10. Java并发编程的魅力之并发一定比顺序编程速度更快么？

并发编程一定比顺序编程速度更快么？
我们来看下案例

public class ConcurrencyTest {

	private static final long count = 10001;

	public static void main(String[] args) throws InterruptedException {
	    serial();//顺序编程
		concurrency();//并发编程
	}

	/**
	 * 顺序编程
	 **/
	private static void serial() {
		long start = System.currentTimeMillis();
		int a = 0;
		for (int i = 0; i < count; i++) {
			a += 5;
		}
		int b = 0;
		for (int i = 0; i < count; i++) {
			b--;
		}
		long time = System.currentTimeMillis() - start;
		System.out.println("Serial:" + time + "ms,b=" + b + ",a=" + a);
	}

	/**
	 * 并发编程
	 */
	private static void concurrency() throws InterruptedException {
		long start = System.currentTimeMillis();
		Thread thread = new Thread(new Runnable() {

			@Override
			public void run() {
				// TODO Auto-generated method stub
				int a = 0;
				for (int i = 0; i < count; i++) {
					a += 5;
				}
			}
		});
		thread.start();
		int b = 0;
		for (int i = 0; i < count; i++) {
			b--;
		}
		long time = System.currentTimeMillis() - start;
		thread.join();
		System.out.println("Concurrency :" + time + "ms,b=" + b);
	}
}

输出结果：

Serial:0ms,b=-10001,a=50005
Concurrency :1ms,b=-10001

该例子测试后可以发现,并发编程比顺序编程速度反而更慢

为什么呢？

• 顺序编程只需要程序执行时间
• 并发编程除了程序执行需要时间，两个线程之间切换也需要时间

因此我们常说：没有阻塞的任务使用多线程就没有意义