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

￡神魔★判官ぃ 2022-09-18 11:57 186阅读 0赞

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;
    #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)