jvm理论-常量池-string-白红宇

jvm理论-常量池-string

阅读量：6273 次

发布时间：2019-06-22

本文共 3710 字，大约阅读时间需要 12 分钟。

字符串常量池-常量项（cp_info）结构

CONSTANT_String_info{    u1 tag=8;    u2 string_index;//存放 CONSTANT_Utf8_info 指针}CONSTANT_Utf8_info{　　u1 tag=1;　　u2 length;//字节数组长度　　u1 bytes[length];//使用utf8编码后的字节数组}

案例1

public class StringDemo {    private String s1="字符串S1";}

javac StringDemo.java

javap -verbose StringDemo.class

#15：CONSTANT_Utf8_info结构体

CONSTANT_Utf8_info{    u1 tag=1;    u2 length;//字节数组长度    u1 bytes[length];//使用utf8编码后的字节数组}

#2：CONSTANT_String_info结构体

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

案例2

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

javac StringDemo.java

javap -verbose StringDemo.class

案例3

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

案例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 StringDemo20.class >d:/1.txt

可以看到 str1、str2、str3、str5都是 ldc #22。

可以看到 str1、str2、str3都是 ldc #22。s5在运行期间，放入运行时常量池。