Talk about Java string constant pool, read your train of thought is clear

Bright future 2020-11-09 14:29:32
talk java string constant pool


Preface

A lot of friends Java The concept of string constant pool has been puzzling for a long time , Recently, I studied jvm Instruction code , Finally, I have a general understanding of it . Before presenting the case , We need to figure out a concept first , as everyone knows ,jvm The memory model is made up of program counters 、 Virtual machine stack 、 Native Method Stack 、 Pile up 、 Meta space ( Method area )、 Direct memory composition . Today we're talking about concepts only with virtual machine stacks 、 Pile up 、 Meta space ( Method area ) of . Let's give an example to illustrate two basic uses of strings :

  • String s =“abc”; During compilation , It's going to put the... To the right of the equal sign “abc” Constants are placed in the constant pool , When the program is running , Will s Variable stack , In the stack s The variable points directly to the string constant pool in the meta space abc term , No heap memory .
  • String s = new String(“abc”); During compilation , It's going to put the... To the right of the equal sign “abc” Constants are placed in the constant pool , When the program is running , First create a... In the heap String object , The contents of this object point to the constant pool “abc” term . And then s Variable stack , In the stack s The variable points to... In the heap String object .

Pass below javap -v xxx.class Order to see class The script of the file , By analyzing these scripts, we can know more exactly what we want to know . Author's jdk yes 1.8 Version of , Different versions may have different effects .

First example :String a = “abc”

First show us our source code

public class StringCodeTest {
public static void main(String[] args) {
String a = "abc";
System.out.println(a);
}
}

We execute javap -v StringCodeTest .class>StringCodeTest.txt command , Put the instruction code in StringCodeTest.txt in .

Classfile /C:/Users/zhiyi/IdeaProjects/springboottest/target/classes/cn/lizy/service/StringCodeTest.class
Last modified 2020-11-7; size 610 bytes
MD5 checksum 2adeba87a0f1b315019efe540bb058cd
Compiled from "StringCodeTest.java"
public class cn.lizy.service.StringCodeTest
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 // abc
#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 // cn/lizy/service/StringCodeTest
#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 Lcn/lizy/service/StringCodeTest;
#14 = Utf8 main
#15 = Utf8 ([Ljava/lang/String;)V
#16 = Utf8 args
#17 = Utf8 [Ljava/lang/String;
#18 = Utf8 a
#19 = Utf8 Ljava/lang/String;
#20 = Utf8 SourceFile
#21 = Utf8 StringCodeTest.java
#22 = NameAndType #7:#8 // "<init>":()V
#23 = Utf8 abc
#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 cn/lizy/service/StringCodeTest
#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 cn.lizy.service.StringCodeTest();
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 Lcn/lizy/service/StringCodeTest;
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 abc
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 5: 0
line 6: 3
line 24: 10
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 args [Ljava/lang/String;
3 8 1 a Ljava/lang/String;
}
SourceFile: "StringCodeTest.java"

The key is to see Constant pool:( This is the constant pool ) and main Method .

main In the method ldc #2 In the constant pool #2 Items are pushed onto the stack , and #2 The associated #23 Namely “abc” Constant . This means that the objects in the stack point directly to the constant pool .String s = “abc” Only one string object is generated ( Objects in the constant pool ) And a reference in the stack

Second example String s = new String(“abc”);

Source code

public class StringCodeTest {
public static void main(String[] args) {
String b = new String("abc");
System.out.println(b)
}
}

To save space , In the second and third examples jvm The script doesn't show up anymore , Just screenshots of key points , You can pass by yourself if you need to javap Command to generate

main First of all new #2, and #2 The associated #25 Namely String object , And then again ldc #3( The associated #26 Namely “abc” Constant ) in other words , When executed , First create... In the heap String object , Then put the constant pool of #3 Items are pushed onto the stack ( The stack here should be a stack of operands , Refer to the top memory model diagram ) for String Object use . therefore , The reference in the stack points to... In the heap String object ,String Object points to... In the constant pool “abc” term ,String s = new String(“abc”) Generated a string object ( Objects in the constant pool )、 A pile of String Object and a reference in the stack .

The third example :String c = new String(“abc”)+“abc”;

Source code

public class StringCodeTest {
public static void main(String[] args) {
String c = new String("abc")+"abc";
System.out.println(c);
}
}

main Method Stringbuiler object , And then created String object ( Namely new String("abc")), Re execution ldc #3 Push the string from the constant pool onto the stack . Finally, do the calculation , The process is similar to the second example , It's just one more Stringbuiler Of append operation .

summary : After understanding the general principle , Meet again like == Judging the equality of strings , Or calculate String s = new String(“abc”) When you create a few objects in memory, you will have a clear idea

Last

Thank you for seeing here , What's wrong with the article , If you think the article is helpful to you, please give me a compliment , Share every day java Related technical articles or industry information , Welcome to follow and forward the article !

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