Dex File Format

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Introduction

This information is based on an old version of the Android SDK from before the G1 was released. The format has changed somewhat since then and I believe Google has released official documentation.

If you've been paying attention to the tech news lately you might have heard about a little something called Android from Google. Android is a new mobile phone platform based on Linux and Java, but unlike other Java platforms Android uses a non-standard JVM called Dalvik. While Google has promised to release much (all?) of Android under an open source license, they haven't done so yet and they also haven't released any documentation on this new VM. Being somewhat impatient, I've taken it upon myself to do some reverse engineering and put together some documentation of my own. This page documents the Dex file format that compiled programs get translated into for use on the Dalvik VM. I hope to write some documentation on the VM itself in the near future.

File Header

Dex files start with a simple header with some checksums and offsets to other structures
OffsetSizeDescription0x08'Magic' value: "dex\n009\0"0x84Checksum0xC20SHA-1 Signature0x204Length of file in bytes0x244Length of header in bytes (currently always 0x5C)0x288Padding (reserved for future use?)0x304Number of strings in the string table0x344Absolute offset of the string table0x384Not sure. String related0x3C4Number of classes in the class list0x404Absolute offset of the class list0x444Number of fields in the field table0x484Absolute offset of the field table0x4C4Number of methods in the method table0x504Absolute offset of the method table0x544Number of class definitions in the class definition table0x584Absolute 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:
OffsetSizeDescription0x04Absolute offset of the string data0x44Length 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:
OffsetSizeDescription0x04String 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:
OffsetSizeDescription0x04Class index of the class this field belongs to0x44String index of the field name0x84String 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:
OffsetSizeDescription0x04Class index of the class this field belongs to0x44String index of the method name0x84String 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:
OffsetSizeDescription0x04Class index0x44Access Flags (not 100% sure what this is for, I think it has to do with private/protected/public status)0x84Index of superclass0xC4Absolute offset of interface list0x104Absolute offset of static field list0x144Absolute offset of instance field list0x184Absolute offset of direct method list0x1C4Absolute 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:
OffsetSizeDescription0x08Index 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.OffsetSizeDescription0x04Method index0x44Access flags (not 100% sure what this is for, I think it has to do with private/protected/public status)0x84Throws list off (no idea what this is)0xC4Absolute offset of header for code that implements the method

Code Header

This header contains information about the code that implements a method.
OffsetSizeDescription0x02Number of registers used by this method0x22Number of inputs this method takes (includes "this" pointer for non-static methods)0x42Output size? (presumably the size of whatever object the method returns)0x62Padding0x84String index of the source file name this method is implemented in0xC4Absolute offset of the actual code that implements this method0x104Absolute offset of the list of exceptions this method can throw (not 100% sure)0x144Absolute offset of the list of address and line number pairs for debugging purposes0x1C4Absolute 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:
OffsetSizeDescription0x04Start (not a clue)0x44End (not a clue)0x84String index of variable name0xC4String index of variable type descriptor0x104Register number this variable will be stored in (not 100% sure)
Notes: This list will include local variables that are arguments to the method as well as the "this" variable for non-static methods.

ToDo

Add documentation on Position list and constant objects for pre-initialized fields.

Questions

If you have any questions about this document feel free to send me an e-mail at [e-mail].