Jar(java archive files)
JAR file is the compressed file format. You can store many files in a JAR file. JAR stands for the Java Archive. This file format is used to distribute a set of java classes. This file helps you to reduce the file size and collect many file in one by compressing files. Downloading the files are become completed in very short duration of time because of reducing the file size. You can make the jar file executable by collecting many class file of your java application in it. The jar file can execute from the javaw (Java Web Start).
The JAR file format is based on the popular ZIP file format. Usually these file format is not only used for archiving and distribution the files, these are also used for implementing various libraries, components and plug-ins in java applications. Compiler and JVMs (Java Virtual Machine) can understand and implement these formats for java application.
For mentioning the product information like vendor name, product version, date of creation of the product and many other things related to the product are mentioned in the manifest file. Such types of files are special which are mentioned in the jar file for making it executable for the application. This file format is to be used for collecting auxiliary files associated with the components.
To perform basic operations for the jar file there has to be used the Java Archive Tool (jar tool). It is provided by the jdk (Java Development Kit). Following are some jar commands which are invoked by the jar tool:
Functions | Command |
creation a jar file viewing contents of a jar file viewing contents with detail of a jar file extract all the files of a jar file extract specific files from the jar file update jar files running a executable packaged jar file | jar cf jar-file-name file-name(s)_or_directory-name jar tf jar-file-name jar tvf jar-file-name jar xf jar-file-name jar xf jar-file-name file-name(s)_from_jar-file jar uf jar-file-name file-name(s)_from_jar-file java -jar jar-file-name |
BLOCK Diagram of JAR File
This is the one of the jar file shown here which contain java file which is convert in to the class file each class file contains HEADER at the top indicating which version of file format your using than its contains Heterogeneous Constant Pool is nothing the private literals used in the class. CLASS is used to access the flags it tells which is current working object with respect to class it used to access super class abstract class and interface. FIELDS is nothing what the parameters we are passing. METHODS what methods we are using in the class.
DEX(Dalvik Executable)
Android programs are compiled into .dex (Dalvik Executable) files, which are in turn zipped into a single .apk file on the device. .dex files can be created by automatically translating compiled applications written in the Java programming language. In the DEX all the class file is compressed in to single file and adding the essential informations which can shared among the other files . The size of the dex file is halved of jar files .
File Header
Dex files start with a simple header with some checksums and offsets to other structures
Offset | Size | Description |
0x0 | 8 | 'Magic' value: "dex\n009\0" |
0x8 | 4 | Checksum |
0xC | 20 | SHA-1 Signature |
0x20 | 4 | Length of file in bytes |
0x24 | 4 | Length of header in bytes (currently always 0x5C) |
0x28 | 8 | Padding (reserved for future use?) |
0x30 | 4 | Number of strings in the string table |
0x34 | 4 | Absolute offset of the string table |
0x38 | 4 | Not sure. String related |
0x3C | 4 | Number of classes in the class list |
0x40 | 4 | Absolute offset of the class list |
0x44 | 4 | Number of fields in the field table |
0x48 | 4 | Absolute offset of the field table |
0x4C | 4 | Number of methods in the method table |
0x50 | 4 | Absolute offset of the method table |
0x54 | 4 | Number of class definitions in the class definition table |
0x58 | 4 | Absolute offset of the class definition table |
Notes: All non-string fields are stored in little-endian format. It would appear that the checksum and signature fields are assumed to be zero when calculating the checksum and signature.
String Table
This table stores the length and offsets for every string in the Dex file including string constants, class names, variable names and more. Each entry has the following format:
Offset | Size | Description |
0x0 | 4 | Absolute offset of the string data |
0x4 | 4 | Length of the string (not including the null-terminator) |
Notes: Although the length of the string is stored in this table. All strings also have C-style null-terminators
Class List
A list of all classes referenced or conatined in this dex file. Each entry has the following format:
Offset | Size | Description |
0x0 | 4 | String index of the name of the class |
Field Table
A table of fields of all classes defined in this dex file. Each entry has the following format:
Offset | Size | Description |
0x0 | 4 | Class index of the class this field belongs to |
0x4 | 4 | String index of the field name |
0x8 | 4 | String index of the field type descriptor |
Method Table
A table of methods of all classes defined in this dex file. Each entry has the following format:
Offset | Size | Description |
0x0 | 4 | Class index of the class this field belongs to |
0x4 | 4 | String index of the method name |
0x8 | 4 | String index of the method type descriptor |
Class Definition Table
A table of class definitions for all classes either defined in this dex file or has a method or field accessed by code in this dex file. Each entry has the following format:
Offset | Size | Description |
0x0 | 4 | Class index |
0x4 | 4 | Access Flags (not 100% sure what this is for, I think it has to do with private/protected/public status) |
0x8 | 4 | Index of superclass |
0xC | 4 | Absolute offset of interface list |
0x10 | 4 | Absolute offset of static field list |
0x14 | 4 | Absolute offset of instance field list |
0x18 | 4 | Absolute offset of direct method list |
0x1C | 4 | Absolute offset of virtual method list |
Notes: Any of the list offset fields can be NULL in which case the class doesn't have any elements of that type. Not every class in the class list will necessarily have an entry in the class definition table.
Field List
Stores data for pre-initialized fields in a class. The list is formed of a 32-bit integer containing the number of entries followed by the entries themselves. Each field has an entry with the following format:
Offset | Size | Description |
0x0 | 8 | Index of string or object constant or literal "primitive" constant |
Notes: If the field does not have a pre-initialized value it will be filled with 0 for primitive types and -1 for object types.
Method List
A list of methods for a particular class. Begins with a 32-bit integer that contains the number of items in the list followed by entries in the following format.
Offset | Size | Description |
0x0 | 4 | Method index |
0x4 | 4 | Access flags (not 100% sure what this is for, I think it has to do with private/protected/public status) |
0x8 | 4 | Throws list off (no idea what this is) |
0xC | 4 | Absolute offset of header for code that implements the method |
Code Header
This header contains information about the code that implements a method.
Offset | Size | Description |
0x0 | 2 | Number of registers used by this method |
0x2 | 2 | Number of inputs this method takes (includes "this" pointer for non-static methods) |
0x4 | 2 | Output size? (presumably the size of whatever object the method returns) |
0x6 | 2 | Padding |
0x8 | 4 | String index of the source file name this method is implemented in |
0xC | 4 | Absolute offset of the actual code that implements this method |
0x10 | 4 | Absolute offset of the list of exceptions this method can throw (not 100% sure) |
0x14 | 4 | Absolute offset of the list of address and line number pairs for debugging purposes |
0x1C | 4 | Absolute offset of the local variable list of this method (includes arguments to the method and "this") |
Notes: The code offset field actually points to a 32-bit integer that contains the number of 16-bit words in the instruction stream. The actual VM instructions follow this integer.
Local Variable List
A list of local variables for a particular method. Begins with a 32-bit integer that contains the number of items in the list. Each entry has the following format:
Offset | Size | Description |
0x0 | 4 | Start (not a clue) |
0x4 | 4 | End (not a clue) |
0x8 | 4 | String index of variable name |
0xC | 4 | String index of variable type descriptor |
0x10 | 4 | Register number this variable will be stored in (not 100% sure) |
BLOCK DIAGRAM OF DEX FILE
At the top of the header is situated which display the version of android api with apk file name
Than there is series of constant pools which have used many times in our application which are different methods, fields, class, strings
Than after that we have class definition
Field list what different fields or data types we used in our application to store values.
Methods list what different methods we used to run our applications.
Local variables which are used for each class only.
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