1、实现硬件访问服务的JNI方法

     ~/android-2.3_r1/frameworks/base/services/jni

     ----com_android_server_FregService.cpp

     ----onload.cpp

     ----Android.mk

    com_android_server_FregService.cpp

#define LOG_TAG "FregServiceJNI"

#include "jni.h"
#include "JNIHelp.h"
#include "android_runtime/AndroidRuntime.h"

#include <utils/misc.h>
#include <utils/Log.h>
#include <hardware/hardware.h>
#include <hardware/freg.h>

#include <stdio.h>

namespace android
{
	static void freg_setVal(JNIEnv* env, jobject clazz, jint ptr, jint value) {//ptr为硬件设备地址，value为要写入的数据
		freg_device_t* device = (freg_device_t*)ptr;
		if(!device) {
			LOGE("Device freg is not open.");
			return;
		}	
	
		int val = value;

		LOGI("Set value %d to device freg.", val);
		
		device->set_val(device, val);//调用硬件抽象层的freg_set_val方法
	}

	static jint freg_getVal(JNIEnv* env, jobject clazz, jint ptr) {//ptr为硬件设备地址
		freg_device_t* device = (freg_device_t*)ptr;
		if(!device) {
			LOGE("Device freg is not open.");
			return 0;
		}

		int val = 0;

		device->get_val(device, &val);//调用硬件抽象层的freg_get_val方法
		
		LOGI("Get value %d from device freg.", val);
	
		return val;
	}

	static inline int freg_device_open(const hw_module_t* module, struct freg_device_t** device) {
		return module->methods->open(module, FREG_HARDWARE_DEVICE_ID, (struct hw_device_t**)device);//调用硬件抽象层的函数freg_device_open根据ID来打开对应的设备，并返回硬件设备地址返回
	}
	
	static jint freg_init(JNIEnv* env, jclass clazz) {
		freg_module_t* module;
		freg_device_t* device;
		
		LOGI("Initializing HAL stub freg......");

		if(hw_get_module(FREG_HARDWARE_MODULE_ID, (const struct hw_module_t**)&module) == 0) {//根据ID获取硬件抽象层模块
			LOGI("Device freg found.");
			if(freg_device_open(&(module->common), &device) == 0) {//调用上面的函数
				LOGI("Device freg is open.");
				return (jint)device;//返回硬件设备地址
			}

			LOGE("Failed to open device freg.");
			return 0;
		}

		LOGE("Failed to get HAL stub freg.");

		return 0;		
	}

	static const JNINativeMethod method_table[] = {//要注册的函数，只有注册才能够供上层调用
		{"init_native", "()I", (void*)freg_init},
		{"setVal_native", "(II)V", (void*)freg_setVal},
		{"getVal_native", "(I)I", (void*)freg_getVal},
	};

	int register_android_server_FregService(JNIEnv *env) {//注册JNI函数
    		return jniRegisterNativeMethods(env, "com/android/server/FregService", method_table, NELEM(method_table));
	}
};

      hw_get_module实现如下：

      ~/android-2.3_r1/hardware/libhardware

      ----hardware.c

.....

#include <hardware/hardware.h>

#include <cutils/properties.h>

#include <dlfcn.h>
#include <string.h>
#include <pthread.h>
#include <errno.h>
#include <limits.h>

#define LOG_TAG "HAL"
#include <utils/Log.h>

/** Base path of the hal modules */
#define HAL_LIBRARY_PATH1 "/system/lib/hw" //编译好的模块文件位于out/target/product/generic/system/lib/hw目录中，而这个目录经过打包后，就对应设备上的/system/lib/hw目录
#define HAL_LIBRARY_PATH2 "/vendor/lib/hw"

....

static const char *variant_keys[] = { //系统属性
    "ro.hardware",  /* This goes first so that it can pick up a different
                       file on the emulator. */
    "ro.product.board",
    "ro.board.platform",
    "ro.arch"
};

static const int HAL_VARIANT_KEYS_COUNT =
    (sizeof(variant_keys)/sizeof(variant_keys[0]));//大小

/**
 * Load the file defined by the variant and if successful
 * return the dlopen handle and the hmi.
 * @return 0 = success, !0 = failure.
 */
static int load(const char *id,
        const char *path,
        const struct hw_module_t **pHmi)
{
    int status;
    void *handle;
    struct hw_module_t *hmi;

    /*
     * load the symbols resolving undefined symbols before
     * dlopen returns. Since RTLD_GLOBAL is not or'd in with
     * RTLD_NOW the external symbols will not be global
     */
    handle = dlopen(path, RTLD_NOW);//path为/system/lib/hw/freg.default.so
    if (handle == NULL) {
        char const *err_str = dlerror();
        LOGE("load: module=%s\n%s", path, err_str?err_str:"unknown");
        status = -EINVAL;
        goto done;
    }

    /* Get the address of the struct hal_module_info. */
    const char *sym = HAL_MODULE_INFO_SYM_AS_STR;//HMI
    hmi = (struct hw_module_t *)dlsym(handle, sym);//根据HMI获取到在硬件抽象层定义的freg_module_t结构体HAL_MODULE_INFO_SYM指针，并转换为一个hw_module_t结构体指针
    if (hmi == NULL) {
        LOGE("load: couldn't find symbol %s", sym);
        status = -EINVAL;
        goto done;
    }

    /* Check that the id matches */
    if (strcmp(id, hmi->id) != 0) {//FREG_HARDWARE_MODULE_ID==FREG_HARDWARE_MODULE_ID
        LOGE("load: id=%s != hmi->id=%s", id, hmi->id);
        status = -EINVAL;
        goto done;
    }

    hmi->dso = handle;//模块句柄值handle保存在hw_module_t结构体指针hmi的成员变量dso中。

    /* success */
    status = 0;

    done:
    if (status != 0) {
        hmi = NULL;
        if (handle != NULL) {
            dlclose(handle);
            handle = NULL;
        }
    } else {
        LOGV("loaded HAL id=%s path=%s hmi=%p handle=%p",
                id, path, *pHmi, handle);
    }

    *pHmi = hmi;//把指向hw_module_t结构体的指针返回去

    return status;
}

int hw_get_module(const char *id, const struct hw_module_t **module) 
{
    int status;
    int i;
    const struct hw_module_t *hmi = NULL;
    char prop[PATH_MAX];
    char path[PATH_MAX];

    /*
     * Here we rely on the fact that calling dlopen multiple times on
     * the same .so will simply increment a refcount (and not load
     * a new copy of the library).
     * We also assume that dlopen() is thread-safe.
     */

    /* Loop through the configuration variants looking for a module */
    for (i=0 ; i<HAL_VARIANT_KEYS_COUNT+1 ; i++) {
        if (i < HAL_VARIANT_KEYS_COUNT) {
            if (property_get(variant_keys[i], prop, NULL) == 0) {//首先获得的系统属性"ro.hardware"值，在Android模拟器中，这个属性的值定义为"goldfish"
                continue;
            }
            snprintf(path, sizeof(path), "%s/%s.%s.so",//path为/system/lib/hw/freg/goldfish.so
                    HAL_LIBRARY_PATH1, id, prop);
            if (access(path, R_OK) == 0) break; //判断是否有对应的文件

            snprintf(path, sizeof(path), "%s/%s.%s.so",//path为/vendor/lib/hw/freg/goldfish.so
                     HAL_LIBRARY_PATH2, id, prop);
            if (access(path, R_OK) == 0) break;//判断是否有对应的文件
        } else {//如果都没有，执行下面代码
            snprintf(path, sizeof(path), "%s/%s.default.so",//path为/system/lib/hw/freg.default.so
                     HAL_LIBRARY_PATH1, id);
            if (access(path, R_OK) == 0) break;//判断是否有对应的文件，此时有。
        }
    }

    status = -ENOENT;
    if (i < HAL_VARIANT_KEYS_COUNT+1) {
        /* load the module, if this fails, we're doomed, and we should not try
         * to load a different variant. */
        status = load(id, path, module);//id为freg,path为/system/lib/hw/freg.default.so,module为hw_module_t指针的指针
    }

    return status;
}

       硬件访问服务FregService的JNI方法编写完成之后，我们还需要修改frameworks/base/services/jni目录下的onLoad.cpp文件，在里面增加register_android_server_FregService函数的声明和调用。

        onload.cpp

namespace android {

      ..............................................................................................

      int register_android_server_FregService(JNIEnv *env);

};

在JNI_onLoad增加register_android_server_FregService函数调用：
extern "C" jint JNI_onLoad(JavaVM* vm, void* reserved)
{
     .................................................................................................
     register_android_server_FregService(env);
     .................................................................................................
}

      Android.mk

LOCAL_SRC_FILES:=       .......
      com_android_server_FregService.cpp /
      onl oad.cpp

2、编译

     硬件访问服务的JNI方法

     编译后得到的libandroid_servers.so位于out/target/product/generic/system/lib，就包含有init_native、setVal_native和getVal_native这三个JNI方法了。

      打包：

      在out/target/product/gerneri目录下会生成Android系统镜像文件system.img。