jvm理论-常量池-string

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。

推荐：

http://www.ciaoshen.com/2016/07/29/string/

http://tangxman.github.io/2015/07/27/the-difference-of-java-string-pool/

Java中几种常量池的区分

Thinking in Java 读书笔记：第十三章 - 字符串

https://www.zhihu.com/question/29884421/answer/113785601

http://rednaxelafx.iteye.com/blog/774673

http://www.javaranch.com/journal/200409/ScjpTipLine-StringsLiterally.html