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时隔多日，终于都抽出时间来写blog了。废话不多说，接着上一篇，这里将介绍如何编写HAL层（硬件抽象层）对应的JNI方法。
这里提到的都是在ICS源码里编译的。
1、定义JNI层接口
进入到android-4.0.4_r1.2/hardware/libhardware/include/hardware目录，并创建 ttt.h 文件，内容如下：
#ifndef ANDROID_TTT_INTERFACE_H
#define ANDROID_TTT_INTERFACE_H
#include <hardware/hardware.h>
__BEGIN_DECLS
// 定义模块ID
#define HELLO_HARDWARE_MODULE_ID "ttt"
// 硬件模块结构体
struct ttt_module_t{
struct hw_module_t common;
};
// hardware interface struct
struct ttt_device_t{
struct hw_device_t common;
int fd;
int(*set_val)(struct ttt_device_t* dev, int val);
int(*get_val)(struct ttt_device_t* dev, int* val);
};
__END_DECLS
#endif
2、实现JNI层接口功能
进入到android-4.0.4_r1.2/frameworks/base/services/jni目录，并创建com_android_server_TTTService.cpp文件，其内容如下：
#define LOG_TAG "TTTService"
#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/ttt.h>
#include <stdio.h>
namespace android
{
struct ttt_device_t* ttt_device = NULL;
// through the HAL interface to set the register value
static void ttt_setVal(JNIEnv* env, jobject clazz, jint value){
int val = value;
LOGI("TTT JNI: set value %d to device.", val);
if(!ttt_device){
LOGI("TTT JNI: device is not open.");
return;
}
ttt_device->set_val(ttt_device, val);
}
// through the HAL interface to read the register value
static jint ttt_getVal(JNIEnv* env, jobject clazz){
int val = 0;
if(!ttt_device){
LOGI("TTT JNI: device is not open.");
return val;
}
ttt_device->get_val(ttt_device, &val);
LOGI("TTT JNI: get value %d from device.", val);
return val;
}
// through the HAL interface to open the hardware device
static inline int ttt_device_open(const hw_module_t* module, struct ttt_device_t** device){
return module->methods->open(module, HELLO_HARDWARE_MODULE_ID, (struct hw_device_t**)device);
}
// throught the hardware module ID to load the HAL module and open the device
static jboolean ttt_init(JNIEnv* env, jclass clazz){
ttt_module_t* module;
LOGI("TTT JNI: initializing...");
if(hw_get_module(HELLO_HARDWARE_MODULE_ID, (const struct hw_module_t**)&module) == 0){
LOGI("TTT JNI: ttt stub found.");
if(ttt_device_open(&(module->common), &ttt_device) == 0){
LOGI("TTT JNI: ttt device is open.");
return 0;
}
LOGE("TTT JNI: failed to open ttt device.");
return -1;
}
LOGE("TTT JNI: failed to get ttt stub module.");
return -1;
}
// JNI methods table
static const JNINativeMethod method_table[] = {
{"init_native", "()Z", (void*)ttt_init},
{"setVal_native", "(I)V", (void*)ttt_setVal},
{"getVal_native", "()I", (void*)ttt_getVal},
};
// regist JNI method
int register_android_server_TTTService(JNIEnv* env){
return jniRegisterNativeMethods(env, "com/android/server/TTTService", method_table, NELEM(method_table));
}
};
3、添加JNI初始化调用
修改android-4.0.4_r1.2/frameworks/base/services/jni目录下的 onload.cpp 文件，在 JNI_OnLoad函数中的return之前添加下面一句：
register_android_server_TTTService(env);
同时，在该文件中的namespace中添加下面一句声明：
int register_android_server_TTTService(JNIEnv* env);
这样，在系统初始化时，就会调用register_android_server_TTTService方法来加载JNI方法了。
4、添加编译JNI的配置
修改android-4.0.4_r1.2/frameworks/base/services/jni目录下的 Android.mk 文件，在 LOCAL_SRC_FILES 变量中添加下面一行：
com_android_server_TTTService.cpp \
这里是添加编译配置。
5、开始编译
root@brantyou-ubuntu:~/workspace/android-4.0.4_r1.2# . build/envsetup.sh including device/moto/stingray/vendorsetup.sh including device/moto/wingray/vendorsetup.sh including device/samsung/crespo4g/vendorsetup.sh including device/samsung/crespo/vendorsetup.sh including device/samsung/maguro/vendorsetup.sh including device/samsung/smdkc110/vendorsetup.sh including device/samsung/smdkv210/vendorsetup.sh including device/samsung/torospr/vendorsetup.sh including device/samsung/toro/vendorsetup.sh including device/samsung/tuna/vendorsetup.sh including device/ti/panda/vendorsetup.sh including sdk/bash_completion/adb.bash root@brantyou-ubuntu:~/workspace/android-4.0.4_r1.2#
root@brantyou-ubuntu:~/workspace/android-4.0.4_r1.2# mmm frameworks/base/services/jni ============================================ PLATFORM_VERSION_CODENAME=REL PLATFORM_VERSION=4.0.4 TARGET_PRODUCT=full TARGET_BUILD_VARIANT=eng TARGET_BUILD_TYPE=release TARGET_BUILD_APPS= TARGET_ARCH=arm TARGET_ARCH_VARIANT=armv7-a HOST_ARCH=x86 HOST_OS=linux HOST_BUILD_TYPE=release BUILD_ID=IMM76I ============================================ make:进入目录'/home/brantyou/workspace/android-4.0.4_r1.2' target thumb C++: libandroid_servers <= frameworks/base/services/jni/com_android_server_HelloService.cpp target thumb C++: libandroid_servers <= frameworks/base/services/jni/com_android_server_TTTService.cpp target thumb C++: libandroid_servers <= frameworks/base/services/jni/onload.cpp make: *** 没有规则可以创建“out/target/product/generic/obj/SHARED_LIBRARIES/libandroid_servers_intermediates/LINKED/libandroid_servers.so”需要的目标“out/target/product/generic/obj/lib/libsystem_server.so”。 停止。 make:离开目录“/home/brantyou/workspace/android-4.0.4_r1.2” root@brantyou-ubuntu:~/workspace/android-4.0.4_r1.2#
矮油~~~这里出错了，提示没有找到 libsystem_server.so 文件。
执行下面的命令，生成 libsystem_server.so 文件：
root@brantyou-ubuntu:~/workspace/android-4.0.4_r1.2# make libsystem_server
生成之后的提示：
Install: out/target/product/generic/system/lib/libvorbisidec.so Install: out/target/product/generic/system/lib/libstagefright_yuv.so Install: out/target/product/generic/system/lib/libdrmframework.so Install: out/target/product/generic/system/lib/libchromium_net.so Install: out/target/product/generic/system/lib/libstagefright_amrnb_common.so Install: out/target/product/generic/system/lib/libstagefright_enc_common.so Install: out/target/product/generic/system/lib/libstagefright_avc_common.so Install: out/target/product/generic/system/lib/libstagefright.so Install: out/target/product/generic/system/lib/libstagefright_omx.so Install: out/target/product/generic/system/lib/libmediaplayerservice.so Install: out/target/product/generic/system/lib/libinput.so Install: out/target/product/generic/system/lib/libsystem_server.so root@brantyou-ubuntu:~/workspace/android-4.0.4_r1.2#
好了，这个问题解决了，我们继续编译这个JNI。
root@brantyou-ubuntu:~/workspace/android-4.0.4_r1.2# mmm frameworks/base/services/jni ============================================ PLATFORM_VERSION_CODENAME=REL PLATFORM_VERSION=4.0.4 TARGET_PRODUCT=full TARGET_BUILD_VARIANT=eng TARGET_BUILD_TYPE=release TARGET_BUILD_APPS= TARGET_ARCH=arm TARGET_ARCH_VARIANT=armv7-a HOST_ARCH=x86 HOST_OS=linux HOST_BUILD_TYPE=release BUILD_ID=IMM76I ============================================ make:进入目录'/home/brantyou/workspace/android-4.0.4_r1.2' target SharedLib: libandroid_servers (out/target/product/generic/obj/SHARED_LIBRARIES/libandroid_servers_intermediates/LINKED/libandroid_servers.so) target Symbolic: libandroid_servers (out/target/product/generic/symbols/system/lib/libandroid_servers.so) target Strip: libandroid_servers (out/target/product/generic/obj/lib/libandroid_servers.so) Install: out/target/product/generic/system/lib/libandroid_servers.so make:离开目录“/home/brantyou/workspace/android-4.0.4_r1.2” root@brantyou-ubuntu:~/workspace/android-4.0.4_r1.2#
很好，这一次已经顺利的编译完了。
下面我们需要重新打包这个 system.img，包我们编写的JNI方法包含进去：
root@brantyou-ubuntu:~/workspace/android-4.0.4_r1.2# make snod ============================================ PLATFORM_VERSION_CODENAME=REL PLATFORM_VERSION=4.0.4 TARGET_PRODUCT=full TARGET_BUILD_VARIANT=eng TARGET_BUILD_TYPE=release TARGET_BUILD_APPS= TARGET_ARCH=arm TARGET_ARCH_VARIANT=armv7-a HOST_ARCH=x86 HOST_OS=linux HOST_BUILD_TYPE=release BUILD_ID=IMM76I ============================================ make snod: ignoring dependencies Target system fs image: out/target/product/generic/system.img out/target/product/generic/system.img total size is 44107008 root@brantyou-ubuntu:~/workspace/android-4.0.4_r1.2#
这样就成功的把我们编写的JNI打包到 system.img中了。
这章到此结束，下一章将介绍，如何基于JNI接口编写对应的服务和编写对应的Android程序来访问服务。