Jvm theory-constant pool-string, jvm constant pool-string

Jvm theory-constant pool-string, jvm constant pool-string

String constant pool-constant item (Cp_info) Structure

CONSTANT_String_info {u1 tag = 8; u2 string_index; // store CONSTANT_Utf8_info pointer} CONSTANT_Utf8_info {u1 tag = 1; u2 length; // byte array length u1 bytes [length]; // UTF-8 encoded byte array}

 

Case 1

Public class StringDemo {private String s1 = "String S1 ";}

Javac StringDemo. java

Javap-verbose StringDemo. class

#15: CONSTANT_Utf8_info struct

CONSTANT_Utf8_info {u1 tag = 1; u2 length; // The length of the byte array u1 bytes [length]; // The byte array encoded with utf8}

#2: CONSTANT_String_info struct

CONSTANT_String_info {u1 tag = 8; u2 string_index; // storage #15}

 

Case 2

public class StringDemo {    private String s1="MyStr1";    private String s2="MyStr1";}

Javac StringDemo. java

Javap-verbose StringDemo. class

 

Case 3

public class StringDemo {    private String s1= new String("MyStr1");}

 

Case 4

public class StringDemo20 {    private String s1="MyStr1";    private String s2="MyStr1";    private String s3="My"+"Str1";    private String s01="My";    private String s02="Str1";    private String s4 = s01+s02;    private String s5 = s4.intern();        private static String str1="MyStr1";    private static String str2="MyStr1";    private static String str3="My"+"Str1";    private static String str01="My";    private static String str02="Str1";    private static String str4 = str01+str02;    private static String str5 = str4.intern();        private static final String fstr1="MyStr1";    private static final String fstr2="MyStr1";    private static final String fstr3="My"+"Str1";    private static final String fstr01="My";    private static final String fstr02="Str1";    private static final String fstr4 = fstr01+fstr02;    private static final String fstr5 = fstr4.intern();        class InnerDemo{        private String s1="MyStr1";        private String s2="MyStr1";        private String s3="My"+"Str1";        private String s01="My";        private String s02="Str1";        private String s4 = s01+s02;        private String s5 = s4.intern();                private static final String ifstr1="MyStr1";        private static final String ifstr2="MyStr1";        private static final String ifstr3="My"+"Str1";        private static final String ifstr01="My";        private static final String ifstr02="Str1";        private static final String ifstr4 = ifstr01+ifstr02;        //private static final String ifstr5 = ifstr4.intern();    }        public static void main(String[] args){        StringDemo20 stringDemo = new StringDemo20();         System.out.println("s1==s2："+(stringDemo.s1==stringDemo.s2));        System.out.println("s1==s3："+(stringDemo.s1==stringDemo.s3));        System.out.println("s1==s4："+(stringDemo.s1==stringDemo.s4));//false        System.out.println("s1==s5："+(stringDemo.s1==stringDemo.s5));                System.out.println("s1==str1："+(stringDemo.s1==StringDemo20.str1));        System.out.println("s1==str3："+(stringDemo.s1==StringDemo20.str3));        System.out.println("s1==str4："+(stringDemo.s1==StringDemo20.str4));//false        System.out.println("s1==str5："+(stringDemo.s1==StringDemo20.str5));                System.out.println("s1==fstr1："+(stringDemo.s1==StringDemo20.fstr1));        System.out.println("s1==fstr3："+(stringDemo.s1==StringDemo20.fstr3));        System.out.println("s1==fstr4："+(stringDemo.s1==StringDemo20.fstr4));        System.out.println("s1==fstr5："+(stringDemo.s1==StringDemo20.fstr5));         System.out.println("s1==innerdemo.fstr1："+(stringDemo.s1==InnerDemo.ifstr1));        System.out.println("s1==innerdemo.fstr3："+(stringDemo.s1==InnerDemo.ifstr3));        System.out.println("s1==innerdemo.fstr4："+(stringDemo.s1==InnerDemo.ifstr4));    }}

Javap-verbose stringdemow.class> d:/1.txt

 

 

We can see that str1, str2, str3, and str5 are all ldc #22.

We can see that str1, str2, and str3 are all ldc #22. During the running of s5, put it into the runtime pool.