String & heap & method area

character string & Pile up & Method area

1. Problem introduction

package com.learn.java;

/** * @author xzy * @date 2020-09-17 22:33 *  explain ： character string  &  Pile up  &  Method area  */
public class Main {
    
    public static String s = "HELLO WORLD!";

    public static void main(String[] args) {
    
        String s1 = "HELLO WORLD!";
        String s2 = "HELLO WORLD!";
        String s3 = new String("HELLO WORLD!");

        System.out.println(s1 == s); // true
        System.out.println(s2 == s); // true
        System.out.println(s1 == s2);// true
        System.out.println(s3 == s); // false
        System.out.println(s3 == s1);// false
        System.out.println(s3 == s2);// false
    }
}

2. Problem analysis

chart 2.1 Java Virtual machine runtime data area

2.1 character string & Method area

class There is a piece in the file called “ Constant pool ” Region , A constant pool can be thought of as class A repository of documents , There are two main types of constants in the constant pool ：

• Literal ： character string 、final Constant ……
• Symbol reference ： class / Fully qualified name of the interface 、 Field / Method name 、 Field / Method descriptor

In the method area, there is a block called “ Runtime constant pool ” Region , When a class is loaded into java Memory space of virtual machine ,class The contents of the file constant pool are loaded into the runtime constant pool .

chart 2.2 The process by which a literal enters the method area

Use javap -v Main.class Command to view the bytecode generated by the compiled program ：

chart 2.3 Constant pool information

2.2 character string & Pile up

Pile up yes java The largest chunk of memory managed by a virtual machine , The only function of this memory area is to store Object instances And an array .

2.3 summary

s、s1、s2 The value in the pointing method area is “Hello WORLD!” Of String example （ Class is created when it is loaded ）;s3 Point to a heap with a value of “Hello WORLD!” Of String example （ Program runtime creates ）.