Android 通过Jni调用Native

     Native是java本地方法的声明关键字,当在方法中调用一些不是由java语言写的代码或要直接操作计算机硬件或要提交某些代码的运行效率时要声明native方法.

     Jni是java native interface的缩写,从字面上来说就是java本地方法的接口,所以他就是提供java native方法的声明和对上层的接口调用.

          1.Jni 编写

       使用native 声明_camera_open 为本地方法,并load与之相对应的动态链接库.

<span style="font-family:Microsoft YaHei;font-size:18px;"><span style="white-space: pre;">	</span>private native int _camera_open(int cameraId);</span><pre name="code" class="java"><span style="font-family:Microsoft YaHei;font-size:18px;">	private final static String studioLib = "studionative";
	
	public static boolean loadLibrarys() {
		Log.i(TAG, "loadLibrarys");
		boolean status = false;
		try{
			System.loadLibrary(studioLib);
			status = true;
		}catch(Exception e){
			Log.e(TAG, "exception:" + e.toString());
		}
		return status;
	}</span>

2.使用javah -jni 命令 进入bin目录下 javah -jni cn.jesse.studo.jni.StudioJni 生成一个cn_jesse_studio_jni_StudioJni.h文件,提供给native 使用.这里不了解为什么在生成的头文件中的方法会被自动加个1.

<span style="font-family:Microsoft YaHei;">/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
#include <stdio.h>
#include <android_runtime/AndroidRuntime.h>
#include <termios.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include "logger.h"
/* Header for class cn_jesse_studio_jni_StudioJni */

#ifndef _Included_cn_jesse_studio_jni_StudioJni
#define _Included_cn_jesse_studio_jni_StudioJni
#ifdef __cplusplus
int register_cn_jesse_studio_jni_studioJni(JNIEnv*);
extern "C" {
#endif
/*
 * Class:     cn_jesse_studio_jni_StudioJni
 * Method:    _camera_open
 * Signature: (I)I
 */
JNIEXPORT jint JNICALL Java_cn_jesse_studio_jni_StudioJni__1camera_1open
  (JNIEnv *, jobject, jint);

/*
 * Class:     cn_jesse_studio_jni_StudioJni
 * Method:    _serial_port_open
 * Signature: (Ljava/lang/String;IIIC)Ljava/io/FileDescriptor;
 */
JNIEXPORT jobject JNICALL Java_cn_jesse_studio_jni_StudioJni__1serial_1port_1open
  (JNIEnv *, jobject, jstring, jint, jint, jint, jchar);

/*
 * Class:     cn_jesse_studio_jni_StudioJni
 * Method:    _serial_port_close
 * Signature: ()I
 */
JNIEXPORT jint JNICALL Java_cn_jesse_studio_jni_StudioJni__1serial_1port_1close
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif</span>

<span style="font-family:Microsoft YaHei;">JNIEXPORT jint JNICALL media_camera_open(JNIEnv *env,jclass thiz,jint cameraId){
    LOGI(CAMERA_TAG, "media_camera_open");
    int status = -1;
    return status;
}</span>

<span style="font-family:Microsoft YaHei;">static const char* const cameraClassPathName = "cn/jesse/studio/jni/StudioJni";
static JNINativeMethod jniMethods[] = {
    {"_camera_open",             "(I)I",                                             (void *) media_camera_open},
    {"_serial_port_open",        "(Ljava/lang/String;IIIC)Ljava/io/FileDescriptor;", (void *) serial_port_open},
    {"_serial_port_close",       "()I",                                              (void *) serial_port_close}
};</span>


<span style="font-family:Microsoft YaHei;">int register_cn_jesse_studio_jni_studioJni(JNIEnv* env){
    return AndroidRuntime::registerNativeMethods(env, cameraClassPathName, jniMethods,sizeof(jniMethods) / sizeof(jniMethods[0]));
}</span>

<span style="font-family:Microsoft YaHei;">static int registerNatives(JNIEnv* env){
    if (register_cn_jesse_studio_jni_studioJni(env) < 0){
        LOGE(JNILOADER_TAG, "ERROR: registerNatives failed\n");
        return JNI_FALSE;
    }

    return JNI_TRUE;
}

jint JNI_OnLoad(JavaVM* vm, void* reserved) {
    JNIEnv* env = NULL;
    jint result = -1;
    if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {
        LOGE(JNILOADER_TAG, "ERROR: GetEnv failed\n");
        goto bail;
    }
    if (env == NULL || registerNatives(env) != JNI_TRUE) {
        LOGE(JNILOADER_TAG, "JniLoad ERROR: registerNatives failed, env=%p", env);
        goto bail;
    }
    LOGI(JNILOADER_TAG, "JniLoad JNI Load Success");
    /* success -- return valid version number */
    result = JNI_VERSION_1_4;

    bail: return result;
}</span>

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