figure-3.13: Result of running ThreadGroupTest application

(3.2) Display all threads in the system

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in DisplayAllThreads as project name.
For Create Main Class field, type in DisplayAllThreads.
Click Finish.

Observe that DisplayAllThreads project appears and IDE generated DisplayAllThreads.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated DisplayAllThreads.java as shown in Code-3.21 below. Study the code by paying special attention to the bold fonted parts.

public class DisplayAllThreads {

    public static void main(String[] args) {

        // Start three threads first. They should belong
        // to a same ThreadsGroup.
        new SimpleThread("Boston").start();
        new SimpleThread("New York").start();
        new SimpleThread("Seoul").start();

        Thread[] tarray = findAllThreads();

        for (int i=0; i<tarray.length;i++){
            System.out.println("Thread " + tarray[i].getName()
            + " in thread group " + tarray[i].getThreadGroup().getName());
        }

    }

    // Create an array of all threads in the system.
    public static Thread[] findAllThreads() {
        ThreadGroup group = Thread.currentThread().getThreadGroup();

        ThreadGroup topGroup = group;

        while (group != null) {
            topGroup = group;
            group = group.getParent();
        }

        int estimatedSize = topGroup.activeCount() * 2;
        Thread[] slackList = new Thread[estimatedSize];

        int actualSize = topGroup.enumerate(slackList);

        Thread[] list = new Thread[actualSize];
        System.arraycopy(slackList, 0, list, 0, actualSize);

        return list;
    }
}

Code-3.21: DisplayAllThreads.java

3. Write SimpleThread.java as shown in Code-3.22 below.

Code-3.22: SimpleThread.java

4. Build and run the project

Right click DisplayAllThreads project and select Run.
Observe the result in the Output window. (Figure-3.23 below)

Thread Reference Handler in thread group system
Thread Finalizer in thread group system
Thread Signal Dispatcher in thread group system
Thread main in thread group main
Thread Boston in thread group main
Thread New York in thread group main
Thread Seoul in thread group main
DONE! New York
DONE! Seoul
DONE! Boston

Figure-1.23: Result of running DisplayAllThreads application

5. For your own exercise, do the following. Build and run the application.

Modify DisplayAllThreads.java to create another (4th) SimpleThread instance using your capital city of your country.

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(3.3) Set thread priority

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in ThreadsPriority as project name.
For Create Main Class field, type in ThreadsPriority.
Click Finish.

Observe that ThreadsPriority project appears and IDE generated ThreadsPriority.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated ThreadsPriority.java as shown in Code-3.31 below. Study the code by paying special attention to the bold fonted parts.

public class ThreadsPriority {

    public static void main(String[] args) {

        Thread t1 = new SimpleThread("Boston");
        t1.start();
        // Set the thread priority to 10(highest)
        t1.setPriority(10);

        Thread t2 = new SimpleThread("New York");
        t2.start();
        // Set the thread priority to 5
        t2.setPriority(5);

        Thread t3 = new SimpleThread("Seoul");
        t3.start();
        // Set the thread priority to 1
        t3.setPriority(1);

    }
}

Code-3.31: ThreadsPriority.java

3. Write SimpleThread.java as shown in Code-3.32 below.

public class SimpleThread extends Thread {

    public SimpleThread(String str) {
        super(str);
    }

    public void run() {
        for (int i = 0; i < 10; i++) {
            System.out.println(i + " " + getName()
                                 + " Priority = " + getPriority());
        }
        System.out.println("Done! " + getName());
    }
}

Code-3.32: SimpleThread.java

4. Build and run the project

Right click ThreadsPriority project and select Run.
Observe the result in the Output window. (Figure-3.33 below)

0 Boston Priority = 10
0 Seoul Priority = 1
0 New York Priority = 5
1 Boston Priority = 10
1 Seoul Priority = 1
1 New York Priority = 5
2 Boston Priority = 10
2 Seoul Priority = 1
3 Boston Priority = 10
2 New York Priority = 5
4 Boston Priority = 10
3 New York Priority = 5
5 Boston Priority = 10
6 Boston Priority = 10
7 Boston Priority = 10
8 Boston Priority = 10
9 Boston Priority = 10
Done! Boston
4 New York Priority = 5
5 New York Priority = 5
6 New York Priority = 5
7 New York Priority = 5
8 New York Priority = 5
9 New York Priority = 5
Done! New York
3 Seoul Priority = 1
4 Seoul Priority = 1
5 Seoul Priority = 1
6 Seoul Priority = 1
7 Seoul Priority = 1
8 Seoul Priority = 1
9 Seoul Priority = 1
Done! Seoul

Figure-3.33: Result of running ThreadsPriority application

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Summary

Exercise 4: Synchronization

(4.1) Build and run a program in which threads are NOT synchronized

In this step, you are going to build an application that displays a result that is not desirable since threads are not synchronized.

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in UnsynchronizedExample as project name.
For Create Main Class field, type in UnsynchronizedExample.
Click Finish.

Observe that UnsynchronizedExample project appears and IDE generated UnsynchronizedExample.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated UnsynchronizedExample.java as shown in Code-4.11 below.

public class UnsynchronizedExample {

    public static void main(String[] args) {
        new PrintStringsThread("Hello ", "there.");
        new PrintStringsThread("How are ", "you?");
        new PrintStringsThread("Thank you ", "very much!");
    }

}

Code-4.11: UnsynchronizedExample.java

3. Write PrintStringsThread.java as shown in Code-4.12 below.

public class PrintStringsThread implements Runnable {

    Thread thread;
    String str1, str2;

    PrintStringsThread(String str1, String str2) {
        this.str1 = str1;
        this.str2 = str2;
        thread = new Thread(this);
        thread.start();
    }

    public void run() {
        TwoStrings.print(str1, str2);
    }

}

Code-4.12: PrintStringsThread.java

4. Write TwoStrings.java as shown in Code-4.13 below. Study the code by paying special attention to the bold fonted parts. Note that the print method is not synchronized.

public class TwoStrings {

    // This method is not synchronized
    static void print(String str1, String str2) {
        System.out.print(str1);
        try {
            Thread.sleep(500);
        } catch (InterruptedException ie) {
        }
        System.out.println(str2);
    }
}

Code-4.13: TwoStrings.java

5. Build and run the project

Right click UnsynchronizedExample project and select Run.
Observe the result in the Output window. (Figure-4.14 below)

Hello How are Thank you there.
very much!
you?

Figure-4.14: Result of running UnsynchronizedExample application

return to top of the exercise

(4.2) Build and run a program in which threads are synchronized through synchronized method

In this step, you are going to build an application that displays a desired result because the threads are synchronized.

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in SynchronizedExample1 as project name.
For Create Main Class field, type in SynchronizedExample1.
Click Finish.

Observe that SynchronizedExample1 project appears and IDE generated SynchronizedExample1.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated SynchronizedExample1.java as shown in Code-4.21 below.

public class SynchronizedExample1 {

    public static void main(String[] args) {
        new PrintStringsThread("Hello ", "there.");
        new PrintStringsThread("How are ", "you?");
        new PrintStringsThread("Thank you ", "very much!");
    }

}

Code-4.21: SynchronizedExample1.java

3. Write PrintStringsThread.java as shown in Code-4.22 below.

Code-4.22: PrintStringsThread.java

4. Write TwoStrings.java as shown in Code-4.23 below. Study the code by paying special attention to the bold fonted parts.

public class TwoStrings {

    // This method is now synchronized
    synchronized static void print(String str1, String str2) {
        System.out.print(str1);
        try {
            Thread.sleep(500);
        } catch (InterruptedException ie) {
        }
        System.out.println(str2);
    }
}

Code-4.23: TwoStrings.java

5. Build and run the project

Right click SynchronizedExample1 project and select Run.
Observe the result in the Output window. (Figure-4.24 below)

How are you?
Thank you very much!
Hello there.

Figure-4.24: Result of running UnSynchronizedExample1 application

return to top of the exercise

(4.3) Build and run a program in which threads are synchronized through synchronized statement on common object

In this step, you are going to build another application that displays a desired result because the threads are synchronized.

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in SynchronizedExample2 as project name.
For Create Main Class field, type in SynchronizedExample2.
Click Finish.

Observe that SynchronizedExample2 project appears and IDE generated SynchronizedExample2.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated SynchronizedExample2.java as shown in Code-4.31 below.

public class SynchronizedExample2 {

    public static void main(String[] args) {

        TwoStrings ts = new TwoStrings();

        new PrintStringsThread("Hello ", "there.", ts);
        new PrintStringsThread("How are ", "you?", ts);
        new PrintStringsThread("Thank you ", "very much!", ts);
    }

}

Code-4.31: SynchronizedExample2.java

3. Write PrintStringsThread.java as shown in Code-4.32 below. Study the code by paying special attention to the bold fonted parts.

public class PrintStringsThread implements Runnable {

    Thread thread;
    String str1, str2;
    TwoStrings ts;

    PrintStringsThread(String str1, String str2,
                       TwoStrings ts) {
        this.str1 = str1;
        this.str2 = str2;
        this.ts = ts;
        thread = new Thread(this);
        thread.start();
    }

    public void run() {
        // Synchronize over TwoString object
        synchronized (ts) {
            ts.print(str1, str2);
        }
    }
}

Code-4.32: PrintStringsThread.java

4. Write TwoStrings.java as shown in Code-4.33 below.

public class TwoStrings {

    static void print(String str1, String str2) {
        System.out.print(str1);
        try {
            Thread.sleep(500);
        } catch (InterruptedException ie) {
        }
        System.out.println(str2);
    }
}

Code-4.33: TwoStrings.java

5. Build and run the project

Right click SynchronizedExample2 project and select Run.
Observe the result in the Output window. (Figure-4.34 below)

How are you?
Thank you very much!
Hello there.

Figure-4.34: Result of running UnSynchronizedExample2 application

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Summary

Exercise 5: Inter-thread communication

(5.1) Producer-Consumer without inter-thread communication

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in ProducerConsumerUnsynchronized as project name.
For Create Main Class field, type in ProducerConsumerUnsynchronized.
Click Finish.

Observe that ProducerConsumerUnsynchronized project appears and IDE generated ProducerConsumerUnsynchronized.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated ProducerConsumerUnsynchronized.java as shown in Code-5.11 below.

public class ProducerConsumerUnsynchronized {

    public static void main(String[] args) {

        CubbyHole c = new CubbyHole();

        Producer p1 = new Producer(c, 1);
        Consumer c1 = new Consumer(c, 1);

        p1.start();
        c1.start();
    }
}

Code-5.11: ProducerConsumerUnsynchronized.java

3. Write CubbyHole.java as shown in Code-5.12 below. Study the code by paying special attention to the bold fonted parts.

// Unsynchronized CubbyHole.
//
// Results are unpredictable; a number may be read before a number has
// been produced or multiple numbers may be produced with only one or
// two being read adding synchronization ensures that a number is first
// produced, then read in the correct order.

public class CubbyHole {
    private int contents;
    private boolean available = false;

    public int get() {
        available = false;
        return contents;
    }

    public void put(int value) {
        contents = value;
        available = true;
    }
}

Code-5.12: CubbyHole.java

4. Write Producer.java as shown in Code-5.13 below.

public class Producer extends Thread {
    private CubbyHole cubbyhole;
    private int number;

    public Producer(CubbyHole c, int number) {
        cubbyhole = c;
        this.number = number;
    }

    public void run() {
        for (int i = 0; i < 10; i++) {
            cubbyhole.put(i);
            System.out.println("Producer #" + this.number
                               + " put: " + i);
            try {
                sleep((int)(Math.random() * 100));
            } catch (InterruptedException e) { }
        }
    }
}

Code-5.13: Producer.java

5. Write Consumer.java as shown in Code-5.14 below.

public class Consumer extends Thread {
    private CubbyHole cubbyhole;
    private int number;

    public Consumer(CubbyHole c, int number) {
        cubbyhole = c;
        this.number = number;
    }

    public void run() {
        int value = 0;
        for (int i = 0; i < 10; i++) {
            value = cubbyhole.get();
            System.out.println("Consumer #" + this.number
                               + " got: " + value);
        }
    }
}

Code-5.14: Consumer.java

6. Build and run the project

Right click ProducerConsumerUnsynchronized project and select Run.
Observe the result in the Output window. (Figure-5.15 below)

Producer #1 put: 0
Consumer #1 got: 0
Consumer #1 got: 0
Consumer #1 got: 0
Consumer #1 got: 0
Consumer #1 got: 0
Consumer #1 got: 0
Consumer #1 got: 0
Consumer #1 got: 0
Consumer #1 got: 0
Consumer #1 got: 0
Producer #1 put: 1
Producer #1 put: 2
Producer #1 put: 3
Producer #1 put: 4
Producer #1 put: 5
Producer #1 put: 6
Producer #1 put: 7
Producer #1 put: 8
Producer #1 put: 9

Figure-5.15: Result of running ProducerConsumerUnsynchronized application

return to top of the exercise

(5.2) Producer-Consumer with inter-thread communication

Summary

Exercise 6: Timer and TimerTask

(6.1) Schedule one-time task

1. Create a new NetBeans project

Select File->New Project (Ctrl+Shift+N). The New Project dialog box appears.
Under Choose Project pane, select Java under Categories and Java Application under Projects.
Click Next.

Under Name and Location pane, for the Project Name field, type in TimerReminder as project name.
For Create Main Class field, type in TimerReminder.
Click Finish.

Observe that TimerReminder project appears and IDE generated TimerReminder.java is displayed in the source editor window of NetBeans IDE.

2. Modify the IDE generated TimerReminder.java as shown in Code-6.11 below. Study the code by paying special attention to the bold fonted parts.

import java.util.Timer;
import java.util.TimerTask;

/**
* Simple demo that uses java.util.Timer to schedule a task to execute
* once 5 seconds have passed.
*/

public class TimerReminder {

    Timer timer;

    public TimerReminder(int seconds) {
        timer = new Timer();
        timer.schedule(new RemindTask(), seconds*1000);
    }

    class RemindTask extends TimerTask {
        public void run() {
            System.out.println("Time's up!");
            timer.cancel(); //Terminate the timer thread
        }
    }

    public static void main(String args[]) {
        System.out.println("About to schedule Reminder task in 5 seconds");
        new TimerReminder(5);
        System.out.println("Task scheduled.");
    }
}

Code-6.11: TimerReminder.java

3. Build and run the project

Right click TimerReminder project and select Run.
Observe the result in the Output window. (Figure-6.12 below)

About to schedule Reminder task in 5 seconds
Task scheduled.
Time's up!

Figure-6.12: Result of running UnTimerReminder application

return to top of the exercise

(6.2) Schedule a repeating task

2. Modify the IDE generated AnnoyingBeep.java as shown in Code-6.21 below. Study the code by paying special attention to the bold fonted parts.

import java.util.Timer;
import java.util.TimerTask;
import java.awt.Toolkit;

/**
* Schedule a task that executes once every second.
* Beep every second.
*/

public class AnnoyingBeep {
    Toolkit toolkit;
    Timer timer;

    public AnnoyingBeep() {
        toolkit = Toolkit.getDefaultToolkit();
        timer = new Timer();
        timer.schedule(new RemindTask(),
                0,        //initial delay
                1*1000); //subsequent rate
    }

    class RemindTask extends TimerTask {
        int numWarningBeeps = 3;

        public void run() {
            if (numWarningBeeps > 0) {
                toolkit.beep();
                System.out.format("Beep!%n");
                numWarningBeeps--;
            } else {
                toolkit.beep();
                System.out.format("Time's up!%n");
                //timer.cancel(); //Not necessary because we call System.exit
                System.exit(0);   //Stops the AWT thread (and everything else)
            }
        }
    }

    public static void main(String args[]) {
        System.out.format("About to schedule task.%n");
        new AnnoyingBeep();
        System.out.format("Task scheduled.%n");
    }
}

Code-6.21: AnnoyingBeep.java

Java Threads

Resources

Exercises

Exercise 1: Extending Thread class

(1.1) The start() method is NOT in the constructor of the subclass

(1.2) The start() method is in the constructor of the subclass

Summary

Exercise 2: Implement Runnable interface

(2.1) Create and start a thread by implementing Runnable interface - start() method is not in the constructor

(2.2) Create and start a thread by implementing Runnable interface - start() method is in the constructor

Summary

Exercise 3: ThreadsGroup, View all threads, ThreadPriority

(3.1) Display threads of a ThreadGroup

(3.2) Display all threads in the system

(3.3) Set thread priority

Summary

Exercise 4: Synchronization

(4.1) Build and run a program in which threads are NOT synchronized

(4.2) Build and run a program in which threads are synchronized through synchronized method

(4.3) Build and run a program in which threads are synchronized through synchronized statement on common object

Summary

Exercise 5: Inter-thread communication

(5.1) Producer-Consumer without inter-thread communication

(5.2) Producer-Consumer with inter-thread communication

Summary

Exercise 6: Timer and TimerTask

(6.1) Schedule one-time task

(6.2) Schedule a repeating task

Summary

Homework