Android JNI（实现自己的JNI_OnLoad函数）

实现JNI中本地函数注册可以两种方式：

（1）采用默认的本地函数注册流程。

（2）自己重写JNI_OnLoad()函数。（本文介绍）（Android中采用这种）

Java端代码：

package com.jni;
public class JavaHello {
public static native String hello();
static {
// load library: libtest.so
try {
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语言代码：

#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"//指定要注册的类

/**
* Table of methods associated with a single class.
*/
static JNINativeMethod gMethods[] = {
{ "hello", "()Ljava/lang/String;", (void*)native_hello },//绑定
};

/*
* 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)) {//注册
return -1;
}
/* success -- return valid version number */
result = JNI_VERSION_1_4;

return result;
}

编译及运行流程：

1 设置三个环境变量：

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 编译JavaHello.java:

javac $JAVA_SRC_PATH/JavaHello.java

3. 编译NativeHello.c，生成共享库

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. 运行

java com/jni/JavaHello