Multithreading

Thread Synchronization

• synchronized keyword is used to achieve thread synchronization. It can be used with a method or a block of code.

• wait() and notify() methods are used to coordinate between threads. wait() method causes a thread to wait until another thread signals (by calling notify() method).

• join() method is used to wait for a thread to finish its execution before moving on to the next task.

• volatile keyword is used to make sure that the changes made to a variable by one thread are immediately visible to other threads.

Thread Pooling

• A thread pool is a group of pre-initialized threads that are ready to execute tasks.

• ExecutorService is an interface that represents an executor that can execute tasks asynchronously.

• ThreadPoolExecutor is a class that provides a thread pool implementation.

• submit() method is used to submit a task to the thread pool.

• shutdown() method is used to shutdown the thread pool after completing all the tasks.

Concurrent Collections

• Java provides thread-safe implementations of common collections in the java.util.concurrent package.

• Examples: ConcurrentHashMap, ConcurrentLinkedQueue, CopyOnWriteArrayList, etc.

• These collections are designed to be used in multi-threaded environments without the need for external synchronization.

Locks

• Java provides a Lock interface which can be used to achieve thread synchronization.

• ReentrantLock is a class that implements the Lock interface.

• tryLock() method is used to try to acquire the lock without waiting. It returns immediately with a boolean value indicating whether the lock was acquired or not.

• lockInterruptibly() method is used to acquire the lock while waiting for the lock to be released. If the thread is interrupted while waiting, an InterruptedException is thrown.

Atomic Variables

• Atomic variables are thread-safe variables that can be updated atomically.

• Java provides a set of classes in the java.util.concurrent.atomic package that implement atomic variables, such as AtomicInteger, AtomicLong, AtomicBoolean, etc.

• Atomic variables are designed to be used in multi-threaded environments without the need for external synchronization.

Thread Safety

• A class is thread-safe if it can be used by multiple threads at the same time without causing any issues.

• Immutable objects are inherently thread-safe because they cannot be modified after they are created.

• Synchronization is required to achieve thread safety for mutable objects.

• Locking can cause contention and degrade performance, so it should be used judiciously.

• Thread-safety can also be achieved using atomic variables and concurrent collections.

Executors

• Executors provide a simple way to create and manage threads.

• Executors provide a number of methods to create different types of thread pools, such as fixed thread pool, cached thread pool, and scheduled thread pool.

• Executors provide methods to execute tasks asynchronously and return the result in the form of a Future object.

• Executors can also be used to schedule tasks for execution at a later time.

Fork/Join Framework

• The fork/join framework is a concurrency mechanism introduced in Java 7.

• The framework is designed for recursive algorithms that can be broken down into smaller subtasks.

• The framework provides a ForkJoinPool class that manages a pool of worker threads and a RecursiveTask class that represents a task that can be broken down into smaller subtasks.

• The invoke() method of the ForkJoinPool class is used to submit a RecursiveTask to the pool for execution.

Thread Local Variables

• Thread local variables are variables that are local to a thread.

• Each thread has its own copy of the thread local variable.

• Thread local variables can be used to store thread-specific information, such as user preferences or database connections.

• Java provides a ThreadLocal class to create thread local variables.

Thread Deadlock

• Thread deadlock occurs when two or more threads are blocked, waiting for each other to release a resource.

• Deadlocks can occur when multiple threads acquire locks in a different order.

• To avoid deadlock, it is important to always acquire locks in the same order.

• Deadlocks can also occur when multiple threads wait indefinitely for a resource that is held by another thread.

Thread Starvation

• Thread starvation occurs when a thread is unable to acquire the resources it needs to continue executing.

• Starvation can occur when a thread is blocked waiting for a lock that is held by another thread.

• To avoid starvation, it is important to ensure that all threads have a fair chance of acquiring the resources they need.

• Java provides a FairLock class that ensures fairness in locking.