[英]Why declaration inside loops seems to be faster then declaration outside loops in Java?
當我檢查此問題時,我不敢相信,因此我進行了測試,似乎是對的。 循環內聲明似乎比循環外聲明快。 有人可以解釋為什么嗎?
這是我的測試代碼:
public class CycleTest {
/**
* @param args the command line arguments
*/
public static void main(String[] args) {
long iterations = 1000000;
warmUp(iterations);
System.out.println("Cycle1");
double individualTime = getAverageTimePerIterationc1(iterations);
iterations = 1000;
double totalTime = getTotalTimec1(iterations);
System.out.println("ns/iteration: " + individualTime);
System.out.println("Total time for " + iterations + " runs: " + totalTime);
System.out.println("Cycle2");
iterations = 1000000;
double individualTime1 = getAverageTimePerIterationc2(iterations);
iterations = 1000;
double totalTime1 = getTotalTimec2(iterations);
System.out.println("ns/iteration: " + individualTime1);
System.out.println("Total time for " + iterations + " runs: " + totalTime1);
}
public static void warmUp(long iterations) {
System.out.println("Starting warmup");
for (int i = 0; i < iterations; i++) {
runCycles();
runCycles1();
}
}
public static double getAverageTimePerIterationc1(long iterations) {
// test
System.out.println("Starting individual time test");
long timeTaken = 0;
for (int i = 0; i < iterations; i++) {
long startTime = System.nanoTime();
runCycles();
timeTaken += System.nanoTime() - startTime;
}
return (double) timeTaken / iterations;
}
public static long getTotalTimec1(long iterations) {
// test
System.out.println("Starting total time test");
long timeTaken = 0;
for (int i = 0; i < iterations; i++) {
long startTime = System.nanoTime();
runCycles();
timeTaken += System.nanoTime() - startTime;
}
return timeTaken;
}
public static double getAverageTimePerIterationc2(long iterations) {
// test
System.out.println("Starting individual time test");
long timeTaken = 0;
for (int i = 0; i < iterations; i++) {
long startTime = System.nanoTime();
runCycles1();
timeTaken += System.nanoTime() - startTime;
}
return (double) timeTaken / iterations;
}
public static long getTotalTimec2(long iterations) {
// test
System.out.println("Starting total time test");
long timeTaken = 0;
for (int i = 0; i < iterations; i++) {
long startTime = System.nanoTime();
runCycles1();
timeTaken += System.nanoTime() - startTime;
}
return timeTaken;
}
private static void runCycles() {
double intermediateResult;
for (int i = 0; i < 1000; i++) {
intermediateResult = i;
intermediateResult += 1;
}
}
private static void runCycles1() {
for (int i = 0; i < 1000; i++) {
double intermediateResult = i;
intermediateResult += 1;
}
}
}
額外的新信息:我已經在Windows計算機上的JDK 1.6.0_27上運行了它。
它們生成相同的代碼。 這兩種方法:
private static void runCycles1() {
double intermediateResult;
for (int i = 0; i < 1000; i++) {
intermediateResult = i;
}
}
private static void runCycles2() {
for (int i = 0; i < 1000; i++) {
double intermediateResult = i;
}
}
在Java 8(jdk1.8.0_51)中生成以下字節碼:
runCycles1() runCycles2()
Code: Code:
0: iconst_0 0: iconst_0 i = 0
1: istore_2 1: istore_0
2: goto 11 2: goto 11 goto 11
5: iload_2 5: iload_0 intermediateResult = (double)i
6: i2d 6: i2d
7: dstore_0 7: dstore_1
8: iinc 2, 1 8: iinc 0, 1 i++
11: iload_2 11: iload_0 if (i < 1000) goto 5
12: sipush 1000 12: sipush 1000
15: if_icmplt 5 15: if_icmplt 5
18: return 18: return return
實際的聲明不會生成任何代碼,因此如果您發現性能有所不同,我會感到驚訝。
如此測試:
for (int j = 0; j < 10; j++) {
long t1 = System.nanoTime();
for (int i = 0; i < 1000_000_000; i++)
runCycles1();
long t2 = System.nanoTime();
for (int i = 0; i < 1000_000_000; i++)
runCycles2();
long t3 = System.nanoTime();
System.out.printf("%d %d%n", t2 - t1, t3 - t2);
}
並得到:
4250095 2020120
4067898 0
3904236 0
0 0
0 0
0 0
0 0
0 0
0 0
0 0
他們最終都編譯成空!!!
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