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© Wang Bin 2004
Java Threads
© Wang Bin 2004
In this lesson, you will learn to:
Define the concepts of threads and multithreading
Identify the functions and syntax of the Thread, Date, Calendar, and GregorianCalendar classes and the methods used to manipulate them
Display the current date and time on an applet
Objectives
© Wang Bin 2004
Definition of a Thread: A process is divided into several tasks. Each task is divided
into smaller units called threads
A thread can be defined as a single sequential flow of control within a program
Every program has at least one thread that is called the primary thread
The microprocessor allocates memory to the processes that you execute. Each process occupies its own address space (memory)
All the threads in a process share the same address space
Threads
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A process that is made of only one thread is said to be single-threaded
A process having more than one thread is said to be multithreaded. The multiple threads in the process run at the same time, perform different tasks, and interact with each other
Java has built-in support for threads. A major portion of the Java architecture is multithreaded
Definition of Multithreading
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An additional feature has to be added to the Customer details applet. The current date and time has to be displayed on the status bar of the applet. Write the code.
Problem Statement
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The Thread Class: The java.lang.Thread class is used to construct
and access the individual threads in a multithreaded application
You can make your applications and classes run in separate threads by extending the Thread class
Syntax:• public class <class_name> extends Thread
Identify the mechanism to be used for displaying the date and time
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The Runnable Interface: When a program needs to inherit from another class
besides the Thread class, you need to implement the Runnable interface
The Runnable interface consists of a single method run(), which is executed when the thread is activated
Syntax for implementing the Runnable interface• public class <class_name> extends
<superclass_name> implements Runnable
Identify the mechanism ...(Contd.)
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Life Cycle of a Thread: When an instance of the Thread class is created, a
thread enters the new thread state
When the start() method of the thread is invoked, the thread enters the runnable state
A thread is said to be in the not runnable state if it is sleeping, waiting, or being blocked by another thread
A thread is put into the sleeping mode with the sleep() method
Identify the mechanism ...(Contd.)
© Wang Bin 2004
Syntax of the sleep() method:
• sleep(long t);
In the syntax, t is the number of milliseconds for which the thread is inactive
A thread object is dead if the loop in run() method is complete or a null value is assigned to the thread
The Date Class The Date class encapsulates the system date and time
information
Identify the mechanism ...(Contd.)
© Wang Bin 2004
The Calendar Class: The Calendar class implements the methods that
were present in the earlier version of the Date class The get() Method:
The get() method of the Calendar class is used to extract the date, the month, or the year from a given date
Syntax of the get() method:• String get(int field);
Identify the mechanism ...(Contd.)
© Wang Bin 2004
Fields in the Calendar Class:
Fields Return Values
static int HOUR Hour of the time
static int MINUTE Minutes of the time
static int SECOND Seconds of the time
static int DATE Date portion of the date
static int MONTH Month portion of the date
static int YEAR Year portion of the date
Identify the mechanism ...(Contd.)
© Wang Bin 2004
The GregorianCalendar Class: The GregorianCalendar class is extended from the
Calendar class
It supports calendar operations for most parts of the world
The setTime() Method: The setTime() method takes a Date object as an
argument
It updates the GregorianCalendar object with the current date
Identify the mechanism ...(Contd.)
© Wang Bin 2004
Syntax of the setTime() method:
• setTime(Date date); Result:
The applet will need to show the date and time simultaneously with the other controls, therefore use the following:• Thread class• Runnable interface• Date class• Calendar class• GregorianCalendar class
Identify the mechanism ...(Contd.)
© Wang Bin 2004
public class CustomerApplet extends JApplet implements Runnable{ Thread datimeThread; Date date; GregorianCalendar calendar; String strDate, strTime, strStatus;
public void init() { createAppletContent(); //Method for Applet Layout is called } protected void createAppletContent() { candidateDetail(); // contents of the applet } public void candidateDetail() { datimeThread = new Thread(this); //Initialize thread datimeThread.start(); //Starting thread } ……}
Codes
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public void run() // body of the thread
{ while(datimeThread != null) { display() ; // This method displays date try { datimeThread.sleep(1000); } catch(InterruptedException e) { showStatus("Thread interrupted"); } } //end of while loop } //end of run method
Codes
© Wang Bin 2004
public void display() //displays date and time on the status bar
{
date = new Date();
calendar = new GregorianCalendar();
calendar.setTime(date);
strTime = calendar.get(Calendar.HOUR)+":" +
calendar.get(Calendar.MINUTE)+":"+
calendar.get(Calendar.SECOND);
strDate = calendar.get(Calendar.MONTH)+"/"+
calendar.get(Calendar.DATE)+"/"+
calendar.get(Calendar.YEAR);
strStatus=strTime+" "+strDate;
showStatus(strStatus);
}
}//end of program
Codes
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When two threads share data, we must guarantee one thread can not change data which another thread use.
We use keyword synchronized to resolve it.
Thread Synchronization
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class MyThread extends Thread{ static String message[] = {"I", "Love","Java","Very","Much."};
public MyThread(String id) { super(id); } public void run() { Sync.displayList(getName(), message); }
Examples
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void waiting() { try { sleep(1000); } catch(InterruptedException e) { System.out.println("Interrupted"); } }}
Examples (Contd.)
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class Sync{ public static synchronized void displayList(String
name, String list[]) { for(int i = 0; i < list.length; ++i) { MyThread thread = (MyThread)Thread.currentThread(); thread.waiting(); System.out.println(name + list[i]); } }}
Examples (Contd.)
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class ThreadSync{ public static void main(String args[]) { MyThread thread1 = new MyThread("Thread 1 : "); MyThread thread2 = new MyThread("Thread 2 : "); thread1.start(); thread2.start(); }}
Examples (Contd.)
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Result
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If no synchronized, Result:
© Wang Bin 2004
In this lesson, you learned that: You can execute multiple tasks within a program
using threads
A thread, like a program, has a beginning, a sequence of steps, and an end. However, it is not a program on its own but runs within a process of a program. A process is an executing instance of a program. A thread is also known as a lightweight process or the execution context
A process having more than one thread is said to be multithreaded. The multiple threads in a process run simultaneously, perform different tasks, and interact with each other
Summary
© Wang Bin 2004
The java.lang.Thread class is used to construct and access the individual threads in a multithreaded application
Applets extend from the JApplet class. Since Java does not support multiple inheritance, you cannot inherit a class from the JApplet as well as the Thread class. Java provides the Runnable interface to solve this problem. The Runnable interface consists of a single method run(), which is executed when the thread is activated
Summary (Contd.)
© Wang Bin 2004
The life cycle of a thread consists of four stages:
• New thread
• Not Runnable
• Runnable
• Dead
When an instance of the Thread class is created, the thread enters the new thread state
The start() method is responsible for starting a thread. When the start() method of the thread is invoked, the thread enters the runnable state
Summary (Contd.)
© Wang Bin 2004
The activities to be performed by a thread are coded in the run() method
A thread is said to be in the not runnable state if it:
• Is sleeping,
• Is waiting, or
• Is being blocked by another thread
A thread is put into the sleeping mode with the sleep() method
Summary (Contd.)
© Wang Bin 2004
Invoking the stop() method of an applet kills a thread. The isAlive() method of the Thread class is used to determine whether a thread has been started or stopped
The Date class is responsible for encapsulating the date and time
The Calendar class implements the date methods that were present earlier in the Date class
The GregorianCalendar class is extended from the Calendar class. It supports calendar operations for most parts of the world
Summary (Contd.)
© Wang Bin 2004
The setTime() method sets the calendar's current time with the given date
The get() method is used to extract the date, month, and year from the Calendar variable
The showStatus() method displays a message on the status bar
Thread Synchronization
Keyword synchronized
Summary (Contd.)
