[Android NDK]Android JNI开发例子 ---2 动态加载JNI lib-蒲公英云

[Android NDK]Android JNI开发例子 ---2 动态加载JNI lib

JNI函数的注册有两种方法，

一种是静态方法，需要用javah为每个声明了native函数的java类编译出的class文件生成一个头文件;

另一种是动态注册，通过数据结构保存关联关系实现注册，这里主要介绍动态注册。

Java 部分与静态加载相同。
JNI部分：

动态注册需要一个数据结构去保存相关的关联关系，这个结构（在jni.h中声明）是：

typedef struct {
const char* name; //java中native函数的名字
const char* signature; //Java函数的签名信息，用字符串表示，是参数类型和返回值类型的组合
void* fnPtr; } JNINativeMethod; //JNI层函数的函数指针，void*类型

在JNI层中是如何使用这个结构体的：

static JNINativeMethod gMethods[] = {
        //{"native method name from Java","(arguments type)return type", "(void*)local_native_name"
        {"nativeGetHelloString", "()Ljava/lang/String;", (void *)nativeGetHelloString},
        {"nativeDrawFrame", "()V", (void *)nativeDrawFrame},
        {"nativeSurfaceChanged", "(II)V", (void *)nativeSurfaceChanged},
        {"nativeSurfaceCreated", "()V", (void *)nativeSurfaceCreated},
};

env是jni.h中定义的一个结构，_JNIEnv中定义了包括registerNativeMethods在内的函数指针。指针在dalvik/vm/Jni.cpp 中具体实现

#if defined(__cplusplus)
typedef _JNIEnv JNIEnv; //cpp中是一个结构
typedef _JavaVM JavaVM;
#else
typedef const struct JNINativeInterface* JNIEnv; //c中是一个指针。
typedef const struct JNIInvokeInterface* JavaVM;
#endif

源码：

#include <string.h>
#include <android/log.h>
#include <jni.h>
#define LOG_TAG "bbaiEGL_Dynamic"
#define LOGD(...) __android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)
#define LOGE(...) __android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)
extern "C" {
/*
 * Class:     com_example_nativeegl_MyRenderer
 * Method:    nativeGetHelloString
 * Signature: ()Ljava/lang/String;
 */
jstring  nativeGetHelloString(JNIEnv *env, jobject obj) {
      return env->NewStringUTF((char*)" This is calling from JNI suckers!");
  }
/*
 * Class:     com_example_nativeegl_MyRenderer
 * Method:    nativeDrawFrame
 * Signature: ()V
 */
void nativeDrawFrame(JNIEnv *env, jobject obj) {
}
/*
 * Class:     com_example_nativeegl_MyRenderer
 * Method:    nativeSurfaceChanged
 * Signature: (II)V
 */
void nativeSurfaceChanged(JNIEnv *env, jobject obj, jint width, jint height){
}
/*
 * Class:     com_example_nativeegl_MyRenderer
 * Method:    nativeSurfaceCreated
 * Signature: ()V
 */
 void nativeSurfaceCreated(JNIEnv *env, jobject obj) {
}
//----------------------------JNI part:Native register------------------------------------------------------
static JNINativeMethod gMethods[] = {
        //{"native method name from Java","(arguments type)return type", "(void*)local_native_name"
        {"nativeGetHelloString", "()Ljava/lang/String;", (void *)nativeGetHelloString},
        {"nativeDrawFrame", "()V", (void *)nativeDrawFrame},
        {"nativeSurfaceChanged", "(II)V", (void *)nativeSurfaceChanged},
        {"nativeSurfaceCreated", "()V", (void *)nativeSurfaceCreated},
};
static const char* className="com/example/nativeegldynamicjni/MyRenderer";
static int registerNativeMethods(JNIEnv *env) {
    jclass clazz;
    clazz = env->FindClass(className);
    if (clazz == NULL) {
        LOGD("failed to load the class %s", className);
        return JNI_FALSE;
    }
    if (env->RegisterNatives(clazz, gMethods, sizeof(gMethods)/sizeof(gMethods[0])) < 0) {
        return JNI_FALSE;
    }
    return JNI_TRUE;
} //end of registerNativeMethods
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("ERROR: GetEnv failed\n");
        goto bail;
    }
    if (registerNativeMethods(env) < 0) {
        LOGE("ERROR: jnitest native registration failed\n");
        goto bail;
    }
    result = JNI_VERSION_1_4;
    bail:
    return result;
}
} //end of extern "C"

MakeFile:

#Description:makefile of Helloworld  
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_CFLAGS    := -Wall
LOCAL_MODULE    := myegl_jni
LOCAL_C_INCLUDES :=$(LOCAL_PATH)/include
LOCAL_CPP_EXTENSION := .cpp
LOCAL_LDLIBS += -llog
LOCAL_SRC_FILES := com_example_nativeegl_MyRenderer.cpp
include $(BUILD_SHARED_LIBRARY)