Decoding Android Runtime (DroidJam 2018)

While writing code, we mostly care about that it should be efficient and should not break. How that code executes at runtime is a completely different story in case of Android. The .dex format has been here for years and everyone is accustomed to it. But there are a lot of complexities going on once the .dex reaches your device. This talk will focussed on analyzing on what happens after your code in Kotlin/Java is compiled, packaged and run on an actual device. We will take a deep dive into .dex format as well as see how runtime has evolved since Android 1.0 and taken a great leap with Android 5.0 and above. In the process, we will also learn how to write runtime efficient code.

Transcript

1. JITIN S HARMA
   D E C O D I N G A N D R O I D
   R U N T I M E
   @js_2892
   

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2. What is Android
   - Some Hardware
   - Some Linux
   - Some C++
   - Some Java
   

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4. Boot-Up
   

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5. Boot-Up
   Hardware Start-Up
   02 Software Start-Up
   ZYGOTE
   03
   01
   

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6. Boot-Up
   init.rc
   AndroidRuntime.cpp Zygote main()
   Transition to
   Java base execution
   Linux Kernel Start
   

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7. Zygote
   - Zygote Process acts a base process from which all processes are forked for subsequent
   applications.
   - This allows Android to run in a Multi-Process environment where all processes run independent of
   each other.
   - All resources preloaded by Zygote are accessed by subsequent applications through shared
   memory.
   

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8. Zygote
   System Server
   Load core libraries
   and resources
   Starts Activity Manager
   Notification Manager, Bluetooth Service,
   Package Manager, Sensor Service…
   

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9. Zygote
   System Server
   Load core libraries
   and resources
   Starts Activity Manager
   Notification Manager, Bluetooth Service,
   Package Manager, Sensor Service…
   

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10. Zygote
    

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11. Phone is ON
    

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12. Building APK
    java/kt files .class files .dex
    javac
    kotlinc
    dx
    APK
    

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13. Installing Application
    

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14. Manifest Parsing
    - All data from AndroidManifest.xml is read and is written to data/system/packages.xml
    - This file acts as a global source for all packages and is used by PackageManager API
    - This happens only once either when device is booted or an application is being installed.
    

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15. Manifest Parsing
    - Identifies various elements within your application
    - At the same time manifest performs checks which can cause installation errors if elements are not
    defined properly.
    

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16. Code Compilation and Other Stuff
    

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17. Code Compilation and Other Stuff
    Dalvik ART
    

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18. dex and Dalvik Era
    Android 1.0 - 4.4
    (2008-2014)
    

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19. .dex
    Dalvik VM
    A register based VM optimised for
    memory constrained device
    .dex -> Dalvik Executable
    

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20. Stack Machines
    value1
    value2
    value3
    value4
    value1 value2
    operation
    pop pop
    value5
    value3
    value4
    push
    

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21. Stack Machines Register Machines
    value1
    value2
    value3
    value4
    value1 value2
    operation
    pop pop
    value5
    value3
    value4
    push
    addr1 value1
    operation
    addr2 value2
    addr3 value3
    

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22. Stack Machines Register Machines
    value1
    value2
    value3
    value4
    value1 value2
    operation
    pop pop
    value5
    value3
    value4
    push
    addr1 value1
    operation
    addr2 value2
    addr3 value3
    ARM Optimized
    

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23. dex
    - Uses 16 bit instruction format.
    - Bytecode tends to be larger due to register based architecture for storing addresses
    

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24. dex
    - Uses 16 bit instruction format.
    - Bytecode tends to be larger due to register based architecture for storing addresses
    ‣header
    ‣string_ids
    ‣type_ids
    ‣proto_ids
    ‣field_ids
    ‣method_ids
    ‣class_defs
    ‣call_site_ids
    ‣method_handles
    ‣data
    ‣link_data
    

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25. dex
    - Uses 16 bit instruction format.
    - Bytecode tends to be larger due to register based architecture for storing addresses
    ‣header
    ‣string_ids
    ‣type_ids
    ‣proto_ids
    ‣field_ids
    ‣method_ids
    ‣class_defs
    ‣call_site_ids
    ‣method_handles
    ‣data
    ‣link_data
    Contains verification
    signatures
    

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26. dex
    

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27. dex
    

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28. public class Data {
    int id;
    String name = "";
    public int getId() {
    return id;
    }
    public void setId(int id) {
    this.id = id;
    }
    public String getName() {
    return name;
    }
    public void setName(String name) {
    this.name = name;
    }
    }
    

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29. public int getId() {
    return id;
    }
    .METHOD getId : int
    .MODIFIERS public
    .REGISTERS 2
    .CODE
    iget v0, v1 field@8046
    return v0
    .METHOD setName : void
    .PARAM java.lang.String
    .MODIFIERS public
    .REGISTERS 2
    .CODE
    iput-object v1, v0 field@8047
    return-void
    public void setName(String name) {
    this.name = name;
    }
    

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30. private fun forEachTest(): Int {
    var sum = 0
    (0..50).forEach {
    sum += it
    }
    return sum
    }
    private fun forTest(): Int {
    var sum = 0
    for (i in 0 until 50) {
    sum += i
    }
    return sum
    }
    VS
    

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31. private fun forTest(): Int {
    var sum = 0
    for (i in 0 until 50) {
    sum += i
    }
    return sum
    }
    .METHOD forTest : int
    .MODIFIERS private final
    .REGISTERS 4
    .CODE
    const/4 v0, #0
    const/4 v1, #0
    const/16 v2, #50
    if-ge v1, v2, 6
    add-int/2addr v1, v0
    add-int/lit8 v1, v1, #1
    goto -7
    return v0
    

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32. private fun forEachTest(): Int {
    var sum = 0
    (0..50).forEach {
    sum += it
    }
    return sum
    }
    .METHOD forEachTest : int
    .MODIFIERS private final
    .REGISTERS 8
    .CODE
    const/4 v0, #0
    new-instance v1, type@2439
    const/4 v2, #0
    const/16 v3, #50
    invoke-direct {v1, v2, v3}, meth@20280
    check-cast v1, type@1851
    move v3, v2
    invoke-interface {v1}, meth@15176
    move-result-object v4
    invoke-interface {v4}, meth@15640
    move-result v5
    if-eqz v5, 13
    move-object v5, v4
    check-cast v5, type@2142
    invoke-virtual {v5}, meth@18564
    move-result v5
    move v6, v5
    add-int/2addr v6, v0
    goto -16
    return v0
    

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33. A.class
    B.class
    Z.class
    classes.dex
    

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34. A.class
    B.class
    Z.class
    classes.dex
    - Reduction in bytecode size during conversion.
    - Duplicate constants from class files are converted to a single one.
    - Each method needs have a reference which is limited by dex instruction format of 16 bits,
    hence total method references which a single dex file can hold is 65,536 (2^16)
    

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35. dex++
    

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36. Multidexing
    classes.dex
    java/kt
    classes2.dex classesN.dex
    …
    

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37. Installation
    data/dalvik-cache
    data/data/com.example.myapplication
    

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38. dexopt
    - Verification and Optimization of contents of .dex file
    - Result differ from device to device.
    - Starts a small version of VM to optimize dex files.
    - Contents of system/framework/ can be used to optimize dex files outside Android OS.
    - Code is optimized for runtime not memory.
    

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39. classes.dex
    My Application
    Zygote
    Process
    

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40. Multidex.install(this)
    classes.dex classes2.dex classes3.dex ….
    System Application
    classesN.dex
    Inject values
    into ClassLoader
    

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41. Multidex.install(this)
    classes.dex classes2.dex classes3.dex ….
    System Application
    Must have Application class
    classesN.dex
    Inject values
    into ClassLoader
    

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42. Multidex.install(this)
    classes.dex classes2.dex classes3.dex ….
    System Application
    Must have Application class
    Library dependencies
    classesN.dex
    Inject values
    into ClassLoader
    

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43. Multidex.install(this)
    classes.dex classes2.dex classes3.dex ….
    System Application
    Must have Application class
    Library dependencies
    (multiDexKeepFile)
    classesN.dex
    Inject values
    into ClassLoader
    

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44. DexClassLoader
    DexClassLoader dexClassLoader = new DexClassLoader(dexFile.getAbsolutePath(),
    getCodeCacheDir().getAbsolutePath(), null, context.getClassLoader());
    

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45. DexClassLoader
    DexClassLoader dexClassLoader = new DexClassLoader(dexFile.getAbsolutePath(),
    getCodeCacheDir().getAbsolutePath(), null, context.getClassLoader());
    Network
    

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46. DexClassLoader
    DexClassLoader dexClassLoader = new DexClassLoader(dexFile.getAbsolutePath(),
    getCodeCacheDir().getAbsolutePath(), null, context.getClassLoader());
    Class> dynamicClass = dexClassLoader.loadClass("com.example.myapplication.runtime");
    Method dynamicMethod = dynamicClass.getDeclaredMethod("executeDataLoad", Integer.class, Integer.class);
    Integer value = (Integer) dynamicMethod.invoke(dynamicClass.newInstance(), 1, 2);
    Network
    

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47. DexClassLoader
    DexClassLoader dexClassLoader = new DexClassLoader(dexFile.getAbsolutePath(),
    getCodeCacheDir().getAbsolutePath(), null, context.getClassLoader());
    Class> dynamicClass = dexClassLoader.loadClass("com.example.myapplication.runtime");
    Method dynamicMethod = dynamicClass.getDeclaredMethod("executeDataLoad", Integer.class, Integer.class);
    Integer value = (Integer) dynamicMethod.invoke(dynamicClass.newInstance(), 1, 2);
    Network
    public Integer executeDataLoad(Integer value1, Integer value2) {
    //
    return result;
    }
    

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48. DexClassLoader
    DexClassLoader dexClassLoader = new DexClassLoader(dexFile.getAbsolutePath(),
    getCodeCacheDir().getAbsolutePath(), null, context.getClassLoader());
    Class> dynamicClass = dexClassLoader.loadClass("com.example.myapplication.runtime");
    Method dynamicMethod = dynamicClass.getDeclaredMethod("executeDataLoad", Integer.class, Integer.class);
    Integer value = (Integer) dynamicMethod.invoke(dynamicClass.newInstance(), 1, 2);
    Network
    public Integer executeDataLoad(Integer value1, Integer value2) {
    //
    return result;
    }
    

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49. JIT
    

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50. JIT
    private void handleIntent() {
    ...
    public RequestQueue getRequestQueue() {
    if (requestQueue == null) {
    requestQueue = Volley.newRequestQueue(this);
    }
    return requestQueue;
    }
    for (String s : networkRequestMap.keySet()) {
    if (BuildConfig.DEBUG) {
    //
    

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51. Tracing and Profiling
    Low Memory
    Optimized
    (100K/200K)
    Faster results
    Google I/O 2010
    

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52. Shortcomings of Dalvik
    Time
    

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53. ART (Android Runtime)
    Android 5.0 - present
    (2014-present)
    

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54. Embracing the GBs…
    

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57. Embracing the GBs…
    

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58. Revamped GC
    - ART keeps track of application going in non-UI state.
    - Seperate types of GC for low memory and non-low memory devices.
    - GC pauses reduced from 2 to 1.
    - Larger objects like bitmap have seperate heap for faster allocation.
    - Young Objects are moved to stack for faster reclamation of memory.
    

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59. Dalvik
    GC GC GC
    1MB 3MB 5MB
    

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60. ART
    GC GC GC
    1MB 3MB 5MB
    

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61. ART
    GC GC GC
    1MB 3MB 5MB
    9MB
    Heap
    Compaction
    

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62. AOT Compilation
    classes.dex
    classes2.dex
    classesN.dex
    .oat
    dex2oat
    - AOT works on principle of compile once and then run.
    - Compilation takes significant time and memory since whole bytecode is converted to machine code
    at installation.
    - AOT compilers do not benefit from profile guided compilation which JIT compilers have instead use
    a restrictive approach to do compilation.
    Android
    Runtime
    VM
    

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63. .oat
    header
    instruction set
    dex file count
    dex2oat cmd line
    code
    data/app/
    

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64. DEX CODE:
    0x0000: 5210 e10f | iget v0, v1, I com.example.myapplication.Data.id // field@4065
    0x0002: 0f00 | return v0
    public int getId() {
    return id;
    }
    Java
    

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65. CODE:
    (code_offset=0x00d6a3fc size_offset=0x00d6a3f8 size=68)...
    0x00d6a3fc: 4885842400E0FFFF testq rax, [rsp + -8192]
    suspend point dex PC: 0x0000
    GC map objects: v1 ([sp + #40])
    0x00d6a404: 4883EC18 subq rsp, 24
    0x00d6a408: 48893C24 movq [rsp], rdi
    0x00d6a40c: 89742428 mov [rsp + 40], esi
    0x00d6a410: 6566833C250000000000 cmpw gs:[0], 0 ; state_and_flags
    0x00d6a41a: 0F8516000000 jnz/ne +22 (0x00d6a436)
    0x00d6a420: 8B442428 mov eax, [rsp + 40]
    0x00d6a424: 8500 test eax, [rax]
    suspend point dex PC: 0x0000
    GC map objects: v1 ([sp + #40])
    0x00d6a426: 8B480C mov ecx, [rax + 12]
    suspend point dex PC: 0x0000
    GC map objects: v1 ([sp + #40])
    0x00d6a429: 894C2410 mov [rsp + 16], ecx
    0x00d6a42d: 8B442410 mov eax, [rsp + 16]
    0x00d6a431: 4883C418 addq rsp, 24
    0x00d6a435: C3 ret
    0x00d6a436: 65FF142538040000 call gs:[1080] ; pTestSuspend
    ……..
    

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66. Problems with AOT compilation
    

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68. JIT/AOT Hybrid
    ‣System boot after OS update will no longer optimise the app, a job solely performed by JIT.
    ‣dex2oat is still used for .oat creation but it uses profile data generated by JIT to do so.
    ‣Code may be executed by directed .dex interpretation or JIT cache or AOT compiled.
    ‣At any time JIT compiled data will be more efficient because of availability of better environment
    data than AOT compiled code, and the same is preferred if both are present.
    

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69. DEX CODE:
    0x0000: 5210 e10f | iget v0, v1, I com.example.myapplication.Data.id // field@4065
    0x0002: 0f00 | return v0
    public int getId() {
    return id;
    }
    Java
    CODE:
    (code_offset=0x00000000 size_offset=0x00aea110 size=0)
    NO CODE!
    

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70. Third party frameworks
    

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71. Future
    

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72. Future
    A.I
    

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73. Thanks
    @js_2892
    

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