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java | StringTable

发表于 分类于 ， ， ， ， Valine：

常量池和串池「StringTable」的区别。

常量池与串池关系

代码如下

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package com.redisc;

import lombok.extern.slf4j.Slf4j;

@Slf4j(topic = "c.Run")
public class Run {

    public static void main(String[] args) {
        String s1 = "a";
        String s2 = "b";
        String s3 = "ab";
    }

}

反编译字节码

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Classfile /Users/licong/java/redisc/target/classes/com/redisc/Run.class
  Last modified 2022年12月18日; size 689 bytes
  SHA-256 checksum 3a85c036643acf659cbc934332903161f9dd77a32b36a01a3d76b735d67c2629
  Compiled from "Run.java"
public class com.redisc.Run
  minor version: 0
  major version: 52
  flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: #8                          // com/redisc/Run
  super_class: #9                         // java/lang/Object
  interfaces: 0, fields: 1, methods: 3, attributes: 1
Constant pool:
   #1 = Methodref          #9.#30         // java/lang/Object."<init>":()V
   #2 = String             #31            // a
   #3 = String             #32            // b
   #4 = String             #33            // ab
   #5 = String             #34            // c.Run
   #6 = Methodref          #35.#36        // org/slf4j/LoggerFactory.getLogger:(Ljava/lang/String;)Lorg/slf4j/Logger;
   #7 = Fieldref           #8.#37         // com/redisc/Run.log:Lorg/slf4j/Logger;
   #8 = Class              #38            // com/redisc/Run
   #9 = Class              #39            // java/lang/Object
  #10 = Utf8               log
  #11 = Utf8               Lorg/slf4j/Logger;
  #12 = Utf8               <init>
  #13 = Utf8               ()V
  #14 = Utf8               Code
  #15 = Utf8               LineNumberTable
  #16 = Utf8               LocalVariableTable
  #17 = Utf8               this
  #18 = Utf8               Lcom/redisc/Run;
  #19 = Utf8               main
  #20 = Utf8               ([Ljava/lang/String;)V
  #21 = Utf8               args
  #22 = Utf8               [Ljava/lang/String;
  #23 = Utf8               s1
  #24 = Utf8               Ljava/lang/String;
  #25 = Utf8               s2
  #26 = Utf8               s3
  #27 = Utf8               <clinit>
  #28 = Utf8               SourceFile
  #29 = Utf8               Run.java
  #30 = NameAndType        #12:#13        // "<init>":()V
  #31 = Utf8               a
  #32 = Utf8               b
  #33 = Utf8               ab
  #34 = Utf8               c.Run
  #35 = Class              #40            // org/slf4j/LoggerFactory
  #36 = NameAndType        #41:#42        // getLogger:(Ljava/lang/String;)Lorg/slf4j/Logger;
  #37 = NameAndType        #10:#11        // log:Lorg/slf4j/Logger;
  #38 = Utf8               com/redisc/Run
  #39 = Utf8               java/lang/Object
  #40 = Utf8               org/slf4j/LoggerFactory
  #41 = Utf8               getLogger
  #42 = Utf8               (Ljava/lang/String;)Lorg/slf4j/Logger;
{
  public com.redisc.Run();
    descriptor: ()V
    flags: (0x0001) 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 11: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/redisc/Run;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=1, locals=4, args_size=1
         0: ldc           #2                  // String a
         2: astore_1
         3: ldc           #3                  // String b
         5: astore_2
         6: ldc           #4                  // String ab
         8: astore_3
         9: return
      LineNumberTable:
        line 14: 0
        line 15: 3
        line 16: 6
        line 17: 9
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      10     0  args   [Ljava/lang/String;
            3       7     1    s1   Ljava/lang/String;
            6       4     2    s2   Ljava/lang/String;
            9       1     3    s3   Ljava/lang/String;

  static {};
    descriptor: ()V
    flags: (0x0008) ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: ldc           #5                  // String c.Run
         2: invokestatic  #6                  // Method org/slf4j/LoggerFactory.getLogger:(Ljava/lang/String;)Lorg/slf4j/Logger;
         5: putstatic     #7                  // Field log:Lorg/slf4j/Logger;
         8: return
      LineNumberTable:
        line 10: 0
}
SourceFile: "Run.java"

说一下，运行时候发生了什么。

  • StringTable 开始的时候是空的，[]
  • 常量池中的信息，都会被加载到运行时常量池中
    • 这时，a、b、ab 都是常量池中的符号，还没有变为 java 字符串对象「懒惰行为」
  • 当用到 a 的时候，也就是 ldc #2「在反编译中」 会把 a 符号变为 a 字符串
    • 这个时候，才会向 StringTable 添加 a
    • 此时 StringTable 为 ["a"]
  • 其他 b、ab 都是如此，只有用到时，才会添加进 StringTable

字符串拼接

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package com.redisc;

import lombok.extern.slf4j.Slf4j;

@Slf4j(topic = "c.Run")
public class Run {

    public static void main(String[] args) {
        String s1 = "a";
        String s2 = "b";
        String s3 = "ab";
        String s4 = s1 + s2; // 经过反编译这段代码实际上为 new StringBuilder().append("a").append("b").toString()

        System.out.println(s3 == s4);
    }

}

经过反编译

  • String s4 = s1 + s2; // 经过反编译这段代码实际上为 new StringBuilder().append(“a”).append(“b”).toString()
  • toString() 相当于 new String("ab")

所以，s4 本质上是 new String("ab") 根据 java | 堆，这种 new 出的对象，最后都放进了堆中。

而，s3 是放进了串池中，所以，输出为 false。

编译器的优化

代码如下

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package com.redisc;

import lombok.extern.slf4j.Slf4j;

@Slf4j(topic = "c.Run")
public class Run {

    public static void main(String[] args) {
        String s1 = "a";
        String s2 = "b";
        String s3 = "ab";
        String s4 = s1 + s2;
        String s5 = "a" + "b";
    }

}

反编译为

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Classfile /Users/licong/java/redisc/target/classes/com/redisc/Run.class
  Last modified 2022年12月18日; size 896 bytes
  SHA-256 checksum 6236b5b6ef4c4af91b0fdc7fcd6fc56790a6366ccfa74f1c78be0521e1914897
  Compiled from "Run.java"
public class com.redisc.Run
  minor version: 0
  major version: 52
  flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: #12                         // com/redisc/Run
  super_class: #13                        // java/lang/Object
  interfaces: 0, fields: 1, methods: 3, attributes: 1
Constant pool:
   #1 = Methodref          #13.#36        // java/lang/Object."<init>":()V
   #2 = String             #37            // a
   #3 = String             #38            // b
   #4 = String             #39            // ab
   #5 = Class              #40            // java/lang/StringBuilder
   #6 = Methodref          #5.#36         // java/lang/StringBuilder."<init>":()V
   #7 = Methodref          #5.#41         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   #8 = Methodref          #5.#42         // java/lang/StringBuilder.toString:()Ljava/lang/String;
   #9 = String             #43            // c.Run
  #10 = Methodref          #44.#45        // org/slf4j/LoggerFactory.getLogger:(Ljava/lang/String;)Lorg/slf4j/Logger;
  #11 = Fieldref           #12.#46        // com/redisc/Run.log:Lorg/slf4j/Logger;
  #12 = Class              #47            // com/redisc/Run
  #13 = Class              #48            // java/lang/Object
  #14 = Utf8               log
  #15 = Utf8               Lorg/slf4j/Logger;
  #16 = Utf8               <init>
  #17 = Utf8               ()V
  #18 = Utf8               Code
  #19 = Utf8               LineNumberTable
  #20 = Utf8               LocalVariableTable
  #21 = Utf8               this
  #22 = Utf8               Lcom/redisc/Run;
  #23 = Utf8               main
  #24 = Utf8               ([Ljava/lang/String;)V
  #25 = Utf8               args
  #26 = Utf8               [Ljava/lang/String;
  #27 = Utf8               s1
  #28 = Utf8               Ljava/lang/String;
  #29 = Utf8               s2
  #30 = Utf8               s3
  #31 = Utf8               s4
  #32 = Utf8               s5
  #33 = Utf8               <clinit>
  #34 = Utf8               SourceFile
  #35 = Utf8               Run.java
  #36 = NameAndType        #16:#17        // "<init>":()V
  #37 = Utf8               a
  #38 = Utf8               b
  #39 = Utf8               ab
  #40 = Utf8               java/lang/StringBuilder
  #41 = NameAndType        #49:#50        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  #42 = NameAndType        #51:#52        // toString:()Ljava/lang/String;
  #43 = Utf8               c.Run
  #44 = Class              #53            // org/slf4j/LoggerFactory
  #45 = NameAndType        #54:#55        // getLogger:(Ljava/lang/String;)Lorg/slf4j/Logger;
  #46 = NameAndType        #14:#15        // log:Lorg/slf4j/Logger;
  #47 = Utf8               com/redisc/Run
  #48 = Utf8               java/lang/Object
  #49 = Utf8               append
  #50 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
  #51 = Utf8               toString
  #52 = Utf8               ()Ljava/lang/String;
  #53 = Utf8               org/slf4j/LoggerFactory
  #54 = Utf8               getLogger
  #55 = Utf8               (Ljava/lang/String;)Lorg/slf4j/Logger;
{
  public com.redisc.Run();
    descriptor: ()V
    flags: (0x0001) 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/redisc/Run;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=6, args_size=1
         0: ldc           #2                  // String a
         2: astore_1
         3: ldc           #3                  // String b
         5: astore_2
         6: ldc           #4                  // String ab
         8: astore_3
         9: new           #5                  // class java/lang/StringBuilder
        12: dup
        13: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V
        16: aload_1
        17: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        20: aload_2
        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        4
        29: ldc           #4                  // String ab
        31: astore        5
        33: return
      LineNumberTable:
        line 9: 0
        line 10: 3
        line 11: 6
        line 12: 9
        line 13: 29
        line 14: 33
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      34     0  args   [Ljava/lang/String;
            3      31     1    s1   Ljava/lang/String;
            6      28     2    s2   Ljava/lang/String;
            9      25     3    s3   Ljava/lang/String;
           29       5     4    s4   Ljava/lang/String;
           33       1     5    s5   Ljava/lang/String;

  static {};
    descriptor: ()V
    flags: (0x0008) ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: ldc           #9                  // String c.Run
         2: invokestatic  #10                 // Method org/slf4j/LoggerFactory.getLogger:(Ljava/lang/String;)Lorg/slf4j/Logger;
         5: putstatic     #11                 // Field log:Lorg/slf4j/Logger;
         8: return
      LineNumberTable:
        line 5: 0
}
SourceFile: "Run.java"

去除杂乱信息

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Code:
  stack=2, locals=6, args_size=1
     0: ldc           #2                  // String a
     2: astore_1
     3: ldc           #3                  // String b
     5: astore_2
     6: ldc           #4                  // String ab
     8: astore_3
     9: new           #5                  // class java/lang/StringBuilder
    12: dup
    13: invokespecial #6                  // Method java/lang/StringBuilder."<init>":()V
    16: aload_1
    17: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
    20: aload_2
    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        4
    29: ldc           #4                  // String ab
    31: astore        5
    33: return

可以看出

  • 29: ldc #4 // String ab
    • 是直接去串池找的 ab，并且其执行的是 #4 指令

和 6 号位置是一样的。

  • 6: ldc #4 // String ab

所以，29 行代码是直接从常量池中拿到的 ab，所以，s3 和 s5 是一个对象。

这是 javac 的一个编译优化。因为，javac 认为 s3 和 s5 都是常量，不应该变化了。

请我喝杯咖啡吧～