Threads And Multithreadingr - Notes By ShariqSP
Threads in Java
In Java, a thread is a lightweight process that runs concurrently with other threads within a single process. Threads allow multiple tasks to be performed simultaneously, providing better utilization of CPU resources and improving the responsiveness of applications.
Types of Threads:
Java supports two types of threads:
- User Threads: These are threads created by the application developer to perform specific tasks.
- Daemon Threads: These are background threads that run in the background and provide services to user threads. They are automatically terminated when all user threads have exited.
Creating Threads in Java:
Threads in Java can be created using two approaches:
- Extending the
Threadclass: By extending theThreadclass and overriding itsrun()method. - Implementing the
Runnableinterface: By implementing theRunnableinterface and providing the implementation for itsrun()method.
Thread States:
A thread in Java can be in one of the following states:
- New: The thread has been created but has not yet started.
- Runnable: The thread is ready to run and waiting for CPU time.
- Blocked: The thread is waiting for a monitor lock to enter a synchronized block or method.
- Waiting: The thread is waiting indefinitely for another thread to perform a particular action.
- Timed Waiting: The thread is waiting for a specified amount of time.
- Terminated: The thread has completed its execution or has been terminated prematurely.
Thread Synchronization:
Thread synchronization is the process of controlling the access to shared resources by multiple threads to prevent data inconsistency and thread interference. Java provides synchronization mechanisms such as synchronized blocks and methods, as well as explicit locks.
Example:
public class Main {
public static void main(String[] args) {
// Creating and starting a new thread
MyThread thread = new MyThread();
thread.start();
}
}
class MyThread extends Thread {
public void run() {
System.out.println("Thread running...");
}
}
In this example, a new thread is created by extending the Thread class and overriding its run() method. The thread is then started using the start() method.
Thread Class and Extending from Thread Class in Java
In Java, threads are lightweight processes that execute independently and concurrently within a single program. The Thread class is a fundamental class in Java's multithreading API that represents a thread of execution. You can create and manage threads in Java by either extending the Thread class or implementing the Runnable interface.
Extending the Thread Class:
To create a new thread by extending the Thread class, you need to:
- Create a new class that extends the
Threadclass. - Override the
run()method to define the code that will be executed by the thread. - Create an instance of the subclass and call its
start()method to start the execution of the thread.
Thread Class Methods:
The Thread class provides several methods to manage threads:
- start(): Starts the execution of the thread by calling its
run()method. - run(): Contains the code that will be executed by the thread. This method needs to be overridden in the subclass to define the thread's behavior.
- sleep(long millis): Causes the current thread to sleep for the specified number of milliseconds.
- join(): Waits for the thread to die.
- setName(String name): Sets the name of the thread.
- getName(): Returns the name of the thread.
- isAlive(): Checks if the thread is alive.
- interrupt(): Interrupts the thread.
- isInterrupted(): Checks if the thread has been interrupted.
- currentThread(): Returns a reference to the currently executing thread.
Example 1: Extending Thread Class
class MyThread extends Thread {
public void run() {
for (int i = 0; i < 5; i++) {
System.out.println("Thread 1: " + i);
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
System.out.println("Thread interrupted.");
}
}
}
}
public class Main {
public static void main(String[] args) {
MyThread thread1 = new MyThread();
thread1.start();
}
}
In this example, a new thread is created by extending the Thread class and overriding its run() method. The thread prints numbers from 0 to 4 with a delay of 1 second between each number.
Example 2: Extending Thread Class
class MyThread extends Thread {
private String message;
public MyThread(String message) {
this.message = message;
}
public void run() {
System.out.println("Message from thread: " + message);
}
}
public class Main {
public static void main(String[] args) {
MyThread thread1 = new MyThread("Hello, world!");
MyThread thread2 = new MyThread("Goodbye, world!");
thread1.start();
thread2.start();
}
}
In this example, two threads are created by extending the Thread class and providing different messages to be printed. Each thread runs concurrently and prints its respective message.
Implementing Runnable Interface in Java
In Java, threads can also be created by implementing the Runnable interface. Implementing Runnable allows for better code organization and flexibility, as it separates the thread's behavior from the thread's execution.
Implementing Runnable Interface:
To create a new thread by implementing the Runnable interface, you need to:
- Create a new class that implements the
Runnableinterface. - Implement the
run()method to define the code that will be executed by the thread. - Create an instance of the class and pass it as a parameter to the
Threadconstructor. - Call the
start()method on theThreadobject to start the execution of the thread.
Runnable Interface Methods:
The Runnable interface has only one method, run(), which must be implemented to define the code that will be executed by the thread.
Example 1: Implementing Runnable Interface
class MyRunnable implements Runnable {
public void run() {
for (int i = 0; i < 5; i++) {
System.out.println("Thread 1: " + i);
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
System.out.println("Thread interrupted.");
}
}
}
}
public class Main {
public static void main(String[] args) {
MyRunnable myRunnable = new MyRunnable();
Thread thread1 = new Thread(myRunnable);
thread1.start();
}
}
In this example, a new thread is created by implementing the Runnable interface and providing the implementation for its run() method. The thread prints numbers from 0 to 4 with a delay of 1 second between each number.
Example 2: Implementing Runnable Interface
class MyRunnable implements Runnable {
private String message;
public MyRunnable(String message) {
this.message = message;
}
public void run() {
System.out.println("Message from thread: " + message);
}
}
public class Main {
public static void main(String[] args) {
MyRunnable myRunnable1 = new MyRunnable("Hello, world!");
MyRunnable myRunnable2 = new MyRunnable("Goodbye, world!");
Thread thread1 = new Thread(myRunnable1);
Thread thread2 = new Thread(myRunnable2);
thread1.start();
thread2.start();
}
}
In this example, two threads are created by implementing the Runnable interface and providing different messages to be printed. Each thread runs concurrently and prints its respective message.
Explanation of run() and start() Methods
In Java multithreading, the run() and start() methods are fundamental for creating and executing threads. These methods are part of the Thread class and are crucial for defining the behavior of a thread and starting its execution, respectively.
The run() Method:
The run() method is the entry point for the code that will be executed by a thread. This method contains the actual logic or tasks that the thread will perform. When you create a new thread by extending the Thread class or implementing the Runnable interface and overriding the run() method, you define what the thread will do when it is started.
It's important to note that the run() method should never be called directly. Instead, it is invoked automatically by the Java Virtual Machine (JVM) when you call the start() method on a thread instance.
The start() Method:
The start() method is used to begin the execution of a thread. When you call the start() method on a thread instance, it causes the JVM to spawn a new thread of execution and invokes the run() method on that thread. The start() method ensures that the thread runs concurrently with other threads in the program.
Attempting to call the run() method directly on a thread instance will not start a new thread but will instead execute the run() method on the current thread, making it behave like a normal method call and not achieving the desired concurrency.
Key Points:
- The
run()method contains the code to be executed by a thread and should be overridden when creating a custom thread. - The
start()method initiates the execution of a thread by spawning a new thread of execution and invoking itsrun()method. - Calling the
start()method is essential for achieving concurrency in multithreaded applications. - Calling the
run()method directly will not start a new thread but will execute the method on the current thread.
Thread Safety in Java
Thread safety in Java refers to the ability of a program to execute multiple threads concurrently without causing unexpected results or data corruption. In multithreaded environments, where multiple threads access shared resources concurrently, it is crucial to ensure that the shared resources are accessed and modified in a safe and consistent manner.
Ensuring Thread Safety:
To ensure thread safety, developers can employ various techniques:
- Synchronization: Synchronizing access to shared resources using the
synchronizedkeyword ensures that only one thread can access the resource at a time, preventing race conditions and data corruption. - Using Thread-Safe Data Structures: Utilizing thread-safe data structures provided by Java's
java.util.concurrentpackage, such asConcurrentHashMaporCopyOnWriteArrayList, ensures safe concurrent access without explicit synchronization. - Atomic Operations: Using atomic operations provided by classes like
AtomicIntegerorAtomicReferenceensures that read-modify-write operations are executed atomically, avoiding data inconsistency issues. - Immutability: Designing classes to be immutable, i.e., once created, their state cannot be modified, eliminates the need for synchronization as immutable objects are inherently thread-safe.
- Thread-Local Variables: Using thread-local variables ensures that each thread has its own copy of a variable, eliminating the need for synchronization when accessing thread-local data.
Advantages of Thread Safety:
Ensuring thread safety offers several benefits:
- Prevents data corruption and race conditions.
- Ensures consistency and integrity of shared data.
- Facilitates safe concurrent execution of multiple threads.
- Enhances the reliability and stability of multithreaded applications.
Limitations of Thread Safety:
While thread safety is essential for multithreaded applications, it comes with some limitations:
- Performance Overhead: Synchronization and other thread safety mechanisms may introduce performance overhead due to locking and contention.
- Complexity: Implementing thread safety measures, such as synchronization, can increase the complexity of code and may lead to potential deadlock or livelock situations if not handled properly.
Explanation of join() Method in Java
The join() method in Java is used to wait for a thread to complete its execution. When a thread calls the join() method on another thread, it waits for that thread to finish executing before proceeding with its own execution.
Usage:
The join() method is typically used in scenarios where one thread depends on the result of another thread's execution or needs to coordinate its execution with another thread.
Syntax:
public final void join() throws InterruptedExceptionThe join() method has an overloaded version that allows specifying a timeout duration:
public final void join(long millis) throws InterruptedExceptionExplanation:
When a thread calls the join() method on another thread, it enters a waiting state until the joined thread completes its execution. The calling thread remains in the waiting state until one of the following conditions occurs:
- The joined thread completes its execution normally.
- The joined thread is interrupted by another thread.
- The specified timeout duration (if provided) elapses.
Once any of these conditions are met, the join() method returns, and the calling thread continues with its execution.
Example:
Thread thread1 = new Thread(() -> {
// Task performed by thread1
});
Thread thread2 = new Thread(() -> {
// Task performed by thread2
});
thread1.start();
thread2.start();
try {
thread1.join(); // Wait for thread1 to finish
thread2.join(); // Wait for thread2 to finish
} catch (InterruptedException e) {
e.printStackTrace();
}
// Code after both threads have finishedIn this example, the main thread waits for thread1 and thread2 to finish their execution using the join() method before continuing with its own execution.
Explanation of Thread Pool in Java
A thread pool in Java is a collection of pre-initialized threads that are available for performing tasks concurrently. Instead of creating and destroying threads for each task, a thread pool manages a pool of threads, reusing them to execute multiple tasks over time.
Key Components of a Thread Pool:
- Thread Pool Size: Specifies the number of threads created and maintained by the thread pool.
- Task Queue: Holds tasks that are submitted to the thread pool for execution. Tasks in the queue are picked up by available threads for execution.
- Thread Factory: Creates new threads when needed. It allows customization of thread creation, such as setting thread names, priority, etc.
- Execution Policy: Determines how tasks are handled when the thread pool's capacity is reached. Common policies include discarding tasks, running tasks in the caller's thread, or blocking until space is available in the task queue.
Advantages of Using a Thread Pool:
- Improved Performance: Thread pools reduce the overhead of thread creation and destruction, resulting in better performance and resource utilization.
- Resource Management: By limiting the number of concurrently executing threads, thread pools prevent resource exhaustion and system overload.
- Concurrency Control: Thread pools provide a convenient way to control the level of concurrency in an application by adjusting the pool size and task queue capacity.
- Enhanced Scalability: Thread pools facilitate scalability by allowing applications to handle a large number of concurrent tasks efficiently without overwhelming system resources.
Common Implementations:
In Java, thread pools are commonly implemented using the ThreadPoolExecutor class from the java.util.concurrent package. This class provides a flexible and customizable framework for creating and managing thread pools.
Example:
// Create a thread pool with fixed size
ExecutorService executor = Executors.newFixedThreadPool(5);
// Submit tasks to the thread pool
for (int i = 0; i < 10; i++) {
executor.submit(new Task());
}
// Shutdown the thread pool after tasks are completed
executor.shutdown();In this example, a thread pool with a fixed size of 5 threads is created using Executors.newFixedThreadPool(). Tasks are submitted to the thread pool using the submit() method, and finally, the thread pool is shutdown gracefully using shutdown() method.
Explanation of Daemon Threads in Java
In Java, a daemon thread is a special type of thread that runs in the background, providing services to other threads without preventing the JVM from exiting. Unlike user threads, which keep the JVM running until they complete their execution, daemon threads do not prevent the JVM from terminating if all user threads have finished execution.
Key Characteristics of Daemon Threads:
- Background Execution: Daemon threads run in the background, performing tasks such as garbage collection, I/O monitoring, or other housekeeping activities.
- Low Priority: Daemon threads typically have lower priority compared to user threads, allowing user threads to receive more CPU time.
- Automatic Termination: Daemon threads automatically terminate when there are no longer any non-daemon threads running or when the
System.exit()method is invoked. - Non-Critical Tasks: Daemon threads are often used for non-critical tasks that can safely terminate if the JVM exits unexpectedly.
Usage:
Daemon threads are commonly used for background tasks that need to run continuously but do not require manual intervention or coordination with other threads. Some examples of daemon threads include:
- Garbage collection thread
- Finalizer thread
- I/O event monitoring thread
- Timer thread
- Shutdown hook thread
Example:
Thread daemonThread = new Thread(() -> {
// Perform background tasks
});
daemonThread.setDaemon(true); // Set the thread as daemon
daemonThread.start();In this example, a daemon thread is created and started to perform background tasks. By calling setDaemon(true), the thread is marked as daemon, allowing the JVM to exit even if this thread is still running.
Explanation of wait(), notify(), and notifyAll() Methods in Java
In Java, the wait(), notify(), and notifyAll() methods are used for inter-thread communication and synchronization. These methods are defined in the Object class and are used to coordinate the execution of multiple threads sharing a common resource.
wait() Method:
The wait() method causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for the same object. It releases the lock on the object and waits indefinitely until it is notified.
Syntax:
public final void wait() throws InterruptedExceptionnotify() Method:
The notify() method wakes up a single thread that is waiting on the object's monitor. If multiple threads are waiting, it is not specified which thread will be awakened. The awakened thread then competes in the usual manner for the object's monitor.
Syntax:
public final void notify()notifyAll() Method:
The notifyAll() method wakes up all threads that are waiting on the object's monitor. It notifies all waiting threads, giving them an opportunity to acquire the object's monitor. The awakened threads then compete in the usual manner for the object's monitor.
Syntax:
public final void notifyAll()Usage:
These methods are typically used in scenarios where threads need to communicate and synchronize their activities. The general workflow involves:
- Threads call
wait()to wait for a condition to be satisfied. - Other threads perform some operation and call
notify()ornotifyAll()to wake up the waiting threads. - Waiting threads are awakened and proceed with their execution.
It's important to note that these methods must be called from synchronized blocks or methods to ensure proper synchronization and avoid IllegalMonitorStateException.
Explanation of volatile Keyword in Java
In Java, the volatile keyword is used to declare a variable as "volatile," indicating that its value may be modified by different threads asynchronously. When a variable is declared as volatile, it ensures that any thread reading the variable always sees the most recent value written to it by any other thread.
Key Characteristics of volatile Keyword:
- Visibility: Ensures that changes to the variable made by one thread are immediately visible to other threads. This prevents the problem of thread-local caching of variables.
- Atomicity: While the volatile keyword guarantees visibility, it does not provide atomicity for compound operations (e.g., increment, decrement). For atomic operations, synchronization mechanisms like
synchronizedorjava.util.concurrent.atomicclasses should be used. - Usage: The volatile keyword is typically used for flags, status flags, or variables that are frequently read but infrequently modified.
Example:
Consider a scenario where multiple threads are accessing a shared variable counter:
public class SharedResource {
private volatile int counter = 0;
public void incrementCounter() {
counter++;
}
public int getCounter() {
return counter;
}
}In this example, the counter variable is declared as volatile, ensuring that changes made to it by one thread are immediately visible to other threads. Without the volatile keyword, the changes made by one thread may not be visible to other threads, leading to potential data inconsistency issues.
Multi-Threading in Java
Multi-threading is a programming concept that allows multiple threads of execution to run concurrently within a single process. In Java, multi-threading is achieved using classes and interfaces provided in the java.lang package, such as the Thread class and the Runnable interface.
Benefits of Multi-Threading:
- Concurrency: Multi-threading allows different parts of a program to execute concurrently, making better use of available resources and potentially improving performance.
- Responsiveness: By running time-consuming tasks in separate threads, the user interface remains responsive and does not freeze while waiting for tasks to complete.
- Resource Utilization: Multi-threading can improve resource utilization by allowing multiple tasks to execute simultaneously on multi-core processors.
- Modularity: Threads enable the decomposition of complex tasks into smaller, more manageable units of work, improving code organization and maintainability.
Creating Threads in Java:
In Java, there are two primary ways to create threads:
- Extending the
Threadclass and overriding itsrun()method. - Implementing the
Runnableinterface and passing it to aThreadconstructor.
Thread Scheduling and Synchronization:
Java's thread scheduler determines the order in which threads are executed based on priority and other factors. Synchronization mechanisms such as locks, semaphores, and monitors are used to coordinate access to shared resources and prevent race conditions and data corruption in multi-threaded programs.
Common Challenges in Multi-Threading:
- Race Conditions: When multiple threads access shared resources concurrently without proper synchronization, race conditions may occur, leading to unpredictable behavior.
- Deadlocks: Deadlocks occur when two or more threads are blocked indefinitely, waiting for each other to release resources.
- Thread Starvation: Thread starvation happens when a thread is unable to gain access to shared resources due to other threads continuously holding them.
- Performance Overhead: The overhead associated with creating and managing threads, context switching, and synchronization can impact the performance of multi-threaded applications.
Best Practices for Multi-Threading:
- Use thread-safe data structures and synchronization mechanisms to prevent data corruption.
- Minimize the use of global variables and shared mutable state to reduce the likelihood of race conditions.
- Keep critical sections of code short and avoid blocking operations that may cause thread starvation.
- Use higher-level concurrency utilities such as
java.util.concurrentfor complex synchronization requirements. - Test and debug multi-threaded code thoroughly to identify and resolve potential concurrency issues.
Synchronization in Java
Synchronization is a technique used in Java to control access to shared resources by multiple threads. It ensures that only one thread can access a shared resource at a time, preventing data corruption and race conditions.
What is Synchronization?
In Java, synchronization is achieved using the synchronized keyword. When a method or block of code is marked as synchronized, only one thread can execute it at a time. Other threads attempting to access the synchronized method or block will be blocked until the owning thread releases the lock.
Advantages of Synchronization:
- Thread Safety: Synchronization ensures that shared resources are accessed in a thread-safe manner, preventing data corruption and maintaining consistency.
- Prevents Race Conditions: By allowing only one thread to access a synchronized block at a time, synchronization helps avoid race conditions and ensures predictable behavior.
- Facilitates Coordination: Synchronization facilitates coordination between threads by allowing them to wait for each other and communicate through shared resources.
Limitations of Synchronization:
- Performance Overhead: Synchronization introduces performance overhead due to the need for acquiring and releasing locks, context switching, and contention for shared resources.
- Potential Deadlocks: Improper use of synchronization can lead to deadlocks, where threads are blocked indefinitely waiting for each other to release locks.
- Complexity: Managing synchronization in large, complex applications can be challenging and prone to errors such as race conditions and deadlocks.
How to Use Synchronization:
Synchronization can be applied at the method level or by using synchronized blocks. Here are two examples:
Example 1: Synchronized Method
class Counter {
private int count = 0;
// Synchronized method
public synchronized void increment() {
count++;
}
public int getCount() {
return count;
}
}
public class Main {
public static void main(String[] args) {
Counter counter = new Counter();
// Create multiple threads to increment the counter concurrently
Thread thread1 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
counter.increment();
}
});
Thread thread2 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
counter.increment();
}
});
thread1.start();
thread2.start();
// Wait for both threads to complete
try {
thread1.join();
thread2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
// Print the final count
System.out.println("Final count: " + counter.getCount());
}
}
In this example, the increment() method of the Counter class is synchronized, ensuring that only one thread can increment the count at a time.
Example 2: Synchronized Block
class SharedResource {
private Object lock = new Object();
private int data = 0;
public void updateData() {
// Synchronized block
synchronized (lock) {
// Critical section of code
data++;
System.out.println("Updated data: " + data);
}
}
}
public class Main {
public static void main(String[] args) {
SharedResource resource = new SharedResource();
// Create multiple threads to update the shared resource concurrently
for (int i = 0; i < 5; i++) {
Thread thread = new Thread(() -> {
resource.updateData();
});
thread.start();
}
}
}
In this example, the critical section of code within the updateData() method of the SharedResource class is synchronized using a synchronized block, ensuring that only one thread can update the data at a time.
Synchronized Block in Java
A synchronized block in Java is a block of code that is marked as synchronized to ensure that only one thread can execute it at a time. Synchronized blocks are used to protect critical sections of code where shared resources are accessed by multiple threads.
Syntax:
synchronized (object) {
// Critical section of code
// Access shared resources here
}
In a synchronized block, object is an object used to provide locking. Only one thread can acquire the lock associated with object and execute the critical section of code at a time.
Working of Synchronized Block:
When a thread enters a synchronized block, it attempts to acquire the lock associated with the specified object. If the lock is available, the thread acquires it and executes the critical section of code. If the lock is already held by another thread, the current thread waits until the lock is released before proceeding.
Advantages of Synchronized Block:
- Granular Locking: Synchronized blocks allow finer-grained locking compared to synchronized methods, enabling more flexibility in controlling access to shared resources.
- Reduced Contention: By synchronizing only critical sections of code, synchronization overhead and contention for locks are minimized, improving performance in multi-threaded applications.
- Dynamic Locking: The object used for synchronization can be dynamically chosen based on runtime conditions, providing more flexibility in locking strategies.
Example:
class SharedResource {
private Object lock = new Object();
private int data = 0;
public void updateData() {
// Synchronized block
synchronized (lock) {
// Critical section of code
data++;
System.out.println("Updated data: " + data);
}
}
}
public class Main {
public static void main(String[] args) {
SharedResource resource = new SharedResource();
// Create multiple threads to update the shared resource concurrently
for (int i = 0; i < 5; i++) {
Thread thread = new Thread(() -> {
resource.updateData();
});
thread.start();
}
}
}
In this example, the updateData() method of the SharedResource class is synchronized using a synchronized block. This ensures that only one thread can execute the critical section of code where the data variable is updated at a time, preventing data corruption and race conditions.
Common Multi-Threading Challenges and Examples
Race Condition:
A race condition occurs when multiple threads access shared resources concurrently without proper synchronization, leading to unpredictable behavior and data corruption. Here's an example:
class Counter {
private int count = 0;
public void increment() {
count++;
}
public int getCount() {
return count;
}
}
public class Main {
public static void main(String[] args) {
Counter counter = new Counter();
// Create two threads to increment the counter concurrently
Thread thread1 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
counter.increment();
}
});
Thread thread2 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
counter.increment();
}
});
thread1.start();
thread2.start();
// Wait for both threads to complete
try {
thread1.join();
thread2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
// Print the final count
System.out.println("Final count: " + counter.getCount());
}
}
In this example, both threads thread1 and thread2 are incrementing the count variable of the Counter class concurrently without synchronization. This can lead to a race condition where the final count may not be what is expected.
Deadlock:
Deadlock occurs when two or more threads are blocked indefinitely, waiting for each other to release resources. Here's an example:
public class DeadlockExample {
private static final Object lock1 = new Object();
private static final Object lock2 = new Object();
public static void main(String[] args) {
Thread thread1 = new Thread(() -> {
synchronized (lock1) {
System.out.println("Thread 1 acquired lock1");
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lock2) {
System.out.println("Thread 1 acquired lock2");
}
}
});
Thread thread2 = new Thread(() -> {
synchronized (lock2) {
System.out.println("Thread 2 acquired lock2");
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lock1) {
System.out.println("Thread 2 acquired lock1");
}
}
});
thread1.start();
thread2.start();
}
}
In this example, thread1 acquires lock1 and waits for lock2, while thread2 acquires lock2 and waits for lock1. Both threads are blocked indefinitely, resulting in a deadlock.
Thread Starvation:
Thread starvation occurs when a thread is unable to gain access to shared resources due to other threads continuously holding them. Here's an example:
public class ThreadStarvationExample {
public static void main(String[] args) {
Object lock = new Object();
Thread greedyThread = new Thread(() -> {
synchronized (lock) {
while (true) {
// Do some work
}
}
});
Thread starvedThread = new Thread(() -> {
synchronized (lock) {
// This thread will never execute because the greedyThread never releases the lock
System.out.println("This will never be printed");
}
});
greedyThread.start();
starvedThread.start();
}
}
In this example, greedyThread continuously holds the lock, preventing starvedThread from executing its critical section. As a result, starvedThread is starved of CPU time and unable to make progress.
Performance Overhead:
The performance overhead associated with multi-threading includes the cost of creating and managing threads, context switching, and synchronization. While multi-threading can improve performance by utilizing multiple CPU cores, excessive thread creation or synchronization can introduce overhead and degrade performance.
Explanation of Executor Framework in Java
The Executor framework in Java provides a higher-level abstraction for managing and executing tasks asynchronously. It decouples task submission from task execution, allowing for more efficient and flexible concurrency control.
Key Components of Executor Framework:
- Executor: Represents an object capable of executing submitted tasks. It typically manages a pool of threads and delegates tasks to them for execution.
- ExecutorService: Extends the Executor interface and provides additional methods for managing the lifecycle of threads and tasks, such as submitting tasks for execution, shutting down the executor, and obtaining the result of a task.
- ThreadPoolExecutor: A concrete implementation of the ExecutorService interface that manages a pool of threads for executing tasks. It provides configurable parameters such as core pool size, maximum pool size, thread keep-alive time, and task queue.
- ScheduledExecutorService: Extends the ExecutorService interface and provides methods for scheduling tasks to run after a specified delay or at fixed intervals.
Advantages of Using Executor Framework:
- Resource Management: Executor framework manages thread creation, lifecycle, and recycling, reducing the overhead of thread creation and destruction.
- Task Management: It provides a flexible mechanism for submitting tasks for execution, allowing for task prioritization, scheduling, and dependency management.
- Concurrency Control: Executor framework enables efficient utilization of system resources and control over the level of concurrency, preventing resource exhaustion and system overload.
- Error Handling: It provides facilities for handling errors and exceptions occurring during task execution, allowing for graceful recovery and error reporting.
Example:
Below is an example demonstrating the use of Executor framework to execute tasks asynchronously:
ExecutorService executor = Executors.newFixedThreadPool(5);
for (int i = 0; i < 10; i++) {
executor.submit(new Task());
}
executor.shutdown();In this example, a fixed-size thread pool with 5 threads is created using Executors.newFixedThreadPool(). Ten tasks are submitted to the executor for execution using the submit() method, and finally, the executor is shut down gracefully using shutdown() method.
Interview Questions on Threads and Multithreading in Java
Interview Questions:
- What is a thread in Java?
- Explain the difference between process and thread.
- What is multithreading?
- What are the advantages of multithreading?
- How do you create a thread in Java?
- Explain the lifecycle of a thread.
- What is the purpose of the start() method in thread execution?
- How do you implement synchronization in Java?
- What are race conditions and how do you prevent them?
- What is thread safety and why is it important?
- Explain the concept of deadlock in multithreading.
- How do you handle exceptions in multithreaded programs?
- What is the purpose of the join() method in Java?
- Explain the concept of thread pool in Java.
- What are daemon threads?
- How do you implement inter-thread communication in Java?
- What is the purpose of the wait(), notify(), and notifyAll() methods?
- Explain the difference between sleep() and wait() methods.
- What are the different ways to achieve synchronization in Java?
- What is the role of the synchronized keyword in Java?
- How do you prevent thread interference and memory consistency errors?
- What is the purpose of the volatile keyword in Java?
- Explain the concept of thread local variables.
- What is the role of the Executor framework in multithreading?
- How do you debug multithreading issues in Java?
Multiple Choice Questions (MCQs):
- Which of the following statements is true about threads in Java?
a) A thread is a lightweight process
b) Threads share the same memory space
c) Threads are independent of each other
d) All of the above
Answer: a) A thread is a lightweight process - What is the purpose of the sleep() method in Java?
a) To pause the execution of the current thread
b) To terminate the current thread
c) To restart the current thread
d) To wait for input from the user
Answer: a) To pause the execution of the current thread - Which method is used to create a new thread in Java?
a) run()
b) start()
c) execute()
d) fork()
Answer: b) start() - What is the purpose of the yield() method in Java?
a) To pause the execution of the current thread
b) To terminate the current thread
c) To restart the current thread
d) To give a hint to the scheduler that the current thread is willing to yield its current use of a processor
Answer: d) To give a hint to the scheduler that the current thread is willing to yield its current use of a processor - Which keyword is used to prevent a method from being accessed by multiple threads simultaneously?
a) synchronized
b) volatile
c) atomic
d) locked
Answer: a) synchronized