FmRadio应用代码结构如下：

一、FMRadio.java通过FmRadioService提供的服务通过JNI访问到HAL层：

FmRadioService实现IfmService的一些接口，以及提供一些供Client调用的接口，接口里面调用到JNI。

例如：

FmRadioService.java中private voidenableFmFacility(boolean bEnable)调用到

enableFmFacility_native。

enableFmFacility_native在com_android_server_FmService.cpp中实现，在映射表中注册，也就是所谓的JNI. 最终调用com_android_server_FmService.cpp中的android_FmService_enableFmFacility。

二、JNI简介：

传统的JNI需要遵循一定的命名规则，而Android改变了这种方式，可以通过以下两种方法把java层与C++层的代码进行映射，如下：

第一个参数是Java层的函数，第二个参数是函数的参数和返回类型也是属于java层，第三个参数是JNI实现的c++函数

static const JNINativeMethodgMethods[] = {

    /* name,                       signature,      funcPtr */

    { "enableFmFacility_native", "(Z)I",         (void*)android_FmService_enableFmFacility  },

    { "setFmCurrentFreq_native",   "(II)I",     (void*)android_FmService_setFmCurrentFreq  },

       { "stationIsAvailable_native", "()Z",          (void*)android_FmService_stationIsAvailable  },

    { "fm_mute_native",                    "(I)I",              (void*)android_FmService_fm_mute  },

};

下面这个函数是将所有的映射函数进行注册

static int registerMethods(JNIEnv*env) {

    static const char* const kClassName =

        "com/rk/FmRadio/FmRadioService";

    jclass clazz;

    /* look up the class */

    clazz = env->FindClass(kClassName);

    if (clazz == NULL) {

        return -1;

    }

    /* register all the methods */

    if(env->RegisterNatives(clazz,gMethods,

            sizeof(gMethods) /sizeof(gMethods[0])) != JNI_OK)

    {

        return -1;

    }

    /* fill out the rest of the ID cache */// .! :在 FM 实现中, 没有必要 cache Java 类 or field 的 ID.

    return cacheIds(env,clazz);

}

然后需要重写JNI_OnLoad函数，这个函数一定要重载，当调用System.loadLibrary("rockchip_radio_jni");的时候，JVM启动的时候就会自动加载，并将我们的函数注册到系统JNI，以便调用。

jintJNI_OnLoad(JavaVM* vm, void* reserved) {

    JNIEnv* env = NULL;

    jint result = -1;

    if (vm->GetEnv((void**) &env,JNI_VERSION_1_4) != JNI_OK) {

        goto bail;

    }

    assert(env != NULL);

    if(registerMethods(env) != 0) {

        goto bail;

    }

    /* success -- return valid version number */

   result = JNI_VERSION_1_4;

bail:

    return result;

}     

或者如果在framework层也可以在framework/base/services/jni/onload.app文件中直接在下面这个方法里面添加我们的注册函数，这样也能将我们自己的函数列表注册到系统JNI里面。

JNI可以参考文档http://blog.csdn.net/zhenyongyuan123/article/details/5862054

最后特别需要注意的是，JNI里面注册的函数列表，在Java除了声明为native函数以外，而且在service里面必须要调用到，否则会出现找不到该函数。

注册好JNI之后，启动service进行访问，要注意的是系统app在Android4.2以后不能直接通过传统的startservice（intent）启动，这样会出现权限问题，因为4.2以后Android引入多用户操作，需要如下方式启动

startServiceAsUser(mServiceIntent,newUserHandle(UserHandle.USER_CURRENT));

参考文档http://xxhalbert.blog.163.com/blog/static/4849663420138127178898/

三、android_FmService_enableFmFacility会调用loadFmHal(&sFmControlDevice)

接口，loadFmHal(&sFmControlDevice)中会调用到\hardware\rk28\fm\fm.c的fm_control_open(&module->common,controlDevice)，从而进入HAL层。

fm_control_open(&module->common,controlDevice)定义如下：

inline intfm_control_open(const struct hw_module_t* module, fm_control_device_t** device)

{

    return module->methods->open(module,

                                FM_HARDWARE_CONTROL,

                                 (structhw_device_t**)device);

}

========================================================================================================================================

类似的，其他从上层的调用也会最终调用到fm.c的其他函数，例如：

static boolean control_context__get_fm_stationIsAvailable(fm_control_device_t*dev)

{

    int result = 0;

    fm_control_context_t* this =(fm_control_context_t*)dev;

    /* 若用户已经禁用了 fm 机构. */

    if ( !(this->is_fm_facility_enabled ) )

    {

        E("fm facility is totallydisabled.");

        result = -1;

        goto EXIT;

    }

    result =get_fm_stationIsAvailable();

EXIT:

    return result;

}

=====================================================================================================================================

int get_fm_stationIsAvailable()

{

    int ret;

    int state;

    if(fm_fd < 0)

    {

        E("no init \n");

        return -1;

    }

    ret =ioctl(fm_fd,FM_STATION_ISAVAILABLE,&state);//对于Kernel，需实现这一类调用对应的驱动

    if(ret < 0)

    {

        E("FM GET stationIsAvailable err,%s\n", strerror(errno));

           return -1;

    }

    return state;

}

========================================================================================================================================

Android.mk中也要设置相应的值（黑体字）：

#

# Copyright (C) 2008 The Android Open Source Project

#

# Licensed under the Apache License, Version 2.0 (the"License");

# you may not use this file except in compliance with the License.

# You may obtain a copy of the License at

#

#     http://www.apache.org/licenses/LICENSE-2.0

#

# Unless required by applicable law or agreed to in writing,software

# distributed under the License is distributed on an "ASIS" BASIS,

# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express orimplied.

# See the License for the specific language governing permissionsand

# limitations under the License.

#

# This makefile supplies the rules for building a library of JNIcode for

# use by our example platform shared library.

LOCAL_PATH:= $(call my-dir)

include $(CLEAR_VARS)

LOCAL_LDLIBS :=  -llog

LOCAL_MODULE_TAGS := eng

# This is the target being built.

LOCAL_MODULE:=librockchip_radio_jni

# All of the source files that we will compile.

LOCAL_SRC_FILES:= \

      com_android_server_FmService.cpp

# All of the shared libraries we link against.

LOCAL_SHARED_LIBRARIES := \

     libandroid_runtime \

     libnativehelper \

     #libdwaharre \

     libcutils \

     libutils \

     liblog \

      libhardware

# No static libraries.

LOCAL_STATIC_LIBRARIES :=

# Also need the JNI headers.

LOCAL_C_INCLUDES += \

     $(JNI_H_INCLUDE)

# No specia compiler flags.

LOCAL_CFLAGS =

# Don't prelink this library. For more efficient code, you may want

# to add this library to the prelink map and set this to true.

LOCAL_PRELINK_MODULE := false

include $(BUILD_SHARED_LIBRARY)

***方法二，将服务和JNI调用写在framework里面***

一、应用层：