Implementing Singleton Pattern
To implement this design pattern we need to consider the following 4 steps:
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Step 1: Provide a default Private constructor
| public class SingletonObjectDemo { // Note that the constructor is private private SingletonObjectDemo() { // Optional Code } } |
Step 2: Create a Method for getting the reference to the Singleton Object
| public class SingletonObjectDemo { private static SingletonObject singletonObject; // Note that the constructor is private private SingletonObjectDemo() { // Optional Code } public static SingletonObjectDemo getSingletonObject() { if (singletonObject == null) { singletonObject = new SingletonObjectDemo(); } return singletonObject; } } |
We write a public static getter or access method to get the instance of the Singleton Object at runtime. First time the object is created inside this method as it is null. Subsequent calls to this method returns the same object created as the object is globally declared (private) and the hence the same referenced object is returned.
Step 3: Make the Access method Synchronized to prevent Thread Problems.
public static synchronized SingletonObjectDemo getSingletonObject()
It could happen that the access method may be called twice from 2 different classes at the same time and hence more than one object being created. This could violate the design patter principle. In order to prevent the simultaneous invocation of the getter method by 2 threads or classes simultaneously we add the synchronized keyword to the method declaration
Step 4: Override the Object clone method to prevent cloning
We can still be able to create a copy of the Object by cloning it using the Object’s clone method. This can be done as shown below
We can still be able to create a copy of the Object by cloning it using the Object’s clone method. This can be done as shown below
SingletonObjectDemo clonedObject = (SingletonObjectDemo) obj.clone();
This again violates the Singleton Design Pattern’s objective. So to deal with this we need to override the Object’s clone method which throws a CloneNotSupportedException exception.
public Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException();
}
throw new CloneNotSupportedException();
}
The below program shows the final Implementation of Singleton Design Pattern in java, by using all the 4 steps mentioned above.
| class SingletonClass { private static SingletonClass singletonObject; /** A private Constructor prevents any other class from instantiating. */ private SingletonClass() { // Optional Code } public static synchronized SingletonClass getSingletonObject() { if (singletonObject == null) { singletonObject = new SingletonClass(); } return singletonObject; } public Object clone() throws CloneNotSupportedException { throw new CloneNotSupportedException(); } } public class SingletonObjectDemo { public static void main(String args[]) { // SingletonClass obj = new SingletonClass(); //Compilation error not allowed SingletonClass obj = SingletonClass.getSingletonObject(); // Your Business Logic System.out.println("Singleton object obtained"); } } |
Download SingletonObjectDemo.java
Another approach
We don’t need to do a lazy initialization of the instance object or to check for null in the get method. We can also make the singleton class final to avoid sub classing that may cause other problems.
| public class SingletonClass { private static SingletonClass ourInstance = new SingletonClass(); public static SingletonClass getInstance() { return singletonObj; } private SingletonClass() { } } |
In Summary, the job of the Singleton class is to enforce the existence of a maximum of one object of the same type at any given time. Depending on your implementation, your class and all of its data might be garbage collected. Hence we must ensure that at any point there must be a live reference to the class when the application is running.
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Java Threads:
Introduction to Threads
Multithreading refers to two or more tasks executing concurrently within a single program. A thread is an independent path of execution within a program. Many threads can run concurrently within a program. Every thread in Java is created and controlled by the java.lang.Thread class. A Java program can have many threads, and these threads can run concurrently, either asynchronously or synchronously.Multithreading has several advantages over Multiprocessing such as;
- Threads are lightweight compared to processes
- Threads share the same address space and therefore can share both data and code
- Context switching between threads is usually less expensive than between processes
- Cost of thread intercommunication is relatively low that that of process intercommunication
- Threads allow different tasks to be performed concurrently.
Thread Creation
There are two ways to create thread in java;
- Implement the Runnable interface (java.lang.Runnable)
- By Extending the Thread class (java.lang.Thread)
Implementing the Runnable Interface
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The Runnable Interface Signature
public interface Runnable {void run();
}
One way to create a thread in java is to implement the Runnable Interface and then instantiate an object of the class. We need to override the run() method into our class which is the only method that needs to be implemented. The run() method contains the logic of the thread.
The procedure for creating threads based on the Runnable interface is as follows:
1. A class implements the Runnable interface, providing the run() method that will be executed by the thread. An object of this class is a Runnable object.2. An object of Thread class is created by passing a Runnable object as argument to the Thread constructor. The Thread object now has a Runnable object that implements the run() method.
3. The start() method is invoked on the Thread object created in the previous step. The start() method returns immediately after a thread has been spawned.
4. The thread ends when the run() method ends, either by normal completion or by throwing an uncaught exception.
Below is a program that illustrates instantiation and running of threads using the runnable interface instead of extending the Thread class. To start the thread you need to invoke the start() method on your object.
class RunnableThread implements Runnable { Thread runner; public RunnableThread() { } public RunnableThread(String threadName) { runner = new Thread(this, threadName); // (1) Create a new thread. System.out.println(runner.getName()); runner.start(); // (2) Start the thread. } public void run() { //Display info about this particular thread System.out.println(Thread.currentThread()); }} public class RunnableExample { public static void main(String[] args) { Thread thread1 = new Thread(new RunnableThread(), "thread1"); Thread thread2 = new Thread(new RunnableThread(), "thread2"); RunnableThread thread3 = new RunnableThread("thread3"); //Start the threads thread1.start(); thread2.start(); try { //delay for one second Thread.currentThread().sleep(1000); } catch (InterruptedException e) { } //Display info about the main thread System.out.println(Thread.currentThread()); }} |
Output
thread3Thread[thread1,5,main]
Thread[thread2,5,main]
Thread[thread3,5,main]
Thread[main,5,main]private
Download Runnable Thread Program Example
This approach of creating a thread by implementing the Runnable Interface must be used whenever the class being used to instantiate the thread object is required to extend some other class.
Extending Thread Class
The procedure for creating threads based on extending the Thread is as follows:
1. A class extending the Thread class overrides the run() method from the Thread class to define the code executed by the thread.2. This subclass may call a Thread constructor explicitly in its constructors to initialize the thread, using the super() call.
3. The start() method inherited from the Thread class is invoked on the object of the class to make the thread eligible for running.
Below is a program that illustrates instantiation and running of threads by extending the Thread class instead of implementing the Runnable interface. To start the thread you need to invoke the start() method on your object.
class XThread extends Thread { XThread() { } XThread(String threadName) { super(threadName); // Initialize thread. System.out.println(this); start(); } public void run() { //Display info about this particular thread System.out.println(Thread.currentThread().getName()); }} public class ThreadExample { public static void main(String[] args) { Thread thread1 = new Thread(new XThread(), "thread1"); Thread thread2 = new Thread(new XThread(), "thread2");// The below 2 threads are assigned default names Thread thread3 = new XThread(); Thread thread4 = new XThread(); Thread thread5 = new XThread("thread5"); //Start the threads thread1.start(); thread2.start(); thread3.start(); thread4.start(); try { //The sleep() method is invoked on the main thread to cause a one second delay. Thread.currentThread().sleep(1000); } catch (InterruptedException e) { } //Display info about the main thread System.out.println(Thread.currentThread()); }} |
Output
Thread[thread5,5,main]thread1
thread5
thread2
Thread-3
Thread-2
Thread[main,5,main]
Download Java Thread Program Example
When creating threads, there are two reasons why implementing the Runnable interface may be preferable to extending the Thread class:
- Extending the Thread class means that the subclass cannot extend any other class, whereas a class implementing the Runnable interface
has this option. - A class might only be interested in being runnable, and therefore, inheriting the full overhead of the Thread class would be excessive.
An example of an anonymous class below shows how to create a thread and start it:
( new Thread() {public void run() {
for(;;) System.out.println(”Stop the world!”);
}
}
).start();
–~~~~~~~~~~~~–
Thread Synchronization
With respect to multithreading, Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access a particular resource at a time.In non synchronized multithreaded application, it is possible for one thread to modify a shared object while
another thread is in the process of using or updating the object’s value. Synchronization prevents such type
of data corruption which may otherwise lead to dirty reads and significant errors.
Generally critical sections of the code are usually marked with synchronized keyword.
Examples of using Thread Synchronization is in “The Producer/Consumer Model”.
Locks are used to synchronize access to a shared resource. A lock can be associated with a shared resource.
Threads gain access to a shared resource by first acquiring the lock associated with the object/block of code.
At any given time, at most only one thread can hold the lock and thereby have access to the shared resource.
A lock thus implements mutual exclusion.
The object lock mechanism enforces the following rules of synchronization:
· A thread must acquire the object lock associated with a shared resource, before it can enter the shared
resource. The runtime system ensures that no other thread can enter a shared resource if another thread
already holds the object lock associated with the shared resource. If a thread cannot immediately acquire
the object lock, it is blocked, that is, it must wait for the lock to become available.
resource. The runtime system ensures that no other thread can enter a shared resource if another thread
already holds the object lock associated with the shared resource. If a thread cannot immediately acquire
the object lock, it is blocked, that is, it must wait for the lock to become available.
· When a thread exits a shared resource, the runtime system ensures that the object lock is also relinquished.
If another thread is waiting for this object lock, it can proceed to acquire the lock in order to gain access
to the shared resource.
Classes also have a class-specific lock that is analogous to the object lock. Such a lock is actually aIf another thread is waiting for this object lock, it can proceed to acquire the lock in order to gain access
to the shared resource.
lock on the java.lang.Class object associated with the class. Given a class A, the reference A.class
denotes this unique Class object. The class lock can be used in much the same way as an object lock to
implement mutual exclusion.
There can be 2 ways through which synchronized can be implemented in Java:
- synchronized methods
- synchronized blocks
executed after a thread has acquired the lock on the object/class referenced in the synchronized statement.
Synchronized Methods
Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method’s object or class. .If the lock is already held by another thread, the calling thread waits. A thread relinquishes the lock simply by returning from the synchronized method, allowing the next thread waiting for this lock to proceed. Synchronized methods are useful in situations where methods can manipulate the state of an object in ways that can corrupt the state if executed concurrently. This is called a race condition. It occurs when two or more threads simultaneously update the same value, and as a consequence, leave the value in an undefined or inconsistent state. While a thread is inside a synchronized method of an object, all other threads that wish to execute this synchronized method or any other synchronized method of the object will have to wait until it gets the lock. This restriction does not apply to the thread that already has the lock and is executing a synchronized method of the object. Such a method can invoke other synchronized methods of the object without being blocked. The non-synchronized methods of the object can of course be called at any time by any thread.Below is an example shows how synchronized methods and object locks are used to coordinate access to a common object by multiple threads. If the ’synchronized’ keyword is removed, the message is displayed in random fashion.
public class SyncMethodsExample extends Thread { static String[] msg = { "Beginner", "java", "tutorial,", ".,", "com", "is", "the", "best" }; public SyncMethodsExample(String id) { super(id); } public static void main(String[] args) { SyncMethodsExample thread1 = new SyncMethodsExample("thread1: "); SyncMethodsExample thread2 = new SyncMethodsExample("thread2: "); thread1.start(); thread2.start(); boolean t1IsAlive = true; boolean t2IsAlive = true; do { if (t1IsAlive && !thread1.isAlive()) { t1IsAlive = false; System.out.println("t1 is dead."); } if (t2IsAlive && !thread2.isAlive()) { t2IsAlive = false; System.out.println("t2 is dead."); } } while (t1IsAlive || t2IsAlive); } void randomWait() { try { Thread.currentThread().sleep((long) (3000 * Math.random())); } catch (InterruptedException e) { System.out.println("Interrupted!"); } } public synchronized void run() { SynchronizedOutput.displayList(getName(), msg); }} class SynchronizedOutput { // if the 'synchronized' keyword is removed, the message // is displayed in random fashion public static synchronized void displayList(String name, String list[]) { for (int i = 0; i < list.length; i++) { SyncMethodsExample t = (SyncMethodsExample) Thread .currentThread(); t.randomWait(); System.out.println(name + list[i]); } }} |
Output
thread1: Beginnerthread1: java
thread1: tutorial,
thread1: .,
thread1: com
thread1: is
thread1: the
thread1: best
t1 is dead.
thread2: Beginner
thread2: java
thread2: tutorial,
thread2: .,
thread2: com
thread2: is
thread2: the
thread2: best
t2 is dead.
Download Synchronized Methods Thread Program Example
Class Locks
Synchronized Blocks
Static methods synchronize on the class lock. Acquiring and relinquishing a class lock by a thread in order to execute a static synchronized method, proceeds analogous to that of an object lock for a synchronized instance method. A thread acquires the class lock before it can proceed with the execution of any static synchronized method in the class, blocking other threads wishing to execute any such methods in the same class. This, of course, does not apply to static, non-synchronized methods, which can be invoked at any time. Synchronization of static methods in a class is independent from the synchronization of instance methods on objects of the class. A subclass decides whether the new definition of an inherited synchronized method will remain synchronized in the subclass.The synchronized block allows execution of arbitrary code to be synchronized on the lock of an arbitrary object.The general form of the synchronized block is as follows:
synchronized (<object reference expression>) {
<code block>
}
A compile-time error occurs if the expression produces a value of any primitive type. If execution of the block completes normally, then the lock is released. If execution of the block completes abruptly, then the lock is released.
A thread can hold more than one lock at a time. Synchronized statements can be nested. Synchronized statements with identical expressions can be nested. The expression must evaluate to a non-null reference value, otherwise, a NullPointerException is thrown.
The code block is usually related to the object on which the synchronization is being done. This is the case with synchronized methods, where the execution of the method is synchronized on the lock of the current object:
public Object method() {
synchronized (this) { // Synchronized block on current object
// method block
}
}
Once a thread has entered the code block after acquiring the lock on the specified object, no other thread will be able to execute the code block, or any other code requiring the same object lock, until the lock is relinquished. This happens when the execution of the code block completes normally or an uncaught exception is thrown.
Object specification in the synchronized statement is mandatory. A class can choose to synchronize the execution of a part of a method, by using the this reference and putting the relevant part of the method in the synchronized block. The braces of the block cannot be left out, even if the code block has just one statement.
class SmartClient {
BankAccount account;
// …
public void updateTransaction() {
synchronized (account) { // (1) synchronized block
account.update(); // (2)
}
}
}
In the previous example, the code at (2) in the synchronized block at (1) is synchronized on the BankAccount object. If several threads were to concurrently execute the method updateTransaction() on an object of SmartClient, the statement at (2) would be executed by one thread at a time, only after synchronizing on the BankAccount object associated with this particular instance of SmartClient.
Inner classes can access data in their enclosing context. An inner object might need to synchronize on its associated outer object, in order to ensure integrity of data in the latter. This is illustrated in the following code where the synchronized block at (5) uses the special form of the this reference to synchronize on the outer object associated with an object of the inner class. This setup ensures that a thread executing the method setPi() in an inner object can only access the private double field myPi at (2) in the synchronized block at (5), by first acquiring the lock on the associated outer object. If another thread has the lock of the associated outer object, the thread in the inner object has to wait for the lock to be relinquished before it can proceed with the execution of the synchronized block at (5). However, synchronizing on an inner object and on its associated outer object are independent of each other, unless enforced explicitly, as in the following code:
class Outer { // (1) Top-level Class
private double myPi; // (2)
protected class Inner { // (3) Non-static member Class
public void setPi() { // (4)
synchronized(Outer.this) { // (5) Synchronized block on outer object
myPi = Math.PI; // (6)
}
}
}
}
Below example shows how synchronized block and object locks are used to coordinate access to shared objects by multiple threads.
public class SyncBlockExample extends Thread { static String[] msg = { "Beginner", "java", "tutorial,", ".,", "com", "is", "the", "best" }; public SyncBlockExample(String id) { super(id); } public static void main(String[] args) { SyncBlockExample thread1 = new SyncBlockExample("thread1: "); SyncBlockExample thread2 = new SyncBlockExample("thread2: "); thread1.start(); thread2.start(); boolean t1IsAlive = true; boolean t2IsAlive = true; do { if (t1IsAlive && !thread1.isAlive()) { t1IsAlive = false; System.out.println("t1 is dead."); } if (t2IsAlive && !thread2.isAlive()) { t2IsAlive = false; System.out.println("t2 is dead."); } } while (t1IsAlive || t2IsAlive); } void randomWait() { try { Thread.currentThread().sleep((long) (3000 * Math.random())); } catch (InterruptedException e) { System.out.println("Interrupted!"); } } public void run() { synchronized (System.out) { for (int i = 0; i < msg.length; i++) { randomWait(); System.out.println(getName() + msg[i]); } } }} |
Output
thread1: Beginnerthread1: java
thread1: tutorial,
thread1: .,
thread1: com
thread1: is
thread1: the
thread1: best
t1 is dead.
thread2: Beginner
thread2: java
thread2: tutorial,
thread2: .,
thread2: com
thread2: is
thread2: the
thread2: best
t2 is dead.
Synchronized blocks can also be specified on a class lock:
synchronized (<class name>.class) { <code block> }
The block synchronizes on the lock of the object denoted by the reference <class name>.class. A static synchronized method
classAction() in class A is equivalent to the following declaration:
static void classAction() {
synchronized (A.class) { // Synchronized block on class A
// …
}
}
In summary, a thread can hold a lock on an object
- by executing a synchronized instance method of the object
- by executing the body of a synchronized block that synchronizes on the object
- by executing a synchronized static method of a class
–~~~~~~~~~~~~–
Thread States
A Java thread is always in one of several states which could be running, sleeping, dead, etc.A thread can be in any of the following states:
- New Thread state (Ready-to-run state)
- Runnable state (Running state)
- Not Runnable state
- Dead state
New Thread
A thread is in this state when the instantiation of a Thread object creates a new thread but does notstart it running. A thread starts life in the Ready-to-run state. You can call only the start() or stop()
methods when the thread is in this state. Calling any method besides start() or stop() causes an
IllegalThreadStateException.
Runnable
When the start() method is invoked on a New Thread() it gets to the runnable state or running state bycalling the run() method. A Runnable thread may actually be running, or may be awaiting its turn to run.
Not Runnable
A thread becomes Not Runnable when one of the following four events occurs:
- When sleep() method is invoked and it sleeps for a specified amount of time
- When suspend() method is invoked
- When the wait() method is invoked and the thread waits for notification of a free resource or waits for
the completion of another thread or waits to acquire a lock of an object. - The thread is blocking on I/O and waits for its completion
Note: Thread.currentThread() may return an output like Thread[threadA,5,main]
The output shown in bold describes
- the name of the thread,
- the priority of the thread, and
- the name of the group to which it belongs.
A thread can be awakened abruptly by invoking the interrupt() method on the sleeping thread object or at the end of the period of time for sleep is over. Whether or not it will actually start running depends on its priority and the availability of the CPU.
Hence I hereby list the scenarios below to describe how a thread switches form a non runnable to a runnable state:
· If a thread has been put to sleep, then the specified number of milliseconds must elapse (or it must be interrupted).
· If a thread has been suspended, then its resume() method must be invoked
· If a thread is waiting on a condition variable, whatever object owns the variable must relinquish it by calling
either notify() or notifyAll().
either notify() or notifyAll().
· If a thread is blocked on I/O, then the I/O must complete.
Dead State
A thread enters this state when the run() method has finished executing or when the stop() method is invoked. Once in this state, the thread cannot ever run again.–~~~~~~~~~~~~–
Thread Priority
In Java we can specify the priority of each thread relative to other threads. Those threads having higherpriority get greater access to available resources then lower priority threads. A Java thread inherits its priority
from the thread that created it. Heavy reliance on thread priorities for the behavior of a program can make the
program non portable across platforms, as thread scheduling is host platform–dependent.
You can modify a thread’s priority at any time after its creation using the setPriority() method and retrieve
the thread priority value using getPriority() method.
The following static final integer constants are defined in the Thread class:
- MIN_PRIORITY (0) Lowest Priority
- NORM_PRIORITY (5) Default Priority
- MAX_PRIORITY (10) Highest Priority
When two or more threads are ready to be executed and system resource becomes available to execute a thread, the runtime system (the thread scheduler) chooses the Runnable thread with the highest priority for execution.
“If two threads of the same priority are waiting for the CPU, the thread scheduler chooses one of them to run in a > round-robin fashion. The chosen thread will run until one of the following conditions is true:
- a higher priority thread becomes Runnable. (Pre-emptive scheduling)
- it yields, or its run() method exits
- on systems that support time-slicing, its time allotment has expired
Thread Scheduler
Schedulers in JVM implementations usually employ one of the two following strategies:Preemptive scheduling
If a thread with a higher priority than all other Runnable threads becomes Runnable, the scheduler willpreempt the running thread (is moved to the runnable state) and choose the new higher priority thread for execution.
Time-Slicing or Round-Robin scheduling
A running thread is allowed to execute for a fixed length of time (a time slot it’s assigned to), after which it moves to the Ready-to-run state (runnable) to await its turn to run again.A thread scheduler is implementation and platform-dependent; therefore, how threads will be scheduled is unpredictable across different platforms.
Yielding
A call to the static method yield(), defined in the Thread class, will cause the current thread in the Running state to move to the Runnable state, thus relinquishing the CPU. The thread is then at the mercy of the thread scheduler as to when it will run again. If there are no threads waiting in the Ready-to-run state, this thread continues execution. If there are other threads in the Ready-to-run state, their priorities determine which thread gets to execute. The yield() method gives other threads of the same priority a chance to run. If there are no equal priority threads in the “Runnable” state, then the yield is ignored.Sleeping and Waking Up
The thread class contains a static method named sleep() that causes the currently running thread to pause its execution and transit to the Sleeping state. The method does not relinquish any lock that the thread might have. The thread will sleep for at least the time specified in its argument, before entering the runnable state where it takes its turn to run again. If a thread is interrupted while sleeping, it will throw an InterruptedException when it awakes and gets to execute. The Thread class has several overloaded versions of the sleep() method.Waiting and Notifying
Waiting and notifying provide means of thread inter-communication that synchronizes on the same object. The threads execute wait() and notify() (or notifyAll()) methods on the shared object for this purpose. The notifyAll(), notify() and wait() are methods of the Object class. These methods can be invoked only from within a synchronized context (synchronized method or synchronized block), otherwise, the call will result in an IllegalMonitorStateException. The notifyAll() method wakes up all the threads waiting on the resource. In this situation, the awakened threads compete for the resource. One thread gets the resource and the others go back to waiting.wait() method signatures
final void wait(long timeout) throws InterruptedExceptionfinal void wait(long timeout, int nanos) throws InterruptedException
final void wait() throws InterruptedException
The wait() call can specify the time the thread should wait before being timed out. An another thread can invoke an interrupt() method on a waiting thread resulting in an InterruptedException. This is a checked exception and hence the code with the wait() method must be enclosed within a try catch block.
notify() method signatures
final void notify()
final void notifyAll()
A thread usually calls the wait() method on the object whose lock it holds because a condition for its continued execution was not met. The thread leaves the Running state and transits to the Waiting-for-notification state. There it waits for this condition to occur. The thread relinquishes ownership of the object lock. The releasing of the lock of the shared object by the thread allows other threads to run and execute synchronized code on the same object after acquiring its lock.
The wait() method causes the current thread to wait until another thread notifies it of a condition change.
A thread in the Waiting-for-notification state can be awakened by the occurrence of any one of these three incidents:
1. Another thread invokes the notify() method on the object of the waiting thread, and the waiting thread is selected as the thread to be awakened.
2. The waiting thread times out.
3. Another thread interrupts the waiting thread.
Notify
Invoking the notify() method on an object wakes up a single thread that is waiting on the lock of this object.A call to the notify() method has no consequences if there are no threads in the wait set of the object.
The notifyAll() method wakes up all threads in the wait set of the shared object.
Below program shows three threads, manipulating the same stack. Two of them are pushing elements on the stack, while the third one is popping elements off the stack. This example illustrates how a thread waiting as a result of calling the wait() method on an object, is notified by another thread calling the notify() method on the same object
class StackClass { private Object[] stackArray; private volatile int topOfStack; StackClass(int capacity) { stackArray = new Object[capacity]; topOfStack = -1; } public synchronized Object pop() { System.out.println(Thread.currentThread() + ": popping"); while (isEmpty()) { try { System.out.println(Thread.currentThread() + ": waiting to pop"); wait(); } catch (InterruptedException e) { e.printStackTrace(); } } Object obj = stackArray[topOfStack]; stackArray[topOfStack--] = null; System.out.println(Thread.currentThread() + ": notifying after pop"); notify(); return obj; } public synchronized void push(Object element) { System.out.println(Thread.currentThread() + ": pushing"); while (isFull()) { try { System.out.println(Thread.currentThread() + ": waiting to push"); wait(); } catch (InterruptedException e) { e.printStackTrace(); } } stackArray[++topOfStack] = element; System.out.println(Thread.currentThread() + ": notifying after push"); notify(); } public boolean isFull() { return topOfStack >= stackArray.length - 1; } public boolean isEmpty() { return topOfStack < 0; }} abstract class StackUser extends Thread { protected StackClass stack; StackUser(String threadName, StackClass stack) { super(threadName); this.stack = stack; System.out.println(this); setDaemon(true); start(); }} class StackPopper extends StackUser { // Stack Popper StackPopper(String threadName, StackClass stack) { super(threadName, stack); } public void run() { while (true) { stack.pop(); } }} class StackPusher extends StackUser { // Stack Pusher StackPusher(String threadName, StackClass stack) { super(threadName, stack); } public void run() { while (true) { stack.push(new Integer(1)); } }} public class WaitAndNotifyExample { public static void main(String[] args) { StackClass stack = new StackClass(5); new StackPusher("One", stack); new StackPusher("Two", stack); new StackPopper("Three", stack); System.out.println("Main Thread sleeping."); try { Thread.sleep(500); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("Exit from Main Thread."); }} |
The field topOfStack in class StackClass is declared volatile, so that read and write operations on this variable will access the master value of this variable, and not any copies, during runtime.
Since the threads manipulate the same stack object and the push() and pop() methods in the class StackClassare synchronized, it means that the threads synchronize on the same object.
How the program uses wait() and notify() for inter thread communication.
(1) The synchronized pop() method - When a thread executing this method on the StackClass object finds that the stack is empty, it invokes the wait() method in order to wait for some other thread to fill the stack by using the synchronized push. Once an other thread makes a push, it invokes the notify method.(2)The synchronized push() method - When a thread executing this method on the StackClass object finds that the stack is full, i t invokes the wait() method to await some other thread to remove an element to provide space for the newly to be pushed element.
Once an other thread makes a pop, it invokes the notify method.
–~~~~~~~~~~~~–
Joining
A thread invokes the join() method on another thread in order to wait for the other thread to complete its execution.Consider a thread t1 invokes the method join() on a thread t2. The join() call has no effect if thread t2 has already completed. If thread t2 is still alive, then thread t1 transits to the Blocked-for-join-completion state.
Below is a program showing how threads invoke the overloaded thread join method.
public class ThreadJoinDemo { public static void main(String[] args) { Thread t1 = new Thread("T1"); Thread t2 = new Thread("T2"); try { System.out.println("Wait for the child threads to finish."); t1.join(); if (!t1.isAlive()) System.out.println("Thread T1 is not alive."); t2.join(); if (!t2.isAlive()) System.out.println("Thread T2 is not alive."); } catch (InterruptedException e) { System.out.println("Main Thread interrupted."); } System.out.println("Exit from Main Thread."); }} |
Output
Wait for the child threads to finish.Thread T1 is not alive.
Thread T2 is not alive.
Exit from Main Thread.
Deadlock
There are situations when programs become deadlocked when each thread is waiting on a resource that cannot become available. The simplest form of deadlock is when two threads are each waiting on a resource that is locked by the other thread. Since each thread is waiting for the other thread to relinquish a lock, they both remain waiting forever in the Blocked-for-lock-acquisition state. The threads are said to be deadlocked.Thread t1 at tries to synchronize first on string o1 and then on string o2. The thread t2 does the opposite. It synchronizes first on string o2 then on string o1. Hence a deadlock can occur as explained above.
Below is a program that illustrates deadlocks in multithreading applications
public class DeadLockExample { String o1 = "Lock "; String o2 = "Step "; Thread t1 = (new Thread("Printer1") { public void run() { while (true) { synchronized (o1) { synchronized (o2) { System.out.println(o1 + o2); } } } } }); Thread t2 = (new Thread("Printer2") { public void run() { while (true) { synchronized (o2) { synchronized (o1) { System.out.println(o2 + o1); }} } } }); public static void main(String[] args) { DeadLockExample dLock = new DeadLockExample(); dLock.t1.start(); dLock.t2.start(); }} |
Note: The following methods namely join, sleep and wait name the InterruptedException in its throws clause and can have a timeout argument as a parameter. The following methods namely wait, notify and notifyAll should only be called by a thread that holds the lock of the instance on which the method is invoked. The Thread.start method causes a new thread to get ready to run at the discretion of the thread scheduler. The Runnable interface declares the run method. The Thread class implements the Runnable interface. Some implementations of the Thread.yield method will not yield to a thread of lower priority. A program will terminate only when all user threads stop running. A thread inherits its daemon status from the thread that created it
Java Collection Framework:
A collection represents a group of objects, known as its elements. This framework is provided in the java.util package. Objects can be stored, retrieved, and manipulated as elements of collections. Collection is a Java Interface. Collections can be used in various scenarios like Storing phone numbers, Employee names database etc. They are basically used to group multiple elements into a single unit. Some collections allow duplicate elements while others do not. Some collections are ordered and others are not.
A Collections Framework mainly contains the following 3 parts
A Collections Framework is defined by a set of interfaces, concrete class implementations for most of the interfaces and a set of standard utility methods and algorithms. In addition, the framework also provides several abstract implementations, which are designed to make it easier for you to create new and different implementations for handling collections of data.
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