在我们的常识里面，用String的+符号也就是字符串的时候，效率会很低，建议使用String builder，那是为什么呢？这次我们通过一个小demo的jvm字节码来分析，首先是demo：
public class TestStringAdd {
public static void f1() {
String src = "";
for(int i=0;i<10;i++) {
//每一次循环都会new一个StringBuilder
src = src + "A";
}
System.out.println(src);
}
public static void f2() {
//只要一个StringBuilder
StringBuilder src = new StringBuilder();
for(int i=0;i<10;i++) {
src.append("A");
}
System.out.println(src);
}
}
解析成字节码之后，就是这样子的：
Classfile /G:/WorkingProjects/learn/project/test/out/production/test/TestStringAdd.class
Last modified 2018-11-19; size 991 bytes
MD5 checksum a8627bc62f24056ffb5ed04ca4386ac2
Compiled from "TestStringAdd.java"
public class TestStringAdd
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #12.#32 // java/lang/Object."<init>":()V
#2 = String #33 //
#3 = Class #34 // java/lang/StringBuilder
#4 = Methodref #3.#32 // java/lang/StringBuilder."<init>":()V
#5 = Methodref #3.#35 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#6 = String #36 // A
#7 = Methodref #3.#37 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#8 = Fieldref #38.#39 // java/lang/System.out:Ljava/io/PrintStream;
#9 = Methodref #40.#41 // java/io/PrintStream.println:(Ljava/lang/String;)V
#10 = Methodref #40.#42 // java/io/PrintStream.println:(Ljava/lang/Object;)V
#11 = Class #43 // TestStringAdd
#12 = Class #44 // java/lang/Object
#13 = Utf8 <init>
#14 = Utf8 ()V
#15 = Utf8 Code
#16 = Utf8 LineNumberTable
#17 = Utf8 LocalVariableTable
#18 = Utf8 this
#19 = Utf8 LTestStringAdd;
#20 = Utf8 f1
#21 = Utf8 i
#22 = Utf8 I
#23 = Utf8 src
#24 = Utf8 Ljava/lang/String;
#25 = Utf8 StackMapTable
#26 = Class #45 // java/lang/String
#27 = Utf8 f2
#28 = Utf8 Ljava/lang/StringBuilder;
#29 = Class #34 // java/lang/StringBuilder
#30 = Utf8 SourceFile
#31 = Utf8 TestStringAdd.java
#32 = NameAndType #13:#14 // "<init>":()V
#33 = Utf8
#34 = Utf8 java/lang/StringBuilder
#35 = NameAndType #46:#47 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#36 = Utf8 A
#37 = NameAndType #48:#49 // toString:()Ljava/lang/String;
#38 = Class #50 // java/lang/System
#39 = NameAndType #51:#52 // out:Ljava/io/PrintStream;
#40 = Class #53 // java/io/PrintStream
#41 = NameAndType #54:#55 // println:(Ljava/lang/String;)V
#42 = NameAndType #54:#56 // println:(Ljava/lang/Object;)V
#43 = Utf8 TestStringAdd
#44 = Utf8 java/lang/Object
#45 = Utf8 java/lang/String
#46 = Utf8 append
#47 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#48 = Utf8 toString
#49 = Utf8 ()Ljava/lang/String;
#50 = Utf8 java/lang/System
#51 = Utf8 out
#52 = Utf8 Ljava/io/PrintStream;
#53 = Utf8 java/io/PrintStream
#54 = Utf8 println
#55 = Utf8 (Ljava/lang/String;)V
#56 = Utf8 (Ljava/lang/Object;)V
{
public TestStringAdd();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 1: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LTestStringAdd;
public static void f1();
descriptor: ()V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=0
0: ldc #2 // String
2: astore_0
3: iconst_0
4: istore_1
5: iload_1
6: bipush 10
8: if_icmpge 37
11: new #3 // class java/lang/StringBuilder
14: dup
15: invokespecial #4 // Method java/lang/StringBuilder."<init>":()V
18: aload_0
19: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
22: ldc #6 // String A
24: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
27: invokevirtual #7 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
30: astore_0
31: iinc 1, 1
34: goto 5
37: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream;
40: aload_0
41: invokevirtual #9 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
44: return
LineNumberTable:
line 3: 0
line 4: 3
line 6: 11
line 4: 31
line 8: 37
line 9: 44
LocalVariableTable:
Start Length Slot Name Signature
5 32 1 i I
3 42 0 src Ljava/lang/String;
StackMapTable: number_of_entries = 2
frame_type = 253 /* append */
offset_delta = 5
locals = [ class java/lang/String, int ]
frame_type = 250 /* chop */
offset_delta = 31
public static void f2();
descriptor: ()V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=0
0: new #3 // class java/lang/StringBuilder
3: dup
4: invokespecial #4 // Method java/lang/StringBuilder."<init>":()V
7: astore_0
8: iconst_0
9: istore_1
10: iload_1
11: bipush 10
13: if_icmpge 29
16: aload_0
17: ldc #6 // String A
19: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
22: pop
23: iinc 1, 1
26: goto 10
29: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream;
32: aload_0
33: invokevirtual #10 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
36: return
LineNumberTable:
line 12: 0
line 13: 8
line 14: 16
line 13: 23
line 16: 29
line 17: 36
LocalVariableTable:
Start Length Slot Name Signature
10 19 1 i I
8 29 0 src Ljava/lang/StringBuilder;
StackMapTable: number_of_entries = 2
frame_type = 253 /* append */
offset_delta = 10
locals = [ class java/lang/StringBuilder, int ]
frame_type = 250 /* chop */
offset_delta = 18
}
SourceFile: "TestStringAdd.java"
我们先看下几个指令的意思，这里中文解释很好：https://www.cnblogs.com/tenghoo/p/jvm_opcodejvm.html
ldc #2：命令负责把数值常量或String常量值（#2）从常量池中推送至栈顶。
if_icmpge 37：意思就是如果相比大于，那就跳到第37行，否则继续往下执行。最后将栈里面的这两个元素移除。
dup： 复制栈顶数值(数值不能是long或double类型的)并将复制值压入栈顶
goto 5：跳到第5行
有了这些前提知识之后，我们可以知道，在本篇文章中的String在调用+之前都会new StringBuilder，然后才append，这是比较耗时间的。