- Java Basic
- Java Object Oriented
- Java Advanced
- Java Multithreading
Java provides built-in support for multithreaded programming. A multithreaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution.
A multithreading is a specialized form of multitasking. Multitasking threads require less overhead than multitasking processes.
I need to define another term related to threads: process: A process consists of the memory space allocated by the operating system that can contain one or more threads. A thread cannot exist on its own; it must be a part of a process. A process remains running until all of the non-daemon threads are done executing.
Multithreading enables you to write very efficient programs that make maximum use of the CPU, because idle time can be kept to a minimum.
Life Cycle of a Thread:
A thread goes through various stages in its life cycle. For example, a thread is born, started, runs, and then dies. Following diagram shows complete life cycle of a thread.
Above mentioned stages are explained here:
- New: A new thread begins its life cycle in the new state. It remains in this state until the program starts the thread. It is also referred to as a born thread.
- Runnable: After a newly born thread is started, the thread becomes runnable. A thread in this state is considered to be executing its task.
- Waiting: Sometimes a thread transitions to the waiting state while the thread waits for another thread to perform a task.A thread transitions back to the runnable state only when another thread signals the waiting thread to continue executing.
- Timed waiting: A runnable thread can enter the timed waiting state for a specified interval of time. A thread in this state transitions back to the runnable state when that time interval expires or when the event it is waiting for occurs.
- Terminated: A runnable thread enters the terminated state when it completes its task or otherwise terminates.
Thread Priorities:
Every Java thread has a priority that helps the operating system determine the order in which threads are scheduled.
Java priorities are in the range between MIN_PRIORITY (a constant of 1) and MAX_PRIORITY (a constant of 10). By default, every thread is given priority NORM_PRIORITY (a constant of 5).
Threads with higher priority are more important to a program and should be allocated processor time before lower-priority threads. However, thread priorities cannot guarantee the order in which threads execute and very much platform dependent.
Creating a Thread:
Java defines two ways in which this can be accomplished:
- You can implement the Runnable interface.
- You can extend the Thread class, itself.
Create Thread by Implementing Runnable:
The easiest way to create a thread is to create a class that implements the Runnable interface.
To implement Runnable, a class need only implement a single method called run( ), which is declared like this:
public void run( )
You will define the code that constitutes the new thread inside run() method. It is important to understand that run() can call other methods, use other classes, and declare variables, just like the main thread can.
After you create a class that implements Runnable, you will instantiate an object of type Thread from within that class. Thread defines several constructors. The one that we will use is shown here:
Thread(Runnable threadOb, String threadName);
Here threadOb is an instance of a class that implements the Runnable interface and the name of the new thread is specified by threadName.
After the new thread is created, it will not start running until you call its start( ) method, which is declared within Thread. The start( ) method is shown here:
void start( );
Example:
Here is an example that creates a new thread and starts it running:
Output:
Create Thread by Extending Thread:
The second way to create a thread is to create a new class that extends Thread, and then to create an instance of that class.
The extending class must override the run( ) method, which is the entry point for the new thread. It must also call start( ) to begin execution of the new thread.
Example:
Here is the preceding program rewritten to extend Thread:
Output:
Thread Synchronization:
When two or more threads need access to a shared resource, they need some way to ensure that the resource will be used by only one thread at a time.
The process by which this synchronization is achieved is called thread synchronization.
The synchronized keyword in Java creates a block of code referred to as a critical section. Every Java object with a critical section of code gets a lock associated with the object. To enter a critical section, a thread needs to obtain the corresponding object's lock.
This is the general form of the synchronized statement:
synchronized(object) { // statements to be synchronized }
Here, object is a reference to the object being synchronized. A synchronized block ensures that a call to a method that is a member of object occurs only after the current thread has successfully entered object's monitor.
Interthread Communication :
Consider the classic queuing problem, where one thread is producing some data and another is consuming it. To make the problem more interesting, suppose that the producer has to wait until the consumer is finished before it generates more data.
In a polling system, the consumer would waste many CPU cycles while it waited for the producer to produce. Once the producer was finished, it would start polling, wasting more CPU cycles waiting for the consumer to finish, and so on. Clearly, this situation is undesirable.
To avoid polling, Java includes an elegant interprocess communication mechanism via the following methods:
- wait( ): This method tells the calling thread to give up the monitor and go to sleep until some other thread enters the same monitor and calls notify( ).
- notify( ): This method wakes up the first thread that called wait( ) on the same object.
- notifyAll( ): This method wakes up all the threads that called wait( ) on the same object.c The highest priority thread will run first.
These methods are implemented as final methods in Object, so all classes have them. All three methods can be called only from within a synchronized context.
These methods are declared within Object. Various forms of wait( ) exist that allow you to specify a period of time to wait.
Example:
The following sample program consists of four classes: Q, the queue that you're trying to synchronize; Producer, the threaded object that is producing queue entries; Consumer, the threaded object that is consuming queue entries; and PC, the tiny class that creates the single Q, Producer, and Consumer.
The proper way to write this program in Java is to use wait( ) and notify( ) to signal in both directions, as shown here:
Output:
Thread Control: Suspend, Stop and Resume
While the suspend( ), resume( ), and stop( ) methods defined by Thread class seem to be a perfectly reasonable and convenient approach to managing the execution of threads, they must not be used for new Java programs and obsolete in newer versions of Java.
The following example illustrates how the wait( ) and notify( ) methods that are inherited from Object can be used to control the execution of a thread.
This example is similar to the program in the previous section. However, the deprecated method calls have been removed. Let us consider the operation of this program.
The NewThread class contains a boolean instance variable named suspendFlag, which is used to control the execution of the thread. It is initialized to false by the constructor.
The run( ) method contains a synchronized statement block that checks suspendFlag. If that variable is true, the wait( ) method is invoked to suspend the execution of the thread. The mysuspend( ) method sets suspendFlag to true. The myresume( ) method sets suspendFlag to false and invokes notify( ) to wake up the thread. Finally, the main( ) method has been modified to invoke the mysuspend( ) and myresume( ) methods.
Example:
Output:
Using Multithreading:
The key to utilizing multithreading support effectively is to think concurrently rather than serially. For example, when you have two subsystems within a program that can execute concurrently, make them individual threads.
With the careful use of multithreading, you can create very efficient programs. A word of caution is in order, however: If you create too many threads, you can actually degrade the performance of your program rather than enhance it.
Remember, some overhead is associated with context switching. If you create too many threads, more CPU time will be spent changing contexts than executing your program!