【java8】为java8的foreach正名
首先为自己没有经过严格测试得出的错误结论感到抱歉，原博文，测试完感觉自己发现了一个新bug，后来思前想后觉得不应该是这样的，如果效率差的这么多，jdk的开发人员会不去优化它吗，但是怎么重复测试任然得到一样的结果，非常疑惑。
我觉得应该是测试方法出问题了，可是怎么也想不到原因，后来了解到jmh，深入研究了一番，觉得jmh的测试值得借鉴，jmh在测试的时候都会先经过预热几遍要测试的代码，示例如下：
public static void main(String[] args) throws RunnerException {
Options opt = new OptionsBuilder()
.include(JMHSample_02_BenchmarkModes.class.getSimpleName())
.warmupIterations(5)
.measurementIterations(5)
.forks(1)
.build();
new Runner(opt).run();
}
我觉得这是有必要的，而我的测试恰恰缺少了这个步骤，随后我在测试中应用了预热处理（均是3次预热，5次运行取平均值），得出的以下结论。
首先是上个测试中被冤枉的java8的foreach循环，测试代码：
package yiwangzhibujian.jmh.test; import org.openjdk.jmh.annotations.*;
import org.openjdk.jmh.runner.Runner;
import org.openjdk.jmh.runner.RunnerException;
import org.openjdk.jmh.runner.options.Options;
import org.openjdk.jmh.runner.options.OptionsBuilder; import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.TimeUnit; @State(Scope.Thread)
@BenchmarkMode(Mode.SingleShotTime)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
public class J8Loop {
public static Map<Integer,List<Dog>> dogMap=new HashMap<>();
static {
dogMap.put(10, getDogs(10));
dogMap.put(100, getDogs(100));
dogMap.put(1000, getDogs(1000));
dogMap.put(10000, getDogs(10000));
dogMap.put(100000, getDogs(100000));
dogMap.put(1000000, getDogs(1000000));
} private static List<Dog> getDogs(int num) {
List<Dog> dogs=new ArrayList<>();
for(int i=0;i<num;i++){
dogs.add(new Dog(i,"dog"+i));
}
return dogs;
} private void loop(List<Dog> list) {
list.forEach(dog->{
dog.hashCode();
});
} @Benchmark
@OperationsPerInvocation(10)
public void measureWrong_10() {
List<Dog> list = dogMap.get(10);
loop(list);
} @Benchmark
@OperationsPerInvocation(100)
public void measureWrong_100() {
List<Dog> list = dogMap.get(100);
loop(list);
} @Benchmark
@OperationsPerInvocation(1000)
public void measureWrong_1000() {
List<Dog> list = dogMap.get(1000);
loop(list);
} @Benchmark
@OperationsPerInvocation(10000)
public void measureWrong_10000() {
List<Dog> list = dogMap.get(10000);
loop(list);
} @Benchmark
@OperationsPerInvocation(100000)
public void measureWrong_100000() {
List<Dog> list = dogMap.get(100000);
loop(list);
} @Benchmark
@OperationsPerInvocation(1000000)
public void measureWrong_1000000() {
List<Dog> list = dogMap.get(1000000);
loop(list);
} /*
* ============================== HOW TO RUN THIS TEST:
* ====================================
*
* You might notice the larger the repetitions count, the lower the
* "perceived" cost of the operation being measured. Up to the point we do
* each addition with 1/20 ns, well beyond what hardware can actually do.
*
* This happens because the loop is heavily unrolled/pipelined, and the
* operation to be measured is hoisted from the loop. Morale: don't overuse
* loops, rely on JMH to get the measurement right.
*
* You can run this test:
*
* a) Via the command line: $ mvn clean install $ java -jar
* target/benchmarks.jar JMHSample_11 -wi 5 -i 5 -f 1 (we requested 5
* warmup/measurement iterations, single fork)
*
* b) Via the Java API: (see the JMH homepage for possible caveats when
* running from IDE: http://openjdk.java.net/projects/code-tools/jmh/)
*/ public static void main(String[] args) throws RunnerException {
Options opt = new OptionsBuilder()
.include(J8Loop.class.getSimpleName())
.warmupIterations(2)
.measurementIterations(2).forks(1).build(); new Runner(opt).run();
} }
class Dog{
private int age;
private String name;
public Dog(int age, String name) {
super();
this.age = age;
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "Dog [age=" + age + ", name=" + name + "]";
}
}
测试结果：
# JMH version: 1.19
# VM version: JDK 1.8.0_51, VM 25.51-b03
# VM invoker: D:\soft\java\java8\jre\bin\java.exe
# VM options: -Dfile.encoding=UTF-8
# Warmup: 3 iterations, single-shot each
# Measurement: 5 iterations, single-shot each
# Timeout: 10 min per iteration
# Threads: 1 thread
# Benchmark mode: Single shot invocation time
# Benchmark: yiwangzhibujian.jmh.test.J8Loop.measureWrong_10 # Run progress: 0.00% complete, ETA 00:00:00
# Fork: 1 of 1
# Warmup Iteration 1: 85.205 ms/op
# Warmup Iteration 2: 0.013 ms/op
# Warmup Iteration 3: 0.012 ms/op
Iteration 1: 0.010 ms/op
Iteration 2: 0.013 ms/op
Iteration 3: 0.009 ms/op
Iteration 4: 0.010 ms/op
Iteration 5: 0.011 ms/op Result "yiwangzhibujian.jmh.test.J8Loop.measureWrong_10":
N = 5
mean = 0.010 ±(99.9%) 0.006 ms/op Histogram, ms/op:
[0.008, 0.009) = 0
[0.009, 0.009) = 1
[0.009, 0.010) = 0
[0.010, 0.010) = 2
[0.010, 0.011) = 0
[0.011, 0.011) = 1
[0.011, 0.012) = 0
[0.012, 0.012) = 0
[0.012, 0.013) = 0
[0.013, 0.013) = 0
[0.013, 0.014) = 1 Percentiles, ms/op:
p(0.0000) = 0.009 ms/op
p(50.0000) = 0.010 ms/op
p(90.0000) = 0.013 ms/op
p(95.0000) = 0.013 ms/op
p(99.0000) = 0.013 ms/op
p(99.9000) = 0.013 ms/op
p(99.9900) = 0.013 ms/op
p(99.9990) = 0.013 ms/op
p(99.9999) = 0.013 ms/op
p(100.0000) = 0.013 ms/op # JMH version: 1.19
# VM version: JDK 1.8.0_51, VM 25.51-b03
# VM invoker: D:\soft\java\java8\jre\bin\java.exe
# VM options: -Dfile.encoding=UTF-8
# Warmup: 3 iterations, single-shot each
# Measurement: 5 iterations, single-shot each
# Timeout: 10 min per iteration
# Threads: 1 thread
# Benchmark mode: Single shot invocation time
# Benchmark: yiwangzhibujian.jmh.test.J8Loop.measureWrong_100 # Run progress: 16.67% complete, ETA 00:00:10
# Fork: 1 of 1
# Warmup Iteration 1: 81.395 ms/op
# Warmup Iteration 2: 0.046 ms/op
# Warmup Iteration 3: 0.070 ms/op
Iteration 1: 0.040 ms/op
Iteration 2: 0.065 ms/op
Iteration 3: 0.023 ms/op
Iteration 4: 0.025 ms/op
Iteration 5: 0.024 ms/op Result "yiwangzhibujian.jmh.test.J8Loop.measureWrong_100":
N = 5
mean = 0.035 ±(99.9%) 0.069 ms/op Histogram, ms/op:
[0.020, 0.025) = 3
[0.025, 0.030) = 0
[0.030, 0.035) = 0
[0.035, 0.040) = 1
[0.040, 0.045) = 0
[0.045, 0.050) = 0
[0.050, 0.055) = 0
[0.055, 0.060) = 0
[0.060, 0.065) = 1 Percentiles, ms/op:
p(0.0000) = 0.023 ms/op
p(50.0000) = 0.025 ms/op
p(90.0000) = 0.065 ms/op
p(95.0000) = 0.065 ms/op
p(99.0000) = 0.065 ms/op
p(99.9000) = 0.065 ms/op
p(99.9900) = 0.065 ms/op
p(99.9990) = 0.065 ms/op
p(99.9999) = 0.065 ms/op
p(100.0000) = 0.065 ms/op # JMH version: 1.19
# VM version: JDK 1.8.0_51, VM 25.51-b03
# VM invoker: D:\soft\java\java8\jre\bin\java.exe
# VM options: -Dfile.encoding=UTF-8
# Warmup: 3 iterations, single-shot each
# Measurement: 5 iterations, single-shot each
# Timeout: 10 min per iteration
# Threads: 1 thread
# Benchmark mode: Single shot invocation time
# Benchmark: yiwangzhibujian.jmh.test.J8Loop.measureWrong_1000 # Run progress: 33.33% complete, ETA 00:00:08
# Fork: 1 of 1
# Warmup Iteration 1: 85.176 ms/op
# Warmup Iteration 2: 0.201 ms/op
# Warmup Iteration 3: 0.183 ms/op
Iteration 1: 0.195 ms/op
Iteration 2: 0.184 ms/op
Iteration 3: 0.566 ms/op
Iteration 4: 0.101 ms/op
Iteration 5: 0.091 ms/op Result "yiwangzhibujian.jmh.test.J8Loop.measureWrong_1000":
N = 5
mean = 0.227 ±(99.9%) 0.751 ms/op Histogram, ms/op:
[0.000, 0.050) = 0
[0.050, 0.100) = 1
[0.100, 0.150) = 1
[0.150, 0.200) = 2
[0.200, 0.250) = 0
[0.250, 0.300) = 0
[0.300, 0.350) = 0
[0.350, 0.400) = 0
[0.400, 0.450) = 0
[0.450, 0.500) = 0
[0.500, 0.550) = 0
[0.550, 0.600) = 1 Percentiles, ms/op:
p(0.0000) = 0.091 ms/op
p(50.0000) = 0.184 ms/op
p(90.0000) = 0.566 ms/op
p(95.0000) = 0.566 ms/op
p(99.0000) = 0.566 ms/op
p(99.9000) = 0.566 ms/op
p(99.9900) = 0.566 ms/op
p(99.9990) = 0.566 ms/op
p(99.9999) = 0.566 ms/op
p(100.0000) = 0.566 ms/op # JMH version: 1.19
# VM version: JDK 1.8.0_51, VM 25.51-b03
# VM invoker: D:\soft\java\java8\jre\bin\java.exe
# VM options: -Dfile.encoding=UTF-8
# Warmup: 3 iterations, single-shot each
# Measurement: 5 iterations, single-shot each
# Timeout: 10 min per iteration
# Threads: 1 thread
# Benchmark mode: Single shot invocation time
# Benchmark: yiwangzhibujian.jmh.test.J8Loop.measureWrong_10000 # Run progress: 50.00% complete, ETA 00:00:06
# Fork: 1 of 1
# Warmup Iteration 1: 85.345 ms/op
# Warmup Iteration 2: 0.698 ms/op
# Warmup Iteration 3: 0.857 ms/op
Iteration 1: 0.866 ms/op
Iteration 2: 0.778 ms/op
Iteration 3: 0.871 ms/op
Iteration 4: 1.047 ms/op
Iteration 5: 0.306 ms/op Result "yiwangzhibujian.jmh.test.J8Loop.measureWrong_10000":
N = 5
mean = 0.774 ±(99.9%) 1.074 ms/op Histogram, ms/op:
[0.300, 0.350) = 1
[0.350, 0.400) = 0
[0.400, 0.450) = 0
[0.450, 0.500) = 0
[0.500, 0.550) = 0
[0.550, 0.600) = 0
[0.600, 0.650) = 0
[0.650, 0.700) = 0
[0.700, 0.750) = 0
[0.750, 0.800) = 1
[0.800, 0.850) = 0
[0.850, 0.900) = 2
[0.900, 0.950) = 0
[0.950, 1.000) = 0
[1.000, 1.050) = 1 Percentiles, ms/op:
p(0.0000) = 0.306 ms/op
p(50.0000) = 0.866 ms/op
p(90.0000) = 1.047 ms/op
p(95.0000) = 1.047 ms/op
p(99.0000) = 1.047 ms/op
p(99.9000) = 1.047 ms/op
p(99.9900) = 1.047 ms/op
p(99.9990) = 1.047 ms/op
p(99.9999) = 1.047 ms/op
p(100.0000) = 1.047 ms/op # JMH version: 1.19
# VM version: JDK 1.8.0_51, VM 25.51-b03
# VM invoker: D:\soft\java\java8\jre\bin\java.exe
# VM options: -Dfile.encoding=UTF-8
# Warmup: 3 iterations, single-shot each
# Measurement: 5 iterations, single-shot each
# Timeout: 10 min per iteration
# Threads: 1 thread
# Benchmark mode: Single shot invocation time
# Benchmark: yiwangzhibujian.jmh.test.J8Loop.measureWrong_100000 # Run progress: 66.67% complete, ETA 00:00:04
# Fork: 1 of 1
# Warmup Iteration 1: 96.430 ms/op
# Warmup Iteration 2: 6.303 ms/op
# Warmup Iteration 3: 1.515 ms/op
Iteration 1: 1.337 ms/op
Iteration 2: 1.260 ms/op
Iteration 3: 1.279 ms/op
Iteration 4: 1.210 ms/op
Iteration 5: 1.107 ms/op Result "yiwangzhibujian.jmh.test.J8Loop.measureWrong_100000":
N = 5
mean = 1.238 ±(99.9%) 0.334 ms/op Histogram, ms/op:
[1.100, 1.125) = 1
[1.125, 1.150) = 0
[1.150, 1.175) = 0
[1.175, 1.200) = 0
[1.200, 1.225) = 1
[1.225, 1.250) = 0
[1.250, 1.275) = 1
[1.275, 1.300) = 1
[1.300, 1.325) = 0
[1.325, 1.350) = 1
[1.350, 1.375) = 0
[1.375, 1.400) = 0 Percentiles, ms/op:
p(0.0000) = 1.107 ms/op
p(50.0000) = 1.260 ms/op
p(90.0000) = 1.337 ms/op
p(95.0000) = 1.337 ms/op
p(99.0000) = 1.337 ms/op
p(99.9000) = 1.337 ms/op
p(99.9900) = 1.337 ms/op
p(99.9990) = 1.337 ms/op
p(99.9999) = 1.337 ms/op
p(100.0000) = 1.337 ms/op # JMH version: 1.19
# VM version: JDK 1.8.0_51, VM 25.51-b03
# VM invoker: D:\soft\java\java8\jre\bin\java.exe
# VM options: -Dfile.encoding=UTF-8
# Warmup: 3 iterations, single-shot each
# Measurement: 5 iterations, single-shot each
# Timeout: 10 min per iteration
# Threads: 1 thread
# Benchmark mode: Single shot invocation time
# Benchmark: yiwangzhibujian.jmh.test.J8Loop.measureWrong_1000000 # Run progress: 83.33% complete, ETA 00:00:02
# Fork: 1 of 1
# Warmup Iteration 1: 155.436 ms/op
# Warmup Iteration 2: 16.059 ms/op
# Warmup Iteration 3: 10.600 ms/op
Iteration 1: 10.347 ms/op
Iteration 2: 10.207 ms/op
Iteration 3: 11.790 ms/op
Iteration 4: 10.717 ms/op
Iteration 5: 10.128 ms/op Result "yiwangzhibujian.jmh.test.J8Loop.measureWrong_1000000":
N = 5
mean = 10.638 ±(99.9%) 2.628 ms/op Histogram, ms/op:
[10.000, 10.125) = 0
[10.125, 10.250) = 2
[10.250, 10.375) = 1
[10.375, 10.500) = 0
[10.500, 10.625) = 0
[10.625, 10.750) = 1
[10.750, 10.875) = 0
[10.875, 11.000) = 0
[11.000, 11.125) = 0
[11.125, 11.250) = 0
[11.250, 11.375) = 0
[11.375, 11.500) = 0
[11.500, 11.625) = 0
[11.625, 11.750) = 0
[11.750, 11.875) = 1 Percentiles, ms/op:
p(0.0000) = 10.128 ms/op
p(50.0000) = 10.347 ms/op
p(90.0000) = 11.790 ms/op
p(95.0000) = 11.790 ms/op
p(99.0000) = 11.790 ms/op
p(99.9000) = 11.790 ms/op
p(99.9900) = 11.790 ms/op
p(99.9990) = 11.790 ms/op
p(99.9999) = 11.790 ms/op
p(100.0000) = 11.790 ms/op # Run complete. Total time: 00:00:13 Benchmark Mode Cnt Score Error Units
J8Loop.measureWrong_10 ss 5 0.010 ± 0.006 ms/op
J8Loop.measureWrong_100 ss 5 0.035 ± 0.069 ms/op
J8Loop.measureWrong_1000 ss 5 0.227 ± 0.751 ms/op
J8Loop.measureWrong_10000 ss 5 0.774 ± 1.074 ms/op
J8Loop.measureWrong_100000 ss 5 1.238 ± 0.334 ms/op
J8Loop.measureWrong_1000000 ss 5 10.638 ± 2.628 ms/op
其中最终的结论是：
Benchmark Mode Cnt Score Error Units
J8Loop.measureWrong_10 ss 5 0.010 ± 0.006 ms/op
J8Loop.measureWrong_100 ss 5 0.035 ± 0.069 ms/op
J8Loop.measureWrong_1000 ss 5 0.227 ± 0.751 ms/op
J8Loop.measureWrong_10000 ss 5 0.774 ± 1.074 ms/op
J8Loop.measureWrong_100000 ss 5 1.238 ± 0.334 ms/op
J8Loop.measureWrong_1000000 ss 5 10.638 ± 2.628 ms/op
然后是表现最好的增强for循环，测试代码：
package yiwangzhibujian.jmh.test; import org.openjdk.jmh.annotations.*;
import org.openjdk.jmh.runner.Runner;
import org.openjdk.jmh.runner.RunnerException;
import org.openjdk.jmh.runner.options.Options;
import org.openjdk.jmh.runner.options.OptionsBuilder; import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.TimeUnit; @State(Scope.Thread)
@BenchmarkMode(Mode.SingleShotTime)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
public class J8Loop {
public static Map<Integer,List<Dog>> dogMap=new HashMap<>();
static {
dogMap.put(10, getDogs(10));
dogMap.put(100, getDogs(100));
dogMap.put(1000, getDogs(1000));
dogMap.put(10000, getDogs(10000));
dogMap.put(100000, getDogs(100000));
dogMap.put(1000000, getDogs(1000000));
} private static List<Dog> getDogs(int num) {
List<Dog> dogs=new ArrayList<>();
for(int i=0;i<num;i++){
dogs.add(new Dog(i,"dog"+i));
}
return dogs;
} private void loop(List<Dog> list) {
for(Dog dog:list){
dog.hashCode();
}
} @Benchmark
@OperationsPerInvocation(10)
public void measureWrong_10() {
List<Dog> list = dogMap.get(10);
loop(list);
} @Benchmark
@OperationsPerInvocation(100)
public void measureWrong_100() {
List<Dog> list = dogMap.get(100);
loop(list);
} @Benchmark
@OperationsPerInvocation(1000)
public void measureWrong_1000() {
List<Dog> list = dogMap.get(1000);
loop(list);
} @Benchmark
@OperationsPerInvocation(10000)
public void measureWrong_10000() {
List<Dog> list = dogMap.get(10000);
loop(list);
} @Benchmark
@OperationsPerInvocation(100000)
public void measureWrong_100000() {
List<Dog> list = dogMap.get(100000);
loop(list);
} @Benchmark
@OperationsPerInvocation(1000000)
public void measureWrong_1000000() {
List<Dog> list = dogMap.get(1000000);
loop(list);
} /*
* ============================== HOW TO RUN THIS TEST:
* ====================================
*
* You might notice the larger the repetitions count, the lower the
* "perceived" cost of the operation being measured. Up to the point we do
* each addition with 1/20 ns, well beyond what hardware can actually do.
*
* This happens because the loop is heavily unrolled/pipelined, and the
* operation to be measured is hoisted from the loop. Morale: don't overuse
* loops, rely on JMH to get the measurement right.
*
* You can run this test:
*
* a) Via the command line: $ mvn clean install $ java -jar
* target/benchmarks.jar JMHSample_11 -wi 5 -i 5 -f 1 (we requested 5
* warmup/measurement iterations, single fork)
*
* b) Via the Java API: (see the JMH homepage for possible caveats when
* running from IDE: http://openjdk.java.net/projects/code-tools/jmh/)
*/ public static void main(String[] args) throws RunnerException {
Options opt = new OptionsBuilder()
.include(J8Loop.class.getSimpleName())
.warmupIterations(3)
.measurementIterations(5).forks(1).build(); new Runner(opt).run();
} }
class Dog{
private int age;
private String name;
public Dog(int age, String name) {
super();
this.age = age;
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "Dog [age=" + age + ", name=" + name + "]";
}
}
测试结果：
# JMH version: 1.19
# VM version: JDK 1.8.0_51, VM 25.51-b03
# VM invoker: D:\soft\java\java8\jre\bin\java.exe
# VM options: -Dfile.encoding=UTF-8
# Warmup: 3 iterations, single-shot each
# Measurement: 5 iterations, single-shot each
# Timeout: 10 min per iteration
# Threads: 1 thread
# Benchmark mode: Single shot invocation time
# Benchmark: yiwangzhibujian.jmh.test.J8Loop.measureWrong_10 # Run progress: 0.00% complete, ETA 00:00:00
# Fork: 1 of 1
# Warmup Iteration 1: 0.047 ms/op
# Warmup Iteration 2: 0.010 ms/op
# Warmup Iteration 3: 0.009 ms/op
Iteration 1: 0.009 ms/op
Iteration 2: 0.011 ms/op
Iteration 3: 0.009 ms/op
Iteration 4: 0.009 ms/op
Iteration 5: 0.010 ms/op Result "yiwangzhibujian.jmh.test.J8Loop.measureWrong_10":
N = 5
mean = 0.010 ±(99.9%) 0.003 ms/op Histogram, ms/op:
[0.008, 0.008) = 0
[0.008, 0.009) = 0
[0.009, 0.009) = 0
[0.009, 0.009) = 2
[0.009, 0.009) = 0
[0.009, 0.010) = 1
[0.010, 0.010) = 0
[0.010, 0.010) = 1
[0.010, 0.010) = 0
[0.010, 0.011) = 0
[0.011, 0.011) = 1 Percentiles, ms/op:
p(0.0000) = 0.009 ms/op
p(50.0000) = 0.009 ms/op
p(90.0000) = 0.011 ms/op
p(95.0000) = 0.011 ms/op
p(99.0000) = 0.011 ms/op
p(99.9000) = 0.011 ms/op
p(99.9900) = 0.011 ms/op
p(99.9990) = 0.011 ms/op
p(99.9999) = 0.011 ms/op
p(100.0000) = 0.011 ms/op # JMH version: 1.19
# VM version: JDK 1.8.0_51, VM 25.51-b03
# VM invoker: D:\soft\java\java8\jre\bin\java.exe
# VM options: -Dfile.encoding=UTF-8
# Warmup: 3 iterations, single-shot each
# Measurement: 5 iterations, single-shot each
# Timeout: 10 min per iteration
# Threads: 1 thread
# Benchmark mode: Single shot invocation time
# Benchmark: yiwangzhibujian.jmh.test.J8Loop.measureWrong_100 # Run progress: 16.67% complete, ETA 00:00:10
# Fork: 1 of 1
# Warmup Iteration 1: 0.068 ms/op
# Warmup Iteration 2: 0.038 ms/op
# Warmup Iteration 3: 0.038 ms/op
Iteration 1: 0.037 ms/op
Iteration 2: 0.025 ms/op
Iteration 3: 0.125 ms/op
Iteration 4: 0.038 ms/op
Iteration 5: 0.038 ms/op Result "yiwangzhibujian.jmh.test.J8Loop.measureWrong_100":
N = 5
mean = 0.053 ±(99.9%) 0.158 ms/op Histogram, ms/op:
[0.000, 0.013) = 0
[0.013, 0.025) = 0
[0.025, 0.038) = 2
[0.038, 0.050) = 2
[0.050, 0.063) = 0
[0.063, 0.075) = 0
[0.075, 0.088) = 0
[0.088, 0.100) = 0
[0.100, 0.113) = 0
[0.113, 0.125) = 0
[0.125, 0.138) = 1
[0.138, 0.150) = 0
[0.150, 0.163) = 0
[0.163, 0.175) = 0
[0.175, 0.188) = 0 Percentiles, ms/op:
p(0.0000) = 0.025 ms/op
p(50.0000) = 0.038 ms/op
p(90.0000) = 0.125 ms/op
p(95.0000) = 0.125 ms/op
p(99.0000) = 0.125 ms/op
p(99.9000) = 0.125 ms/op
p(99.9900) = 0.125 ms/op
p(99.9990) = 0.125 ms/op
p(99.9999) = 0.125 ms/op
p(100.0000) = 0.125 ms/op # JMH version: 1.19
# VM version: JDK 1.8.0_51, VM 25.51-b03
# VM invoker: D:\soft\java\java8\jre\bin\java.exe
# VM options: -Dfile.encoding=UTF-8
# Warmup: 3 iterations, single-shot each
# Measurement: 5 iterations, single-shot each
# Timeout: 10 min per iteration
# Threads: 1 thread
# Benchmark mode: Single shot invocation time
# Benchmark: yiwangzhibujian.jmh.test.J8Loop.measureWrong_1000 # Run progress: 33.33% complete, ETA 00:00:08
# Fork: 1 of 1
# Warmup Iteration 1: 0.266 ms/op
# Warmup Iteration 2: 0.310 ms/op
# Warmup Iteration 3: 0.635 ms/op
Iteration 1: 0.297 ms/op
Iteration 2: 0.255 ms/op
Iteration 3: 0.425 ms/op
Iteration 4: 0.413 ms/op
Iteration 5: 0.317 ms/op Result "yiwangzhibujian.jmh.test.J8Loop.measureWrong_1000":
N = 5
mean = 0.341 ±(99.9%) 0.287 ms/op Histogram, ms/op:
[0.200, 0.225) = 0
[0.225, 0.250) = 0
[0.250, 0.275) = 1
[0.275, 0.300) = 1
[0.300, 0.325) = 1
[0.325, 0.350) = 0
[0.350, 0.375) = 0
[0.375, 0.400) = 0
[0.400, 0.425) = 2
[0.425, 0.450) = 0
[0.450, 0.475) = 0 Percentiles, ms/op:
p(0.0000) = 0.255 ms/op
p(50.0000) = 0.317 ms/op
p(90.0000) = 0.425 ms/op
p(95.0000) = 0.425 ms/op
p(99.0000) = 0.425 ms/op
p(99.9000) = 0.425 ms/op
p(99.9900) = 0.425 ms/op
p(99.9990) = 0.425 ms/op
p(99.9999) = 0.425 ms/op
p(100.0000) = 0.425 ms/op # JMH version: 1.19
# VM version: JDK 1.8.0_51, VM 25.51-b03
# VM invoker: D:\soft\java\java8\jre\bin\java.exe
# VM options: -Dfile.encoding=UTF-8
# Warmup: 3 iterations, single-shot each
# Measurement: 5 iterations, single-shot each
# Timeout: 10 min per iteration
# Threads: 1 thread
# Benchmark mode: Single shot invocation time
# Benchmark: yiwangzhibujian.jmh.test.J8Loop.measureWrong_10000 # Run progress: 50.00% complete, ETA 00:00:06
# Fork: 1 of 1
# Warmup Iteration 1: 2.482 ms/op
# Warmup Iteration 2: 2.617 ms/op
# Warmup Iteration 3: 2.814 ms/op
Iteration 1: 2.656 ms/op
Iteration 2: 2.848 ms/op
Iteration 3: 2.636 ms/op
Iteration 4: 1.800 ms/op
Iteration 5: 0.927 ms/op Result "yiwangzhibujian.jmh.test.J8Loop.measureWrong_10000":
N = 5
mean = 2.173 ±(99.9%) 3.101 ms/op Histogram, ms/op:
[0.000, 0.250) = 0
[0.250, 0.500) = 0
[0.500, 0.750) = 0
[0.750, 1.000) = 1
[1.000, 1.250) = 0
[1.250, 1.500) = 0
[1.500, 1.750) = 0
[1.750, 2.000) = 1
[2.000, 2.250) = 0
[2.250, 2.500) = 0
[2.500, 2.750) = 2 Percentiles, ms/op:
p(0.0000) = 0.927 ms/op
p(50.0000) = 2.636 ms/op
p(90.0000) = 2.848 ms/op
p(95.0000) = 2.848 ms/op
p(99.0000) = 2.848 ms/op
p(99.9000) = 2.848 ms/op
p(99.9900) = 2.848 ms/op
p(99.9990) = 2.848 ms/op
p(99.9999) = 2.848 ms/op
p(100.0000) = 2.848 ms/op # JMH version: 1.19
# VM version: JDK 1.8.0_51, VM 25.51-b03
# VM invoker: D:\soft\java\java8\jre\bin\java.exe
# VM options: -Dfile.encoding=UTF-8
# Warmup: 3 iterations, single-shot each
# Measurement: 5 iterations, single-shot each
# Timeout: 10 min per iteration
# Threads: 1 thread
# Benchmark mode: Single shot invocation time
# Benchmark: yiwangzhibujian.jmh.test.J8Loop.measureWrong_100000 # Run progress: 66.67% complete, ETA 00:00:04
# Fork: 1 of 1
# Warmup Iteration 1: 17.857 ms/op
# Warmup Iteration 2: 7.360 ms/op
# Warmup Iteration 3: 2.186 ms/op
Iteration 1: 1.542 ms/op
Iteration 2: 1.419 ms/op
Iteration 3: 2.023 ms/op
Iteration 4: 1.382 ms/op
Iteration 5: 1.448 ms/op Result "yiwangzhibujian.jmh.test.J8Loop.measureWrong_100000":
N = 5
mean = 1.563 ±(99.9%) 1.017 ms/op Histogram, ms/op:
[1.300, 1.350) = 0
[1.350, 1.400) = 1
[1.400, 1.450) = 2
[1.450, 1.500) = 0
[1.500, 1.550) = 1
[1.550, 1.600) = 0
[1.600, 1.650) = 0
[1.650, 1.700) = 0
[1.700, 1.750) = 0
[1.750, 1.800) = 0
[1.800, 1.850) = 0
[1.850, 1.900) = 0
[1.900, 1.950) = 0
[1.950, 2.000) = 0
[2.000, 2.050) = 1 Percentiles, ms/op:
p(0.0000) = 1.382 ms/op
p(50.0000) = 1.448 ms/op
p(90.0000) = 2.023 ms/op
p(95.0000) = 2.023 ms/op
p(99.0000) = 2.023 ms/op
p(99.9000) = 2.023 ms/op
p(99.9900) = 2.023 ms/op
p(99.9990) = 2.023 ms/op
p(99.9999) = 2.023 ms/op
p(100.0000) = 2.023 ms/op # JMH version: 1.19
# VM version: JDK 1.8.0_51, VM 25.51-b03
# VM invoker: D:\soft\java\java8\jre\bin\java.exe
# VM options: -Dfile.encoding=UTF-8
# Warmup: 3 iterations, single-shot each
# Measurement: 5 iterations, single-shot each
# Timeout: 10 min per iteration
# Threads: 1 thread
# Benchmark mode: Single shot invocation time
# Benchmark: yiwangzhibujian.jmh.test.J8Loop.measureWrong_1000000 # Run progress: 83.33% complete, ETA 00:00:02
# Fork: 1 of 1
# Warmup Iteration 1: 78.570 ms/op
# Warmup Iteration 2: 21.032 ms/op
# Warmup Iteration 3: 10.005 ms/op
Iteration 1: 12.503 ms/op
Iteration 2: 10.849 ms/op
Iteration 3: 12.988 ms/op
Iteration 4: 12.030 ms/op
Iteration 5: 11.531 ms/op Result "yiwangzhibujian.jmh.test.J8Loop.measureWrong_1000000":
N = 5
mean = 11.980 ±(99.9%) 3.206 ms/op Histogram, ms/op:
[10.000, 10.250) = 0
[10.250, 10.500) = 0
[10.500, 10.750) = 0
[10.750, 11.000) = 1
[11.000, 11.250) = 0
[11.250, 11.500) = 0
[11.500, 11.750) = 1
[11.750, 12.000) = 0
[12.000, 12.250) = 1
[12.250, 12.500) = 0
[12.500, 12.750) = 1 Percentiles, ms/op:
p(0.0000) = 10.849 ms/op
p(50.0000) = 12.030 ms/op
p(90.0000) = 12.988 ms/op
p(95.0000) = 12.988 ms/op
p(99.0000) = 12.988 ms/op
p(99.9000) = 12.988 ms/op
p(99.9900) = 12.988 ms/op
p(99.9990) = 12.988 ms/op
p(99.9999) = 12.988 ms/op
p(100.0000) = 12.988 ms/op # Run complete. Total time: 00:00:12 Benchmark Mode Cnt Score Error Units
J8Loop.measureWrong_10 ss 5 0.010 ± 0.003 ms/op
J8Loop.measureWrong_100 ss 5 0.053 ± 0.158 ms/op
J8Loop.measureWrong_1000 ss 5 0.341 ± 0.287 ms/op
J8Loop.measureWrong_10000 ss 5 2.173 ± 3.101 ms/op
J8Loop.measureWrong_100000 ss 5 1.563 ± 1.017 ms/op
J8Loop.measureWrong_1000000 ss 5 11.980 ± 3.206 ms/op
其中最终的结论是：
Benchmark Mode Cnt Score Error Units
J8Loop.measureWrong_10 ss 5 0.010 ± 0.003 ms/op
J8Loop.measureWrong_100 ss 5 0.053 ± 0.158 ms/op
J8Loop.measureWrong_1000 ss 5 0.341 ± 0.287 ms/op
J8Loop.measureWrong_10000 ss 5 2.173 ± 3.101 ms/op
J8Loop.measureWrong_100000 ss 5 1.563 ± 1.017 ms/op
J8Loop.measureWrong_1000000 ss 5 11.980 ± 3.206 ms/op
如果用折线图对比的话：
会发现java8的foreach循环效率会更高一筹，真是冤枉了它。
其他几种循环就没必要测试了，用这两种我觉得就差不多了，如果你写代码思维不够清晰，需要经常debug，建议使用增强for循环，方便调试。
然后说一下有可能让人困惑的预热，我自己也做过测试，这个java8的foreach预热是jvm级别的，只要预热一次，以后速度都是这样的，至于深层次的原因我后续会做研究的。
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