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。