JVM Class（基于openjdk17）

Parse Class File

The key and the entry method of parsing class file is located in src/hotspot/share/classfile/classFileParser.cpp.

// src/hotspot/share/classfile/classFileParser.cpp

void ClassFileParser::parse_stream(const ClassFileStream* const stream,
                                   TRAPS) {

  assert(stream != NULL, "invariant");
  assert(_class_name != NULL, "invariant");

  // BEGIN STREAM PARSING
  stream->guarantee_more(8, CHECK);  // magic, major, minor
  // Magic value
  const u4 magic = stream->get_u4_fast();
  guarantee_property(magic == JAVA_CLASSFILE_MAGIC,
                     "Incompatible magic value %u in class file %s",
                     magic, CHECK);

At the very first, parse_stream reads the first four bytes from class file, and compare to the JAVA_CLASSFILE_MAGIC CAFEBABE.

This is a very well-known magic number in Java world, when people talk about class file.

If you open class file in hex:

hexdump -C Main.class | head -n 1
00000000  ca fe ba be 00 00 00 3c  00 2f 0a 00 02 00 03 07  |.......<./......|

You can see the class file starts with "cafebabe".

Parse Method

ClassFileParser::parse_stream
  ClassFileParser::parse_methods
    ClassFileParser::parse_method
      ...
      ClassFileParser::parse_localvariable_table // for LocalVariableTable(LVT)
      ClassFileParser::parse_localvariable_table // for LocalVariableTypeTable(LVTT)
      ...

TODOs

ClassLoader::load_class

src/hotspot/share/classfile/classFileParser.cpp

src/hotspot/share/classfile/javaClasses.hpp

// Read and parse class file and create Klass instance
src/hotspot/share/classfile/klassFactory.hpp

src/hotspot/share/oops/symbol.hpp

References

Chapter 4. The class File Format
Java class file