1.2 Threads and Multithreading

Creating a Thread

A thread in Java begins as an instance of java.lang.Thread. You’ll find methods in the Thread class for managing threads including creating, starting, and pausing them. They are:
start()
yield()
sleep()
run()

All the awesome action happens in the run() method. Think of the code you want to execute in a separate thread as the job to do. Lets say, you have some work that needs to be done, in the background while other things are happening in the program, so what you really want is that work to be executed in its own thread. All that code you want executed in a separate thread goes into the run() method.

Ex:
public void run() {
// your job code goes here
}

The run() method will call other methods, of course, but the thread of execution—the new call stack—always begins by invoking run(). So where does the run() method go? In one of the two classes you can use to define your thread job.

You can define and instantiate a thread in one of two ways:
• Extend the java.lang.Thread class.
• Implement the Runnable interface.

You need to know about both for the exam, although I would personally recommend that you implement Runnable than extend Thread. Extending the Thread class is the easiest, but it’s usually not a good OO practice.

Now, you may ask me “Why? Why should I choose the Runnable over Thread?”
Well, if you thought of the why before reading the preceding line, give yourself a pat on the back. Well done!

Because, if you extend the Thread class what would you do if you have to extend another concrete parent class that has vital/important behavior that you need in your class? Get the point? Java does not support direct multiple inheritance and hence, once you extend the Thread class, you are done, for good. You cannot extend any other class. That is why I recommended the Runnable interface. Even if you implement the interface, you are free to extend any class you want. Convenient, right?

Defining a Thread

To define a thread, you need a place to put your run() method, and as we just discussed, you can do that by extending the Thread class or by implementing the Runnable interface. We’ll look at both in this section.

Extending java.lang.Thread
The simplest way to define code to run in a separate thread is to
• Extend the java.lang.Thread class.
• Override the run() method.
It looks like this:
class MyFirstThread extends Thread {
public void run() {
System.out.println("Important job running in MyFirstThread");
}
}
The limitation with this approach is that if you extend Thread, you can’t extend anything else. And it’s not as if you really need that inherited Thread class behavior, because in order to use a thread you’ll need to instantiate one anyway.

Keep in mind that you’re free to overload the run() method in your Thread subclass:
class MyFirstThread extends Thread {
public void run() {
System.out.println("Important job running in MyFirstThread");
}
public void run(String s) {
System.out.println("String running is " + s);
}
}

Note: The overloaded run(String s) method will be ignored by the Thread class unless you call it yourself. The Thread class expects a run() method with no arguments, and it will execute this method for you in a separate call stack after the thread has been started. With a run(String s) method, the Thread class won’t call the method for you, and even if you call the method directly yourself, execution won’t happen in a new thread of execution with a separate call stack. It will just happen in the same call stack as the code that you made the call from, just like any other normal method call.

1.1 Threads and Multithreading

This chapter and the subsequent ones on Threads and Multi-threading is probably by far the most complex topic we will be covering as part of our aim at getting a SCJP Certification.
Before we begin – let me put out a Disclaimer…

Disclaimer: This chapter makes almost no attempt to teach you how to design a good, safe, multithreaded application. The concepts covered in this article are just the tip of the iceberg and you’re here only to learn the basics of threading, and what you need to get through the thread questions on the exam. Before you can write decent multithreaded code, however, you really need to study more on the complexities and subtleties of multithreaded code.

In a single-threaded runtime environment, actions execute one after another. The next action can happen only when the previous one is finished. If task A takes half an hour, and the user wants to do something else, he is doomed to wait until task A is over.

Ideally, the user should be able to carry on with his other task until his first task is complete. Imagine having to wait till your eclipse build all your source code before you could check your email? Thankfully windows uses multiple threads to run your eclipse and outlook. So you don't have to go through this pain. Similarly java has options wherein the programmer can have multiple threads of execution running.

Now you are ready to dive into the magical or rather complicated world of threads.

Lets get started!!!

What is a Thread?

In Java, “thread” means:
• A thread of execution

A thread of execution is an individual process that has its own call stack. In Java, there is one thread per call stack or, to think of it in reverse, one call stack per thread. Even if you don’t create any new threads in your program, threads are there running without your knowledge.
If you are curious and ask me how? Just stop for a moment and think how you would execute a java program? You’ll write a main method and then execute it from the command line using the “java” command. Well, you have already answered the question because, when your main method starts, the JVM assigns a thread for this main method and assigns it a call stack. So, eventhough you did not intentionally create a thread, the system did one for you.

The most important concept to understand from this entire chapter is this:

When it comes to threads, very little is guaranteed.

So be very cautious about interpreting the behavior you see on one machine as “the way threads work.” The exam expects you to know what is and is not guaranteed behavior, so that you can design your program in such a way that it will work regardless of the underlying JVM.

NEXT

Creating a Thread

Creating a Thread is fairly simple. The complexities get introduced once you have your threads running and then you see that the output isn’t what you exactly expected. Well, I don’t want to jump ahead too much. We will learn all those complexities one by one. For now lets keep our focus on how to create threads.

You can create threads in two ways:

1. By Extending the Thread Class
2. By Implementing the Runnable Interface

Common sense related to Java Programming tells us that using the interface may be a better option because, your class may need to inherit features from its parent class or such a feature may be required in future. Either ways, it would be a better idea to implement the Runnable Interface so that our classes can do, what they were designed/created to do.

understanding threads in java-what the heck is thread

A thread is an application task that is executed by a host computer. The notion of a task should be familiar to you even if the terminology is not.

Suppose you have a Java program to do some task:

public class DoSomething{
public static void main(String[] args) {
Task 1;
Task 2;
Task 3;
…
Task N;
}
}

When your computer runs this application, it executes a sequence of commands. At an abstract level, that list of commands looks like this:

• Do Task 1
• Do Task 2
• Continue on until you reach Task N
• Exit the Program

Behind the scenes, what happens is somewhat more complicated since the instructions that are executed are actually machine-level assembly instructions; each of our logical steps requires many machine instructions to execute. But the principle is the same: an application is executed as a series of instructions. The execution path of these instructions is a thread.

Consequently, every computer program has at least one thread: the thread that executes the body of the application. In a Java application, that thread is called the main thread, and it begins executing statements with the first statement of the main() method of your class. In other programming languages, the starting point may be different, and the terminology may be different, but the basic idea is the same.

Trivia:
For Java applications, execution begins with the main() method of the class being run. What about other Java programs?
In applets, servlets, and other J2EE programs, execution still begins with the main() method of the program, but in this case, the main() method belongs to the Java plug-in or J2EE container. Those containers then call your code through predetermined, well-known locations. An applet is called via its init() and start() methods; a servlet is called through its doGet() and doPost() methods, and so on.
In any case, the procedure is the same: execution of your code begins with the first statements and proceeds by a single thread sequentially.

In a Java program, it so happens that every program has more than one thread. Many of these are threads that developers are unaware of, such as threads that perform garbage collection and compile Java bytecodes into machine-level instructions. In a graphical application, other threads handle input from the mouse and keyboard and play audio. Your Java application is highly threaded, whether you program & code additional threads into it or not.

Returning to our example, let's suppose that we wrote a program that performed two tasks: one calculated the factorial of a number and one calculated the square root of that number. These are two separate tasks, and so you could choose to write them as two separate threads. Now how would your application run?

The answer to that depends on the conditions under which the application is run. The Java virtual machine now has two distinct lists of instructions to execute. One list calculates the factorial of a number, and the other list calculates the square root of the number. The Java virtual machine executes both of these lists almost simultaneously.

Although you may not have thought about it in these terms, this situation should also be familiar to you from the computer on which you normally do your work. The program you use to read your email is a list of instructions that the computer executes. So too is the program that you use to listen to music. You're able to read email and listen to music at the same time because the computer executes both lists of instructions at about the same time.

In fact, what happens is that the computer executes a handful of instructions from the email application and then executes a handful of instructions from the music program. It continues this procedure, switching back and forth between lists of instructions, and it does that quickly enough so that both programs appear to be executing at the same time. Quickly enough, in fact, that there are no gaps in the music.

If you happen to have more than one CPU on your computer, the lists of instructions can execute at exactly the same time: one list can execute on each CPU. But multiple CPUs aren't necessary to give the appearance of simultaneous execution or to exploit the power of threading. A single CPU can appear to execute both lists of instructions in parallel, letting you read your email and listen to music simultaneously.

Threads behave exactly the same way. In our case, the Java virtual machine executes a handful of the instructions to calculate the factorial and then executes a handful of instructions to calculate the square root, and so on.

So threads are simply tasks that you want to execute at roughly the same time. Why, then, write an application with multiple threads? Why not just write multiple applications? The answer lies in the fact that because threads are running in the same application, they share the same memory space in the computer. This allows them to share information seamlessly. Your email program and your music application don't communicate very well. At best, you can copy and paste some data (like the name of a file) between the two. That allows you to double-click on an MP3 attachment in your email and play it in your music application, but the only information that is shared between the two is the name of the MP3 file.

In a multitasking environment, data in the programs is separated by default: each has its own stack for local variables, and each has its own area for objects and other data. All the programs can access various types of shared memory (including the name of the MP3 file that you clicked on in your email program). The shared memory is restricted to information put there by other programs, and the APIs to access it are usually quite different than the APIs used to access other data in the program.

This type of data sharing is fine for dissimilar programs, but it is inadequate for other programs that need to communicate with one another. Consider a network server that sends stock quotes to multiple clients. Sending a quote to a client is a discrete task and may be done in a separate thread. In fact, if the client must acknowledge the quote, then sending the data in separate threads is highly recommended: you don't want all clients to wait for a particularly slow client to respond. Here the data to be sent to the clients is the same; you don't want each client to require a separate server process which must then replicate all the data held by every other server process. Instead, you want multiple threads in one program so that they may share data and each perform discrete tasks on that data.

Conceptually, the threads seem to be the same as programs. The key difference here is that the global memory is the entire Java heap: threads can transparently share access between any object in the heap. Each thread still has its own space for local variables (variables specific to the method the thread is executing). But objects are shared automatically and transparently.

A thread, then, is a discrete task that operates on data shared with other threads.

understanding threads in java-Intoroduction

Threads are an important and interesting aspect of the Java Programming Language. If you are an experienced Java Programmer, you are invariably expected to know the intricacies of multi-threading. Well, this series of chapters is aimed at helping you understand this interesting and complicated concept.

So, lets get started!!!

Why Use Threads?

The notion of threading is so ingrained in Java that it's almost impossible to write even the simplest programs in Java without creating and using threads. And many of the classes in the Java API are already threaded, so often you are using multiple threads without realizing it.

The biggest question you might have is “Why use Threads?”

Well, my friend, it is not so simple to explain the reason in a couple of lines. Maybe a 100 page thesis would sound more appropriate. But still, am gonna try explaining in a short and concise manner.

First and foremost – threads increase performance. Given the processing power of the modern day processors, irrespective of how much code you write, the processor is going to finish running them in a jiffy and will remain underutilized almost 99% or more times. By creating multiple threads in your program, you are trying to utilize the processor effectively trying to keep it as little time as possible “idle”

Some of the significant reasons for using Threads are explained in the subsequent sections of this article.

Nonblocking I/O

In Java, as in most programming languages, when you try to get input from the user, you execute a read() method specifying the user's terminal (System.in in Java). When the program executes the read() method, the program typically waits until the user types at least one character before it continues and executes the next statement. This type of I/O is called blocking I/O : the program blocks until some data is available to satisfy the read() method.

This type of behavior is often undesirable. If you're reading data from a network socket, that data is often not available when you want to read it: the data may have been delayed in transit over the network, or you may be reading from a network server that sends data only periodically. If the program blocks when it tries to read from the socket, it's unable to do anything else until the data is actually available. If the program has a user interface that contains a button and the user presses the button while the program is executing the read() method, nothing happens: the program is unable to handle the mouse events and execute the event processing method associated with the button. This can be very frustrating for the user, who thinks the program has hung.

This kind of situation is where Threads in Java look the most promising solution.

Independent Tasks

A Java program is often called on to perform independent tasks. In the simplest case, a single applet may perform two independent animations for a web page. A more complex program would be a calculation server that performs calculations on behalf of several clients simultaneously. In either case, while it is possible to write a single-threaded program to perform multiple tasks, it's easier and more elegant to place each task in its own thread.

Parallelizable Algorithms

With the advent of virtual machines that can use multiple CPUs simultaneously, Java has become a useful platform for developing programs that use algorithms that can be parallelized; that is, running one iteration of the loop on one CPU while another iteration of the loop is simultaneously running on another CPU. Dependencies between the data that each iteration of the loop needs may prohibit a particular loop from being parallelized, and there may be other reasons why a loop should not be parallelized. But for many programs with CPU-intensive loops, parallelizing the loop greatly speeds up the execution of the program when it is run on a machine with multiple processors.

Now that we know why and how threads are useful for us as programmers, lets dig deep into the magical world of Threads!!!

adding user to mysql on the command line

login to admin as root
go to start-->cmd prompt, type following command
c:>mysql - u root - p           (it will ask for the password)
Enter the password:****     (give the passworg which is generally root)

now you will inter into mysql command line interface

mysql> create user 'newuser'@'localhost';     (it will create you a new user which is myuser)

mysql> set password for 'newuser'@'localhost' =PASSWORD('myuser');
                                                                    (it will set the password for myser as myuser)

you can again login to the mysql with the create user as
 c:>mysql  -u newuser -p
Enter password:newuser 

Hibernate Tutorial For Beginners

Open eclipse framework,
File-->New-->Other-->Java Project-->Give name of the project-->Finish

Go to http://www.hibernate.org/downloads download the release bundles save it.  

Right click the created project-->propertires-->java Bulid path-->Add Library-->user library-->Next-->user libraries-->New-->User Library Name-->Give name HibernateLib-->Ok

Select Created library that is HibernateLib-->Add jars.
1) hibernate3.jar
2) All jars in requireed folder.
3) hibernate-jpa-2.0 from lib/jpa folder
4) javassist-3.12.0.GA from lib/bytecode/javaassist folder
5) one extra jar which you can add externally ,drivers i.e mysql connector.
total 9 jars are ther have a look.

Right click the created project-->propertires-->java Bulid path-->Add Library-->user library-->Next-->choose the create HibernateLib-->select it-->finish.

You are all set to go.
Your Hibernate enviornment is all set

Next tutorial we will have our first hibernate program.
(next tutorial if you want,so give your feedback...thank you)