跨平台开发-从C中调用Android代码

从C调用Android代码，需要用到JNI函数。需要将Android平台上的某些操作用C函数进行封装。下面就几个关键点给出说明。有不对的地方，也欢迎大家指正。

JNI_OnLoad

这个函数在加载.so时会被调用，我们要在这个函数中保存JavaVM指针。如果有全局初始化需要做，也可以放在这里面。函数的声明在jni.h中，这里给出一个简单的示例。

#define NZLOGI(fmt, args...) __android_log_print(ANDROID_LOG_INFO,  "NZ", fmt, ##args)
#define NZLOGD(fmt, args...) __android_log_print(ANDROID_LOG_DEBUG, "NZ", fmt, ##args)
#define NZLOGE(fmt, args...) __android_log_print(ANDROID_LOG_ERROR, "NZ", fmt, ##args)

static JavaVM *g_javavm = nullptr;

jobject android_app_Application_globaljobject = nullptr;

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM* vm, void* reserved)
{
    NZLOGI("JNI_OnLoad");
    NZLOGI("__ANDROID_API__ %d", __ANDROID_API__);

    g_javavm = vm;
    (void)reserved;

    JNIEnv *env = NZJNI_GetEnv();
    if (env) {
        jobject application_jobject = NZJNI_GetApplication();
        NZJNI_ClearException(env);
        if (application_jobject) {
            android_app_Application_globaljobject = env->NewGlobalRef(application_jobject);
            env->DeleteLocalRef(application_jobject);
        }
    }

    return JNI_VERSION_1_6;
}

JNI_OnUnload

这个函数在卸载.so时会被调用，我们只需要把在OnLoad中申请的资源释放掉就可以了。示例如下：

JNIEXPORT void JNICALL JNI_OnUnload(JavaVM* vm, void* reserved)
{
    NZLOGI("JNI_OnUnload");

    (void)vm;
    (void)reserved;

    JNIEnv *env = NZJNI_GetEnv();
    if (env) {
        if (android_app_Application_globaljobject) {
            env->DeleteGlobalRef(android_app_Application_globaljobject);
            android_app_Application_globaljobject = nullptr;
        }
    }
}

JNIEnv

这个是JNIEnv是可以用来新建Java对象实例并调用对象方法的。值得注意的地方有两个：

1. 这个JNIEnv必须每次调用时都要重新获取。
2. 在C环境下创建的子线程中，获取JNIEnv必须要AttachCurrentThread
   例子如下：

JNIEnv *NZJNI_GetEnv()
{
    JNIEnv *env = nullptr;
    if (g_javavm) {
        if (g_javavm->GetEnv((void **)(&env), JNI_VERSION_1_6) != JNI_OK) {
            NZLOGE("NZJNI_GetEnv can't get the enviroument");
        }
    } else {
        NZLOGE("NZJNI_GetEnv null javavm");
    }
    return env;
}

JNIEnv *NZJNI_AttachCurrentThread()
{
    JNIEnv *env = nullptr;
    if (g_javavm) {
        if (g_javavm->AttachCurrentThread(&env, nullptr) != JNI_OK) {
            NZLOGE("NZJNI_AttachCurrentThread can't get the enviroument");
        }
    } else {
        NZLOGE("NZJNI_AttachCurrentThread null javavm");
    }
    return env;
}

JNIEnv *NZJNI_AutoAttachAndGetEnv(bool *newAttached)
{
    JNIEnv *env = nullptr;
    if (g_javavm) {
        jint result = g_javavm->GetEnv((void **) (&env), JNI_VERSION_1_6);
        if (result == JNI_OK) {
            *newAttached = false;
            return env;
        } else if (result == JNI_EDETACHED) {
            if (g_javavm->AttachCurrentThread(&env, nullptr) == JNI_OK) {
                *newAttached = true;
                return env;
            } else {
                NZLOGE("NZJNI_AutoAttachAndGetEnv can't AttachCurrentThread ");
                *newAttached = false;
                return env;
            }
        } else {
            NZLOGE("NZJNI_AutoAttachAndGetEnv can't GetEnv");
            *newAttached = false;
            return env;
        }
    } else {
        NZLOGE("NZJNI_AutoAttachAndGetEnv null javavm");
        return env;
    }
}

void NZJNI_DetachCurrentThread()
{
    if (g_javavm) {
        if (g_javavm->DetachCurrentThread() != JNI_OK) {
            NZLOGE("NZJNI_DetachCurrentThread failure");
        }
    } else {
        NZLOGE("NZJNI_DetachCurrentThread null javavm");
    }
}

调用示例，获取Application对象

主要就是调用FindClass，GetMethodID，CallMethod等函数。
示例如下：

jobject NZJNI_GetApplication()
{
    JNIEnv *env = nullptr;
    jclass ActivityThreadClass = nullptr;
    jmethodID currentActivityThreadMethod = nullptr;
    jobject currentActivityThread_jobject = nullptr;
    jmethodID getApplicationMethod = nullptr;
    jobject application_jobject = nullptr;

    env = NZJNI_GetEnv();
    if (!env) {
        NZLOGE("NZJNI_GetApplication NZJNI_GetEnv failure");
        goto delete_localref_and_return;
    }
    ActivityThreadClass = env->FindClass("android/app/ActivityThread");
    if (!ActivityThreadClass) {
        NZLOGE("NZJNI_GetApplication null ActivityThreadClass");
        goto delete_localref_and_return;
    }
    currentActivityThreadMethod = env->GetStaticMethodID(ActivityThreadClass, "currentActivityThread", "()Landroid/app/ActivityThread;");
    if (!currentActivityThreadMethod) {
        NZLOGE("NZJNI_GetApplication null currentActivityThreadMethod");
        goto delete_localref_and_return;
    }
    currentActivityThread_jobject = env->CallStaticObjectMethod(ActivityThreadClass, currentActivityThreadMethod);
    if (!currentActivityThread_jobject) {
        NZLOGE("NZJNI_GetApplication null currentActivityThread_jobject");
        goto delete_localref_and_return;
    }
    getApplicationMethod = env->GetMethodID(ActivityThreadClass, "getApplication", "()Landroid/app/Application;");
    if (!getApplicationMethod) {
        NZLOGE("NZJNI_GetApplication null getApplicationMethod");
        goto delete_localref_and_return;
    }
    application_jobject = env->CallObjectMethod(currentActivityThread_jobject, getApplicationMethod);
    if (!application_jobject) {
        NZLOGE("NZJNI_GetApplication null application_jobject");
        goto delete_localref_and_return;
    }
    goto delete_localref_and_return;

delete_localref_and_return:
    if (env) {
        NZJNI_ClearException(env);
        if (ActivityThreadClass)
            env->DeleteLocalRef(ActivityThreadClass);
        if (currentActivityThread_jobject)
            env->DeleteLocalRef(currentActivityThread_jobject);
    }
    return application_jobject;
}

bool NZJNI_ClearException(JNIEnv *env)
{
    if (env) {
        jthrowable obj = env->ExceptionOccurred();
        if (obj) {
            env->ExceptionDescribe();
            env->ExceptionClear();
            env->DeleteLocalRef(obj);
            return true;
        }
    }
    return false;
}