Java 空检查性能

Question

我想知道在 java 中通过直接比较或使用 Objects.isNull() 方法检查对象是否为 null 是否有任何显着差异。

public class Test {

  public final static Long ITERATIONS = 100000000L; 

  @Test
  public void noFnCalls() {
    balong startTime = System.currentTimeMillis();
    Object x = new Object();
    Long i;
    for (i = 0L; i < ITERATIONS; i++) {
      boolean t = x == null;
    }
    long estimatedTime = System.currentTimeMillis() - startTime;
    System.out.println("noFnCalls ellapsed time: " + estimatedTime);
  }

  @Test
  public void withFnCalls() {
    long startTime = System.currentTimeMillis();
    Object x = new Object();
    Long i;
    for (i = 0L; i < ITERATIONS; i++) {
      boolean t = Objects.isNull(x);
    }
    long estimatedTime = System.currentTimeMillis() - startTime;
    System.out.println("withFnCalls ellapsed time: " + estimatedTime);
  }
}

令人惊讶的是，至少对我而言，完成“noFnCalls”总是需要更多时间。我期待的结果恰恰相反，因为它会导致使用堆栈进行方法调用。

这是输出：（显然，每次都会更改，但“noFnCalls”总是更高）

noFnCalls ellapsed time: 583

withFnCalls ellapsed time: 463

为什么会产生这个？

Answer 1

您看到的结果可能是由于运行 "noFnCalls" 首先，没有在测试和测量之前引入适当的预热。

我明白了：

withFnCalls ellapsed time: 444
noFnCalls ellapsed time: 471
withFnCalls ellapsed time: 334
noFnCalls ellapsed time: 331
withFnCalls ellapsed time: 330
noFnCalls ellapsed time: 325
withFnCalls ellapsed time: 331
noFnCalls ellapsed time: 326
withFnCalls ellapsed time: 326
noFnCalls ellapsed time: 328

正在使用

import java.util.Objects;

public class Test {

  public final static Long ITERATIONS = 100000000L; 

  public static void main(String args[]) {
    withFnCalls();
    noFnCalls();
    withFnCalls();
    noFnCalls();
    withFnCalls();
    noFnCalls();
    withFnCalls();
    noFnCalls();
    withFnCalls();
    noFnCalls();
  }
  public static void noFnCalls() {
    long startTime = System.currentTimeMillis();
    Object x = new Object();
    Long i;
    for (i = 0L; i < ITERATIONS; i++) {
      boolean t = x == null;
    }
    long estimatedTime = System.currentTimeMillis() - startTime;
    System.out.println("noFnCalls ellapsed time: " + estimatedTime);
  }

  public static void withFnCalls() {
    long startTime = System.currentTimeMillis();
    Object x = new Object();
    Long i;
    for (i = 0L; i < ITERATIONS; i++) {
      boolean t = Objects.isNull(x);
    }
    long estimatedTime = System.currentTimeMillis() - startTime;
    System.out.println("withFnCalls ellapsed time: " + estimatedTime);
  }
}

和

withFnCalls ellapsed time: 3618
noFnCalls ellapsed time: 3361
withFnCalls ellapsed time: 3445
noFnCalls ellapsed time: 3278
withFnCalls ellapsed time: 3350
noFnCalls ellapsed time: 3292
withFnCalls ellapsed time: 3309
noFnCalls ellapsed time: 3262
withFnCalls ellapsed time: 3293
noFnCalls ellapsed time: 3261

如果我增加到 1000000000L 次迭代。这是通过 Java 9 64 位服务器 jvm，构建 9+181，由 Oracle，运行在 Windows 10 上使用一台装有 Intel i5-2600 cpu 的机器完成的。

正如其他人所说，微基准测试很难很多不同的因素都会影响结果。您不应该通过此类测试得出结论。这种测试并不能说明什么 - 任何差异都容易在彼此如此接近的噪声测量代码中丢失。

java 中有关微基准测试的强制性推荐线程：How do I write a correct micro-benchmark in Java?。

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