1. Mobile Handset Virtual Machine

2. Outline
Virtual Machine
Java Virtual Machine
Dalvik Virtual Machine

3. Concept
A virtual machine (VM) is a software emulation of a real machine so that
another operating system can run in the simulated machine
more than one different operating systems can run at the same time
Fundamental idea: abstract the real hardware into different execution environments

4. VM Components (1) Three components of a virtual machine
Host: underlying hardware system
Virtual machine manager (VMM): creates and runs different virtual machines
Guest: the emulated operating system on the host system

5. VM Components (2)
Guest
VMM
Host
Non-virtual machine
Virtual machine

6. System/Process VM
Two major kinds of virtual machines based on their emulation degree of real machines
System virtual machine: it provides a complete system platform to support a complete operating system
Process virtual machine: it runs a single program and supports a single process

7. System VM
It usually emulates a platform to run programs where the real hardware is not available to use (for example, running Mac OS on a windows computer)
Multiple OSes can co-exist on the same physical hardware and are strongly isolated from each other
Popular virtual machine manager products: Xen, VMware Workstation, Sun Virtualbox

8. Process VM
It is created when the process is started and destroyed when the process exits. Its purpose is to provide a platform-independent programming environment so that a program can execute in a same way on any platform
For example, Java bytecode can run on any platform where there is a Java Virtual Machine and the code does not need to be recompiled.

9. Outline
Virtual Machine
Java Virtual Machine
Dalvik Virtual Machine

10. Java Virtual Machine
Java Virtual machine (JVM) is the virtual machine that can execute Java bytecode. It is the execution part of the Java platform
It helps Java to achieve its goal “write once, run anywhere” by hiding the differences between different operating systems

11. Java Bytecode
Java bytecode is the instruction set for Java virtual machine
Each bytecode consists of two parts
One or two bytes that represent the instruction
Zero or more bytes for parameters
Java compiler compiles Java code into Java bytecode. The Java programmer does not need to be aware of or understand Java bytecode

12. Example of Java Bytecode
Java Code
Java Compiler
instruction
Fucntion
iconst_n
Push the integer constant n onto the stack
istore_n
Store an integer value into the variable of index n
iload_n
Load an integer value from variable of index n
sipush
Push a short integer onto the stack
iinc n i
Increase variable of index n by i

13. JVM Workflow (1)
Java source code is compiled into Java bytecode which is stored within .class files
Each class in Java source code will be compiled into one .class file.
The .class files are read and interpreted by JVM

14. JVM Workflow (2) A.java A.class Compile source code
Java Compiler
A.class
Compile source code
Java Virtual Machine
Class Loader
Java API
Bytecode Interpreter
B.class
Host system
(Windows, Linux, etc)

15. JVM Components (1) Class loader Loads .class file into memory
Verifies bytecode instructions
Allocates memory for the program

16. JVM Components (2) Runtime data area
Method area: it stores class and method codes
Heap: it is the place where Java objects are created
Java stacks: they are places where Java methods are executed
Program counter (PC) registers: they store memory addresses of the instructions which to be executed
Native method stacks: they are places where native methods (e.g. C programs) are executed. Native method is a function which is written in another language other than Java

17. JVM Components (3)
Native method interface: it is a program that connects native method libraries with JVM for executing native methods
Native method library
Execution engine: it contains the interpreter which converts Java bytecode into machine code

18. JVM Architecture

19. Java Runtime Environment
We usually install the Java Runtime Environment (JRE) on our computers to use the Java platform
JRE consists of the JVM and Java APIs

20. Outline
Virtual Machine
Java Virtual Machine
Dalvik Virtual Machine

21. Introduction
Dalvik virtual machine (DVM) is the process virtual machine in Google’s Android operating system. It executes applications written for Android.
It is open-source software which was originally written by Dan Bornstein, who named it after the fishing village of Dalvik in Iceland

22. Why DVM not JVM
When Google selected Java as the language for developing Android applications, it chose DVM instead of JVM for several reasons:
Though JVM is free, it was under GPL license, which is not good for Android as most the Android is under Apache license
JVM was designed for desktops and it is too heavy for embedded devices
DVM takes less memory, runs and loads faster compared to JVM

23. Android Architecture
Virtual Machine
C/C++

24. Dalvik Bytecode
Dalvik bytecode is the instruction set for Dalvik virtual machine
Android programs are firstly compiled into Java bytecode which is in .class files. A tool called dx then converts Java bytecode into Dalvik bytecode

25. Dalvik Bytecode VS Java Bytecode
Java source code
Java bytecode
Dalvik bytecode

26. Dalvik Bytecode VS Java Bytecode

27. DEX File
A .dex (Dalvik EXecutable) file is used to store the Dalvik bytecode. It is converted from .class files and executed on the DVM
The .dex file has been optimized for memory usage and the design principle is sharing of data
It uses shared, type-specific constant pools as its primary mechanism for saving memory

28. Constant Pool Concept
A constant pool is a table stores all constant values (e.g. string constants, field constants, class constants, etc.) used within a Java class.
Constant values are referred to by their index in the constant pool rather than stored throughout the class
Constant pool is the biggest part of the Java class file and takes up 61% of the file size

29. Constant Pool Optimization (1)
Although average size of one .class file is quite small, the size still matters because the time to read file from storage is a dominant factor in VM startup time
.dex file optimizes the constant pool when converted from .class files

30. Constant Pool Optimization (2)
In the .class file, each class has its own private, heterogeneous constant pool. It is heterogeneous because all types of constants (field, string, class, etc.) are mixed together
In the .dex file, all classes share the same type-specific constant pools. Duplication of constant values across different classes is eliminated

31. Constant Pool Optimization (3)
.class
dx tool
Heterogeneous Constant Pool
.dex
Strings Constant Pool
Other Data
Java Compiler
Class
Constant Pool
.class
Java Source Code
(.java files)
Heterogeneous Constant Pool
Field
Constant Pool
Other Data
Method Constant Pool
.class
Other Data
Heterogeneous Constant Pool
Other Data

32. Memory Saving Evaluation
The Android team found that .dex file format cuts the size in half of some common system libraries and applications within Android system
Code
Size of .class Files (bytes)
Size of .dex File
(bytes)
Common System Libraries
21,445,320 (100%)
10,311,972 (48%)
Web Browser App
470,312 (100%)
209,248 (44%)
Alarm Clock App
119,200 (100%)
53,020 (44%)

33. Zygote (1)
Zygote is another concept used by Android to speedup VM performance
Every Android application runs in its own instance of the VM, so VM instances should be able to start quickly when a new application is launched
Zygote enables code sharing across different VM instances and provide fast startup time for new VM instances

34. Zygote (2)
Zygote is a VM process which starts at system boot time. When Zygote starts, it initializes a VM instance and preloads core library classes which are good candidates for sharing across processes
Zygote will sit and wait for socket requests from other processes who need new VM instances
Cold starting VM takes a long time. Once a request occurs, Zygote will fork a new VM instance from itself and the startup time will be minimized

35. Zygote (3)

36. Android Runtime
Android Runtime (ART) is an application runtime environment used by Android operating system
It is designed to replace Dalvik virtual machine. It supports standard .dex file to maintain backward compatibility
ART came out as an alternative runtime environment in Android 4.4
Dalvik virtual machine was entirely replace by ART in Android 5.0

37. Java Source Code to Android App
Java Compiler
Java Bytecode
Java Source Code
(.java files)
.class file
.class file
.class file
.class file
.class file
dx tool
Dalvik Bytecode
.dex file
Resource files
Package Builder
Android App
.apk file

38. Android Application Launch Procedure
Many things happen in the background when a user clicks on an icon and launch a new application
The click event gets routed to activity manager.
The activity manager sends parameters to Zygote process over the socket connection and creates a new process
Zygote forks itself and returns the new process ID
The activity manager attaches the new process to the application and the application’s classes will be loaded into the process’s memory
The application is launched

39. References (1) http://en.wikipedia.org/wiki/Virtual_machine

40. References (2)