StringBuilder有多消耗性能?
在看KMP算法时,想要简单的统计一下执行时间和性能。
得出的结论是： Java的String的indexOf方法性能最好，其次是KMP算法，其次是传统的BF算法，当然，对比有点牵强，SUN的算法也使用Java来实现、用的看着不像是KMP，还需要详细研究一下。
测试代码如下所示：
package com.test.test.kmp;
import java.util.Random;
public class KMPTest {
public static void main(String[] args) {
test();
}
//
public static void test() {
//
int allLen = 8000000;
int subLen = 11;
int charLen = 4;
String cl = buildString(charLen);
int times = 50;
//
testMatch(allLen, subLen, cl, times);
}
public static void testMatch(int allLen, int subLen, String cl, int times){
int allBF = 0;
int allString = 0;
int allKMP = 0;
int allGC = 0;
int allbuild = 0;
int alltoArray = 0;
long start = System.currentTimeMillis();
System.out.println("--------------------------");
for (int i = 0; i < times; i++) {
// 1. 构造字符串的损耗
long buildStart = System.currentTimeMillis();
String sub = buildString(subLen, cl);
String all = buildString(allLen, sub) +sub;
long buildEnd = System.currentTimeMillis();
allbuild += (buildEnd - buildStart);
// 2. toCharArray的损耗
long toArrayStart = System.currentTimeMillis();
char[] allArray = all.toCharArray();
char[] subArray = sub.toCharArray();
long toArrayEnd = System.currentTimeMillis();
alltoArray += (toArrayEnd - toArrayStart);
//
long startBF = System.currentTimeMillis();
int indexBF = bfIndexOf(all, sub);
long endBF = System.currentTimeMillis();
//
long timeoutBF = endBF - startBF;
allBF += timeoutBF;
System.gc();
allGC += (System.currentTimeMillis() - endBF);
//
long startString = System.currentTimeMillis();
int indexString = stringIndexOf(all, sub);
long endString = System.currentTimeMillis();
//
long timeoutString = endString - startString;
allString += timeoutString;
System.gc();
allGC += (System.currentTimeMillis() - endString);
//
long startKMP = System.currentTimeMillis();
//int indexKMP = kmpIndexOf(all, sub);
int indexKMP = KMP_Index(allArray, subArray);
long endKMP = System.currentTimeMillis();
//
long timeoutKMP = endKMP - startKMP;
allKMP += timeoutKMP;
System.gc();
allGC += (System.currentTimeMillis() - endKMP);
//
//System.out.println("all="+all.substring(0,100)+" ......");
//System.out.println("sub="+sub);
//
//System.out.println("indexBF="+indexBF+";耗时: "+timeoutBF+"ms");
//System.out.println("indexString="+indexString+";耗时: "+timeoutString+"ms");
if(indexBF == indexString && indexKMP == indexString){
//System.out.println("!!!!对比相等。");
} else {
throw new RuntimeException("对比出错.");
}
//
if(i % 20 == 10){
System.out.println(i+"次bfIndexOf= "+allBF+"ms");
System.out.println(i+"次stringIndexOf= "+allString+"ms");
System.out.println(i+"次KMPIndexOf= "+allKMP+"ms");
System.out.println(i+"次allbuild= "+allbuild+"ms");
System.out.println(i+"次alltoArray= "+alltoArray+"ms");
System.out.println(i+"*3次,GC= "+allGC+"ms");
long end = System.currentTimeMillis();
long allTime = end-start;
long lossTime = allTime - (allBF+allString+allKMP+allGC);
System.out.println(i+"次,所有总计耗时: "+(allTime)+"ms; 损耗:" + lossTime +"ms; 损耗比: " +((0.0+lossTime)/allTime * 100)+"%");
System.out.println("--------------------------");
}
}
//
System.out.println(times+"次bfIndexOf;总计耗时: "+allBF+"ms");
System.out.println(times+"次stringIndexOf;总计耗时: "+allString+"ms");
System.out.println(times+"次KMPIndexOf;总计耗时: "+allKMP+"ms");
System.out.println(times+"次allbuild= "+allbuild+"ms");
System.out.println(times+"次alltoArray= "+alltoArray+"ms");
System.out.println(times+"*3次,GC;总计耗时: "+allGC+"ms");
long end = System.currentTimeMillis();
long allTime = end-start;
long lossTime = allTime - (allBF+allString+allKMP+allGC);
System.out.println(times+"次,所有总计耗时: "+(allTime)+"ms; 损耗:" + lossTime +"ms; 损耗比: " +((0.0+lossTime)/allTime * 100)+"%");
System.out.println("--------------------------");
}
//
// 构建字符
public static String buildString(int len){
return buildString(len, null);
}
public static String buildString(int len, String sub){
//
final int TYPE_NUMBER = 0;
final int TYPE_LOWERCASE = 1;
final int TYPE_UPPERCASE = 2;
//
long seed = System.nanoTime();
Random random = new Random(seed);
//
//
StringBuilder builder = new StringBuilder();
for (int i = 0; i < len; i++) {
//
int type = random.nextInt(3);// 0-2
int cvalue = 0;
if(TYPE_NUMBER == type){
cvalue = '0' + random.nextInt(10);// 0~(n-1)
} else if(TYPE_LOWERCASE == type){
cvalue = 'a' + random.nextInt(26);// 0~(n-1)
} else if(TYPE_UPPERCASE == type){
cvalue = 'A' + random.nextInt(26);// 0~(n-1)
} else {
throw new RuntimeException(Random.class.getName()+"#newxtInt(int);Wrong!");
}
//
//cvalue = 'A' + random.nextInt(26);// 0~(n-1)
//
char c = (char)cvalue;
if(null != sub && sub.length() > 1){
int index = random.nextInt(sub.length());
c = sub.charAt(index);
}
//String s = String.valueOf(c);
builder.append(c);
//
}
//
return builder.toString();
}
/**
* Java自带的方法,内部实现了
* @param all
* @param sub
* @return
*/
public static int stringIndexOf(String all, String sub){
// 防御式编程
if(null == all || null== sub){
return -1;
}
// 调用Java的String方法
return all.indexOf(sub);
}
/**
* BF算法
* @param all
* @param sub
* @return
*/
public static int bfIndexOf(String all, String sub){
// 防御式编程
if(null == all || null== sub){
return -1;
}
//
int lenAll = all.length();
int lenSub = sub.length();
int i = 0;
while (i < lenAll) {
// 从i下标开始对比
int j = 0;
while (j < lenSub) {
//
char all_i = all.charAt(i);
char sub_j = sub.charAt(j);
if(all_i == sub_j){
// 相等则继续对比下一个字符
i++;
j++; // 这个增长很重要
} else {
// 否则跳出内层循环
break;
}
}
// 如果 j 增长到和 lenSub相等,则匹配成功
if(lenSub == j){
return i - lenSub;
} else {
i = 1+i - j; // 回溯 i
}
}
//
return -1;
}
/**
* KMP 算法
* @param all
* @param sub
* @return
*/
public static int kmpIndexOf(String all, String sub){
//
char[] allArray = all.toCharArray();
char[] subArray = sub.toCharArray();
//
return KMP_Index(allArray, subArray);
}
/**
* 获得字符串的next函数值
*
* @param t
* 字符串
* @return next函数值
*/
public static int[] next(char[] t) {
int[] next = new int[t.length];
next[0] = -1;
int i = 0;
int j = -1;
while (i < t.length - 1) {
if (j == -1 || t[i] == t[j]) {
i++;
j++;
if (t[i] != t[j]) {
next[i] = j;
} else {
next[i] = next[j];
}
} else {
j = next[j];
}
}
return next;
}
/**
* KMP匹配字符串
*
* @param allArray
* 主串
* @param subArray
* 模式串
* @return 若匹配成功，返回下标，否则返回-1
*/
public static int KMP_Index(char[] allArray, char[] subArray) {
int[] next = next(subArray);
int i = 0;
int j = 0;
while (i <= allArray.length - 1 && j <= subArray.length - 1) {
if (j == -1 || allArray[i] == subArray[j]) {
i++;
j++;
} else {
j = next[j];
}
}
if (j < subArray.length) {
return -1;
} else
return i - subArray.length; // 返回模式串在主串中的头下标
}
}
测试的一个结果如下所示：
--------------------------
10次bfIndexOf= 94ms
10次stringIndexOf= 56ms
10次KMPIndexOf= 76ms
10次allbuild= 5870ms
10次alltoArray= 137ms
10*3次,GC= 155ms
10次,所有总计耗时: 6388ms; 损耗:6007ms; 损耗比: 94.03569192235442%
--------------------------
30次bfIndexOf= 365ms
30次stringIndexOf= 222ms
30次KMPIndexOf= 295ms
30次allbuild= 16452ms
30次alltoArray= 395ms
30*3次,GC= 452ms
30次,所有总计耗时: 18184ms; 损耗:16850ms; 损耗比: 92.66388033435987%
--------------------------
50次bfIndexOf;总计耗时: 550ms
50次stringIndexOf;总计耗时: 335ms
50次KMPIndexOf;总计耗时: 450ms
50次allbuild= 26501ms
50次alltoArray= 637ms
50*3次,GC;总计耗时: 734ms
50次,所有总计耗时: 29211ms; 损耗:27142ms; 损耗比: 92.91705179555647%
--------------------------
从中可以看出来，使用StringBuilder构造字符串，800万*50次append大约消耗了26秒。换算下来每秒钟是1600万次。。。
看来是需要写一个专门计时的函数，本来Junit是有自己的统计的，但是样本不太好做。
如此看来，数据的准备，也就是测试用例的选取很关键,可能需要先生成并写入到文本文件中。