看到網上有人已經做過對比，並且貼出了代碼，然后我運行了之后發現跟我分析的結論差距很大。發現他的代碼有個問題，UUID.randomUUID() 首次調用耗時會很高，這個耗時被計算給了String，這對String是不公平的。
原始代碼參見：http://www.codes51.com/article/detail_99554.html
修改后的測試代碼如下：
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耗時：" + (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 + "次耗時：" + (new Date().getTime() - start.getTime())); start = new Date(); for (int i = 0; i < testLength; i++) { str = testStr + testStr; } System.out.println("String 拼接測試,測試長度" + testLength + ",測試字符串數組長度" + arr.length + ",完成時間" + (new Date().getTime() - start.getTime())); start = new Date(); for (int i = 0; i < testLength; i++) { str = testStr.concat(testStr); } System.out.println("String.concat 拼接測試,測試長度" + testLength + ",測試字符串數組長度" + arr.length + ",完成時間" + (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 拼接測試,測試長度" + testLength + ",測試字符串數組長度" + arr.length + ",完成時間" + (new Date().getTime() - start.getTime())); } }
測試結果：
1. 測試字符串數組長度10
首次生成randomUUID耗時：290
非首次生成randomUUID 10000次耗時：44
String 拼接測試,測試長度10000,測試字符串數組長度10,完成時間14
String 使用循環 拼接測試,測試長度10000,測試字符串數組長度10,完成時間66
String.concat 拼接測試,測試長度10000,測試字符串數組長度10,完成時間14
StringBuilder 拼接測試,測試長度10000,測試字符串數組長度10,完成時間14
2. 測試字符串數組長度5
首次生成randomUUID耗時：287
非首次生成randomUUID 10000次耗時：48
String 拼接測試,測試長度10000,測試字符串數組長度5,完成時間11
String 使用循環 拼接測試,測試長度10000,測試字符串數組長度5,完成時間20
String.concat 拼接測試,測試長度10000,測試字符串數組長度5,完成時間9
StringBuilder 拼接測試,測試長度10000,測試字符串數組長度5,完成時間10
3. 測試字符串數組長度3
首次生成randomUUID耗時：308
非首次生成randomUUID 10000次耗時：35
String 拼接測試,測試長度10000,測試字符串數組長度3,完成時間10
String 使用循環 拼接測試,測試長度10000,測試字符串數組長度3,完成時間21
String.concat 拼接測試,測試長度10000,測試字符串數組長度3,完成時間6
StringBuilder 拼接測試,測試長度10000,測試字符串數組長度3,完成時間11
4. 測試字符串數組長度2
首次生成randomUUID耗時：298
非首次生成randomUUID 10000次耗時：70
String 拼接測試,測試長度10000,測試字符串數組長度2,完成時間10
String 使用循環 拼接測試,測試長度10000,測試字符串數組長度2,完成時間8
String.concat 拼接測試,測試長度10000,測試字符串數組長度2,完成時間3
StringBuilder 拼接測試,測試長度10000,測試字符串數組長度2,完成時間7
5. 測試字符串數組長度1
首次生成randomUUID耗時：278
非首次生成randomUUID 10000次耗時：71
String 拼接測試,測試長度10000,測試字符串數組長度1,完成時間1
String 使用循環 拼接測試,測試長度10000,測試字符串數組長度1,完成時間8
String.concat 拼接測試,測試長度10000,測試字符串數組長度1,完成時間3
StringBuilder 拼接測試,測試長度10000,測試字符串數組長度1,完成時間4
到此，可以看出，絕大多數情況下StringBuilder妥妥的比String 使用循環快，但是跟String直接相加差不多，String concat效率跟StringBuilder差不多，很多時候還要快一些，這些都是為什么呢？
javap -c StringTest.class 看看Java編譯器都做了什么：
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耗時： 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 次耗時： 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 拼接測試,測試長度 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 ,測試字符串數組長度 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 ,完成時間 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 使用循環 拼接測試,測試長度 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 ,測試字符串數組長度 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 ,完成時間 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 拼接測試,測試長度 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 ,測試字符串數組長度 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 ,完成時間 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 拼接測試,測試長度 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 ,測試字符串數組長度 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 ,完成時間 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直接相加已經都被編譯器優化成StringBuilder了，只是循環里優化的不太合理。所以，String相加快還是StringBuilder快，其實只是StringBuilder調用方式對比。。。
String.concat為什么快？
看看jdk1.8里這個方法的源代碼就知道了：
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); }
簡單粗暴，直接Arrays.copyOf，直接內存復制，這根StringBuilder原理類似，但是它不用初始化StringBuilder對象，只是每次concat都會創建一個新的String對象，所以在有些情況下它比StringBuilder要快一點。
結論：簡單場景里，直接用+好了，反正編譯器默認會優化成StringBuilder，畢竟對一般人來說加號可讀性高一點。但是在循環中使用或者是比較復雜的應用場景里，還是盡量自己直接用StringBuilder或String concat。