Java Concurrency 简明教程

Java Concurrency - AtomicInteger Class

java.util.concurrent.atomic.AtomicInteger 类提供对底层 int 值的操作，该值可以原子地读写，还包含高级原子操作。AtomicInteger 支持对底层 int 变量的原子操作。它有 get 和 set 方法，就像对 volatile 变量进行读写一样。也就是说，set 与同一变量上的任何后续 get 都有 happens-before 关系。atomic compareAndSet 方法也有这些内存一致性特性。

A java.util.concurrent.atomic.AtomicInteger class provides operations on underlying int value that can be read and written atomically, and also contains advanced atomic operations. AtomicInteger supports atomic operations on underlying int variable. It have get and set methods that work like reads and writes on volatile variables. That is, a set has a happens-before relationship with any subsequent get on the same variable. The atomic compareAndSet method also has these memory consistency features.

AtomicInteger Methods

以下是 AtomicInteger 类中可用的重要方法的列表。

Following is the list of important methods available in the AtomicInteger class.

Sr.No.

Method & Description

1

public int addAndGet(int delta) Atomically adds the given value to the current value.

2

public boolean compareAndSet(int expect, int update) Atomically sets the value to the given updated value if the current value is same as the expected value.

3

public int decrementAndGet() Atomically decrements by one the current value.

4

public double doubleValue() Returns the value of the specified number as a double.

5

public float floatValue() Returns the value of the specified number as a float.

6

public int get() Gets the current value.

7

public int getAndAdd(int delta) Atomiclly adds the given value to the current value.

8

public int getAndDecrement() Atomically decrements by one the current value.

9

public int getAndIncrement() Atomically increments by one the current value.

10

public int getAndSet(int newValue) Atomically sets to the given value and returns the old value.

11

public int incrementAndGet() Atomically increments by one the current value.

12

public int intValue() Returns the value of the specified number as an int.

13

public void lazySet(int newValue) Eventually sets to the given value.

14

public long longValue() Returns the value of the specified number as a long.

15

public void set(int newValue) Sets to the given value.

16

public String toString() Returns the String representation of the current value.

17

public boolean weakCompareAndSet(int expect, int update) Atomically sets the value to the given updated value if the current value is same as the expected value.

Example

以下 TestThread 程序显示了在线程环境中计数器的非安全实现。

The following TestThread program shows a unsafe implementation of counter in thread based environment.

public class TestThread {

   static class Counter {
      private int c = 0;

      public void increment() {
         c++;
      }

      public int value() {
         return c;
      }
   }

   public static void main(final String[] arguments) throws InterruptedException {
      final Counter counter = new Counter();

      //1000 threads
      for(int i = 0; i < 1000 ; i++) {

         new Thread(new Runnable() {

            public void run() {
               counter.increment();
            }
         }).start();
      }
      Thread.sleep(6000);
      System.out.println("Final number (should be 1000): " + counter.value());
   }
}

这可能会根据计算机速度和线程交错产生以下结果。

This may produce the following result depending upon computer’s speed and thread interleaving.

Output

Final number (should be 1000): 1000

Example

import java.util.concurrent.atomic.AtomicInteger;

public class TestThread {

   static class Counter {
      private AtomicInteger c = new AtomicInteger(0);

      public void increment() {
         c.getAndIncrement();
      }

      public int value() {
         return c.get();
      }
   }

   public static void main(final String[] arguments) throws InterruptedException {
      final Counter counter = new Counter();

      //1000 threads
      for(int i = 0; i < 1000 ; i++) {

         new Thread(new Runnable() {
            public void run() {
               counter.increment();
            }
         }).start();
      }
      Thread.sleep(6000);
      System.out.println("Final number (should be 1000): " + counter.value());
   }
}

这将产生以下结果。

This will produce the following result.

Output

Final number (should be 1000): 1000