Constant pool = constant pool = constant pool = constant pool = constant pool = constant pool

Constant pool

package com.example.demo;

/ * * *@author zinsanity
 * @dateThe 2020-11-21 16:06 *@desc* /
public class Review {
    public static void main(String[] args) {
        String info = "hello world";
        int a = Awesome!;
        final int b = 66;
        int c = 36728; }}Copy the code

After this code is compiled, use the javap -v review.class command to view the bytecode

Classfile /D:/idea_projects/demo/target/classes/com/example/demo/Review.class
  Last modified 2020-11-21; size 561 bytes
  MD5 checksum 09914fecad4a776e7ac86d1a6fd43baa
  Compiled from "Review.java"
public class com.example.demo.Review
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #5.#26         // java/lang/Object."<init>":()V
   #2 = String             #27            // hello world
   #3 = Integer            36728
   #4 = Class              #28            // com/example/demo/Review
   #5 = Class              #29            // java/lang/Object
   #6 = Utf8               <init>
   #7 = Utf8               ()V
   #8 = Utf8               Code
   #9 = Utf8               LineNumberTable
  #10 = Utf8               LocalVariableTable
  #11 = Utf8               this
  #12 = Utf8               Lcom/example/demo/Review;
  #13 = Utf8               main
  #14 = Utf8               ([Ljava/lang/String;)V
  #15 = Utf8               args
  #16 = Utf8               [Ljava/lang/String;
  #17 = Utf8               info
  #18 = Utf8               Ljava/lang/String;
  #19 = Utf8               a
  #20 = Utf8               I
  #21 = Utf8               b
  #22 = Utf8               c
  #23 = Utf8               MethodParameters
  #24 = Utf8               SourceFile
  #25 = Utf8               Review.java
  #26 = NameAndType        #6:#7          // "<init>":()V
  #27 = Utf8               hello world
  #28 = Utf8               com/example/demo/Review
  #29 = Utf8               java/lang/Object
{
  public com.example.demo.Review();
    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 8: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/example/demo/Review;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=1, locals=5, args_size=1
         0: ldc           #2                  // String hello world
         2: astore_1
         3: sipush        666
         6: istore_2
         7: bipush        66
         9: istore_3
        10: ldc           #3                  // int 36728
        12: istore        4
        14: return
      LineNumberTable:
        line 10: 0
        line 11: 3
        line 12: 7
        line 13: 10
        line 14: 14
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      15     0  args   [Ljava/lang/String;
            3      12     1  info   Ljava/lang/String;
            7       8     2     a   I
           10       5     3     b   I
           14       1     4     c   I
    MethodParameters:
      Name                           Flags
      args
}
SourceFile: "Review.java"
Copy the code

You can see the Constant Pool in the bytecode file. That is, every class file has a constant pool. Let’s start with this bytecode, which is the bytecode compiled by our main method, where 0: LDC #2 means to fetch the second location from the constant pool and store it in astore_1. So let’s go to the constant pool

public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=1, locals=5, args_size=1
         0: ldc           #2                  // String hello world
         2: astore_1
         3: sipush        666
         6: istore_2
         7: bipush        66
         9: istore_3
        10: ldc           #3                  // int 36728
        12: istore        4
        14: return
Copy the code

In a bytecode file, the following snippet represents the constant pool. #2 corresponds to String, and then you have to find #27, which corresponds to Hello Word. So, 0: LDC #2 finally takes a Hello Word String of type String from the constant pool and stores it into ASTore_1. Sipush stores 666 to istore_2, bipush stores 66 to istore_3, and LDC goes to the constant pool to store data at #3.

Constant pool: #1 = Methodref #5.#26 // java/lang/Object."<init>":()V ...... # 29Copy the code

First question: why are all types of int data using different instructions? According to the SPECIFICATION of the JVM, different instructions are used to push int values according to the int value range. Second question: why are 666 and 66 not in the constant pool? Int = 36728; int = 36728; int = 36728;Copy the code

Run-time constant pool

In the constant pool, you can see that these are all temporary symbols like #1, #2, and #3. When a program is run, the JVM adds all the bytecode files to memory and, after linking and validating, converts the #1, #2, and #3 symbols into the actual address in memory and runs them in the runtime constant pool. The run-time constant pool is held in the method area, which is globally unique, and is an area shared by all classes.

String constant pool

For instance

String a = "test1";
String b = "test2";
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The strings “test1” and “test2” are first stored in the constant pool after compilation. When the program runs, the string is loaded into the string constant pool after loading into memory. After jdk1.7, the string constant pool is in the heap.