java String、String.concat和StringBuilder性能对比
看到网上有人已经做过对比，并且贴出了代码，然后我运行了之后发现跟我分析的结论差距很大。发现他的代码有个问题，UUID.randomUUID() 首次调用耗时会很高，这个耗时被计算给了String，这对String是不公平的。
原始代码参见：http://www.codes51.com/article/detail_99554.html
修改后的测试代码如下：
import java.util.Date;
import java.util.UUID; public class StringTest { public static void main(String[] args) {
int testLength = 10000;
String[] arr = new String[2];
String str = ""; Date start = new Date();
String testStr = UUID.randomUUID().toString();
System.out.println("首次生成randomUUID耗时：" + (new Date().getTime() - start.getTime())); start = new Date();
for (int i = 0; i < testLength; i++) {
testStr = UUID.randomUUID().toString();
}
System.out.println("非首次生成randomUUID " + testLength + "次耗时：" + (new Date().getTime() - start.getTime())); start = new Date();
for (int i = 0; i < testLength; i++) {
str = testStr + testStr;
}
System.out.println("String 拼接测试,测试长度" + testLength + ",测试字符串数组长度" + arr.length + ",完成时间" + (new Date().getTime() - start.getTime())); start = new Date();
for (int i = 0; i < testLength; i++) {
str = testStr.concat(testStr);
}
System.out.println("String.concat 拼接测试,测试长度" + testLength + ",测试字符串数组长度" + arr.length + ",完成时间" + (new Date().getTime() - start.getTime())); start = new Date();
StringBuilder sb;
for (int i = 0; i < testLength; i++) {
str = "";
sb = new StringBuilder();
for (int j = 0; j < arr.length; j++) {
sb.append(testStr);
}
str = sb.toString();
}
System.out.println("StringBuilder 拼接测试,测试长度" + testLength + ",测试字符串数组长度" + arr.length + ",完成时间" + (new Date().getTime() - start.getTime()));
}
}
测试结果：
1. 测试字符串数组长度10
首次生成randomUUID耗时：290
非首次生成randomUUID 10000次耗时：44
String 拼接测试,测试长度10000,测试字符串数组长度10,完成时间14
String 使用循环 拼接测试,测试长度10000,测试字符串数组长度10,完成时间66
String.concat 拼接测试,测试长度10000,测试字符串数组长度10,完成时间14
StringBuilder 拼接测试,测试长度10000,测试字符串数组长度10,完成时间14
2. 测试字符串数组长度5
首次生成randomUUID耗时：287
非首次生成randomUUID 10000次耗时：48
String 拼接测试,测试长度10000,测试字符串数组长度5,完成时间11
String 使用循环 拼接测试,测试长度10000,测试字符串数组长度5,完成时间20
String.concat 拼接测试,测试长度10000,测试字符串数组长度5,完成时间9
StringBuilder 拼接测试,测试长度10000,测试字符串数组长度5,完成时间10
3. 测试字符串数组长度3
首次生成randomUUID耗时：308
非首次生成randomUUID 10000次耗时：35
String 拼接测试,测试长度10000,测试字符串数组长度3,完成时间10
String 使用循环 拼接测试,测试长度10000,测试字符串数组长度3,完成时间21
String.concat 拼接测试,测试长度10000,测试字符串数组长度3,完成时间6
StringBuilder 拼接测试,测试长度10000,测试字符串数组长度3,完成时间11
4. 测试字符串数组长度2
首次生成randomUUID耗时：298
非首次生成randomUUID 10000次耗时：70
String 拼接测试,测试长度10000,测试字符串数组长度2,完成时间10
String 使用循环 拼接测试,测试长度10000,测试字符串数组长度2,完成时间8
String.concat 拼接测试,测试长度10000,测试字符串数组长度2,完成时间3
StringBuilder 拼接测试,测试长度10000,测试字符串数组长度2,完成时间7
5. 测试字符串数组长度1
首次生成randomUUID耗时：278
非首次生成randomUUID 10000次耗时：71
String 拼接测试,测试长度10000,测试字符串数组长度1,完成时间1
String 使用循环 拼接测试,测试长度10000,测试字符串数组长度1,完成时间8
String.concat 拼接测试,测试长度10000,测试字符串数组长度1,完成时间3
StringBuilder 拼接测试,测试长度10000,测试字符串数组长度1,完成时间4
到此，可以看出，绝大多数情况下StringBuilder妥妥的比String 使用循环快，但是跟String直接相加差不多，String concat效率跟StringBuilder差不多，很多时候还要快一些，这些都是为什么呢？
javap -c StringTest.class 看看Java编译器都做了什么：
Compiled from "StringTest.java"
public class StringTest {
public StringTest();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return public static void main(java.lang.String[]);
Code:
0: sipush 10000
3: istore_1
4: iconst_2
5: anewarray #2 // class java/lang/String
8: astore_2
9: ldc #3 // String
11: astore_3
12: new #4 // class java/util/Date
15: dup
16: invokespecial #5 // Method java/util/Date."<init>":()V
19: astore 4
21: invokestatic #6 // Method java/util/UUID.randomUUID:()Ljava/util/UUID;
24: invokevirtual #7 // Method java/util/UUID.toString:()Ljava/lang/String;
27: astore 5
29: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream;
32: new #9 // class java/lang/StringBuilder
35: dup
36: invokespecial #10 // Method java/lang/StringBuilder."<init>":()V
39: ldc #11 // String 首次生成randomUUID耗时：
41: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
44: new #4 // class java/util/Date
47: dup
48: invokespecial #5 // Method java/util/Date."<init>":()V
51: invokevirtual #13 // Method java/util/Date.getTime:()J
54: aload 4
56: invokevirtual #13 // Method java/util/Date.getTime:()J
59: lsub
60: invokevirtual #14 // Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
63: invokevirtual #15 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
66: invokevirtual #16 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
69: new #4 // class java/util/Date
72: dup
73: invokespecial #5 // Method java/util/Date."<init>":()V
76: astore 4
78: iconst_0
79: istore 6
81: iload 6
83: iload_1
84: if_icmpge 101
87: invokestatic #6 // Method java/util/UUID.randomUUID:()Ljava/util/UUID;
90: invokevirtual #7 // Method java/util/UUID.toString:()Ljava/lang/String;
93: astore 5
95: iinc 6, 1
98: goto 81
101: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream;
104: new #9 // class java/lang/StringBuilder
107: dup
108: invokespecial #10 // Method java/lang/StringBuilder."<init>":()V
111: ldc #17 // String 非首次生成randomUUID
113: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
116: iload_1
117: invokevirtual #18 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
120: ldc #19 // String 次耗时：
122: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
125: new #4 // class java/util/Date
128: dup
129: invokespecial #5 // Method java/util/Date."<init>":()V
132: invokevirtual #13 // Method java/util/Date.getTime:()J
135: aload 4
137: invokevirtual #13 // Method java/util/Date.getTime:()J
140: lsub
141: invokevirtual #14 // Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
144: invokevirtual #15 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
147: invokevirtual #16 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
150: new #4 // class java/util/Date
153: dup
154: invokespecial #5 // Method java/util/Date."<init>":()V
157: astore 4
159: iconst_0
160: istore 6
162: iload 6
164: iload_1
165: if_icmpge 195
168: new #9 // class java/lang/StringBuilder
171: dup
172: invokespecial #10 // Method java/lang/StringBuilder."<init>":()V
175: aload 5
177: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
180: aload 5
182: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
185: invokevirtual #15 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
188: astore_3
189: iinc 6, 1
192: goto 162
195: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream;
198: new #9 // class java/lang/StringBuilder
201: dup
202: invokespecial #10 // Method java/lang/StringBuilder."<init>":()V
205: ldc #20 // String String 拼接测试,测试长度
207: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
210: iload_1
211: invokevirtual #18 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
214: ldc #21 // String ,测试字符串数组长度
216: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
219: aload_2
220: arraylength
221: invokevirtual #18 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
224: ldc #22 // String ,完成时间
226: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
229: new #4 // class java/util/Date
232: dup
233: invokespecial #5 // Method java/util/Date."<init>":()V
236: invokevirtual #13 // Method java/util/Date.getTime:()J
239: aload 4
241: invokevirtual #13 // Method java/util/Date.getTime:()J
244: lsub
245: invokevirtual #14 // Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
248: invokevirtual #15 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
251: invokevirtual #16 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
254: new #4 // class java/util/Date
257: dup
258: invokespecial #5 // Method java/util/Date."<init>":()V
261: astore 4
263: iconst_0
264: istore 6
266: iload 6
268: iload_1
269: if_icmpge 317
272: ldc #3 // String
274: astore_3
275: iconst_0
276: istore 7
278: iload 7
280: aload_2
281: arraylength
282: if_icmpge 311
285: new #9 // class java/lang/StringBuilder
288: dup
289: invokespecial #10 // Method java/lang/StringBuilder."<init>":()V
292: aload_3
293: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
296: aload 5
298: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
301: invokevirtual #15 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
304: astore_3
305: iinc 7, 1
308: goto 278
311: iinc 6, 1
314: goto 266
317: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream;
320: new #9 // class java/lang/StringBuilder
323: dup
324: invokespecial #10 // Method java/lang/StringBuilder."<init>":()V
327: ldc #23 // String String 使用循环 拼接测试,测试长度
329: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
332: iload_1
333: invokevirtual #18 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
336: ldc #21 // String ,测试字符串数组长度
338: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
341: aload_2
342: arraylength
343: invokevirtual #18 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
346: ldc #22 // String ,完成时间
348: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
351: new #4 // class java/util/Date
354: dup
355: invokespecial #5 // Method java/util/Date."<init>":()V
358: invokevirtual #13 // Method java/util/Date.getTime:()J
361: aload 4
363: invokevirtual #13 // Method java/util/Date.getTime:()J
366: lsub
367: invokevirtual #14 // Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
370: invokevirtual #15 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
373: invokevirtual #16 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
376: new #4 // class java/util/Date
379: dup
380: invokespecial #5 // Method java/util/Date."<init>":()V
383: astore 4
385: iconst_0
386: istore 6
388: iload 6
390: iload_1
391: if_icmpge 408
394: aload 5
396: aload 5
398: invokevirtual #24 // Method java/lang/String.concat:(Ljava/lang/String;)Ljava/lang/String;
401: astore_3
402: iinc 6, 1
405: goto 388
408: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream;
411: new #9 // class java/lang/StringBuilder
414: dup
415: invokespecial #10 // Method java/lang/StringBuilder."<init>":()V
418: ldc #25 // String String.concat 拼接测试,测试长度
420: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
423: iload_1
424: invokevirtual #18 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
427: ldc #21 // String ,测试字符串数组长度
429: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
432: aload_2
433: arraylength
434: invokevirtual #18 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
437: ldc #22 // String ,完成时间
439: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
442: new #4 // class java/util/Date
445: dup
446: invokespecial #5 // Method java/util/Date."<init>":()V
449: invokevirtual #13 // Method java/util/Date.getTime:()J
452: aload 4
454: invokevirtual #13 // Method java/util/Date.getTime:()J
457: lsub
458: invokevirtual #14 // Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
461: invokevirtual #15 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
464: invokevirtual #16 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
467: new #4 // class java/util/Date
470: dup
471: invokespecial #5 // Method java/util/Date."<init>":()V
474: astore 4
476: iconst_0
477: istore 7
479: iload 7
481: iload_1
482: if_icmpge 533
485: ldc #3 // String
487: astore_3
488: new #9 // class java/lang/StringBuilder
491: dup
492: invokespecial #10 // Method java/lang/StringBuilder."<init>":()V
495: astore 6
497: iconst_0
498: istore 8
500: iload 8
502: aload_2
503: arraylength
504: if_icmpge 521
507: aload 6
509: aload 5
511: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
514: pop
515: iinc 8, 1
518: goto 500
521: aload 6
523: invokevirtual #15 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
526: astore_3
527: iinc 7, 1
530: goto 479
533: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream;
536: new #9 // class java/lang/StringBuilder
539: dup
540: invokespecial #10 // Method java/lang/StringBuilder."<init>":()V
543: ldc #26 // String StringBuilder 拼接测试,测试长度
545: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
548: iload_1
549: invokevirtual #18 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
552: ldc #21 // String ,测试字符串数组长度
554: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
557: aload_2
558: arraylength
559: invokevirtual #18 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
562: ldc #22 // String ,完成时间
564: invokevirtual #12 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
567: new #4 // class java/util/Date
570: dup
571: invokespecial #5 // Method java/util/Date."<init>":()V
574: invokevirtual #13 // Method java/util/Date.getTime:()J
577: aload 4
579: invokevirtual #13 // Method java/util/Date.getTime:()J
582: lsub
583: invokevirtual #14 // Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
586: invokevirtual #15 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
589: invokevirtual #16 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
592: return
}
String直接相加已经都被编译器优化成StringBuilder了，只是循环里优化的不太合理。所以，String相加快还是StringBuilder快，其实只是StringBuilder调用方式对比。。。
String.concat为什么快？
看看jdk1.8里这个方法的源代码就知道了：
public String concat(String str) {
int otherLen = str.length();
if (otherLen == 0) {
return this;
}
int len = value.length;
char buf[] = Arrays.copyOf(value, len + otherLen);
str.getChars(buf, len);
return new String(buf, true);
}
简单粗暴，直接Arrays.copyOf，直接内存复制，这根StringBuilder原理类似，但是它不用初始化StringBuilder对象，只是每次concat都会创建一个新的String对象，所以在有些情况下它比StringBuilder要快一点。
结论：简单场景里，直接用+好了，反正编译器默认会优化成StringBuilder，毕竟对一般人来说加号可读性高一点。但是在循环中使用或者是比较复杂的应用场景里，还是尽量自己直接用StringBuilder或String concat。
java String、String.concat和StringBuilder性能对比的更多相关文章
- Java中String、StringBuffer、StringBuilder区别与理解
一.先比较String.StringBuffer.StringBuilder变量的HashCode值 使用System.out.println(obj.hashcode())输出的时对象的哈希码, 而 ...
- java中String StringBuilder StringBuffer比较和效率（性能）测试
string stringbuilder stringbuffer三者的区别 从JDK源码看,String.StringBuilder.StringBuffer都是存放在char[] 数组字符串. 简 ...
- 不同Framework下StringBuilder和String的性能对比，及不同Framework性能比（附Demo）
本文版权归mephisto和博客园共有,欢迎转载,但须保留此段声明,并给出原文链接,谢谢合作. 文章是哥(mephisto)写的,SourceLink 阅读目录 介绍 环境搭建 测试用例 MSDN说明 ...
- StringBuilder和string.Format性能对比
本文由博主(YinaPan)原创,转载请注明出处:http://www.cnblogs.com/YinaPan/p/sbformat.html StringBuilder的性能优于string.For ...
- 【Java必修课】判断String是否包含子串的四种方法及性能对比
1 简介 判断一个字符串是否包含某个特定子串是常见的场景,比如判断一篇文章是否包含敏感词汇.判断日志是否有ERROR信息等.本文将介绍四种方法并进行性能测试. 2 四种方法 2.1 JDK原生方法St ...
- Java中的常用类：包装类、String、StringBuffer、StringBuilder、Math、System、Arrays、BigInteger、BigDecimal、Data、Calendar
一.包装类 √ 二.String类 ★ 三.StringBuffer和StringBuilder类 ★ 四.Math类 五.System类 六.Arrays类 七.BigInteger类和BigDec ...
- 探秘Java中String、StringBuilder以及StringBuffer
探秘Java中String.StringBuilder以及StringBuffer 相信String这个类是Java中使用得最频繁的类之一,并且又是各大公司面试喜欢问 到的地方,今天就来和大家一起学习 ...
- java中String类、StringBuilder类和StringBuffer类详解
本位转载自http://www.cnblogs.com/dolphin0520/p/3778589.html 版权声明如下: 作者:海子 出处:http://www.cnblogs.com/dolp ...
- Java中String、StringBuilder以及StringBuffer
原文出处: 海子 相信String这个类是Java中使用得最频繁的类之一,并且又是各大公司面试喜欢问到的地方,今天就来和大家一起学习一下String.StringBuilder和StringBuffe ...
随机推荐
- [LeetCode] 450. Delete Node in a BST 删除二叉搜索树中的节点
Given a root node reference of a BST and a key, delete the node with the given key in the BST. Retur ...
- [LeetCode] 12. Integer to Roman 整数转化成罗马数字
Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M. Symbol Value I 1 ...
- API数据加密框架monkey-api-encrypt
之前有写过一篇加密的文章<前后端API交互如何保证数据安全性>.主要是在Spring Boot中如何对接口的数据进行自动加解密操作,通过注解的方式来指定是否需要加解密. 原理也很简单,通过 ...
- 从GopherChina 2019看当前的go语言
GopherChina 2019大会4月底刚刚结束,大会上使用的PPT也放了出来(大会情况及PPT在https://mp.weixin.qq.com/s/_oVpIcBMVIKVzQn6YrkAJw) ...
- logback&log4j异步日志配置
logback 原始配置 配置 appender, 控制文件的滚动方式,日志的输出格式. <appender name="method-time-appender" clas ...
- c# 在DataTable的第一列（指定列）的前面添加一列
c# 在DataTable的第一列(指定列)的前面添加一列 dt.Columns.Add("ID").SetOrdinal(0)
- Charles设置断点- （超详解）
1.选择你要设置断点的接口 2.右键选择 Breakpoints 3.断点的相关配置, Proxy ——>Breakpoint Settings 5.双击刚刚已经设置的断点接口,进行设置 6. ...
- C#循环 — break VS continue
一.简介 1.break语句:循环-循环中断并停止,退出当前循环: 流程图: 2.continue:循环-循环下一次迭代继续执行. 流程图: 执行过程:立即结果本次循环,判断循环条件,如果成立,则进入 ...
- WebService--导出excel并将excel发送到邮箱
1.利用NPOI生成EXCEL 2.导出excel之后发送邮件 //导出excel //path :保存路径 //dt:要导出的数据 public static bool DataTableToExc ...
- 简单的sql语句汇总（sqlserver）
1.修改字段的默认值 alter table 表名 add default 默认值 for 字段名称 例子: for Age; alter table 表名 add constraint DF_TAB ...