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。