JNI Hello World (Android KW)

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JNI @jessewilson

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Java • Productive • Lots of great libraries • Fast enough

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JVM • Architecture neutral • Garbage collected • Fast Java Virtual Machine

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JVM • Architecture neutral • Garbage collected • Fast Java Virtual Machine just-in-time compiled

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JVM • Architecture neutral • Garbage collected • Fast Java Virtual Machine just-in-time compiled interpreted

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ART • Architecture neutral • Garbage collected • Fast Android Runtime

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ART • Architecture neutral • Garbage collected • Fast Android Runtime ahead-of- time compiled

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ART • Architecture neutral • Garbage collected • Fast Android Runtime just-in-time compiled ahead-of- time compiled

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ART • Architecture neutral • Garbage collected • Fast Android Runtime just-in-time compiled interpreted ahead-of- time compiled

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Architecture Neutral

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Popular Architectures • Desktops: x86, x64 • Sma rt phones: ARM, MIPS

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Architecture Neutral • You ship one app • The runtime adapts it to the host computer’s OS and architecture

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Operating System Computer Application

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Operating System Computer Application architecture speci fi c

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Operating System Computer JVM / ART Application architecture speci fi c

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Operating System Computer JVM / ART Application architecture speci fi c architecture neutral

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART Application

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Operating System Computer JVM / ART App Java

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++ architecture neutral

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Operating System Computer JVM / ART App Java App C/C++ architecture neutral architecture speci fi c

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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App Java ARM C/C++

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App Java ARM C/C++ foo.apk

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App Java ARM C/C++ foo.apk Image Image Image Image Image Image Image Image Layout Layout Resource Resource Translation XML Translation XML

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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

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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!

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Garbage Collected

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Garbage Collection • You solve application problems • The runtime worries about allocating and freeing memory • Why make a clumsy human do a computer’s job?

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List characters = new ArrayList<>();   characters.add("grant");   characters.add("hammond");   characters.add("malcolm");

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String String String ArrayList Object[] char[] char[] char[]

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String String String ArrayList Object[] char[] char[] char[] 0x00 0x10 0x20 0x30 0x40 0x50

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String String String ArrayList Object[] char[] char[] char[] 0x00 0x10 0x20 0x30 0x40 0x50

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String String String ArrayList Object[] char[] char[] char[] 0x00 0x10 0x20 0x30 0x40 0x50

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String String String ArrayList Object[] char[] char[] char[] uint struct void* unsigned int* struct struct uint struct char* void* struct

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String String String ArrayList Object[] char[] char[] char[] uint struct void* unsigned int* struct struct uint struct char* void* struct native heap

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String String String ArrayList Object[] char[] char[] char[] uint struct void* unsigned int* struct struct uint struct char* void* struct native heap managed heap

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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

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Moving References • You need to be careful about pointers into the managed heap • If an object is moved, your pointers will break!

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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!

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Fast Enough

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A Langauge of Abstraction • List vs. array • Methods are vi rt ual by default • Strict encapsulation

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A Dynamic Runtime • Re fl ection • Runtime code loading • Memory managed

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Native Code

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Typical Native Libraries • Cryptography • Databases • PDF viewers

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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

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Why Not Native? • “faster”

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Operating System Computer JVM / ART App Java App C/C++

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Operating System Computer JVM / ART App Java App C/C++

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“We rewrote our program in and it was 5x faster” C

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“We rewrote our program in and it was 5x faster” Java

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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

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Lots of fast Java programs • javac vs. gcj • IntelliJ vs. Visual Studio

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How to JNI

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http://tinyurl.com/jni-example

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public final class Hello {   static {   System.loadLibrary("jniexample");   }     public static native String greet(String name);   }

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#include   #include     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);   }

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http://tinyurl.com/jnidocs

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JNI Reference • Mapping Java types to C types • References • Calling into Java • Exceptions • Synchronization