多線程併發一定比單線程快嗎？

很多時候我們都以為要想處理速度更快,那就多開幾個線程跑!

確實多線程在一定情況下比單線程更快。

下面的代碼演示串行和併發執行並累加操作的時間，請分析：下面的代碼併發執行一定比串行執行快嗎？當count的數量增加 1萬 -> 10萬 -> 100萬 -> 1000萬 -> 1億

public class ConcurrencyTest {
 private static final long count = 10000l;
 public static void main(String[] args) throws InterruptedException {
 for(int i=0; i<10; i++) {
 concurrency();
 serial();
 System.out.println("------------------------------------------");
 }
 }
 private static void concurrency() throws InterruptedException {
 long start = System.currentTimeMillis();
 Thread thread = new Thread(new Runnable() {
 @Override
 public void run() {
 int a = 0;
 for (long i = 0; i < count; i++) {
 a += 5;
 }
 }
 });
 thread.start();
 int b = 0;
 for (long i = 0; i < count; i++) {
 b--;
 }
 long time = System.currentTimeMillis() - start;
 thread.join();
 System.out.println("concurrency :" + time + "ms,b=" + b);
 }
 private static void serial() {
 long start = System.currentTimeMillis();
 int a = 0;
 for (long i = 0; i < count; i++) {
 a += 5;
 }
 int b = 0;
 for (long i = 0; i < count; i++) {
 b--; 

 }
 long time = System.currentTimeMillis() - start;
 System.out.println("serial:" + time + "ms,b=" + b + ",a=" + a);
 }
}

測試結果

這是count=1萬的結果，基本上串行要快些

併發 :1ms,b=-10000
串行:0ms,b=-10000,a=50000
------------------------------------------
併發 :0ms,b=-10000
串行:1ms,b=-10000,a=50000
------------------------------------------
併發 :0ms,b=-10000
串行:1ms,b=-10000,a=50000
------------------------------------------
併發 :0ms,b=-10000
串行:0ms,b=-10000,a=50000
------------------------------------------
併發 :1ms,b=-10000
串行:0ms,b=-10000,a=50000
------------------------------------------
併發 :1ms,b=-10000
串行:0ms,b=-10000,a=50000
------------------------------------------
併發 :0ms,b=-10000
串行:1ms,b=-10000,a=50000
------------------------------------------
併發 :0ms,b=-10000
串行:0ms,b=-10000,a=50000
------------------------------------------
併發 :1ms,b=-10000
串行:0ms,b=-10000,a=50000
------------------------------------------

這是count=10萬l的結果，基本上串行要快點點

併發 :4ms,b=-100000
串行:3ms,b=-100000,a=500000
------------------------------------------
併發 :0ms,b=-100000
串行:1ms,b=-100000,a=500000
------------------------------------------
併發 :1ms,b=-100000
串行:0ms,b=-100000,a=500000
------------------------------------------
併發 :0ms,b=-100000
串行:1ms,b=-100000,a=500000
------------------------------------------
併發 :1ms,b=-100000
串行:0ms,b=-100000,a=500000
------------------------------------------
併發 :0ms,b=-100000
串行:0ms,b=-100000,a=500000
------------------------------------------
併發 :1ms,b=-100000
串行:0ms,b=-100000,a=500000
------------------------------------------
併發 :0ms,b=-100000
串行:0ms,b=-100000,a=500000
------------------------------------------
併發 :0ms,b=-100000
串行:1ms,b=-100000,a=500000
------------------------------------------
併發 :0ms,b=-100000
串行:0ms,b=-100000,a=500000
------------------------------------------

這是count=100萬l的結果，基本上併發就要比串行要快了

併發 :5ms,b=-1000000
串行:5ms,b=-1000000,a=5000000
------------------------------------------
併發 :3ms,b=-1000000
串行:7ms,b=-1000000,a=5000000
------------------------------------------
併發 :1ms,b=-1000000
串行:7ms,b=-1000000,a=5000000 

------------------------------------------
併發 :1ms,b=-1000000
串行:2ms,b=-1000000,a=5000000
------------------------------------------
併發 :1ms,b=-1000000
串行:2ms,b=-1000000,a=5000000
------------------------------------------
併發 :2ms,b=-1000000
串行:2ms,b=-1000000,a=5000000
------------------------------------------
併發 :1ms,b=-1000000
串行:3ms,b=-1000000,a=5000000
------------------------------------------
併發 :1ms,b=-1000000
串行:2ms,b=-1000000,a=5000000
------------------------------------------
併發 :1ms,b=-1000000
串行:3ms,b=-1000000,a=5000000
------------------------------------------
併發 :1ms,b=-1000000
串行:2ms,b=-1000000,a=5000000
------------------------------------------

餘下的讀者可以自己去調試哦。

結論

1. 在數量比較小的時候，很明顯串行要比並發更快。
2. 因為數量較小的時候，併發會花費很多時間在線程調度上。

最後

需要面試題和架構資料的可以私信【面試題】或【架構】可獲取Java架構資源合集以及600道面試題+答案。

最後祝福所有遇到瓶疾且不知道怎麼辦的Java程序員們，在往後的工作與面試中一切順利。