Android JNI（實現自己的JNI_OnLoad函數）

實現JNI中本地函數註冊可以兩種方式：
（1）採用預設的本地函數註冊流程。
（2）自己重寫JNI_OnLoad()函數。（本文介紹）（Android中採用這種）

Java端代碼：

package com.jni;public class JavaHello {public static native String hello();static {// load library: libtest.sotry {System.loadLibrary("test");} catch (UnsatisfiedLinkError ule) {System.err.println("WARNING: Could not load library!");}}public static void main(String[] args) {String s = new JavaHello().hello();System.out.println(s);}}

本地C語言代碼：

#include <stdlib.h>#include <string.h>#include <stdio.h>#include <jni.h>#include <assert.h>JNIEXPORT jstring JNICALL native_hello(JNIEnv *env, jclass clazz){printf("hello in c native code./n");return (*env)->NewStringUTF(env, "hello world returned.");}#define JNIREG_CLASS "com/jni/JavaHello"//指定要註冊的類/*** Table of methods associated with a single class.*/static JNINativeMethod gMethods[] = {{ "hello", "()Ljava/lang/String;", (void*)native_hello },//綁定};/** Register several native methods for one class.*/static int registerNativeMethods(JNIEnv* env, const char* className,        JNINativeMethod* gMethods, int numMethods){jclass clazz;clazz = (*env)->FindClass(env, className);if (clazz == NULL) {return JNI_FALSE;}if ((*env)->RegisterNatives(env, clazz, gMethods, numMethods) < 0) {return JNI_FALSE;}return JNI_TRUE;}/** Register native methods for all classes we know about.*/static int registerNatives(JNIEnv* env){if (!registerNativeMethods(env, JNIREG_CLASS, gMethods, 
                                 sizeof(gMethods) / sizeof(gMethods[0])))return JNI_FALSE;return JNI_TRUE;}/** Set some test stuff up.** Returns the JNI version on success, -1 on failure.*/JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM* vm, void* reserved){JNIEnv* env = NULL;jint result = -1;if ((*vm)->GetEnv(vm, (void**) &env, JNI_VERSION_1_4) != JNI_OK) {return -1;}assert(env != NULL);if (!registerNatives(env)) {//註冊return -1;}/* success -- return valid version number */result = JNI_VERSION_1_4;return result;}

編譯及運行流程：

1 設定三個環境變數：
export JAVA_HOME:=/usr/lib/jvm/java-6-sun-1.6.0.15
export JAVA_SRC_PATH:=/home/kortide/Jackey/jni/jni_onload/com/jfo
export NATIVE_SRC_PATH:=/home/kortide/Jackey/jni/jni_onload/jni

2 編譯JavaHello.java:
javac $JAVA_SRC_PATH/JavaHello.java

3. 編譯NativeHello.c，產生共用庫
gcc -I $JAVA_HOME/include -I $JAVA_HOME/include/linux -c -o $NATIVE_SRC_PATH/NativeHello.o  $NATIVE_SRC_PATH/NativeHello.c

gcc -fPIC -I $JAVA_HOME/include -I $JAVA_HOME/include/linux -shared -o $NATIVE_SRC_PATH/libtest.so $NATIVE_SRC_PATH/NativeHello.o

4. 運行
java com/jni/JavaHello