String str =“a“ + “b“ 与 String str = new String(“a“) + new String(“b“) 的区别（jdk1.8环境利用javap命令探究）

喜欢ヅ旅行 2022-10-02 10:59 131阅读 0赞

今天，又在思考一个常见的问题，那就是字符串创建对象系列。发现了一个很多人都模糊不清的问题，那就是  
String str =“a” + “b” 与 Strin str = new String(“c”) + new String(“d”) 把对象创建在了哪里？创建了几个字符串对象？  
话不多出，show my code：

### String str =“a” + “b” ###

package com.bestqiang.set;
    
    /** * @author BestQiang */
    public class Test2 { 
        public static void main(String[] args) { 
            String str = "a" + "b";
        }
    }

使用 javap进行解析：

Classfile /D:/workspace_idea/ data_structure/out/production/Container/com/bestqiang/set/Test2.class
      Last modified 2019-6-18; size 446 bytes
      MD5 checksum a78553412dae26fae704645a9bf0b82b
      Compiled from "Test2.java"
    public class com.bestqiang.set.Test2
      minor version: 0
      major version: 52
      flags: ACC_PUBLIC, ACC_SUPER
    Constant pool:
       #1 = Methodref          #4.#20         // java/lang/Object."<init>":()V
       #2 = String             #21            // ab
       #3 = Class              #22            // com/bestqiang/set/Test2
       #4 = Class              #23            // java/lang/Object
       #5 = Utf8               <init>
       #6 = Utf8               ()V
       #7 = Utf8               Code
       #8 = Utf8               LineNumberTable
       #9 = Utf8               LocalVariableTable
      #10 = Utf8               this
      #11 = Utf8               Lcom/bestqiang/set/Test2;
      #12 = Utf8               main
      #13 = Utf8               ([Ljava/lang/String;)V
      #14 = Utf8               args
      #15 = Utf8               [Ljava/lang/String;
      #16 = Utf8               str
      #17 = Utf8               Ljava/lang/String;
      #18 = Utf8               SourceFile
      #19 = Utf8               Test2.java
      #20 = NameAndType        #5:#6          // "<init>":()V
      #21 = Utf8               ab
      #22 = Utf8               com/bestqiang/set/Test2
      #23 = Utf8               java/lang/Object
    {
      public com.bestqiang.set.Test2();
        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 6: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       5     0  this   Lcom/bestqiang/set/Test2;
    
      public static void main(java.lang.String[]);
        descriptor: ([Ljava/lang/String;)V
        flags: ACC_PUBLIC, ACC_STATIC
        Code:
          stack=1, locals=2, args_size=1
             0: ldc           #2                  // String ab
             2: astore_1
             3: return
          LineNumberTable:
            line 8: 0
            line 11: 3
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       4     0  args   [Ljava/lang/String;
                3       1     1   str   Ljava/lang/String;
    }
    SourceFile: "Test2.java"
    
    Process finished with exit code 0

你认为 String str = “a” + “b”;这是创建了几个对象呢？没错，你没看错，就是只创建了一个对象

*  String常量的累加操作中，编译器会进行优化，如果是 String str = “a” + “b”,那相当于String str = “ab”; ，常量池中会创建一个ab的对象。 ldc \#2 // String ab 然后 将int,float或String型常量值从常量池中推送至栈顶即可。等于只在常量池中创建了一个对象，jdk1.8的常量池在java堆中，等于在java堆的常量池创建了一个对象。

### String str = new String(“a”) + new String(“b”) ###

package com.bestqiang.set;
    
    /** * @author BestQiang */
    public class Test2 { 
        public static void main(String[] args) { 
           
             String str1 = new String("a") + new String("b");
          
        }
    }

javap：

Classfile /D:/workspace_idea/ data_structure/out/production/Container/com/bestqiang/set/Test2.class
      Last modified 2019-6-18; size 686 bytes
      MD5 checksum 14d133b0ba6751a29914b05da34af598
      Compiled from "Test2.java"
    public class com.bestqiang.set.Test2
      minor version: 0
      major version: 52
      flags: ACC_PUBLIC, ACC_SUPER
    Constant pool:
       #1 = Methodref          #11.#27        // java/lang/Object."<init>":()V
       #2 = Class              #28            // java/lang/StringBuilder
       #3 = Methodref          #2.#27         // java/lang/StringBuilder."<init>":()V
       #4 = Class              #29            // java/lang/String
       #5 = String             #30            // a
       #6 = Methodref          #4.#31         // java/lang/String."<init>":(Ljava/lang/String;)V
       #7 = Methodref          #2.#32         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
       #8 = String             #33            // b
       #9 = Methodref          #2.#34         // java/lang/StringBuilder.toString:()Ljava/lang/String;
      #10 = Class              #35            // com/bestqiang/set/Test2
      #11 = Class              #36            // java/lang/Object
      #12 = Utf8               <init>
      #13 = Utf8               ()V
      #14 = Utf8               Code
      #15 = Utf8               LineNumberTable
      #16 = Utf8               LocalVariableTable
      #17 = Utf8               this
      #18 = Utf8               Lcom/bestqiang/set/Test2;
      #19 = Utf8               main
      #20 = Utf8               ([Ljava/lang/String;)V
      #21 = Utf8               args
      #22 = Utf8               [Ljava/lang/String;
      #23 = Utf8               str1
      #24 = Utf8               Ljava/lang/String;
      #25 = Utf8               SourceFile
      #26 = Utf8               Test2.java
      #27 = NameAndType        #12:#13        // "<init>":()V
      #28 = Utf8               java/lang/StringBuilder
      #29 = Utf8               java/lang/String
      #30 = Utf8               a
      #31 = NameAndType        #12:#37        // "<init>":(Ljava/lang/String;)V
      #32 = NameAndType        #38:#39        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      #33 = Utf8               b
      #34 = NameAndType        #40:#41        // toString:()Ljava/lang/String;
      #35 = Utf8               com/bestqiang/set/Test2
      #36 = Utf8               java/lang/Object
      #37 = Utf8               (Ljava/lang/String;)V
      #38 = Utf8               append
      #39 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
      #40 = Utf8               toString
      #41 = Utf8               ()Ljava/lang/String;
    {
      public com.bestqiang.set.Test2();
        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 6: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       5     0  this   Lcom/bestqiang/set/Test2;
    
      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 a
            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: new           #4                  // class java/lang/String
            22: dup
            23: ldc           #8                  // String b
            25: invokespecial #6                  // Method java/lang/String."<init>":(Ljava/lang/String;)V
            28: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
            31: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
            34: astore_1
            35: return
          LineNumberTable:
            line 9: 0
            line 11: 35
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0      36     0  args   [Ljava/lang/String;
               35       1     1  str1   Ljava/lang/String;
    }
    SourceFile: "Test2.java"
    
    Process finished with exit code 0

从javap的解析结果可以看出，在常量池中生成了 a 和 b两个字符串对象，并没有ab这个字符串对象。在堆中用两个new创建了两个String对象，然后通过StringBuilder对象调用append方法进行两次字符串的添加操作，最后用StringBuilder的toString方法进行返回了一个堆中的字符串对象。  
![在这里插入图片描述][20190618140510357.png]  
所以，Strin str = new String(“a”) + new String(“b”) 在堆中创建了三个字符串对象（a字符串对象，b字符串对象，ab字符串对象），在堆中的常量池中创建了2个字符串对象（a字符串对象，b字符串对象），一共创建了5个字符串对象。

**总结：**

*  String str =“a” + “b” 创建了1个字符串对象，在java堆中的常量池中
 *  String str = new String(“a”) + new String(“b”) 创建了5个字符串对象，3个在java堆中，2个在java堆的常量池中。

希望大家能够从上面的分析有所收获，上面的分析如有谬论，请大家指出，共同探究。

[20190618140510357.png]: /images/20220112/8cdd6fd618df4a7d8afb58b73be9d16f.png