JVM Basic - Compiler

2020-07-17
2 min read
java , jvm

Class 文件的结构

Class File

ClassFile structure

ClassFile {
    u4 magic;
    u2 minor_version;
    u2 major_version;
    u2 constant_pool_count;
    cp_info constant_pool[constant_pool_count-1];
    u2 access_flags;
    u2 this_class;
    u2 super_class;
    u2 interfaces_count;
    u2 interfaces[interfaces_count];
    u2 fields_count;
    field_info fields[fields_count];
    u2 methods_count;
    method_info methods[methods_count];
    u2 attributes_count;
    attribute_info attributes[attributes_count];
}

The constant_pool is a table of structures representing various string constants, class and interface names, field names, and other constants that are referred to within the ClassFile structure and its substructures. The format of each constant_pool table entry is indicated by its first “tag” byte.

constant_pool 是一个结构表，表示字符串常量、class 和 interface 名称、field 名称以及 ClassFile 结构和其子结构中引用的常量。每个 constant_pool 表条目的格式由其 “tag” 字节标识。

constant pool

Jvm 指令不依赖于类，接口，类实例或数组，相反，指令引用了 constant_pool 表中的符号信息（symbolic info）。常量池中的条目有如下的结构：

cP_info {
    u1 tag;
    u1 info[];
}

tag 表示常量的类型，info 存储常量的值

tag 类型和对应的值：

Constant Type Value
CONSTANT_Class 7
CONSTANT_Fieldref 9
CONSTANT_Methodref 10
CONSTANT_InterfaceMethodref 11
CONSTANT_String 8
CONSTANT_Integer 3
CONSTANT_Float 4
CONSTANT_Long 5
CONSTANT_Double 6
CONSTANT_NameAndType 12
CONSTANT_Utf8 1

Fields

field_info 的结构：

field_info {
    u2 access_flags;
    u2 name_index;
    u2 descriptor_index;
    u2 attributes_count;
    attribute_info attributes[attributes_count];
}

Field access_flags

Flag Name Value Interpretation
ACC_PUBLIC 0x0001 Declared public; may be accessed from outside its package.
ACC_PRIVATE 0x0002 Declared private; usable only within the defining class.
ACC_PROTECTED 0x0004 Declared protected; may be accessed within subclasses.
ACC_STATIC 0x0008 Declared static.
ACC_FINAL 0x0010 Declared final; no further assignment after initialization.
ACC_VOLATILE 0x0040 Declared volatile; cannot be cached.
ACC_TRANSIENT 0x0080 Declared transient; not written or read by a persistent object manager.

Methods

method_info 的结构：

method_info {
    u2 access_flags;
    u2 name_index;
    u2 descriptor_index;
    u2 attributes_count;
    attribute_info attributes[attributes_count];
}

Method access_flags

Flag Name Value Interpretation
ACC_PUBLIC 0x0001 Declared public; may be accessed from outside its package.
ACC_PRIVATE 0x0002 Declared private; accessible only within the defining class.
ACC_PROTECTED 0x0004 Declared protected; may be accessed within subclasses.
ACC_STATIC 0x0008 Declared static.
ACC_FINAL 0x0010 Declared final; may not be overridden.
ACC_SYNCHRONIZED 0x0020 Declared synchronized; invocation is wrapped in a monitor lock.
ACC_NATIVE 0x0100 Declared native; implemented in a language other than Java.
ACC_ABSTRACT 0x0400 Declared abstract; no implementation is provided.
ACC_STRICT 0x0800 Declared strictfp; floating-point mode is FP-strict

Interpreter vs Compilers

Interpreter

Fetch next bytecode, execute, repeat

Compilers

HotSpot has two compilers: JIT compiler and server compiler

  • Convert entire methods into native code
  • Optimize(inlining, unboxing, loop optimization, etc)
  • Code calls back to “runtime” when necessary

Compiling for JVM

编译是将 Java 源码编译成 Jvm 指令集（instruction set）的过程，编译器（compiler）是实现这一过程的工具，有些场景编译器指代将 Jvm 指令集转到到特定 CPU 指令集的工具。

#compiler
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