java StringString.concat和StringBuilder性能对比
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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。
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