Java Atomic Variables用法及代码示例

在多线程，共享实体主要会在以下情况下导致问题：并发性被合并。共享实体，例如，可变的对象或变量可能会被更改，这可能会导致程序的不一致或数据库。因此，在并发访问时处理共享实体变得至关重要。一个原子变量可以是这种情况下的替代方案之一。

Java提供原子类如AtomicInteger,AtomicLong,AtomicBoolean和AtomicReference。这些类的对象代表原子变量分别为 int、long、boolean 和对象引用。这些类包含以下方法。

设置(整数值)：设置为给定值
get():获取当前值
惰性设置(int值)：最终设置为给定值
CompareAndSet(int期望，int更新)：如果当前值 == 预期值，则自动将该值设置为给定的更新值
addAndGet(int delta):以原子方式将给定值添加到当前值
decrementAndGet():以原子方式将当前值减一

例子：

// Atomic Variable
AtomicInteger var;

需要原子变量
考虑下面的例子：

Java


class Counter extends Thread {
    // Counter Variable
    int count = 0;
    // method which would be called upon
    // the start of execution of a thread
    public void run()
    {
        int max = 1_000_00_000;
        // incrementing counter
        // total of max times
        for (int i = 0; i < max; i++) {
            count++;
        }
    }
}
public class UnSafeCounter {
    public static void main(String[] args)
        throws InterruptedException
    {
        // Instance of Counter Class
        Counter c = new Counter();
        // Defining Two different threads
        Thread first = new Thread(c, "First");
        Thread second = new Thread(c, "Second");
        // Threads start executing
        first.start();
        second.start();
        // main thread will wait for
        // both threads to get completed
        first.join();
        second.join();
        // Printing final value of count variable
        System.out.println(c.count);
    }
}

输出：

137754082

在单线程环境中，上述类只会给出预期的结果。但当涉及到多线程环境时，可能会导致结果不一致。发生这种情况是因为更新 “var” 是通过三个步骤完成的：读取、更新和写入。如果两个或更多线程尝试同时更新该值，则它可能无法正确更新。

这个问题可以使用 Lock and Synchronization 解决，但效率不高。

1.使用锁类比或同步：同步或锁定可以解决我们的问题，但它会损害时间效率或性能。首先，它要求资源和线程调度程序来控制锁。其次，当多个线程尝试获取锁时，只有一个线程获胜，其余线程被挂起或阻塞。挂起或阻塞线程会对性能产生巨大影响。

Java


import java.io.*;
import java.util.concurrent.locks.*;
class Counter extends Thread {
    // Counter Variable
    int count = 0;
    // method which would be called upon
    // the start of execution of a thread
    public synchronized void run()
    {
        int max = 1_000_00_000;
        // incrementing counter total of max times
        for (int i = 0; i < max; i++) {
            count++;
        }
    }
}
public class SynchronizedCounter {
    public static void main(String[] args)
        throws InterruptedException
    {
        // Instance of Counter Class
        Counter c = new Counter();
        // Defining Two different threads
        Thread first = new Thread(c, "First");
        Thread second = new Thread(c, "Second");
        // Threads start executing
        first.start();
        second.start();
        // main thread will wait for both
        // threads to complete execution
        first.join();
        second.join();
        // Printing final value of count variable
        System.out.println(c.count);
    }
}

输出：

200000000

2.使用原子变量：

Java


import java.util.concurrent.atomic.AtomicInteger;
class Counter extends Thread {
    // Atomic counter Variable
    AtomicInteger count;
    // Constructor of class
    Counter()
    {
        count = new AtomicInteger();
    }
    // method which would be called upon
    // the start of execution of a thread
    public void run()
    {
        int max = 1_000_00_000;
        // incrementing counter total of max times
        for (int i = 0; i < max; i++) {
            count.addAndGet(1);
        }
    }
}
public class AtomicCounter {
    public static void main(String[] args)
        throws InterruptedException
    {
        // Instance of Counter Class
        Counter c = new Counter();
        // Defining Two different threads
        Thread first = new Thread(c, "First");
        Thread second = new Thread(c, "Second");
        // Threads start executing
        first.start();
        second.start();
        // main thread will wait for both
        // threads to complete execution
        first.join();
        second.join();
        // Printing final value of count variable
        System.out.println(c.count);
    }
}

输出：

200000000

相关用法

注：本文由纯净天空筛选整理自NiravTalaviya大神的英文原创作品 Atomic Variables in Java with Examples。非经特殊声明，原始代码版权归原作者所有，本译文未经允许或授权，请勿转载或复制。