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关于java中字符串联接使用 Stirng+String+. 效率的一点想法

分类: IT文章 • 2024-07-14 13:44:12
关于java中字符串连接使用 Stirng+String+... 效率的一点想法
记得自己还是个连菜鸟都算不上的coder的时候，老鸟们总教育我：代码里不要出现String+String+...这种代码，非多线程的情况下要用StringBuilder效率才高
当时没啥感觉，最近闲来无事，想看看java编译器都对String+String+...这种代码怎么处理的呢？于是发现javac也是用StringBuilder来处理的，所以我感觉这两者效率上是没有区别的，如本人理解有错，望老鸟给予指正...
请看代码吧，主要代码我都加上了注释
package test;

public class Test {

    public static void main(String[] args) {

        String s1 = new String("小菜鸟");
        String s2 = new String("大菜鸟");
        
        String resultString = s1 + s2 + "老鸟";
    }

}


以上代码编译成class文件后
用javap Test -verbose 命令看下字节码文件中的内容：
注意下面代码第100行，重点所在

D:\Workspace\MyEclipse\workspace\Test\bin\test>javap Test -verbose
Compiled from "Test.java"
public class test.Test extends java.lang.Object
  SourceFile: "Test.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = class        #2;     //  test/Test
const #2 = Asciz        test/Test;
const #3 = class        #4;     //  java/lang/Object
const #4 = Asciz        java/lang/Object;
const #5 = Asciz        <init>;
const #6 = Asciz        ()V;
const #7 = Asciz        Code;
const #8 = Method       #3.#9;  //  java/lang/Object."<init>":()V
const #9 = NameAndType  #5:#6;//  "<init>":()V
const #10 = Asciz       LineNumberTable;
const #11 = Asciz       LocalVariableTable;
const #12 = Asciz       this;
const #13 = Asciz       Ltest/Test;;
const #14 = Asciz       main;
const #15 = Asciz       ([Ljava/lang/String;)V;
const #16 = class       #17;    //  java/lang/String
const #17 = Asciz       java/lang/String;
const #18 = String      #19;    //  小菜鸟
const #19 = Asciz       小菜鸟;
const #20 = Method      #16.#21;        //  java/lang/String."<init>":(Ljava/lan
g/String;)V
const #21 = NameAndType #5:#22;//  "<init>":(Ljava/lang/String;)V
const #22 = Asciz       (Ljava/lang/String;)V;
const #23 = String      #24;    //  大菜鸟
const #24 = Asciz       大菜鸟;
const #25 = class       #26;    //  java/lang/StringBuilder
const #26 = Asciz       java/lang/StringBuilder;
const #27 = Method      #16.#28;        //  java/lang/String.valueOf:(Ljava/lang
/Object;)Ljava/lang/String;
const #28 = NameAndType #29:#30;//  valueOf:(Ljava/lang/Object;)Ljava/lang/Strin
g;
const #29 = Asciz       valueOf;
const #30 = Asciz       (Ljava/lang/Object;)Ljava/lang/String;;
const #31 = Method      #25.#21;        //  java/lang/StringBuilder."<init>":(Lj
ava/lang/String;)V
const #32 = Method      #25.#33;        //  java/lang/StringBuilder.append:(Ljav
a/lang/String;)Ljava/lang/StringBuilder;
const #33 = NameAndType #34:#35;//  append:(Ljava/lang/String;)Ljava/lang/String
Builder;
const #34 = Asciz       append;
const #35 = Asciz       (Ljava/lang/String;)Ljava/lang/StringBuilder;;
const #36 = String      #37;    //  老鸟
const #37 = Asciz       老鸟;
const #38 = Method      #25.#39;        //  java/lang/StringBuilder.toString:()L
java/lang/String;
const #39 = NameAndType #40:#41;//  toString:()Ljava/lang/String;
const #40 = Asciz       toString;
const #41 = Asciz       ()Ljava/lang/String;;
const #42 = Asciz       args;
const #43 = Asciz       [Ljava/lang/String;;
const #44 = Asciz       s1;
const #45 = Asciz       Ljava/lang/String;;
const #46 = Asciz       s2;
const #47 = Asciz       resultString;
const #48 = Asciz       SourceFile;
const #49 = Asciz       Test.java;

{
public test.Test();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #8; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 3: 0

  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      5      0    this       Ltest/Test;


public static void main(java.lang.String[]);
  Code:
   Stack=3, Locals=4, Args_size=1
   0:   new     #16; //class java/lang/String |我加上的：创建一个新对象，对象类型存储在常量池#16
   3:   dup				//我加上的：复制栈顶部一个字长的内容
   4:   ldc     #18; //String 小菜鸟 |我加上的：把常量池中索引为#18的值压入栈
   6:   invokespecial   #20; //Method java/lang/String."<init>":(Ljava/lang/String;)V |我加上的：调用初始化方法
   9:   astore_1		//我加上的：将引用类型存入局部变量1
   10:  new     #16; //class java/lang/String |我加上的：10-19同上实例化另一个String对象(大菜鸟)
   13:  dup
   14:  ldc     #23; //String 大菜鸟
   16:  invokespecial   #20; //Method java/lang/String."<init>":(Ljava/lang/String;)V
   19:  astore_2
   20:  new     #25; //class java/lang/StringBuilder |我加上的：创建一个StringBuilder对象
   23:  dup
   24:  aload_1			//我加上的：从局部变量1中装载引用类型
   25:  invokestatic    #27; //Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;
   28:  invokespecial   #31; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
   31:  aload_2			//我加上的：从局部变量2中装载引用类型
   //哈哈，看到这里了吧，javac(我的jdk版本是1.6)编译后的字节码对于String+String+...的处理也是使用StringBuilder的append的
   32:  invokevirtual   #32; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   35:  ldc     #36; //String 老鸟
   //同理，连接字符串“老鸟”
   37:  invokevirtual   #32; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   40:  invokevirtual   #38; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   43:  astore_3
   44:  return
  LineNumberTable:
   line 7: 0
   line 8: 10
   line 10: 20
   line 11: 44

  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      45      0    args       [Ljava/lang/String;
   10      35      1    s1       Ljava/lang/String;
   20      25      2    s2       Ljava/lang/String;
   44      1      3    resultString       Ljava/lang/String;


}


D:\Workspace\MyEclipse\workspace\Test\bin\test>

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