Multithreading in Java

• Multithreading in Java enables the concurrent execution of program parts, improving CPU use.
• A thread is a part of this program.
• Threads are lightweight processes within a process, and they share memory.

Core Concepts of Multithreading

• Process: An isolated execution space with its own memory.
• Thread: A subprocess that shares resources within a process.
• Concurrency: Multiple threads progress, but they don't necessarily run at the same time.
• Parallelism: Multiple threads execute simultaneously, often on different CPU cores.

Creating Threads

• Extending the Thread Class: A class extends java.lang.Thread and overrides the run() method for thread execution code.
• Implementing the Runnable Interface: A class implements java.lang.Runnable with a run() method implementation. A Thread object is instantiated with an instance of the Runnable class.

Thread Lifecycle

• New: The thread is created, but not started.
• Runnable: The thread is ready and waiting for CPU time.
• Running: The thread is executing.
• Blocked/Waiting: The thread is suspended, awaiting a resource or event.
• Terminated: The thread has finished executing.

Thread Synchronization

• Synchronization: Manages shared resource access by multiple threads, preventing race conditions and data corruption.
• synchronized Keyword: It creates synchronized methods or blocks, allowing one thread to execute the code at a time.
• Monitors: Each Java object has a monitor acquired by a thread with the synchronized keyword.
• Intrinsic Locks: Locks associated with synchronized methods or blocks.
• Reentrant Locks: A thread holding a lock can re-enter the same synchronized method/block without re-acquiring the lock.

Common Synchronization Issues

• Race Condition: Multiple threads access and change data concurrently, causing unpredictable results.
• Deadlock: Threads are blocked indefinitely, each waiting for the other to release resources.
• Livelock: Threads repeatedly change state in response to each other, preventing progress.
• Starvation: A thread is continuously denied access to a resource or CPU time.

Inter-Thread Communication

• wait(), notify(), notifyAll(): Object class methods for thread communication.
• wait(): Releases the lock and waits until another thread calls notify() or notifyAll() on the same object.
• notify(): Wakes up a single thread waiting on the object's monitor.
• notifyAll(): Wakes up all threads waiting on the object's monitor.

Concurrent Collections

• java.util.concurrent Package: Provides thread-safe collections for concurrent access.
• ConcurrentHashMap: A thread-safe implementation of the Map interface.
• CopyOnWriteArrayList: A thread-safe ArrayList where mutations copy the underlying array.
• BlockingQueue: An interface for queues that block on empty retrieval or full storage attempts.

Executors and Thread Pools

• Executor Framework: Manages threads via abstraction.
• Executor: An interface to submit tasks for execution.
• ExecutorService: Extends Executor to manage thread lifecycles and shut down the executor.
• ThreadPoolExecutor: An ExecutorService implementation managing a pool of threads.
• FixedThreadPool: Creates a thread pool with a fixed number of threads.
• CachedThreadPool: Creates a thread pool, reusing threads or creating new ones as needed.
• ScheduledThreadPoolExecutor: An ExecutorService to schedule tasks periodically or after a delay.

Locks

• java.util.concurrent.locks Package: Offers flexible locking than synchronized blocks/methods.
• Lock Interface: Methods for acquiring and releasing locks.
• ReentrantLock: A mutual exclusion Lock with similar behavior to implicit monitors via synchronized, but with extended capabilities.
• ReadWriteLock: An interface for read-only and writing operation locks.
• ReentrantReadWriteLock: Allows multiple readers or one writer at a time.

Atomic Variables

• java.util.concurrent.atomic Package: Classes for lock-free thread-safe single variable programming.
• AtomicInteger, AtomicLong, AtomicBoolean: Classes for atomic operations on integer, long, and boolean values.
• AtomicReference: A class for atomic operations on object references .

Fork/Join Framework

• Fork/Join Framework: Parallelizes recursive algorithms by dividing tasks into subtasks.
• ForkJoinPool: An ExecutorService for executing ForkJoinTasks.
• ForkJoinTask: An abstract class for tasks that can be split.
• RecursiveTask: Returns a result; a subclass of ForkJoinTask.
• RecursiveAction: A subclass of ForkJoinTask that does not return a result.

ThreadLocal

• ThreadLocal Class: Provides thread-local variables, where each thread has its own independent copy of the variable.
• It avoids sharing mutable data between threads.

Best Practices for Multithreading

• Minimize Shared Mutable State: Limit shared, modifiable data.
• Use Immutable Objects: Immutable objects are inherently thread-safe.
• Avoid Excessive Locking: Prevents performance bottlenecks.
• Use Concurrent Collections: Thread-safe collections from java.util.concurrent should be preferred.
• Properly Handle Exceptions: Prevents unexpected behavior.
• Monitor Thread Performance: Identifies and addresses performance issues.
• Avoid Deadlocks: Prevents circular dependencies between locks.
• Use Thread Pools: Manages threads to avoid creation/destruction overhead.
• Prefer Higher-Level Abstractions: Use executors, futures, and concurrent collections.

Volatile Keyword

• volatile Keyword: Guarantees visibility of variable changes across threads.
• Each read/write is done from/to main memory.
• No atomicity; suitable for single-variable updates.

Memory Model

• Java Memory Model (JMM): Determines how threads interact with memory.
• It establishes rules for when and how threads can observe changes to shared variables.
• Happens-Before Relationship: Defines the ordering of operations between threads.

Callable and Future

• Callable Interface: Like Runnable, but returns a result and throws exceptions.
• Future Interface: Represents the result of an asynchronous computation.
• Used with ExecutorService to execute tasks and retrieve results asynchronously.

Semaphores

• Semaphore Class: It controls access to shared resources among threads.
• Maintains permits; threads acquire before access and release afterward.
• It limits concurrent resource accesses.

Explore multithreading in Java, which allows concurrent execution of multiple program parts to boost CPU usage. Learn how threads, lightweight subprocesses, share memory within a process. Understand the core concepts of concurrency and parallelism.