Java-Vector vs ArrayList性能测试
每个人都说应该使用vector是因为它的性能(因为vector在每次操作和内容之后都会同步)。我写了一个简单的测试:Java-Vector vs ArrayList性能测试,java,performance,vector,arraylist,Java,Performance,Vector,Arraylist,每个人都说应该使用vector是因为它的性能(因为vector在每次操作和内容之后都会同步)。我写了一个简单的测试: import java.util.ArrayList; import java.util.Date; import java.util.Vector; public class ComparePerformance { public static void main(String[] args) { ArrayList<Integer> l
import java.util.ArrayList;
import java.util.Date;
import java.util.Vector;
public class ComparePerformance {
public static void main(String[] args) {
ArrayList<Integer> list = new ArrayList<Integer>();
Vector<Integer> vector = new Vector<Integer>();
int size = 10000000;
int listSum = 0;
int vectorSum = 0;
long startList = new Date().getTime();
for (int i = 0; i < size; i++) {
list.add(new Integer(1));
}
for (Integer integer : list) {
listSum += integer;
}
long endList = new Date().getTime();
System.out.println("List time: " + (endList - startList));
long startVector = new Date().getTime();
for (int i = 0; i < size; i++) {
vector.add(new Integer(1));
}
for (Integer integer : list) {
vectorSum += integer;
}
long endVector = new Date().getTime();
System.out.println("Vector time: " + (endVector - startVector));
}
}
基于此,
Vector
在迭代和读取方面的性能似乎稍好一些。也许这是一个愚蠢的问题,或者我做了错误的假设——有人能解释一下吗?你写了一个天真的微基准。JVM上的微基准标记是一项非常棘手的业务,列举所有陷阱甚至都不容易,但这里有一些经典的陷阱:
System.currentTimeMillis
是不精确的,但您似乎连这个方法都不知道(您的new Date().getTime()
是等效的,但速度较慢)jmh
工具或Google的卡钳
我的测试结果
因为我有点想亲自看看这些数字,下面是jmh
的输出。首先,测试代码:
public class Benchmark1
{
static Integer[] ints = new Integer[0];
static {
final List<Integer> list = new ArrayList(asList(1,2,3,4,5,6,7,8,9,10));
for (int i = 0; i < 5; i++) list.addAll(list);
ints = list.toArray(ints);
}
static List<Integer> intList = Arrays.asList(ints);
static Vector<Integer> vec = new Vector<Integer>(intList);
static List<Integer> list = new ArrayList<Integer>(intList);
@GenerateMicroBenchmark
public Vector<Integer> testVectorAdd() {
final Vector<Integer> v = new Vector<Integer>();
for (Integer i : ints) v.add(i);
return v;
}
@GenerateMicroBenchmark
public long testVectorTraverse() {
long sum = (long)Math.random()*10;
for (int i = 0; i < vec.size(); i++) sum += vec.get(i);
return sum;
}
@GenerateMicroBenchmark
public List<Integer> testArrayListAdd() {
final List<Integer> l = new ArrayList<Integer>();
for (Integer i : ints) l.add(i);
return l;
}
@GenerateMicroBenchmark
public long testArrayListTraverse() {
long sum = (long)Math.random()*10;
for (int i = 0; i < list.size(); i++) sum += list.get(i);
return sum;
}
}
注意以下几点:
- 在
方法中,我正在创建一个新的本地集合。JIT编译器使用了这一事实,避免了对…add
方法的锁定,因此性能几乎相同李>Vector
- 在
…遍历
方法中,我从一个全局集合中读取;锁不能被忽略,这就是真正的性能损失
Vector
主要的收获应该是:JVM上的性能模型非常复杂,有时甚至不稳定。从微基准点进行推断,即使非常小心,也可能导致对生产系统性能的危险错误预测。我做了您的测试,ArrayList比1000000大小的Vector更快
public static void main(String[] args) {
ArrayList<Integer> list = new ArrayList<Integer>();
Vector<Integer> vector = new Vector<Integer>();
int size= 1000000;
int listSum = 0;
int vectorSum = 0;
long startList = System.nanoTime();
for (int i = 0; i < size; i++) {
list.add(Integer.valueOf(1));
}
for (Integer integer : list) {
listSum += integer;
}
long endList = System.nanoTime();
System.out.println("List time: " + (endList - startList)/1000000);
//
// long startVector = System.nanoTime();
// for (int i = 0; i < size; i++) {
// vector.add(Integer.valueOf(1));
// }
// for (Integer integer : list) {
// vectorSum += integer;
// }
// long endVector = System.nanoTime();
// System.out.println("Vector time: " + (endVector - startVector)/1000000);
}
}
正如Marko Topolnik所说,很难写出正确的微基准并正确解释结果。关于这个主题的好文章很多 根据我的经验和我对实施的了解,我使用以下经验法则:
- 使用ArrayList
- 如果集合中有许多元素,并且列表中经常有插入或删除操作(不是在末尾),则使用LinkedList
尽可能使用列表界面隐藏实现细节,并尝试添加注释,说明选择特定实现的原因。我同意Marko关于使用Caliper的看法,它是一个很棒的框架 但是,如果您能更好地组织基准测试,您可以自己完成一部分工作:
public class ComparePerformance {
private static final int SIZE = 1000000;
private static final int RUNS = 500;
private static final Integer ONE = Integer.valueOf(1);
static class Run {
private final List<Integer> list;
Run(final List<Integer> list) {
this.list = list;
}
public long perform() {
long oldNanos = System.nanoTime();
for (int i = 0; i < SIZE; i++) {
list.add(ONE);
}
return System.nanoTime() - oldNanos;
}
}
public static void main(final String[] args) {
long arrayListTotal = 0L;
long vectorTotal = 0L;
for (int i = 0; i < RUNS; i++) {
if (i % 50 == 49) {
System.out.println("Run " + (i + 1));
}
arrayListTotal += new Run(new ArrayList<Integer>()).perform();
vectorTotal += new Run(new Vector<Integer>()).perform();
}
System.out.println();
System.out.println("Runs: "+RUNS+", list size: "+SIZE);
output(arrayListTotal, "List");
output(vectorTotal, "Vector");
}
private static void output(final long value, final String name) {
System.out.println(name + " total time: " + value + " (" + TimeUnit.NANOSECONDS.toMillis(value) + " " + "ms)");
long avg = value / RUNS;
System.out.println(name + " average time: " + avg + " (" + TimeUnit.NANOSECONDS.toMillis(avg) + " " + "ms)");
}
}
我认为这应该让您了解性能差异。1)不建议使用Vector 2)您的测试应该在2次内运行。。不是一个接一个你不知道JIT magic做什么,使用System.nanotime()所以,有两个注释:(1)由于同步,使用Vector会带来轻微的开销,这可能会使它比ArrayList慢。(2) 在这里,您实际上是在试验一个VM预热,它使ArrayList的µ-基准测试比Vector测试稍微慢一点。尝试交换两个基准测试,看看会发生什么!不知道你在哪里听到的,看看。也许您应该在
C++
中使用它,但不要在Java
中使用它。请看,您的代码中似乎有一个输入错误,您在其中使用了向量遍历,非常感谢!如果我不早点做,我会“接受”你的@德斯特朗扎克很公平。在我写这篇文章的过程中,被接受的答案得到了改进,而且是最好的。
public static void main(String[] args) {
ArrayList<Integer> list = new ArrayList<Integer>();
Vector<Integer> vector = new Vector<Integer>();
int size= 1000000;
int listSum = 0;
int vectorSum = 0;
long startList = System.nanoTime();
for (int i = 0; i < size; i++) {
list.add(Integer.valueOf(1));
}
for (Integer integer : list) {
listSum += integer;
}
long endList = System.nanoTime();
System.out.println("List time: " + (endList - startList)/1000000);
//
// long startVector = System.nanoTime();
// for (int i = 0; i < size; i++) {
// vector.add(Integer.valueOf(1));
// }
// for (Integer integer : list) {
// vectorSum += integer;
// }
// long endVector = System.nanoTime();
// System.out.println("Vector time: " + (endVector - startVector)/1000000);
}
}
Code : list time 83
vector time 113
public class ComparePerformance {
private static final int SIZE = 1000000;
private static final int RUNS = 500;
private static final Integer ONE = Integer.valueOf(1);
static class Run {
private final List<Integer> list;
Run(final List<Integer> list) {
this.list = list;
}
public long perform() {
long oldNanos = System.nanoTime();
for (int i = 0; i < SIZE; i++) {
list.add(ONE);
}
return System.nanoTime() - oldNanos;
}
}
public static void main(final String[] args) {
long arrayListTotal = 0L;
long vectorTotal = 0L;
for (int i = 0; i < RUNS; i++) {
if (i % 50 == 49) {
System.out.println("Run " + (i + 1));
}
arrayListTotal += new Run(new ArrayList<Integer>()).perform();
vectorTotal += new Run(new Vector<Integer>()).perform();
}
System.out.println();
System.out.println("Runs: "+RUNS+", list size: "+SIZE);
output(arrayListTotal, "List");
output(vectorTotal, "Vector");
}
private static void output(final long value, final String name) {
System.out.println(name + " total time: " + value + " (" + TimeUnit.NANOSECONDS.toMillis(value) + " " + "ms)");
long avg = value / RUNS;
System.out.println(name + " average time: " + avg + " (" + TimeUnit.NANOSECONDS.toMillis(avg) + " " + "ms)");
}
}
Runs: 500, list size: 1000000
List total time: 3524708559 (3524 ms)
List average time: 7049417 (7 ms)
Vector total time: 6459070419 (6459 ms)
Vector average time: 12918140 (12 ms)