금요일, 9월 17
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#025 Java Threads

Lab Exercises

 

Exercise 1: Extending Thread class

In this exercise, you are going to learn how to create and start a thread execution by writing a class that extends Thread class.  You will learn how to start the thread either by not having the start() method in the constructor of the subclass or having it in the constructor method of the subclass.

  1. The start() method is not in the constructor of the subclass
  2. The start() method is in the constructor of the subclass

(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.

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();

}
}

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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/ExtendThreadClassTest0.  You can just open it and run it.

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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/ExtendThreadClassTest1.  You can just open it and run it.

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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/ExtendThreadClassTest2.  You can just open it and run it.

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”

Summary

In this exercise, you have learned how to create and start a thread by extending Thread class.

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.

  1. Create and start a thread by implementing Runnable interface – start() method is not in the constructor
  2. Create and start a thread by implementing Runnable interface – start() method is in the constructor

(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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/RunnableThreadTest1.  You can just open it and run it.

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.

(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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/RunnableThreadTest2.  You can just open it and run it.

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

In this exercise, you are going to learn how to display information on a ThreadGroup, how to set a thread priority, and so on.

 

(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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/ThreadGroupTest.  You can just open it and run it.

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

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

(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.

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-3.22: 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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/DisplayAllThreads.  You can just open it and run it.

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.

(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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/ThreadsPriority.  You can just open it and run it.

Summary

In this exercise, you have learned how to retrieve information on a ThreadGroup.

 

 

Exercise 4: Synchronization

In this exercise, you are going to exercise how to do synchronization among threads.

(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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/UnsynchronizedExample.  You can just open it and run it.

(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.

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-4.22: 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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/SynchronizedExample1.  You can just open it and run it.

(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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/SynchronizedExample2.  You can just open it and run it.

Summary

In this exercise, you have learned how to use synchronization.

 

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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/ProducerConsumerUnsynchronized.  You can just open it and run it.

(5.2) Producer-Consumer with 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 ProducerConsumerSynchronized as project name.
  • For Create Main Class field, type in ProducerConsumerSynchronized.
  • Click Finish.
  • Observe that ProducerConsumerSynchronized project appears and IDE generated ProducerConsumerSynchronized.java is displayed in the source editor window of NetBeans IDE.
2. Modify the IDE generated ProducerConsumerSynchronized.java as shown in Code-5.21 below.

public class ProducerConsumerSynchronized {

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-5.21: ProducerConsumerSynchronized.java

3. Write CubbyHole.java as shown in Code-5.22 below.  Study the code by paying special attention to the bold fonted parts.

public class CubbyHole {

private int contents;
private boolean available = false;

public synchronized int get(int who) {
while (available == false) {
try {
wait();
} catch (InterruptedException e) { }
}
available = false;
System.out.format(“Consumer %d got: %d%n”, who, contents);
notifyAll();
return contents;
}

public synchronized void put(int who, int value) {
while (available == true) {
try {
wait();
} catch (InterruptedException e) { }
}
contents = value;
available = true;
System.out.format(“Producer %d put: %d%n”, who, contents);
notifyAll();
}
}

Code-5.22: CubbyHole.java

4. Write Producer.java as shown in Code-5.23 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(number, i);
try {
sleep((int)(Math.random() * 100));
} catch (InterruptedException e) { }
}
}
}

Code-5.23: Producer.java

5. Write Consumer.java as shown in Code-5.24 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(number);
}
}
}

Code-5.24: Consumer.java

6. Build and run the project

  • Right click ProducerConsumerSynchronized project and select Run.
  • Observe the result in the Output window. (Figure-5.25 below)
Producer 1 put: 0
Consumer 1 got: 0
Producer 1 put: 1
Consumer 1 got: 1
Producer 1 put: 2
Consumer 1 got: 2
Producer 1 put: 3
Consumer 1 got: 3
Producer 1 put: 4
Consumer 1 got: 4
Producer 1 put: 5
Consumer 1 got: 5
Producer 1 put: 6
Consumer 1 got: 6
Producer 1 put: 7
Consumer 1 got: 7
Producer 1 put: 8
Consumer 1 got: 8
Producer 1 put: 9
Consumer 1 got: 9
Figure-5.25: Result of running ProducerConsumerSynchronized application

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/ProducerConsumerSynchronized.  You can just open it and run it.

Summary

In this exercise, you have learned how to perform inter-thread commmunication by the usage of wait(), notify(), and notifyAll() methods.

Exercise 6: Timer and TimerTask

In this exercise, you will learn how to use Timer and TimerTask to schedule a single or repeating task.

(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

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/TimerReminder.  You can just open it and run it.

(6.2) Schedule a repeating 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 AnnoyingBeep as project name.
  • For Create Main Class field, type in AnnoyingBeep.
  • Click Finish.
  • Observe that AnnoyingBeep project appears and IDE generated AnnoyingBeep.java is displayed in the source editor window of NetBeans IDE.
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

3 Build and run the project
  • Right click AnnoyingBeep project and select Run.
  • Observe the result in the Output window. (Figure-6.22 below)
About to schedule task.
Task scheduled.
Beep!
Beep!
Beep!
Time’s up!

Figure-6.22: Result

Solution: This exercise up to this point is provided as a ready-to-open-and-run NetBeans project as part of hands-on lab zip file. You can find it as <LAB_UNZIPPED_DIRECTORY>/javathreads/samples/AnnoyingBeep.  You can just open it and run it.

Summary

In this exercise, you have learned how to use Timer and TimerTask classes to schedule one-time or repeating tasks.

 

Homework exercise (for people who are taking Sang Shin’s “Java Programming online course”)

 

1. The homework is to create a new NetBeans project called MyRunnableProject as following.
  • Create a class called MyCurrentDate that implements Runnable interface.
  • The MyCurrentDate class displays the current date and time 10 times, with 100 milli seconds interval – use sleep() method for this interval.
  • Create a class called MyMain, which contans main() method, in which 3 instances of MyCurrentDate threads are being run.
2. Send the following files to javaprogramminghomework@sun.com with Subject as JavaIntro-javathreads.
  • Zip file of the the MyRunnableProject NetBeans project.  (Someone else should be able to open and run it as a NetBeans project.)  You can use your favorite zip utility or you can use “jar” utility that comes with JDK as following.
    • cd <parent directory that contains MyRunnableProject directory> (assuming you named your project as MyRunnableProject)
    • jar cvf MyRunnableProject.zip MyRunnableProject (MyRunnableProject should contain nbproject directory)
  • Captured output screen  – name it as JavaIntro-javathreads.gif or JavaIntro-javathreads.jpg (or JavaIntro-javathreads.<whatver graphics format>)
    • Any screen capture that shows that your program is working is good enough.  No cosmetic polishment is required.
  • If you decide to use different IDE other than NetBeans, the zip file should contain all the files that are needed for rebuilding the project.

 

 

 

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