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Java .class File Format

陳正佳

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Java Virtual Machine

• the cornerstone of Sun's Java programming language. • a component of the Java technology responsible for A. Java's cross-platform delivery, B. the small size of its compiled code, C. Java's ability to protect users from malicious programs. • JVM knows nothing of the Java programming language, only of a particular file format, the class file format.

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Class file format

A class file contains » 1. Java Virtual Machine instructions (bytecodes)» 2. a symbol table (Constant pool)» 3. other ancillary information.

Javap options» 1. -c bytecode» 2. -s internal type signatures» 3. -verbose stack size and number of local

variables and args.

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Class file format

Each class file contains one Java type, either a class or an interface.

A class file consists of a stream of 8-bit bytes.

All 16-bit, 32-bit, and 64-bit quantities are constructed by reading in two, four, and eight consecutive 8-bit bytes, respectively.

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Class File Format

Multibyte data items are always stored in big-endian order, where the high bytes come first.

u1, u2,and u4 represent an unsigned one-, two-, or four-byte quantity, respectively.

These types may be read by methods such as

readUnsignedByte, readUnsignedShort, and

readInt of the interface java.io.DataInput.

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class SumI {

public static void main (String[] args) {int count=10;int sum =0;for (int index=1;index<count;index++)

sum=sum+index;System.out.println("Sum="+sum);

} // method main

}

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Class File 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];

}

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Magic

magic :u4

The magic item supplies the magic

number identifying the class file format;

it has the value 0xCAFEBABE.

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Version

minor_version :u2, major_version:u2

The values of the minor_version and

major_version items are the minor

and major version numbers of the

compiler that produced this class file.

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Constant Pool

constant_pool_count :u2» must > 0.» indicates the number of entries in the

constant_pool table of the class file,» where the constant_pool entry at index zero

is included in the count but is not present in the constant_pool table of the class file.

» i.e., if count = 13 => pool[1] … pool[12]

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Constant Pool

constant_pool[]» a table of variable-length structures» representing various

– numeric literals– string constants, – class/interface/type names, – field reference,– method reference and – other constants that are referred to within the

ClassFile structure and its substructures.

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Constant Pool

constant_pool[0], » reserved for internal use by a JVM implementation. That

entry is not present in the class file.

The first entry in the class file is constant_pool[1]. Each constant_pool[i] is a variable-length structure

whose format is indicated by its first "tag" byte.

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Access Flags

access_flags :u2» a mask of modifiers used with class and

interface declarations.

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Access Flags

The access_flags modifiers Flag Name Value Meaning Used By ACC_PUBLIC 0x0001 Is public; Class, interface ACC_FINAL 0x0010 Is final; Class ACC_SUPER 0x0020 superclass Class, =1 for new JVM interface ACC_INTERFACE 0x0200 interface. Interface ACC_ABSTRACT 0x0400 Is abstract; Class, interface

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Access Flags

The access_flags modifiers Flag Name Value Meaning Used By ACC_PUBLIC 0x0001 Is public; Class, interface ACC_FINAL 0x0010 Is final; Class ACC_SUPER 0x0020 superclass Class, =1 for new JVM interface ACC_INTERFACE 0x0200 interface. Interface ACC_ABSTRACT 0x0400 Is abstract; Class, interface

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Access Flags (continued)

The access_flags modifiers Flag Name Value Meaning Used By

ACC_SYNTHETIC 0x1000 synthetic

; Not present in the source code.

ACC_ANNOTATION 0x2000 annotation type.

ACC_ENUM 0x4000 enum type.

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this class

this_class :u2» a valid index into the constant_pool table. » The entry at that index must be a

CONSTANT_Class_info structure representing the class or interface defined by this class file.

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super_class

super_class» For a class, the value of the super_class

item either must be zero or must be a valid index into the constant_pool table.

» If the value of the super_class item is nonzero, the constant_pool entry at that index must be a CONSTANT_Class_info structure representing the superclass of the class defined by this class file.

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super_class

super_class» Neither the superclass nor any of its

superclasses may be a final class.» If the value of super_class is zero, then this

class file must represent the class java.lang.Object,

» the only class or interface without a superclass.

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interfaces

interfaces_count» the number of direct superinterfaces of this class or

interface type.

interfaces[]» Each value in the interfaces array must be a valid

index into the constant_pool table. » The constant_pool entry at each value of

interfaces[i], where i < interfaces_count, must be

a CONSTANT_Class_info structure

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Fields

fields_count» gives the number of field_info structures in the fields

table.

fields[]» Each entry a variable-length field_info

structure giving a complete description of a field in the class or interface type.

» includes only those fields that are declared by this class or interface.

» does not include fields inherited from superclasses or superinterfaces.

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Methods

methods_count» gives the number of method_info structures in the

methods table.

methods[]» Each entry a variable-length method_info

structure giving a complete description of Java Virtual Machine code for a method in the class or interface.

» The method_info structures represent all methods, both instance methods, class (static) methods, and constructor methods declared by this class or interface type.

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Methods

methods[]» includes only those methods explicitly declared

by this class.» Interfaces have only the single method <clinit>,

the interface initialization method. » Constructor methods have the a common name

<init>.» does not include items representing methods

that are inherited from superclasses or superinterfaces.

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Class Attributes

attributes_count » the number of attributes in the attributes

table of this class

attributes[]» Each value of the attributes table must be a

variable-length attribute structure. » A ClassFile structure can have any number

of attributes associated with it.

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Internal form of fully qualified names

Replace all dots with /.

EX: a.b.C ==> a/b/C

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Descriptor A descriptor is a string representing the

type of a field or method. Descriptors are represented in the class

file format using UTF-8 strings. Grammar:

FieldType ::=       BaseType|      ObjectType

  |     ArrayType

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Fields Descriptors BaseType ::= B | C | D | F | I | J | S | Z

» B for byte; Z for boolean» C D F I J S for char, double, float, int, long

and short, respectively. ObjectType ::= Lclassname; ArrayType ::= [ComponentType Ex:

» int[][] ==> [[I» Object[] ==> [java/lang/Object;

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Field and method descriptors

FieldDescriptor ::= FieldType ComponentType ::= FieldType MethodDescriptor ::=    ( ParameterDescriptor* ) ReturnDescriptor ParameterDescriptor ::= FieldDescriptor ReturnDescriotor ::= FieldDescriptor | V

» V for void method Ex: 1. Object mymethod(int i, double d, Thread t) ==> (IDLjava/lang/Thread;)Ljava/lang/Object; 2. void com.Clazz.m(int i) ==> (I)V.

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The Constant Pool JVM instructions do not rely on the runtime

layout of classes, interfaces, class instances, or arrays.

Instead, instructions refer to symbolic information in the constant_pool table.

All constant_pool table entries have the following general format:

 cp_info { u1 tag;      u1 info[]; }

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

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The CONSTANT_Utf8_info Structure

Used to represent constant string values. utf-8 encoding :1. 1~127 (xxx xxxx) ==> 1 bytes : 0xxx xxxx 2. 0, \u0080~\u07ff ( xxx xxxx xxxx) ==> 2 bytes: 110x xxxx 10xx xxxx 0 => 1100 0000 1000 00003. \u0800 ~ \uffff (xxxx xxxx xxxx xxxx)

==> 3 bytes: 1110 xxxx 10xx xxxx 10xx xxxx

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Format CONSTANT_Utf8_info {

     u1 tag;      // = 1

u2 length;  

    u1 bytes[length];     }

Notes: tag = 1. length : number of bytes in the encoded utf8 string.

bytes[] bytes[] : contains the bytes of the string. No byte may have the value (byte)0 or lie in the range

(byte)0xf0-(byte)0xff.

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The CONSTANT_String_info Structure

 CONSTANT_String_info {      u1 tag;     // = 8u2 string_index;     }

string_index : » a valid index into a CONSTANT_Utf8_info

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CONSTANT_Integer_info and CONSTANT_Float_info

 CONSTANT_Integer_info {      u1 tag;    // = 3

u4 bytes;     }

CONSTANT_Float_info {      u1 tag;    // =4  u4 bytes; }

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CONSTANT_Long_info and CONSTANT_Double_info

 CONSTANT_Long_info {      u1 tag;    // = 5

u8 bytes;     }

CONSTANT_Double_info {      u1 tag;    // =6  u8 bytes; }

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CONSTANT_Class_info Used to represent a class or an interface:

CONSTANT_Class_info {      u1 tag;      // = 7u2 name_index; }

name_index is an index into an utf-8 entry encoding a fully qualified class name.

include also array type: [[I, [Lcom/Cl;, etc.

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CONSTANT_NameAndType_info

used to represent a field or method, without indicating which class or interface type it belongs to:

CONSTANT_NameAndType_info {      u1 tag;      // = 12 u2 name_index;  // index into utf8 string of a simple name u2 descriptor_index;  // index into utf8 string

of a filed/method descriptor   }

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Constant fieldref, methodRef

and interfaceMethodRef info CONSTANT_Fieldref_info {   u1 tag; // = 9 u2 class_index;  // index into a class_info

entry  u2 name_and_type_index;     } CONSTANT_Methodref_info {   u1 tag;   // = 10   u2 class_index;      u2 name_and_type_index;     } CONSTANT_InterfaceMethodref_info { u1 tag;

//11    u2 class_index;      u2 name_and_type_index;     }

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Fields Each field is described by a field_info structure. No two fields in one class file may have the same

name and descriptor. Format: field_info {     

» u2 access_flags; » u2 name_index;     // index to utf8 entry for name» u2 descriptor_index;// index to uft8 entry for field

type    » u2 attributes_count;      » attribute_info attributes[attributes_count];     }

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Methods Each method, including each instance

initialization method(<init>) and the class or interface initialization method(<clinit>) , is described by a method_info structure.

Format: method_info {      u2 access_flags;      u2 name_index;  // index into utf8 entry for

name  u2 descriptor_index;  // index into utf8 entry  

u2 attributes_count;      attribute_info attributes[attributes_count];     }

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AttributesAttributes are used in the ClassFile,

field_info, method_info, and Code_attribute structures of the class file format.

General Format: Attribute_info {      u2 attribute_name_index;      // into utf8 u4 attribute_length;    // excluding init 6

bytes  u1 info[attribute_length]; }

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Types of attributes class attributes:

» synthetic, deprecated, sourceFile» innerClasses

field attributes:» constantValue, deprecated

method attributes:» code attributes, deprecated, » Exception attributes

code attributes:» lineNumberTable, localVariableTable

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ConstantValue Attribute ConstantValue_attribute {      u2 attribute_name_index; // index to

utf8 : “conatantValue” u4 attribute_length;  // = 2    u2 constantvalue_index; // into primitive(Integer_entry,…) // or string_entry     }

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Code Attribute a variable-length attribute used in the

attribute table of method_info structures. A Code attribute contains

» the Java virtual machine instructions and » auxiliary information for a single method, » instance initialization method , or class or

interface initialization method. Every JVM implementation must recognize

Code attributes. native or abstract, => no this attribute in

its method_info structure» O/W=> has exactly one Code attribute.

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Code_attribute {      u2 attribute_name_index;      // into utf8:

“Code” u4 attribute_length; // exclude initial 6

bytes     u2 max_stack;// max # of stack slots needed   u2 max_locals;

// maximum # of local variables needed // Note: double and long count 2.     u4 code_length;      u1 code[code_length];     

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u2 exception_table_length;      { u2 start_pc;    u2 end_pc; //exclusive     u2 handler_pc;      u2 catch_type;

// index to cp entry f type constant_class_info     // zero means called by all exceptions// used to implement finally-clause

} exception_table[exception_table_length];      u2 attributes_count;      attribute_info attributes[attributes_count];     }

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The (uncaught checked)Exception Attribute

appear in the attribute table of a method_info structure.

indicates which checked exceptions a method may throw.

1 Exception attribute in each method_info structure.

Format: Exceptions_attribute {     

u2 attribute_name_index;  // into utf8:”Exception”  u4 attribute_length;  u2 number_of_exceptions;      u2 exception_index_table[number_of_exceptions];

// each entry an index to cp of class_info     }

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The InnerClasses Attribute a variable-length attribute in the attributes table of

the ClassFile structure. record any inner class/interface referred (or

declared) in this class/interface» If the constant pool of a class or interface refers to any

class or interface that is not a member of a package, its ClassFile structure must have exactly one InnerClasses attribute in its attributes table.

» If a class has members that are classes or interfaces, its constant_pool table (and hence its InnerClasses attribute) must refer to each such member, even if that member is not otherwise mentioned by the class.

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Format:InnerClasses_attribute {      u2 attribute_name_index; // to utf8: “innerClasses u4 attribute_length; u2 number_of_classes;      { u2 inner_class_info_index;  // into class_info // for each innerclass C in

cp  u2 outer_class_info_index; // into class_info  // containing class of C 

u2 inner_name_index; // into utf8 for simple // name (no / no $); anonymous => zero    

u2 inner_class_access_flags; } classes[number_of_classes];     }

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The Syntheticand Deprecated Attribute

A class member that does not appear in the source code must be marked using a Synthetic attribute.

Synthetic_attribute {      » u2 attribute_name_index;  // utf8(“synthetic”)   » u4 attribute_length;     // = zero}

The Deprecated attribute has the following format:

Deprecated_attribute {      » u2 attribute_name_index;   // utf8(“Deprecated”)   » u4 attribute_length;   // = 0    }

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The SourceFile Attribute

 SourceFile_attribute {     u2 attribute_name_index; // into utf8(“SourceFile”)   u4 attribute_length;    // = 2  u2 sourcefile_index;   // into utf8 entry  }

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LineNumberTable Attribute LineNumberTable_attribute {  

» u2 attribute_name_index ; // utf8(“LineNumberTable”)

» u4 attribute_length;      » u2 line_number_table_length;     » { u2 start_pc;      » u2 line_number;      }

line_number_table[line_number_table_length];»     }

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The LocalVariableTable Attribute

 LocalVariableTable_attribute {      » u2 attribute_name_index;  //

utf8(“LocalVariableTable”)    » u4 attribute_length;      » u2 local_variable_table_length;      » { u2 start_pc;      u2 length;     // scope must have» // value in [start_pc, start_pc + length] » u2 name_index; // utf8 for var name     » u2 descriptor_index;  // uft8 for type»    u2 index; // local var index»      } local_variable_table[local_variable_table_length];

    }