Thread Synchronization & Inter Thread Communication

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 block statement:

synchronized(object) { // statements to be synchronized }

We also have synchronized methods which are used as below:

synchronized void m1() {
   // method body to be synchronized
}

  
A synchronized block/method 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.

Thread synchronization also includes a topic related to

INTER THREAD 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.

Thread Priorities, Groups and Methods of Thread Class

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 dependentant.

Thread Group:

A ThreadGroup is used to represent a group of Threads. A tree structure can be formed from ThreadsGroups containing other ThreadGroups & Threads. Threads can only access the ThreadGroup to which they belong. This means that access to a ThreadGroups parent or other ThreadGroup is not permitted. Once a ThreadGroup is created various information can be obtained such as the ThreadGroups name, how many threads are active, the maximum priority that can be contained within the group and a host of other information. 

Thread Methods:

Following is the list of important methods available in the Thread class.

SN	Methods with Description
1	public void start() Starts the thread in a separate path of execution, then invokes the run() method on this Thread object.
2	public void run() If this Thread object was instantiated using a separate Runnable target, the run() method is invoked on that Runnable object.
3	public final void setName(String name) Changes the name of the Thread object. There is also a getName() method for retrieving the name.
4	public final void setPriority(int priority) Sets the priority of this Thread object. The possible values are between 1 and 10.
5	public final void setDaemon(boolean on) A parameter of true denotes this Thread as a daemon thread.
6	public final void join(long millisec) The current thread invokes this method on a second thread, causing the current thread to block until the second thread terminates or the specified number of milliseconds passes.
7	public void interrupt() Interrupts this thread, causing it to continue execution if it was blocked for any reason.
8	public final boolean isAlive() Returns true if the thread is alive, which is any time after the thread has been started but before it runs to completion.

The previous methods are invoked on a particular Thread object. The following methods in the Thread class are static. Invoking one of the static methods performs the operation on the currently running thread

SN	Methods with Description
1	public static void yield() Causes the currently running thread to yield to any other threads of the same priority that are waiting to be scheduled
2	public static void sleep(long millisec) Causes the currently running thread to block for at least the specified number of milliseconds
3	public static boolean holdsLock(Object x) Returns true if the current thread holds the lock on the given Object.
4	public static Thread currentThread() Returns a reference to the currently running thread, which is the thread that invokes this method.
5	public static void dumpStack() Prints the stack trace for the currently running thread, which is useful when debugging a multithreaded application

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.

Multi Threading in Java

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.

Difference Between Process and Thread:

1.Threads share the address space of the process that created it;

 processes have their own address.

2.Threads have direct access to the data segment of its process; 

processes have their own copy of the data segment of the parent process. 

3.Threads can directly communicate with other threads of its process; 

processes must use interprocess communication to communicate with sibling processes(child process) 

4.Threads have almost no overhead; processes have considerable overhead.

(Overhead related to context switching)

5.New threads are easily created; new processes require lot of new allocation

 by operating System

6.Threads can exercise considerable control over threads of the same process; 

processes can only exercise control over child processes. 

7.Changes to the main thread (cancellation, priority change, etc.) may affect the 

behavior of the other threads of the process; 

changes to the parent process does not affect child processes.