Synchronization Under Concurrency Control in Java
Table of Contents:
- Introduction
- The Importance of Synchronization in Java
- Key Concepts of Synchronization
- Threads and Concurrency
- Race Conditions
- Shared Resources
- Understanding the Main Code Example
- Code Walkthrough
- Explanation of Key Components
- Synchronization in Practice
- Adding Synchronized Blocks
- Thread Safety Techniques
- Conclusion
- Keywords
1. Introduction
In Java, concurrency control is a critical aspect of developing multithreaded applications, where multiple threads run in parallel and interact with shared resources. Synchronization becomes essential to avoid issues such as race conditions, where threads try to modify the same resource simultaneously.
In this eBook, we will explore the concept of synchronization under concurrency control in Java, its importance, and practical implementation using a real-world example. The Java code presented demonstrates how multiple threads can increment a shared counter and the issues that arise without synchronization.
2. The Importance of Synchronization in Java
Synchronization ensures that only one thread can access a shared resource at a time, preventing unpredictable outcomes that may arise from concurrent access.
When and Where to Use Synchronization
- When: Use synchronization when multiple threads need to access or modify shared resources.
- Where: Critical sections in code where shared resources like variables or objects are being modified.
Pros and Cons of Synchronization
| Pros | Cons |
|---|---|
| Prevents race conditions | May lead to deadlock if not handled properly |
| Ensures data consistency | Can reduce performance due to thread locking |
| Simplifies debugging for thread issues | Requires careful design and testing |
3. Key Concepts of Synchronization
Threads and Concurrency
Java uses threads to execute multiple tasks in parallel, improving performance in multi-core systems. However, without proper control, threads may interfere with one another when accessing shared resources.
Race Conditions
A race condition occurs when two or more threads can access shared data and try to change it simultaneously. Without proper synchronization, this can lead to inconsistent or incorrect results.
Shared Resources
In the given Java code, a shared variable counter is used, and two threads increment it simultaneously. Without synchronization, the final value of counter may not be what is expected.
4. Understanding the Main Code Example
Below is the core Java code from the project file that demonstrates the issue of concurrency without synchronization:
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package org.studyeasy; public class Main { public static int counter = 0; public static void main(String[] args) throws InterruptedException { new Thread(new Runnable() { @Override public void run() { for (int i = 1; i <= 100000; i++) { Main.counter++; } System.out.println("The loop in thread 1 is over"); } }).start(); new Thread(new Runnable() { @Override public void run() { for (int i = 1; i <= 100000; i++) { Main.counter++; } System.out.println("The loop in thread 2 is over"); } }).start(); Thread.sleep(200); System.out.println(counter); } } |
Code Walkthrough
- Two threads are created using anonymous Runnable classes.
- Both threads attempt to increment the shared variable counter 100,000 times.
- After both threads complete their execution, the final value of counter is printed.
Expected vs. Actual Output
- Expected Output: 200,000 (as each thread increments counter by 100,000).
- Actual Output: The final value of counter may be less than 200,000 due to race conditions.
5. Synchronization in Practice
To prevent race conditions, we can modify the code by adding a synchronized block:
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public class Main { public static int counter = 0; public static synchronized void incrementCounter() { counter++; } public static void main(String[] args) throws InterruptedException { new Thread(() -> { for (int i = 1; i <= 100000; i++) { incrementCounter(); } System.out.println("The loop in thread 1 is over"); }).start(); new Thread(() -> { for (int i = 1; i <= 100000; i++) { incrementCounter(); } System.out.println("The loop in thread 2 is over"); }).start(); Thread.sleep(200); System.out.println(counter); } } |
Explanation
Synchronized Method: The incrementCounter() method is marked as synchronized, ensuring that only one thread can access this method at a time.
Thread Safety: By using synchronization, we prevent both threads from incrementing counter simultaneously, ensuring the final value of counter will always be 200,000.
6. Conclusion
Synchronization plays a crucial role in Java’s concurrency model, ensuring thread safety when multiple threads interact with shared resources. By understanding and applying synchronization, developers can prevent issues such as race conditions and ensure data consistency in multithreaded environments.