How to Fix: Java Multithreading Error: Race Condition

How to solve: Java multithreading error: race condition

Introduction:
In Java multithreaded programming, race conditions are a common problem. It refers to the fact that when multiple threads access and modify shared data at the same time, it may lead to indeterminate program results. This article introduces the concept of race conditions and provides some methods for resolving race conditions.

1. What are competition conditions?
A race condition means that when multiple threads are executing code, they read and write shared data, but the order and time of execution cannot be determined, resulting in uncertainty in the results. Specifically, the following conditions need to be met to generate a race condition:

1. Multiple threads access shared data at the same time.
2. At least one thread writes shared data.
3. The execution order and time between threads cannot be determined.

2. Examples of race conditions
The following example code shows a classic race condition problem: multiple threads increment a shared variable at the same time.

public class RaceConditionDemo {
    private static int count = 0;
    
    public static void increment() {
        count++;
    }
    
    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                increment();
            }
        });
        
        Thread t2 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                increment();
            }
        });
        
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        
        System.out.println("Count: " + count);
    }
}

The above code creates two threads t1 and t2, which increment the shared variable count. However, since the execution order and timing between threads cannot be determined, a race condition may occur when two threads are performing increment operations at the same time. Without the correct synchronization mechanism to ensure atomicity of the operation, the final result may be less than the expected value of 2000.

3. Methods to solve competition conditions
To solve the problem of competition conditions in Java multi-threading, you can use the following methods:

1. Use the synchronized keyword
   The synchronized keyword ensures that only one thread can enter a code block or method marked synchronized at the same time. The above code can be modified as follows:

public class SynchronizedDemo {
    private static int count = 0;
    
    public synchronized static void increment() {
        count++;
    }
    
    // 省略其他代码
    
}

By marking the increment() method as synchronized, we can ensure that only one thread can execute this method at any time. This approach can effectively eliminate race conditions and ensure the atomicity of operations.

2. Using the Lock interface
   In addition to using the synchronized keyword, we can also use the Lock interface to control access to shared resources. The following is the improved sample code:

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class LockDemo {
    private static int count = 0;
    private static Lock lock = new ReentrantLock();
    
    public static void increment() {
        lock.lock();
        try {
            count++;
        } finally {
            lock.unlock();
        }
    }
    
    // 省略其他代码
    
}

In this example, we create a Lock object to control access to shared variables by calling the lock() and unlock() methods. Using the Lock interface can provide finer-grained control and is more flexible than synchronized.

3. Using atomic classes
   Java provides some atomic classes, such as AtomicInteger, which can be used to implement thread-safe increment operations. Here is an improved example code using AtomicInteger:

import java.util.concurrent.atomic.AtomicInteger;

public class AtomicDemo {
    private static AtomicInteger count = new AtomicInteger(0);
    
    public static void increment() {
        count.incrementAndGet();
    }
    
    // 省略其他代码
    
}

Using the AtomicInteger class ensures that the incrementing operation on count is atomic and will not be affected by race conditions.

Summary:
Race conditions are a common problem in Java multi-threaded programming, which may lead to uncertainty in the results of the program. In order to solve the problem of race conditions, we can use methods such as synchronized keyword, Lock interface or atomic class to ensure that access to shared resources is thread-safe. By using these techniques appropriately, we can reduce problems caused by race conditions and improve the performance and reliability of multi-threaded programs.

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