Summary of JUC related knowledge points

Java JUC（) is the core toolkit for Java concurrent programming, providing a rich variety of concurrent tool classes and frameworks. The following are the main knowledge points of JUC, classified by difficulty level for your reference:

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1. Basic concepts and tools

1.1 Concurrency and Parallelism (Easy)

• content: Understand the difference between concurrency and parallelism.
• importance: Basic concepts, the premise of understanding concurrent programming.

1.2 Threads and thread pools (middle)

• content：
  • Thread creation and startup (Thread、Runnable）。
  • Thread pool (ExecutorService、ThreadPoolExecutor）。
  • Thread pool parameters (number of core threads, maximum number of threads, queue type, rejection policy).
• importance: Thread pools are the core tool of concurrent programming and must be mastered.

1.3 Thread life cycle and state (Easy)

• content: 6 states of threads (NEW、RUNNABLE、BLOCKED、WAITING、TIMED_WAITING、TERMINATED）。
• importance: Understanding thread state is the basis for debugging concurrency problems.

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2. Synchronization and locking

2.1 synchronizedKeywords (Easy)

• content：
  • Method synchronization is synchronized with code blocks.
  • Optimize the granularity and performance of the lock.
• importance: The most basic synchronization mechanism.

2.2 ReentrantLock(middle)

• content：
  • Basic use of reentrant locks.
  • Fair lock and unfair lock.
  • tryLock、lockInterruptiblyAdvanced features.
• importance:ComparesynchronizedMore flexible locking mechanism.

2.3 ReadWriteLock(middle)

• content：
  • Read and write lock (ReentrantReadWriteLock）。
  • The separation of read lock and write lock.
• importance: Suitable for scenarios where more reads, less writes.

2.4 StampedLock(Disaster)

• content：
  • Optimistic reading lock and pessimistic reading lock.
  • Upgrade and downgrade of locks.
• importance: High-performance lock, suitable for specific scenarios.

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3. Atomic operation class

3.1 AtomicInteger、AtomicLongWait (Easy)

• content：
  • Basic use of atomic operations.
  • compareAndSet(CAS) principle.
• importance: The basics of lock-free programming.

3.2 AtomicReference、AtomicStampedReference(middle)

• content：
  • Atomic operation of reference type.
  • Solve ABA problems.
• importance: Suitable for atomic operations of complex objects.

3.3 LongAdder、DoubleAdder(middle)

• content：
  • Accumulator in high concurrency scenarios.
  • Segment lock mechanism.
• importance:CompareAtomicLongHigher performance.

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4. Concurrent collection

4.1 ConcurrentHashMap(middle)

• content：
  • Segment locking and CAS mechanism.
  • Performance optimization under high concurrency.
• importance: The most commonly used concurrent collection.

4.2 CopyOnWriteArrayList、CopyOnWriteArraySet(easy)

• content：
  • Copy on write mechanism.
  • Applicable scenarios and performance characteristics.
• importance: Suitable for scenarios where more reads, less writes.

4.3 BlockingQueueIts implementation class (middle)

• content：
  • ArrayBlockingQueue、LinkedBlockingQueue。
  • PriorityBlockingQueue、SynchronousQueue。
  • put、takeSo for blocking operations.
• importance: The basis for implementing the producer-consumer model.

4.4 ConcurrentLinkedQueue、ConcurrentSkipListMap(Disaster)

• content：
  • Lockless queue and jump table.
  • Performance advantages in high concurrency scenarios.
• importance: Suitable for high-performance lock-free scenarios.

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5. Synchronization tool class

5.1 CountDownLatch(easy)

• content：
  • Wait for multiple threads to complete the task.
• importance: The basic tool for multi-threaded collaboration.

5.2 CyclicBarrier(middle)

• content：
  • Multithreading continues to execute after reaching the barrier.
  • andCountDownLatchThe difference.
• importance: Suitable for phased tasks.

5.3 Semaphore(middle)

• content：
  • Controls the number of concurrent threads.
  • Basic use of semaphores.
• importance: The core tool for resource pool management.

5.4 Phaser(Disaster)

• content：
  • Multi-stage task synchronization.
  • Dynamically adjust the number of participating threads.
• importance: Suitable for complex task scheduling.

5.5 Exchanger(Disaster)

• content：
  • Inter-thread data exchange.
• importance: Suitable for specific scenarios.

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6. Asynchronous programming

6.1 FutureandFutureTask(middle)

• content：
  • Result acquisition of asynchronous tasks.
  • getBlocking characteristics of the method.
• importance: The basics of asynchronous programming.

6.2 CompletableFuture(Disaster)

• content：
  • Chain calls and combination operations.
  • thenApply、thenAccept、thenCombineetc.
• importance: The core tool of modern asynchronous programming.

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7. Thread Scheduling and Timed Tasks

7.1 ScheduledExecutorService(middle)

• content：
  • Timed and periodic tasks.
• importance: SubstitutionTimerRecommended tool.

7.2 ForkJoinPool(Disaster)

• content：
  • Dividing and conquer algorithm and work theft.
  • RecursiveTaskandRecursiveAction。
• importance: Suitable for computing-intensive tasks.

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8. Other advanced features

8.1 ThreadLocal(middle)

• content：
  • Thread-local variables.
  • Memory leak problem.
• importance: A tool for data isolation between threads.

8.2 AQS(AbstractQueuedSynchronizer) (Difficult)

• content：
  • The underlying implementation of the synchronizer.
  • Custom locks and synchronization tools.
• importance: The key to understanding the underlying mechanism of JUC.

8.3 Memory model andvolatile(middle)

• content：
  • Visibility, orderliness, atomicity.
  • happens-beforein principle.
• importance: Understand the basics of concurrent programming.

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Study Suggestions

1. From easy to difficult: First master the basic concepts and tools, and then learn advanced features in depth.
2. Hands-on practice: Deepen understanding by writing code, especially concurrent collections and synchronization tool classes.
3. Read the source code:in particularAQS、ConcurrentHashMapSource codes such as core categories.
4. Debugging and testing: Use tools (such as JConsole, VisualVM) to analyze concurrency problems.

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By systematically learning the above knowledge points, you will fully grasp the core content of Java JUC and be able to cope with concurrent programming challenges in actual development.

Demonstrate the above functions in detail

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Complete summary of Java JUC knowledge points

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1. Basic concepts and tools

1.1 Concurrency and Parallel

describe：

• concurrent: Multiple tasks are executed alternately, suitable for single-core CPU or multi-threaded tasks.
• parallel: Multiple tasks are executed simultaneously, requiring multi-core CPU support.
• Scope of application: Understanding the difference between concurrency and parallelism is the basis of concurrent programming.
• Note: Parallel requires hardware support (multi-core CPU).
• Implementation principle: Concurrency is implemented through thread switching and parallelism is implemented through multi-core CPU.

Code Example：

public class ConcurrencyVsParallelism {
     public static void main(String[] args) {
         // Concurrency: multiple tasks are executed alternately
         Runnable task = () -> (().getName() + " is running");
         new Thread(task).start(); // Start thread 1
         new Thread(task).start(); // Start thread 2

         // Parallel: multiple tasks are executed simultaneously (requires multi-core CPU support)
         ExecutorService executor = (2); // Create a fixed-size thread pool
         (task); // Submit task 1
         (task); // Submit task 2
         (); // Close the thread pool
     }
 }

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1.2 Threads and thread pools

describe：

• Thread: The most basic concurrency unit in Java.
• Thread pool: Manage the life cycle of threads and avoid frequent creation and destruction of threads.
• Scope of application: Scenarios where threads need to be created frequently.
• Note: Thread pool parameters (number of core threads, maximum number of threads, queue type, rejection policy) need to be configured reasonably.
• Implementation principle: The thread pool realizes task scheduling through task queues and worker threads.

Code Example：

public class ThreadPoolExample {
     public static void main(String[] args) {
         ExecutorService executor = (2); // Create a fixed-size thread pool
         (() -> ("Task 1")); // Submit task 1
         (() -> ("Task 2")); // Submit task 2
         (); // Close the thread pool
     }
 }

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1.3 Thread life cycle and state

describe：

• Thread status：NEW、RUNNABLE、BLOCKED、WAITING、TIMED_WAITING、TERMINATED。
• Scope of application: Debugging and analyzing thread behavior.
• Note: Thread state is managed by JVM and cannot be directly controlled by the developer.
• Implementation principle: JVM manages thread state through internal state machine.

Code Example：

public class ThreadStateExample {
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(() -> {
            try {
                (1000); // TIMED_WAITING
            } catch (InterruptedException e) {
                ();
            }
        });
        (()); // NEW
        ();
        (()); // RUNNABLE
        ();
        (()); // TERMINATED
    }
}

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2. Synchronization and locking

2.1 synchronized

describe：

• Scope of application: Simple thread synchronization scenario.
• Note: The particle size of the lock should be as small as possible to avoid performance problems.
• Implementation principle: Based on the JVM built-in lock mechanism, throughmonitorenterandmonitorexitInstruction implementation.

Code Example：

public class SynchronizedExample {
     private static class Counter {
         private int count = 0;
         public synchronized void increment() {
             count++; // Critical area operation
         }
         public int getCount() {
             return count;
         }
     }

     public static void main(String[] args) throws InterruptedException {
         Counter counter = new Counter();
         Runnable task = () -> {
             for (int i = 0; i < 1000; i++) {
                 (); // Call the synchronization method
             }
         };
         Thread thread1 = new Thread(task); // Create thread 1
         Thread thread2 = new Thread(task); // Create thread 2
         (); // Start thread 1
         (); // Start thread 2
         (); // Wait for thread 1 to complete
         (); // Wait for thread 2 to complete
         ("Final count: " + ()); // Output the final result
     }
 }

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2.2 ReentrantLock

describe：

• Scope of application: More flexible lock control is required (such as interrupt locks and timeout locks).
• Note: The lock must be released manually, otherwise it will cause a deadlock.
• Implementation principle:based onAQS(AbstractQueuedSynchronizer) implementation.

Code Example：

import ;

 public class ReentrantLockExample {
     private static class Counter {
         private int count = 0;
         private final ReentrantLock lock = new ReentrantLock(); // Create ReentrantLock
         public void increment() {
             (); // Get the lock
             try {
                 count++; // Critical area operation
             } finally {
                 (); // Release the lock
             }
         }
         public int getCount() {
             return count;
         }
     }

     public static void main(String[] args) throws InterruptedException {
         Counter counter = new Counter();
         Runnable task = () -> {
             for (int i = 0; i < 1000; i++) {
                 (); // Call the synchronization method
             }
         };
         Thread thread1 = new Thread(task); // Create thread 1
         Thread thread2 = new Thread(task); // Create thread 2
         (); // Start thread 1
         (); // Start thread 2
         (); // Wait for thread 1 to complete
         (); // Wait for thread 2 to complete
         ("Final count: " + ()); // Output the final result
     }
 }

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3. Atomic operation class

3.1 AtomicInteger

describe：

• Scope of application: Lockless thread-safe counter.
• Note: Suitable for simple atomic operations, complex scenarios may require locks.
• Implementation principle: Based on CAS (Compare-And-Swap).

Code Example：

import ;

 public class AtomicIntegerExample {
     public static void main(String[] args) {
         AtomicInteger atomicInt = new AtomicInteger(0); // The initialization value is 0
         (); // Atomic operation: self-increase and return a new value
         (5); // Atomic operation: add the specified value and return the new value
         ("Final value: " + ()); // Output the final value
     }
 }

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4. Concurrent collection

4.1 ConcurrentHashMap

describe：

• Scope of application: Key-value pair storage in high concurrency scenarios.
• Note: Not supportednullKeys and values.
• Implementation principle: Segment lock + CAS mechanism.

Code Example：

import ;

 public class ConcurrentHashMapExample {
     public static void main(String[] args) {
         ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>(); // Create ConcurrentHashMap
         ("a", 1); // Insert key-value pair
         ("b", 2); // Insert key-value pairs
         ("Value for key 'a': " + ("a")); // Get the value
     }
 }

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5. Synchronization tool class

5.1 CountDownLatch

describe：

• Scope of application: Wait for multiple threads to complete the task.
• Note: The counter cannot be reset.
• Implementation principle: Based on AQS implementation.

Code Example：

import ;

 public class CountDownLatchExample {
     public static void main(String[] args) throws InterruptedException {
         CountDownLatch latch = new CountDownLatch(2); // Initialize counter to 2
         new Thread(() -> {
             ("Task 1 completed");
             (); // Counter is reduced by 1
         }).start();
         new Thread(() -> {
             ("Task 2 completed");
             (); // Counter is reduced by 1
         }).start();
         (); // Block until the counter is 0
         ("All tasks completed");
     }
 }

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6. Asynchronous programming

6.1 Future

describe：

• Scope of application: Obtain the result of asynchronous task.
• Note：get()The method blocks until the task is completed.
• Implementation principle:based onRunnableandCallableaccomplish.

Code Example：

import .*;

 public class FutureExample {
     public static void main(String[] args) throws ExecutionException, InterruptedException {
         ExecutorService executor = (); // Create a single-threaded thread pool
         Future<Integer> future = (() -> 1 + 1); // Submit task
         ("Task result: " + ()); // Get task results
         (); // Close the thread pool
     }
 }

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7. Thread Scheduling and Timed Tasks

7.1 ScheduledExecutorService

describe：

• Scope of application: Timed tasks and periodic tasks.
• Note: Too long task execution time will affect subsequent tasks.
• Implementation principle: Based on thread pool and task queue.

Code Example：

import ;
 import ;
 import ;

 public class ScheduledExecutorServiceExample {
     public static void main(String[] args) {
         ScheduledExecutorService scheduler = (1); // Create a scheduled thread pool
         (() -> ("Task executed"), 1, ); // Delay execution
         (); // Close the thread pool
     }
 }

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8. Other advanced features

8.1 ThreadLocal

describe：

• Scope of application: Data isolation between threads.
• Note: It may cause memory leakage and needs to be cleaned in time.
• Implementation principle: Each thread maintains an independentThreadLocalMap。

Code Example：

public class ThreadLocalExample {
     private static final ThreadLocal<String> threadLocal = (() -> "Initial Value");

     public static void main(String[] args) {
         new Thread(() -> {
             ("Thread 1 Value"); // Set thread local variables
             ("Thread 1: " + ()); // Get thread local variable
         }).start();
         new Thread(() -> {
             ("Thread 2 Value"); // Set thread local variables
             ("Thread 2: " + ()); // Get thread-local variables
         }).start();
     }
 }

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The above is a complete summary of Java JUC knowledge points, including code examples, implementation principles and attention points. If further supplements or adjustments are needed, please feel free to let me know!