String 拼接字符串效率低？是真的吗？

作者：红袖添香

2023-12-05
北京
本文字数：4478 字
阅读完需：约 15 分钟

前言

又刷到这篇文章

网上有很多文章用 JUnit 进行测试验证，可以网上搜一下，这里不做赘述。今天我们从反编译和字节码的角度分析字符串拼接的时候底层到底做了什么

场景

拼接 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();    }
}

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反编译结果：

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

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Java 编译器优化（JDK 版本相关），编译结果可以直接看出， String a = "aaa" + "bbb" + "ccc" 执行效率更高；

拼接 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());    }
}

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反编译结果：

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());  }  }

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这一步结果不明显

接下来，我们看一下字节码信息

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}

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注意：

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

调用了 StringBuilder 构造函数，初始化了一个 StringBuilder 对象（循环中初始化对象）

总结：

根据字节码改写代码

    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);    }

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字节码文件：

public void add2();    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     46      11: new           #3                  // class java/lang/StringBuilder      14: dup      15: invokespecial #12                 // Method java/lang/StringBuilder."<init>":()V      18: aload_1      19: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;      22: ldc           #13                 // String -      24: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;      27: invokestatic  #14                 // Method java/util/UUID.randomUUID:()Ljava/util/UUID;      30: invokevirtual #15                 // Method java/util/UUID.toString:()Ljava/lang/String;      33: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;      36: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;      39: astore_1      40: iinc          2, 1      43: goto          5      46: getstatic     #16                 // Field java/lang/System.out:Ljava/io/PrintStream;      49: aload_1      50: invokevirtual #17                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V      53: return

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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

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从字节码文件可以看出，add2() 和 add3() 基本上等价的。

所以循环中拼接字符串更高效的做法是将 StringBulider 放在循环外。

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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发布于: 2023-12-05阅读数: 19

原文链接:【http://xie.infoq.cn/article/3daf161f2af4b4dc8159fb0a8】。文章转载请联系作者。

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String 拼接字符串效率低？是真的吗？

前言

场景

拼接 1：

拼接 2：

总结：

红袖添香

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