Demystifying the Java Memory Model: Mastering the Secrets Behind Multi-Threaded Programming

php Xiaobian Yuzai takes you to reveal the Java memory model and gain an in-depth understanding of the mysteries behind multi-threaded programming. Multi-threaded programming is an important skill in Java development, and understanding the memory model is crucial to writing efficient and reliable multi-threaded programs. Let's explore the Java memory model together and uncover the mystery of multi-threaded programming!

The main goal of JMM is to ensure the correctness and predictability of multi-threaded programs. It prevents data races and memory consistency issues by defining a set of rules to regulate thread access to shared memory. The basic principles of JMM include:

• Visibility: Modifications of shared variables by threads must be reflected in the visible range of other threads in a timely manner.
• Atomicity: Read and write operations on shared variables are atomic, that is, uninterruptible.
• Ordering: The order of thread access to shared variables must be consistent with the order of execution in the program.

In order to realize these basic principles, JMM introduces the following key concepts:

• Main memory (main memory): Main memory is the physical memory space shared by all threads.
• Working memory: Each thread has its own working memory, which stores a copy of the thread's private variables.
• cache coherence protocol: Cache The coherence protocol is a protocol used to ensure that data in the caches of multiple processors remains consistent.

When a thread modifies a shared variable, it writes the modified value to main memory. Other threads can get the latest value by reading the value in main memory. However, other threads may not see the modified value immediately due to the latency of the cache coherence protocol. To solve this problem, JMM introduces the concept of memory barrier. A memory barrier forces a thread to immediately write modified values ​​to main memory and ensures that other threads can see the modified values.

The Java language provides two keywords, synchronized and volatile, to achieve thread synchronization and visibility. The synchronized keyword can ensure that access to shared variables is atomic, and the volatile keyword can ensure that modifications to shared variables are visible.

Here is some demo code showing how to use the synchronized and volatile keywords to achieve thread synchronization and visibility:

class SharedCounter {
private int count = 0;

public synchronized void increment() {
count++;
}

public int getCount() {
return count;
}
}

public class Main {
public static void main(String[] args) {
SharedCounter counter = new SharedCounter();

Thread thread1 = new Thread(() -> {
for (int i = 0; i < 10000; i++) {
counter.increment();
}
});

Thread thread2 = new Thread(() -> {
for (int i = 0; i < 10000; i++) {
counter.increment();
}
});

thread1.start();
thread2.start();

try {
thread1.join();
thread2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}

System.out.println("Final count: " + counter.getCount());
}
}

In this example, we use the synchronized keyword to ensure that access to the count variable is atomic, thus avoiding data race issues.

class SharedCounter {
private volatile int count = 0;

public void increment() {
count++;
}

public int getCount() {
return count;
}
}

public class Main {
public static void main(String[] args) {
SharedCounter counter = new SharedCounter();

Thread thread1 = new Thread(() -> {
for (int i = 0; i < 10000; i++) {
counter.increment();
}
});

Thread thread2 = new Thread(() -> {
for (int i = 0; i < 10000; i++) {
System.out.println("Current count: " + counter.getCount());
}
});

thread1.start();
thread2.start();

try {
thread1.join();
thread2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

In this example, we use the volatile keyword to ensure that modifications to the count variable are visible, so that Thread 2 can see Thread 1's changes to ## in a timely manner #count Modification of variables.

A deep understanding of the Java memory model is critical to solving

problems in concurrent programming . By mastering the basic principles and key concepts of JMM, programmers can write more robust and predictable multi-threaded programs.

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