JNI Hello World (Android KW)

Video: https://www.youtube.com/watch?v=uglvahwOvIM

A quick look at what JNI is, how it works, and how you can use it to be awesome.

Transcript

1. JNI @jessewilson
2. Java • Productive • Lots of great libraries • Fast enough
3. JVM • Architecture neutral • Garbage collected • Fast Java Virtual Machine
4. JVM • Architecture neutral • Garbage collected • Fast Java Virtual Machine just-in-time compiled
5. JVM • Architecture neutral • Garbage collected • Fast Java Virtual Machine just-in-time compiled interpreted
6. ART • Architecture neutral • Garbage collected • Fast Android Runtime
7. ART • Architecture neutral • Garbage collected • Fast Android Runtime ahead-of- time compiled
8. ART • Architecture neutral • Garbage collected • Fast Android Runtime just-in-time compiled ahead-of- time compiled
9. ART • Architecture neutral • Garbage collected • Fast Android Runtime just-in-time compiled interpreted ahead-of- time compiled
10. Architecture Neutral
11. Popular Architectures • Desktops: x86, x64 • Sma rt phones: ARM, MIPS
12. Architecture Neutral • You ship one app • The runtime adapts it to the host computer’s OS and architecture
13. Operating System Computer Application
14. Operating System Computer Application architecture speci fi c
15. Operating System Computer JVM / ART Application architecture speci fi c
16. Operating System Computer JVM / ART Application architecture speci fi c architecture neutral
17. Operating System Computer JVM / ART Application
18. Operating System Computer JVM / ART Application
19. Operating System Computer JVM / ART Application
20. Operating System Computer JVM / ART Application
21. Operating System Computer JVM / ART Application
22. Operating System Computer JVM / ART Application
23. Operating System Computer JVM / ART Application
24. Operating System Computer JVM / ART Application
25. Operating System Computer JVM / ART Application
26. Operating System Computer JVM / ART Application
27. Operating System Computer JVM / ART Application
28. Operating System Computer JVM / ART Application
29. Operating System Computer JVM / ART Application
30. Operating System Computer JVM / ART Application
31. Operating System Computer JVM / ART Application
32. Operating System Computer JVM / ART App Java
33. Operating System Computer JVM / ART App Java App C/C++
34. Operating System Computer JVM / ART App Java App C/C++ architecture neutral
35. Operating System Computer JVM / ART App Java App C/C++ architecture neutral architecture speci fi c
36. Operating System Computer JVM / ART App Java App C/C++
37. Operating System Computer JVM / ART App Java App C/C++
38. Operating System Computer JVM / ART App Java App C/C++
39. Operating System Computer JVM / ART App Java App C/C++
40. Operating System Computer JVM / ART App Java App C/C++
41. Operating System Computer JVM / ART App Java App C/C++
42. Operating System Computer JVM / ART App Java App C/C++
43. Operating System Computer JVM / ART App Java App C/C++
44. Operating System Computer JVM / ART App Java App C/C++
45. Operating System Computer JVM / ART App Java App C/C++
46. Operating System Computer JVM / ART App Java App C/C++
47. Operating System Computer JVM / ART App Java App C/C++
48. Operating System Computer JVM / ART App Java App C/C++
49. Operating System Computer JVM / ART App Java App C/C++
50. App Java ARM C/C++
51. App Java ARM C/C++ foo.apk
52. App Java ARM C/C++ foo.apk Image Image Image Image Image Image Image Image Layout Layout Resource Resource Translation XML Translation XML
53. App Java ARM C/C++ x64 C/C++ x86 C/C++ MIPS C/C++ foo.apk Image Image Image Image Image Image Image Image Layout Layout Resource Resource Translation XML Translation XML
54. JNI & Architecture • Build & ship a JNI library for every CPU+OS pair you suppo rt • That means you need to decide which CPU+OS pairs to suppo rt ! • Fo rt unately, Android is a single OS, but there are still several CPUs!
55. Garbage Collected
56. Garbage Collection • You solve application problems • The runtime worries about allocating and freeing memory • Why make a clumsy human do a computer’s job?
57. List<String> characters = new ArrayList<>(); 
 characters.add("grant"); 
 characters.add("hammond"); 
 characters.add("malcolm");
58. String String String ArrayList Object[] char[] char[] char[]
59. String String String ArrayList Object[] char[] char[] char[] 0x00 0x10 0x20 0x30 0x40 0x50
60. String String String ArrayList Object[] char[] char[] char[] 0x00 0x10 0x20 0x30 0x40 0x50
61. String String String ArrayList Object[] char[] char[] char[] 0x00 0x10 0x20 0x30 0x40 0x50
62. String String String ArrayList Object[] char[] char[] char[] uint struct void* unsigned int* struct struct uint struct char* void* struct
63. String String String ArrayList Object[] char[] char[] char[] uint struct void* unsigned int* struct struct uint struct char* void* struct native heap
64. String String String ArrayList Object[] char[] char[] char[] uint struct void* unsigned int* struct struct uint struct char* void* struct native heap managed heap
65. Garbage Collector • Within a managed heap, an object’s identity and location in memory are decoupled • A compacting collector defragments the heap by moving objects around
66. Moving References • You need to be careful about pointers into the managed heap • If an object is moved, your pointers will break!
67. Tracking References • You need to keep the garbage collector informed about what objects you’re using • Otherwise they could be collected out from under you!
68. Fast Enough
69. A Langauge of Abstraction • List vs. array • Methods are vi rt ual by default • Strict encapsulation
70. A Dynamic Runtime • Re fl ection • Runtime code loading • Memory managed
71. Native Code
72. Typical Native Libraries • Cryptography • Databases • PDF viewers
73. Why Native? • Cross-platform • Historical accident • Inef fi cient on Java’s pe rf ormance model • Bene fi ts from access to fancy operating system features
74. Why Not Native? • “faster”
75. Operating System Computer JVM / ART App Java App C/C++
76. Operating System Computer JVM / ART App Java App C/C++
77. “We rewrote our program in and it was 5x faster” C
78. “We rewrote our program in and it was 5x faster” Java
79. Java, Fast & Slow • Most programs are slow because they use inappropriate datastructures, algorithms, and abstractions • Because Java has many rich APIs, Java programs are often faster
80. Lots of fast Java programs • javac vs. gcj • IntelliJ vs. Visual Studio
81. How to JNI
82. http://tinyurl.com/jni-example
85. public final class Hello { 
 static { 
 System.loadLibrary("jniexample"); 
 } 
 
 public static native String greet(String name); 
 }
88. #include <string.h> 
 #include <jni.h> 
 
 JNIEXPORT jstring JNICALL 
 Java_com_squareup_jniexample_Hello_greet(JNIEnv *env, jclass type, jstring name_) { 
 const char *name = (*env)->GetStringUTFChars(env, name_, 0); 
 
 char *greeting; 
 if (strcmp(name, "Jesse") == 0) { 
 greeting = "Yo dog"; 
 } else { 
 greeting = "Hello"; 
 } 
 
 (*env)->ReleaseStringUTFChars(env, name_, name); 
 
 return (*env)->NewStringUTF(env, greeting); 
 }
89. http://tinyurl.com/jnidocs
90. JNI Reference • Mapping Java types to C types • References • Calling into Java • Exceptions • Synchronization
91. Tips • Android Studio • ProGuard • CheckJNI
92. Building JNI • Much improved in 2015 • Gradle! • Testing on desktop JVM
93. Testing JNI • You’ll want a separate Gradle module
94. Questions?