[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。

1. Java 部分与静态加载相同。

2. 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;#elsetypedef 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 registerNativeMethodsjint 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 $(CLEAR_VARS)LOCAL_CFLAGS    := -WallLOCAL_MODULE    := myegl_jniLOCAL_C_INCLUDES :=$(LOCAL_PATH)/includeLOCAL_CPP_EXTENSION := .cppLOCAL_LDLIBS += -llogLOCAL_SRC_FILES := com_example_nativeegl_MyRenderer.cppinclude $(BUILD_SHARED_LIBRARY)

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