Java 中的原子变量示例
原文:https://www . geesforgeks . org/atomic-variables-in-Java-with-examples/
在 多线程 中，共享实体在合并并发时大多会导致问题。共享实体，例如，可变的对象或变量，可能会被更改，这可能会导致程序或数据库的不一致。因此，处理并发访问的共享实体变得至关重要。在这种情况下，原子变量可以是备选方案之一。
Java 提供原子类，如 AtomicInteger 、 AtomicLong 、 AtomicBoolean 和 AtomicReference 。这些类的对象分别表示 int、long、boolean 和 object reference 的原子变量。这些类包含以下方法。
- *设置(int 值):*设置给定值
- *get():* 获取当前值
- *lazySet(int 值):*最终设置为给定值
- *比较数据集(int expect，int update):* 如果当前值==期望值，则自动将该值设置为给定的更新值
- *addAndGet(int delta):* 自动将给定值加到当前值上
- *递减和获取():*自动将当前值递减 1
*示例:*
// Atomic Variable
AtomicInteger var;
*需要原子变量* 考虑下面的例子:
Java 语言(一种计算机语言，尤用于创建网站)
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);
}
}
**Output:
137754082
```**
**在单线程环境中，上述类只会给出预期的结果。但是当涉及多线程环境时，可能会导致不一致的结果。发生这种情况是因为更新“var”分三步完成:读取、更新和写入。如果两个或多个线程试图同时更新该值，则它可能无法正确更新。**
**使用[锁定和同步](https://www.geeksforgeeks.org/lock-framework-vs-thread-synchronization-in-java/)可以解决这个问题，但是效率不高。**
****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器控制锁。第二，当多个线程试图获取锁时，只有一个线程获胜，其余的线程被挂起或阻塞。挂起或阻塞线程会对性能产生巨大影响。**
## **Java 语言(一种计算机语言，尤用于创建网站)**
```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);
}
}
**Output:
200000000
```**
****2。使用原子变量:****
## **Java 语言(一种计算机语言，尤用于创建网站)**
```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);
}
}
**Output:
java
200000000**
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