两个字符串相加究竟发生了什么

首先从一张图开始，图中对字符串相加和StringBuild.append()做了性能对比。

有人说字符串每次相加都新建了一个对象所以慢，事情真的是这样吗？

先要了解真相最好的方法当然是反编译了，使用 javap -c 反编译的结果

public void test();
    Code:
       0: ldc           #2                  // String s1
       2: astore_1
       3: ldc           #3                  // String s2
       5: astore_2
       6: invokestatic  #4                  // Method java/lang/System.currentTimeMillis:()J
       9: lstore_3
      10: aload_1
      11: invokevirtual #5                  // Method java/lang/String.length:()I
      14: ldc           #6                  // int 100000
      16: if_icmpge     41
　　　　
　　　　//虽然是字符串相加但是仍然创建了StringBuilder对象
      19: new           #7                  // class java/lang/StringBuilder
      22: dup
      23: invokespecial #8                  // Method java/lang/StringBuilder."<init>":()V
      26: aload_1
      27: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      30: aload_2
      31: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
　　　　//将拼接好的字符串重新复制给s1的时候 toString
      34: invokevirtual #10                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
　　　　//赋值给s1
      37: astore_1
　　　　回到10，循环 继续创建新的StringBuilder。然后toString
      38: goto          10
      41: getstatic     #11                 // Field java/lang/System.out:Ljava/io/PrintStream;
      44: invokestatic  #4                  // Method java/lang/System.currentTimeMillis:()J
      47: lload_3
      48: lsub
      49: invokevirtual #12                 // Method java/io/PrintStream.println:(J)V



　　　　//　下面是SringBuilder
      52: invokestatic  #4                  // Method java/lang/System.currentTimeMillis:()J
      55: lstore        5
　　　　// 生成一个StringBuilder
      57: new           #7                  // class java/lang/StringBuilder
      60: dup
      61: ldc           #2                  // String s1
      63: invokespecial #13                 // Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
      66: astore        7
      68: aload         7
      70: invokevirtual #14                 // Method java/lang/StringBuilder.length:()I
      73: ldc           #6                  // int 100000
      75: if_icmpge     88
      78: aload         7
      80: aload_2
　　　　//调用append方法，不新建对象

　　　 81: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      84: pop
      85: goto          68
      88: getstatic     #11                 // Field java/lang/System.out:Ljava/io/PrintStream;
      91: invokestatic  #4                  // Method java/lang/System.currentTimeMillis:()J
      94: lload         5
      96: lsub
      97: invokevirtual #12                 // Method java/io/PrintStream.println:(J)V
     100: return

这个时候你可能有疑问，这个循环执行了50000次，创建了50000个StringBuilder对象会慢这么多吗？

其实StringBuilder的toString方法会调用new String()也就是说会创建十万个对象，并且量变会引起质变，

十万个对象产生的耗时不仅是创建对象的耗时，还有gc的耗时，下面是gc日志

[GC (Allocation Failure) [PSYoungGen: 33280K->1576K(38400K)] 33280K->1584K(125952K), 0.0262659 secs] [Times: user=0.03 sys=0.02, real=0.03 secs] 
[GC (Allocation Failure) [PSYoungGen: 34856K->1224K(71680K)] 34864K->1240K(159232K), 0.0019681 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 67784K->1158K(71680K)] 67800K->1174K(159232K), 0.0094555 secs] [Times: user=0.03 sys=0.00, real=0.01 secs] 
[GC (Allocation Failure) [PSYoungGen: 67718K->1118K(138240K)] 67734K->1134K(225792K), 0.0287607 secs] [Times: user=0.06 sys=0.00, real=0.03 secs] 
[GC (Allocation Failure) [PSYoungGen: 134238K->1155K(138240K)] 134254K->1171K(225792K), 0.0214991 secs] [Times: user=0.01 sys=0.00, real=0.02 secs] 
[GC (Allocation Failure) [PSYoungGen: 134275K->1109K(268288K)] 134291K->1125K(355840K), 0.0133364 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
[GC (Allocation Failure) [PSYoungGen: 267349K->69K(268288K)] 267365K->1163K(355840K), 0.0325131 secs] [Times: user=0.02 sys=0.02, real=0.03 secs] 
[GC (Allocation Failure) [PSYoungGen: 266309K->119K(534528K)] 267403K->1213K(622080K), 0.0109569 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
[GC (Allocation Failure) [PSYoungGen: 532599K->86K(534528K)] 533693K->1180K(622080K), 0.0294418 secs] [Times: user=0.00 sys=0.02, real=0.03 secs] 
[GC (Allocation Failure) [PSYoungGen: 532566K->221K(689152K)] 533660K->1315K(776704K), 0.0138167 secs] [Times: user=0.02 sys=0.01, real=0.01 secs] 
[GC (Allocation Failure) [PSYoungGen: 687325K->177K(689152K)] 688419K->1271K(776704K), 0.0188002 secs] [Times: user=0.02 sys=0.00, real=0.02 secs] 
[GC (Allocation Failure) [PSYoungGen: 687281K->113K(689152K)] 688375K->1207K(776704K), 0.0177870 secs] [Times: user=0.03 sys=0.00, real=0.02 secs] 
[GC (Allocation Failure) [PSYoungGen: 687217K->121K(689152K)] 688311K->1215K(776704K), 0.0004647 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 687225K->128K(689152K)] 688319K->1222K(776704K), 0.0187945 secs] [Times: user=0.03 sys=0.00, real=0.02 secs] 
[GC (Allocation Failure) [PSYoungGen: 687232K->238K(689152K)] 688326K->1332K(776704K), 0.0005238 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 687342K->360K(689152K)] 688436K->1454K(776704K), 0.0004996 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 687464K->262K(689152K)] 688558K->1356K(776704K), 0.0010731 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 687366K->395K(688128K)] 688460K->1489K(775680K), 0.0014061 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 687499K->158K(688640K)] 688593K->1252K(776192K), 0.0160660 secs] [Times: user=0.05 sys=0.00, real=0.02 secs] 
[GC (Allocation Failure) [PSYoungGen: 687262K->427K(689664K)] 688356K->1520K(777216K), 0.0006749 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 688555K->305K(689664K)] 689648K->1398K(777216K), 0.0026644 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 688433K->314K(689664K)] 689526K->1408K(777216K), 0.0006023 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 688442K->178K(689664K)] 689536K->1271K(777216K), 0.0143623 secs] [Times: user=0.01 sys=0.00, real=0.01 secs] 
[GC (Allocation Failure) [PSYoungGen: 688306K->333K(689664K)] 689399K->1426K(777216K), 0.0022489 secs] [Times: user=0.01 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 688461K->341K(689664K)] 689554K->1435K(777216K), 0.0230670 secs] [Times: user=0.05 sys=0.00, real=0.02 secs] 
[GC (Allocation Failure) [PSYoungGen: 688469K->350K(688640K)] 689563K->1444K(776192K), 0.0205516 secs] [Times: user=0.03 sys=0.00, real=0.02 secs] 
[GC (Allocation Failure) [PSYoungGen: 688478K->195K(689152K)] 689572K->1289K(776704K), 0.0035319 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 688323K->366K(690176K)] 689417K->1460K(777728K), 0.0115201 secs] [Times: user=0.02 sys=0.00, real=0.01 secs] 
[GC (Allocation Failure) [PSYoungGen: 689518K->546K(690176K)] 690612K->1639K(777728K), 0.0176273 secs] [Times: user=0.03 sys=0.00, real=0.02 secs] 
[GC (Allocation Failure) [PSYoungGen: 689698K->557K(690176K)] 690791K->1651K(777728K), 0.0018610 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 689709K->210K(690176K)] 690803K->1304K(777728K), 0.0134489 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
[GC (Allocation Failure) [PSYoungGen: 689362K->580K(690176K)] 690456K->1673K(777728K), 0.0004401 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 689732K->218K(690176K)] 690825K->1311K(777728K), 0.0015334 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 689370K->411K(690176K)] 690463K->1505K(777728K), 0.0006384 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 689563K->611K(690176K)] 690657K->1705K(777728K), 0.0005230 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 

　　

总结：

两个字符串相加底层会生成StringBuilder对象，然后使用append来拼接，如果不在循环中使用，可以不用StringBuilder，

但是养成使用StringBuilder的习惯能避免出现问题，如果使用idea的话，万能的alt+enter会立刻把字符串相加变成

StringBuilder。