Android.mk文件用来向编译系统描述如何编译你的源代码。在编译整个工程的情况下，系统所找到的所有的Android.mk将会先存入subdir_makefiles变量中，随后一次性一次性include进整个编译文件中。
(1)Android.mk文件首先需要指定LOCAL_PATH变量，用于查找源文件。
由于一般情况下Android.mk和需要编译的源文件在同一目录下，所以定义成如下形式：
LOCAL_PATH:=$(call my-dir)
上面的语句的意思是将LOCAL_PATH变量定义成本文件所在目录路径。
(2)Android.mk中可以定义多个编译模块，每个编译模块都是以include $(CLEAR_VARS)开始以include $(BUILD_XXX)结束。
①include $(CLEAR_VARS)
CLEAR_VARS的定义在build/core/clear_vars.mk中，它清除了上百个除LOCAL_PATH外的变量，因而LOCAL_PATH通常被认为是一个编译模块的开始标志。
②include $(BUILD_XXX)
这个是Android.mk的重点，它表示将源码编译成什么目标文件，如动态库、可执行程序、APK等，它一般被认为是一个编译模块的结束标志。
(3)编译一个应用程序(APK)
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
# Build all java files in the java subdirectory
LOCAL_SRC_FILES := $(call all-subdir-java-files)
# Name of the APK to build
LOCAL_PACKAGE_NAME := LocalPackage
# Tell it to build an APK
include $(BUILD_PACKAGE)
(4)编译一个依赖于静态Java库(static.jar)的应用程序
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
# List of static libraries to include in the package
LOCAL_STATIC_JAVA_LIBRARIES := static-library
# Build all java files in the java subdirectory
LOCAL_SRC_FILES := $(call all-subdir-java-files)
# Name of the APK to build
LOCAL_PACKAGE_NAME := LocalPackage
# Tell it to build an APK
include $(BUILD_PACKAGE)
(5)编译一个需要用平台的key签名的应用程序
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
# Build all java files in the java subdirectory
LOCAL_SRC_FILES := $(call all-subdir-java-files)
# Name of the APK to build
LOCAL_PACKAGE_NAME := LocalPackage
LOCAL_CERTIFICATE := platform
# Tell it to build an APK
include $(BUILD_PACKAGE)
(6)编译一个需要用特定key前面的应用程序
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
# Build all java files in the java subdirectory
LOCAL_SRC_FILES := $(call all-subdir-java-files)
# Name of the APK to build
LOCAL_PACKAGE_NAME := LocalPackage
LOCAL_CERTIFICATE := vendor/example/certs/app
# Tell it to build an APK
include $(BUILD_PACKAGE)
(7)添加一个预编译应用程序
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
# Module name should match apk name to be installed.
LOCAL_MODULE := LocalModuleName
LOCAL_SRC_FILES := $(LOCAL_MODULE).apk
LOCAL_MODULE_CLASS := APPS
LOCAL_MODULE_SUFFIX := $(COMMON_ANDROID_PACKAGE_SUFFIX)
include $(BUILD_PREBUILT)
(8)添加一个静态JAVA库
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
# Build all java files in the java subdirectory
LOCAL_SRC_FILES := $(call all-subdir-java-files)
# Any libraries that this library depends on
LOCAL_JAVA_LIBRARIES := android.test.runner
# The name of the jar file to create
LOCAL_MODULE := sample
# Build a static jar file.
include $(BUILD_STATIC_JAVA_LIBRARY)
(9)编译生成可执行文件
LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE_TAGS := optional
LOCAL_C_INCLUDES += \
$(LOCAL_PATH)
LOCAL_SRC_FILES:= \
gpio-server.c
LOCAL_SHARED_LIBRARIES := \
libcurl \
libcutils
LOCAL_MODULE:= gpio-server
include $(BUILD_EXECUTABLE)
include $(call all-makefiles-under,$(LOCAL_PATH))
(10)编译生成动态库
LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)
LOCAL_SRC_FILES:= \
com_android_server_AlarmManagerService.cpp \
com_android_server_BatteryService.cpp \
com_android_server_InputApplicationHandle.cpp \
com_android_server_InputManager.cpp \
com_android_server_InputWindowHandle.cpp \
com_android_server_LightsService.cpp \
com_android_server_PowerManagerService.cpp \
com_android_server_SystemServer.cpp \
com_android_server_UsbDeviceManager.cpp \
com_android_server_UsbHostManager.cpp \
com_android_server_VibratorService.cpp \
com_android_server_location_GpsLocationProvider.cpp \
com_android_server_connectivity_Vpn.cpp \
com_android_server_ConnectivityService.cpp \
onload.cpp
LOCAL_C_INCLUDES += \
$(JNI_H_INCLUDE) \
frameworks/base/services \
frameworks/base/core/jni \
external/skia/include/core
LOCAL_SHARED_LIBRARIES := \
libandroid_runtime \
libcutils \
libhardware \
libhardware_legacy \
libnativehelper \
libsystem_server \
libutils \
libui \
libinput \
libskia \
libgui \
libusbhost
ifeq ($(WITH_MALLOC_LEAK_CHECK),true)
LOCAL_CFLAGS += -DMALLOC_LEAK_CHECK
endif
LOCAL_MODULE:= libandroid_servers
include $(BUILD_SHARED_LIBRARY)
(11)通过Android.mk添加一个编译模块到系统中的顺序如下
①LOCAL_PATH
②CLEAR_VARS
③LOCAL_SRC_FILES
④LOCAL_CFLAGS(可选)
⑤LOCAL_MODULE
⑥LOCAL_STATIC_LIBRARIES(可选)
⑦BUILD_XXX
(12)Android.mk常用的变量
<360>变量名 说明
LOCAL_PATH 用于确定源码所在的目录，最好把它放在CLEAR_VARS变量引用的前面，因为它不会被清除，每个Android.mk只需要定义
一次即可。
CLEAR_VARS 它清空了很多以LOCAL_开头的变量（LOCAL_PATH除外）。由于所有的MakeFile都是在一个编译环境下执行的，因此
变量的定义理论上是全局的，在每个模块编译开始前进行清理工作是必要的。
LOCAL_MODULE 模块名，需保证在整个编译系统中是唯一存在的，而且中间不可以有空格
LOCAL_MODULE_PATH 模块的输出路径
LOCAL_SRC_FILES 模块编译过程中涉及到的源文件。如果是JAVA程序，可以考虑调用all-subdir-java-files来一次性添加目录（包括子目录）下所有的java文件
因为有LOCAL_PATH，这里只需要给出文件名（相对路径）即可；而且编译系统有比较强的推导功能，可以自动计算依赖关系。
LOCAL_CC 用于指定C编译器
LOCAL_CXX 用于指定C++编译器
LOCAL_CPP_EXTENSION 用于指定特殊的C++文件后缀名
LOCAL_CFLAGS C语言编译时的额外选项
LOCAL_CXXFLAGS C++语言编译时的额外选项
LOCAL_C_INCLUDE 编译C和C++程序所需要的额外头文件
LOCAL_STATIC_LIBRARIES 编译所需的静态库列表
LOCAL_SHARED_LIBRARIES 编译时所需要的动态库列表
LOCAL_JAVA_LIBRARIES 编译时所需要的JAVA类库
LOCAL_LDLIBS 编译时所需要的链接选项
LOCAL_COPY_HEADERS 安装应用程序时所需要复制的头文件列表，需要和LOCAL_COPY_HEARERS_TO变量配合使用
LOCAL_MODULE_CLASS 标识所编译模块最后放置的位置。ETC表示放置在/system/etc.目录下，APPS表示放置在/system/app目录下，SHARED_LIBRARIES表示放置在/system/lib目录下。如果具体指定，则编译的模块不会放到编译系统中，最后会在out对应product的obj目录下的对应目录中。
LOCAL_MODULE_TAGS 当前模块所包含的标签，一个模块可以包含多个标签。标签的值可能是eng、user、debug、development、optional。其中，optional是默认标签。
LOCAL_DEX_PREOPT apk的odex优化开关，默认是false
$(call my-dir) 获取当前文件夹的路径
$(call all-java-files-under,<src>) 获取指定目录下的所有java文件
$(call all-c-files-under, <src>) 获取指定目录下的所有c文件
$(call all-Iaidl-files-under, <src>) 获取指定目录下的所有AIDL文件
$(call all-makefiles-under, <folder>) 获取指定目录下的所有Make文件
$(call intermediates-dir-for, <class>, <app_name>, <host or target>, <common?>) 获取Build输入的目标文件夹路径
BUILD_HOST_STATIC_LIBRARY
BUILD_HOST_SHARED_LIBRARY
BUILD_STATIC_LIBRARY
BUILD_RAW_STATIC-LIBRARY
BUILD_SHARED_LIBRARY
BUILD_EXECUTABLE
BUILD_RAW_EXECUTABLE
BUILD_HOST_EXECUTABLE
BUILD_PACKAGE
BUILD_HOST_PREBUILT
BUILD_PREBUILT
BUILD_MULTI_PREBUILT
BUILD_JAVA_LIBRARY
BUILD_STATIC_JAVA_LIBRARY
BUILD_HOST_JAVA_LIBRARY
BUILD_DROIDDOC
BUILD_COPY_HEADERS
BUILD_KEY_CHAR_MAP 各种形式的编译模板，如静态、动态库文件，可执行文件，文档等
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