山东前网站建设,中山企业网站建设,中国最好的编程学校,互联网站的建设维护营销前言: 近段时间因为项目的需求,需要使用JNI,所以下载了Google的Ndk-Sample学习下,准备记录 下来,留给后期自己查看 问题点一:JNI_OnLoad方法必须返回JNI的版本
JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved) {JNIEnv *env;memset(g_ctx, 0, sizeof(g_…前言: 近段时间因为项目的需求,需要使用JNI,所以下载了Google的Ndk-Sample学习下,准备记录 下来,留给后期自己查看 问题点一:JNI_OnLoad方法必须返回JNI的版本
JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved) {JNIEnv *env;memset(g_ctx, 0, sizeof(g_ctx));g_ctx.javaVM vm;if ((*vm)-GetEnv(vm, (void **)env, JNI_VERSION_1_6) ! JNI_OK) {return JNI_ERR; // JNI version not supported.}jclass clz (*env)-FindClass(env, com/example/hellojnicallback/JniHandler);g_ctx.jniHelperClz (*env)-NewGlobalRef(env, clz);jmethodID jniHelperCtor (*env)-GetMethodID(env, g_ctx.jniHelperClz, init, ()V);jobject handler (*env)-NewObject(env, g_ctx.jniHelperClz, jniHelperCtor);g_ctx.jniHelperObj (*env)-NewGlobalRef(env, handler);queryRuntimeInfo(env, g_ctx.jniHelperObj);g_ctx.done 0;g_ctx.mainActivityObj NULL;return JNI_VERSION_1_6;
}
上面是hello-jniCallback项目中的原始文件hello-jnicallback.c中相关代码,为了慢慢学习知识点,我自己没有使用hello-jnicallback.c文件,而是自己创建了自己的学习文件,hello_jni.h 跟hello_jni.c.
根据原始sample的中的文件,大概代码可以简化为如下所示,返回0
jint JNI_OnLoad(JavaVM *vm, void *reserved)
{LOGI(enter JNI_OnLoad);JNIEnv* env;if ((*vm)-GetEnv(vm, (void **)env, JNI_VERSION_1_6) ! JNI_OK){LOGE(Failed to get GetEnv);return JNI_ERR;}LOGI(leave JNI_OnLoad);return 0;
}
但是运行程序之后,程序发生了错误,通过LOG信息可以发现,错误提示是错误的Jni 版本 估计是这里返回值是要返回一个JNI的对应版本,尝试了下返回JNI_VERSIOn_1_2,JNI_VERSIOn_1_4, JNI_VERSIOn_1_6都没有问题
jint JNI_OnLoad(JavaVM *vm, void *reserved)
{LOGI(enter JNI_OnLoad);JNIEnv* env;......LOGI(leave JNI_OnLoad);return JNI_VERSION_1_2;
} 问题点二:JNI如何调用java中的方法
JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved) {JNIEnv *env;memset(g_ctx, 0, sizeof(g_ctx));g_ctx.javaVM vm;if ((*vm)-GetEnv(vm, (void **)env, JNI_VERSION_1_6) ! JNI_OK) {return JNI_ERR; // JNI version not supported.}jclass clz (*env)-FindClass(env, com/example/hellojnicallback/JniHandler);g_ctx.jniHelperClz (*env)-NewGlobalRef(env, clz);jmethodID jniHelperCtor (*env)-GetMethodID(env, g_ctx.jniHelperClz, init, ()V);jobject handler (*env)-NewObject(env, g_ctx.jniHelperClz, jniHelperCtor);g_ctx.jniHelperObj (*env)-NewGlobalRef(env, handler);queryRuntimeInfo(env, g_ctx.jniHelperObj);g_ctx.done 0;g_ctx.mainActivityObj NULL;return JNI_VERSION_1_6;
}void queryRuntimeInfo(JNIEnv *env, jobject instance) {// Find out which OS we are running on. It does not matter for this app// just to demo how to call static functions.// Our java JniHelper class id and instance are initialized when this// shared lib got loaded, we just directly use them// static function does not need instance, so we just need to feed// class and method id to JNIjmethodID versionFunc (*env)-GetStaticMethodID(env, g_ctx.jniHelperClz, getBuildVersion, ()Ljava/lang/String;);if (!versionFunc) {LOGE(Failed to retrieve getBuildVersion() methodID line %d, __LINE__);return;}jstring buildVersion (*env)-CallStaticObjectMethod(env, g_ctx.jniHelperClz, versionFunc);const char *version (*env)-GetStringUTFChars(env, buildVersion, NULL);if (!version) {LOGE(Unable to get version string line %d, __LINE__);return;}LOGI(Android Version - %s, version);(*env)-ReleaseStringUTFChars(env, buildVersion, version);// we are called from JNI_OnLoad, so got to release LocalRef to avoid leaking(*env)-DeleteLocalRef(env, buildVersion);// Query available memory size from a non-static public function// we need use an instance of JniHelper class to call JNIjmethodID memFunc (*env)-GetMethodID(env, g_ctx.jniHelperClz,getRuntimeMemorySize, ()J);if (!memFunc) {LOGE(Failed to retrieve getRuntimeMemorySize() methodID line %d,__LINE__);return;}jlong result (*env)-CallLongMethod(env, instance, memFunc);LOGI(Runtime free memory size: % PRId64, result);(void)result; // silence the compiler warning
} java中的JniHandler的相关方法如下
public class JniHandler {private static final String TAG hello-jniCallback;/** Print out status to logcat*/Keepprivate void updateStatus(String msg) {if (msg.toLowerCase().contains(error)) {Log.e(JniHandler, Native Err: msg);} else {Log.i(JniHandler, Native Msg: msg);}}/** Return OS build version: a static function*/Keepstatic public String getBuildVersion() {return Build.VERSION.RELEASE;}/** Return Java memory info*/Keeppublic long getRuntimeMemorySize() {return Runtime.getRuntime().freeMemory();}
} Log信息如下: 通过上面的代码分析:这里有两种方法的调用,一种是CallStaicObjectMethod方法,另外一种是CalllongMethod方法,以为我之前接触过一点JNI的知识,了解JNI跟java的交互分为对象的调用跟类的调用 1.JNI如何调用java静态方法
经过代码分析,大概的调用流程如下所示
findClassNewGlobalRefGetMethodIdCallMeehod
jclass clz (*env)-FindClass(env, com/example/hellojnicallback/JniHandler);g_ctx.jniHelperClz (*env)-NewGlobalRef(env, clz);jmethodID jniHelperCtor (*env)-GetMethodID(env, g_ctx.jniHelperClz, init, ()V);jobject handler (*env)-NewObject(env, g_ctx.jniHelperClz, jniHelperCtor);g_ctx.jniHelperObj (*env)-NewGlobalRef(env, handler);jstring buildVersion (*env)-CallStaticObjectMethod(env, g_ctx.jniHelperClz, versionFunc);const char *version (*env)-GetStringUTFChars(env, buildVersion, NULL);if (!version) {LOGE(Unable to get version string line %d, __LINE__);return;}LOGI(Android Version - %s, version);(*env)-ReleaseStringUTFChars(env, buildVersion, version);// we are called from JNI_OnLoad, so got to release LocalRef to avoid leaking(*env)-DeleteLocalRef(env, buildVersion);
代码修改为如下
jint JNI_OnLoad(JavaVM *vm, void *reserved)
{LOGI(enter JNI_OnLoad);JNIEnv* env;if ((*vm)-GetEnv(vm, (void **)env, JNI_VERSION_1_6) ! JNI_OK){LOGE(Failed to get GetEnv);return JNI_ERR;}LOGI(leave JNI_OnLoad);jclass clz (*env)-FindClass(env, com/example/hellojnicallback/JniHandler);jobject objRef (*env)-NewGlobalRef(env, clz);jmethodID versionFun (*env)-GetStaticMethodID(env, objRef, getBuildVersion, ()Ljava/lang/String;);jstring buildVersion (*env)-CallStaticObjectMethod(env, objRef, versionFun);const char* version (*env)-GetStringUTFChars(env, buildVersion, NULL);if (version NULL){LOGE(Failed to get version);return JNI_ERR;}LOGI(java build version - %s, version);return JNI_VERSION_1_6;
}
经过测试,这样是没有问题的, 但是上面有一个NewGlobalRef的调用通过jni.h的查询发现,jclass跟jobject其实是同一个类型void* 准备测试下,如果不调用这个这个NewGlobalRef能不能正常运行,通过这个方法描述,这个好像只是一个全局的引用,修改为如下所示:
jint JNI_OnLoad(JavaVM *vm, void *reserved)
{LOGI(enter JNI_OnLoad);JNIEnv* env;if ((*vm)-GetEnv(vm, (void **)env, JNI_VERSION_1_6) ! JNI_OK){LOGE(Failed to get GetEnv);return JNI_ERR;}LOGI(leave JNI_OnLoad);jclass clz (*env)-FindClass(env, com/example/hellojnicallback/JniHandler);jmethodID versionFun (*env)-GetStaticMethodID(env, clz, getBuildVersion, ()Ljava/lang/String;);jstring buildVersion (*env)-CallStaticObjectMethod(env, clz, versionFun);const char* version (*env)-GetStringUTFChars(env, buildVersion, NULL);if (version NULL){LOGE(Failed to get version);return JNI_ERR;}LOGI(java build version - %s, version);return JNI_VERSION_1_6;
}
经过测试发现,没有任何问题 总结:JNI调用java中的静态方法的流程是
FindClassGetStaticMethodIDCallStaticobjectMethod 2.JNI如何调用java的对象方法
通过跟上面调用JNI调用静态方法的对比,猜测调用对象方法,应该是一样的
在 java中的JniHandler中添加一个新方法,如下所示
Keeppublic String getBuildVersion2() {return BuildVersion2;}
新的代码如下所示 但是发现报错了,错误提示信息如下 JVM object referenced by clz is of type ClassJniHandler and it does not have access to method getBuildVersion2() declared in class JniHandler. 猜测是类型不匹配,这里应该是一个jobject类型,而不是jclass类型 ,在上面的步骤中,不是可以通过调用NewGlobalR来获取jobject类型吗,修改如下, 信心满满,这下应该没有问题吧,现实是残酷的,啪啪打脸.c代码发生crash了 再对比原始的方法,发现有一段这样的代码调用
jmethodID jniHelperCtor (*env)-GetMethodID(env, g_ctx.jniHelperClz, init, ()V);jobject handler (*env)-NewObject(env, g_ctx.jniHelperClz, jniHelperCtor);
应该需要通过先调用java的非静态方法,然后再根据生成的jmethodID去调用NewObject.开始编写代码.大概是如下所示 在编写过程中, 在传入参数,versionFun的时候, Android Studio报错了,提示 Not a constructor. 不是一个构造期,难道需要跟java反射一样吗.需要搞一个构造器,再次去看了下原来的代码,发现有如下几行代码 jmethodID jniHelperCtor (*env)-GetMethodID(env, g_ctx.jniHelperClz, init, ()V);
jobject handler (*env)-NewObject(env, g_ctx.jniHelperClz, jniHelperCtor); 根据名字猜测JnihelperCtor,这个难道是需要调用构造器吗,立马行动起来,修改如下
jint JNI_OnLoad(JavaVM *vm, void *reserved)
{LOGI(enter JNI_OnLoad);JNIEnv* env;if ((*vm)-GetEnv(vm, (void **)env, JNI_VERSION_1_6) ! JNI_OK){LOGE(Failed to get GetEnv);return JNI_ERR;}LOGI(leave JNI_OnLoad);jclass clz (*env)-FindClass(env, com/example/hellojnicallback/JniHandler);jmethodID ctor (*env)-GetMethodID(env, clz, init, ()V);jobject obj (*env)-NewObject(env, clz, ctor);jmethodID versionFun (*env)-GetMethodID(env, clz, getBuildVersion2, ()Ljava/lang/String;);jstring buildVersion (*env)-CallObjectMethod(env, obj, versionFun);const char* version (*env)-GetStringUTFChars(env, buildVersion, NULL);if (version NULL){LOGE(Failed to get version);return JNI_ERR;}LOGI(java build version - %s, version);return JNI_VERSION_1_6;
} 运行测试,发现可以了 考虑了下,调用流程应该跟java的反射一样,需要调用构造器函数,调用流程如下
FindClassGeMethodID(必须是构造器方法)NewObjectGetMethodID(你想调用的java非静态方法) CallObjectMethod
