從java反編譯及字節(jié)碼角度探索分析String拼接字符串效率

更新時(shí)間：2023年12月11日 10:29:57 作者：紅袖添香

這篇文章主要介紹了從java反編譯及字節(jié)碼角度探索分析String拼接字符串效率，有需要的朋友可以借鑒參考下,希望能夠有所幫助,祝大家多多進(jìn)步,早日升職加薪

前言

又刷到這篇文章

為什么idea建議去掉StringBuilder，使用“+”拼接字符串

網(wǎng)上有很多文章用 JUnit 進(jìn)行測(cè)試驗(yàn)證，可以網(wǎng)上搜一下，這里不做贅述。今天我們從反編譯和字節(jié)碼的角度分析字符串拼接的時(shí)候底層到底做了什么

拼接場(chǎng)景

示例 1：

public class Hello2 {
    public static void main(String[] args) {
    }
    public void add1() {
        String a = "aaa" + "bbb" + "ccc";
        String b = new StringBuilder("aaa").append("bbb").append("ccc").toString();
    }
}

反編譯結(jié)果：

package com.noah.nowcoder;
public class Hello2 {
  public static void main(String[] args) {}
  public void add1() {
    String a = "aaabbbccc";
    String b = "aaa" + "bbb" + "ccc";
  }
}

Java 編譯器優(yōu)化（JDK 版本相關(guān)），編譯結(jié)果可以直接看出， String a = "aaa" + "bbb" + "ccc" 執(zhí)行效率更高；

示例 2：

public class Hello {
    public static void main(String[] args) {
        String str1 = "";
        for (int i = 0; i < 100000; i++) {
            str1 += "-" + UUID.randomUUID().toString();
        }
        System.out.println(str1);
        StringBuilder stringBuilder = new StringBuilder();
        for(int i = 0; i < 100000; ++i) {
            stringBuilder.append("-").append(UUID.randomUUID().toString());
        }
        System.out.println(stringBuilder.toString());
    }
}

反編譯結(jié)果：

package com.noah.nowcoder;
import java.util.UUID;
public class Hello {
  public static void main(String[] args) {
    String str1 = "";
    for (int i = 0; i < 100000; i++)
      str1 = str1 + "-" + UUID.randomUUID().toString(); 
    System.out.println(str1);
    StringBuilder stringBuilder = new StringBuilder();
    for (int j = 0; j < 100000; j++)
      stringBuilder.append("-").append(UUID.randomUUID().toString()); 
    System.out.println(stringBuilder.toString());
  }
}

這一步結(jié)果不明顯

字節(jié)碼信息

接下來(lái)，我們看一下字節(jié)碼信息

Compiled from "Hello.java"
public class com.noah.nowcoder.Hello {
  public com.noah.nowcoder.Hello();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return
  public static void main(java.lang.String[]);
    Code:
       0: ldc           #2                  // String
       2: astore_1
       3: iconst_0
       4: istore_2
       5: iload_2
       6: ldc           #3                  // int 100000
       8: if_icmpge     46
      11: new           #4                  // class java/lang/StringBuilder
      14: dup
      15: invokespecial #5                  // Method java/lang/StringBuilder."<init>":()V
      18: aload_1
      19: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      22: ldc           #7                  // String -
      24: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      27: invokestatic  #8                  // Method java/util/UUID.randomUUID:()Ljava/util/UUID;
      30: invokevirtual #9                  // Method java/util/UUID.toString:()Ljava/lang/String;
      33: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      36: invokevirtual #10                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      39: astore_1
      40: iinc          2, 1
      43: goto          5
      46: getstatic     #11                 // Field java/lang/System.out:Ljava/io/PrintStream;
      49: aload_1
      50: invokevirtual #12                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      53: new           #4                  // class java/lang/StringBuilder
      56: dup
      57: invokespecial #5                  // Method java/lang/StringBuilder."<init>":()V
      60: astore_2
      61: iconst_0
      62: istore_3
      63: iload_3
      64: ldc           #3                  // int 100000
      66: if_icmpge     91
      69: aload_2
      70: ldc           #7                  // String -
      72: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      75: invokestatic  #8                  // Method java/util/UUID.randomUUID:()Ljava/util/UUID;
      78: invokevirtual #9                  // Method java/util/UUID.toString:()Ljava/lang/String;
      81: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      84: pop
      85: iinc          3, 1
      88: goto          63
      91: getstatic     #11                 // Field java/lang/System.out:Ljava/io/PrintStream;
      94: aload_2
      95: invokevirtual #10                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      98: invokevirtual #12                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
     101: return
}

注意：

15: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V

調(diào)用了 StringBuilder 構(gòu)造函數(shù)，初始化了一個(gè) StringBuilder 對(duì)象（循環(huán)中初始化對(duì)象）

總結(jié)

根據(jù)字節(jié)碼改寫代碼

    public void add2() {
        String str1 = "";
        for (int i = 0; i < 100000; i++) {
            str1 += "-" + UUID.randomUUID().toString();
        }
        System.out.println(str1);
    }
    public void add3() {
        String str1 = "";
        for (int i = 0; i < 100000; i++) {
            StringBuilder stringBuilder = new StringBuilder();
            str1 = stringBuilder.append(str1).append("-").append(UUID.randomUUID().toString()).toString();
        }
        System.out.println(str1);
    }

字節(jié)碼文件：

public void add3();
    Code:
       0: ldc           #10                 // String
       2: astore_1
       3: iconst_0
       4: istore_2
       5: iload_2
       6: ldc           #11                 // int 100000
       8: if_icmpge     48
      11: new           #3                  // class java/lang/StringBuilder
      14: dup
      15: invokespecial #12                 // Method java/lang/StringBuilder."<init>":()V
      18: astore_3
      19: aload_3
      20: aload_1
      21: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      24: ldc           #13                 // String -
      26: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      29: invokestatic  #14                 // Method java/util/UUID.randomUUID:()Ljava/util/UUID;
      32: invokevirtual #15                 // Method java/util/UUID.toString:()Ljava/lang/String;
      35: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      38: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      41: astore_1
      42: iinc          2, 1
      45: goto          5
      48: getstatic     #16                 // Field java/lang/System.out:Ljava/io/PrintStream;
      51: aload_1
      52: invokevirtual #17                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      55: return

從字節(jié)碼文件可以看出，add2() 和 add3() 基本上等價(jià)的。

所以循環(huán)中拼接字符串更高效的做法是將 StringBulider 放在循環(huán)外。

如下：

  public static void main(String[] args) {
    StringBuilder stringBuilder = new StringBuilder();
    for (int j = 0; j < 100000; j++)
      stringBuilder.append("-").append(UUID.randomUUID().toString()); 
    System.out.println(stringBuilder.toString());
  }

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