String We often use it in development and learning , But yes String The value range of type is not clear .

String The bottom is char Array , The length limit is not indicated .java You can specify the length of the array , If it is not specified, it is specified with the actual element 

private final char value[];

From the bottom method length() It can be seen that , The return is int type .

public int length()

And in the java in ,int The maximum value of 2^31-1 = 2147483647, It's also value[] The length of the array is 2^31-1 .

But when defining a 10 Ten thousand character string post compilation ,jvm It will prompt for an error ：java: Constant string too long

Because jvm The compilation specification makes restrictions , At compile time ,jvm Will put literal strings into the constant pool , and JVM in class The document is standardized .

All entries in the constant pool have the following format .

cp_info{

    u1 tag;

    u2 info[];

}

among u1 tag Indicates a flag , Represents the type of the current constant ;u2 info[] from tag Determine the contents of the array .（ See 《Java Virtual machine specification 》 4.4 chapter ）

notes ： stay class In file , There are special data types used to represent Class The content of the document , Include u1、u2、u4, Represent the 1、2、4 Unsigned number of bytes ; Every Class All documents are made by 8 A byte stream in bytes , all 16 、 23 、 64 Bit length data will be constructed as 2 individual 、 4 individual 、 8 individual 8 In byte units （ See 《Java Virtual machine specification 》 The first 4 Chapter ）

tag The corresponding items are .

constant type value
CONSTANT_Class7
CONSTANT_Fieldref9
CONSTANT_Methodref10
CONSTANT_InterfaceMethodref11
CONSTANT_String8
CONSTANT_Integer3
CONSTANT_Float4
CONSTANT_Long5
CONSTANT_Double6
CONSTANT_NameAndType12
CONSTANT_Utf81
CONSTANT_MethodHandle15
CONSTANT_MethodType16
CONSTANT_InvokeDynamic18

String The corresponding is CONSTANT_String term , The specific definition format is 

CONSTANT_String_info{

    u1 tag;

    u2 string_index;

}

u1 tag The value of is CONSTANT_String(8);u2 string_index The value of must be a valid index to the constant pool table , And the entry at this index must be CONSTANT_Utf8_info structure

String is based on CONSTANT_Utf8 The type is represented in the constant pool .class In file CONSTANT_Utf8 Specific definition format ,

CONSTANT_Utf8_info {

    u1 tag;

    u2 length;

    u1 bytes[length];

}

u1 tag The value of is CONSTANT_Utf8(1);CONSTANT_Utf8_info The content of u2 length Property to determine the length ;u1 bytes[length] Representing a string value byte Array

among u2 representative 2 Unsigned byte units , and 1 Bytes 8 position (bit),2 One is 16 position ,16 The highest bit indicates 2^16 - 1 = 65535, And because virtual machines need 1 An instruction of bytes is used as the end instruction , So its real effective range is 【0-65564】. Of course, this range is the valid range of compilation time , The string spliced at runtime is in the maximum range of shaping .

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