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Lecture notes for cmsc 212 - computer science course, specifically for lecture 24 on java native interface (jni). The notes cover goals, keys ideas, jni definition, process of compiling using jni, using jni functions, accessing java data from c, accessing java arrays, and calling java methods from c.
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Helloworld.java Java Compiler (javac)
gcc
Helloworld.class
Helloworld.h
HelloworldC.c
Javah โjni
stdio.h
jni.h
Hello.so
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#include <jni.h> #include "HelloWorld.h" #include <stdio.h>
JNIEXPORT void JNICALL Java_HelloWorld_displayHelloWorld(JNIEnv *env, jobject obj) { printf("Hello world!\n"); return; }
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Mapping Types
Long Jlong 64
Int Jint 32
Short Jshort 16
Char Jchar 16
Boolean Jboolean 8, unsigned
Byte Jbyte 8
Java Type Native Type Size (bits)
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The String object in the Java language, which is represented as jstring in Java Native Interface (JNI), is a 16 bit unicode string. In C a string is by default constructed from 8 bit characters.
To access a Java language String object passed to a C or C++ function or return a C or C++ string to a Java language method, you need to use JNI conversion functions in your native method implementation. The following converts the java string to an array of C characters:
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Need to call special functions to get data and size Example: JNIEXPORT jfloat JNICALL Java_FloatArray_sumArray(JNIEnv *env, jobject obj, jfloatArray arr) { jfloat *body, sum = 0; jsize I, len;
len = (env)->GetArrayLength(env, arr); body = (env)->GetFloatArrayElements(env, arr, 0); for (i=0; i<len; i++) { sum += body[i]; } (*env)->ReleaseFloatArrayElements(env, arr, body, 0); return sum; }
