從字符串拼接之StringBuilder到String str = new String("a" + "b") + "a" + "b"創(chuàng)建了多少個(gè)字符串對(duì)象

1.StringBuilder

??我們?cè)趯?duì)做字符串拼接時(shí)，如果采用如下操作：

String result = "";
for(String str : strArr){
    result += str;
}

??確實(shí)能夠達(dá)到拼接字符串的目的，但是會(huì)在字符串常量池中創(chuàng)建多個(gè)字符串對(duì)象，會(huì)浪費(fèi)大量的內(nèi)存，并且影響GC（垃圾收集）效率。為了解決這個(gè)問題，Java標(biāo)準(zhǔn)庫提供了StringBuilder類以實(shí)現(xiàn)字符串拼接。StringBuilder與String不同，它的對(duì)象是可變對(duì)象，并且會(huì)預(yù)分配緩沖區(qū)，如此就不會(huì)創(chuàng)建多個(gè)對(duì)象，只用創(chuàng)建一個(gè)StringBuilder對(duì)象即可。
??一般情況下，對(duì)于字符串間的+操作，并不需要用StringBuilder。因?yàn)镴ava編譯時(shí)會(huì)把字符串+操作編譯為StringConcatFactory的操作，自動(dòng)將字符串拼接優(yōu)化為數(shù)組復(fù)制或StringBuilder的操作。

2.從下面兩道面試題展開討論：

1 String str = "a" + "b";此條語句創(chuàng)建了多少個(gè)對(duì)象？
2 String str = new String("a" + "b") + "a" + "b";此條語句創(chuàng)建了多少個(gè)對(duì)象？

??第一條語句，Java編譯時(shí)會(huì)將"a"+"b"直接編譯為"ab"，如此原語句等同于String str = "ab";，這樣會(huì)在字符串常量池中創(chuàng)建一個(gè)字符串對(duì)象。
??對(duì)于本例，我們編寫代碼并使用javap進(jìn)行解析：

public class cp1 {
    public static void main(String[] args){
        String s = "a" + "b";
        System.out.println(s);
    }
}

??使用javac -g cp1.java編譯源碼之后，使用javap -v cp1查看解析結(jié)果：

Classfile /E:/Java學(xué)習(xí)/src/cp1.class
  Last modified 2021-10-19; size 544 bytes
  MD5 checksum e771532b8aacc4a59518aae9ac7faddd
  Compiled from "cp1.java"
public class cp1
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #6.#22         // java/lang/Object."<init>":()V
   #2 = String             #23            // ab
   #3 = Fieldref           #24.#25        // java/lang/System.out:Ljava/io/PrintStream;
   #4 = Methodref          #26.#27        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #5 = Class              #28            // cp1
   #6 = Class              #29            // java/lang/Object
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Utf8               LineNumberTable
  #11 = Utf8               LocalVariableTable
  #12 = Utf8               this
  #13 = Utf8               Lcp1;
  #14 = Utf8               main
  #15 = Utf8               ([Ljava/lang/String;)V
  #16 = Utf8               args
  #17 = Utf8               [Ljava/lang/String;
  #18 = Utf8               s
  #19 = Utf8               Ljava/lang/String;
  #20 = Utf8               SourceFile
  #21 = Utf8               cp1.java
  #22 = NameAndType        #7:#8          // "<init>":()V
  #23 = Utf8               ab
  #24 = Class              #30            // java/lang/System
  #25 = NameAndType        #31:#32        // out:Ljava/io/PrintStream;
  #26 = Class              #33            // java/io/PrintStream
  #27 = NameAndType        #34:#35        // println:(Ljava/lang/String;)V
  #28 = Utf8               cp1
  #29 = Utf8               java/lang/Object
  #30 = Utf8               java/lang/System
  #31 = Utf8               out
  #32 = Utf8               Ljava/io/PrintStream;
  #33 = Utf8               java/io/PrintStream
  #34 = Utf8               println
  #35 = Utf8               (Ljava/lang/String;)V
{
  public cp1();
    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 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcp1;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=2, args_size=1
         0: ldc           #2                  // String ab
         2: astore_1
         3: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
         6: aload_1
         7: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        10: return
      LineNumberTable:
        line 13: 0
        line 14: 3
        line 15: 10
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      11     0  args   [Ljava/lang/String;
            3       8     1     s   Ljava/lang/String;
}
SourceFile: "cp1.java"

??可以看到在Constant pool中新創(chuàng)建了一個(gè)字符串對(duì)象"ab"，在堆中并沒有創(chuàng)建。總計(jì)只創(chuàng)建了一個(gè)字符串對(duì)象。
??第二條語句，new String("a" + "b") + "a" + "b";中，同樣的編譯器將"a"+"b"直接編譯為"ab"，會(huì)在字符串常量池中創(chuàng)建"ab"一個(gè)字符串對(duì)象，然后在堆中創(chuàng)建一個(gè)"ab"字符串對(duì)象。然后調(diào)用StringBuilder的append()方法拼接字符串后，調(diào)用toString()方法返回了一個(gè)字符串對(duì)象"abab"。
??對(duì)第二例采用相同的方法進(jìn)行解析：

// 源碼
public class cp1 {
        String s = new String("a" + "b") + "a" + "b";
        System.out.println(s);
    }
}

// javap 解析結(jié)果
Classfile /E:/Java學(xué)習(xí)/src/cp1.class
  Last modified 2021-10-19; size 746 bytes
  MD5 checksum 4d1188a58dd3d189626ca06c1a5f9e18
  Compiled from "cp1.java"
public class cp1
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #12.#28        // java/lang/Object."<init>":()V
   #2 = Class              #29            // java/lang/StringBuilder
   #3 = Methodref          #2.#28         // java/lang/StringBuilder."<init>":()V
   #4 = Class              #30            // java/lang/String
   #5 = String             #31            // ab
   #6 = Methodref          #4.#32         // java/lang/String."<init>":(Ljava/lang/String;)V
   #7 = Methodref          #2.#33         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   #8 = Methodref          #2.#34         // java/lang/StringBuilder.toString:()Ljava/lang/String;
   #9 = Fieldref           #35.#36        // java/lang/System.out:Ljava/io/PrintStream;
  #10 = Methodref          #37.#38        // java/io/PrintStream.println:(Ljava/lang/String;)V
  #11 = Class              #39            // cp1
  #12 = Class              #40            // java/lang/Object
  #13 = Utf8               <init>
  #14 = Utf8               ()V
  #15 = Utf8               Code
  #16 = Utf8               LineNumberTable
  #17 = Utf8               LocalVariableTable
  #18 = Utf8               this
  #19 = Utf8               Lcp1;
  #20 = Utf8               main
  #21 = Utf8               ([Ljava/lang/String;)V
  #22 = Utf8               args
  #23 = Utf8               [Ljava/lang/String;
  #24 = Utf8               s
  #25 = Utf8               Ljava/lang/String;
  #26 = Utf8               SourceFile
  #27 = Utf8               cp1.java
  #28 = NameAndType        #13:#14        // "<init>":()V
  #29 = Utf8               java/lang/StringBuilder
  #30 = Utf8               java/lang/String
  #31 = Utf8               ab
  #32 = NameAndType        #13:#41        // "<init>":(Ljava/lang/String;)V
  #33 = NameAndType        #42:#43        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #34 = NameAndType        #44:#45        // toString:()Ljava/lang/String;
  #35 = Class              #46            // java/lang/System
  #36 = NameAndType        #47:#48        // out:Ljava/io/PrintStream;
  #37 = Class              #49            // java/io/PrintStream
  #38 = NameAndType        #50:#41        // println:(Ljava/lang/String;)V
  #39 = Utf8               cp1
  #40 = Utf8               java/lang/Object
  #41 = Utf8               (Ljava/lang/String;)V
  #42 = Utf8               append
  #43 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #44 = Utf8               toString
  #45 = Utf8               ()Ljava/lang/String;
  #46 = Utf8               java/lang/System
  #47 = Utf8               out
  #48 = Utf8               Ljava/io/PrintStream;
  #49 = Utf8               java/io/PrintStream
  #50 = Utf8               println
{
  public cp1();
    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 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcp1;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=4, locals=2, args_size=1
         0: new           #2                  // class java/lang/StringBuilder
         3: dup
         4: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V
         7: new           #4                  // class java/lang/String
        10: dup
        11: ldc           #5                  // String ab
        13: invokespecial #6                  // Method java/lang/String."<init>":(Ljava/lang/String;)V
        16: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        19: ldc           #5                  // String ab
        21: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        24: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        27: astore_1
        28: getstatic     #9                  // Field java/lang/System.out:Ljava/io/PrintStream;
        31: aload_1
        32: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        35: return
      LineNumberTable:
        line 13: 0
        line 14: 28
        line 15: 35
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      36     0  args   [Ljava/lang/String;
           28       8     1     s   Ljava/lang/String;
}
SourceFile: "cp1.java"

??可以看到在常量池中創(chuàng)建了一個(gè)字符串對(duì)象"ab"，在堆中創(chuàng)建了一個(gè)StringBuilder對(duì)象，一個(gè)字符串對(duì)象"ab"，并且調(diào)用StringBuilder對(duì)象的toString()方法返回了一個(gè)字符串對(duì)象"abab"?？傆?jì)創(chuàng)建了三個(gè)字符串對(duì)象，一個(gè)StringBuilder對(duì)象。

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