Android Container 系列：Container Switch

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原文链接：http://condroid.github.io/2014/08/18/container-switch/

文章标签：

#android #container #容器 #lxc

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本文介绍了一个虚拟设备/dev/containerdevice的功能及其实现原理。该设备通过ioctl操作实现容器的注册、获取当前活动容器、设置活动容器等功能。文章还详细介绍了C++层和Java层的接口封装，并展示了容器管理服务(ContainerManagerService)的工作流程。

/dev/container device

Device Drive

/dev/container device is a virtual one,device driver code is in drivers/staging/android/container.c in kernel
Container device provide the function of register and switch for container
By ioctl operating on the device,we can finish specified function

ioctl Operation

CONTAINER_REGISTER:its parameter is a pointer to int;to register the container's ID to which the pointer points
CONTAINER_GET_FRONT_ID:parameter is a pointer to int;to write in running container's ID
CONTAINER_GET_STACK_POS:parameter is a pointer to int;to write in container's ID which need get position before calling;to write in the container's position in stack after calling
CONTAINER_SET_FRONT_ID:parameter is a pointer to int;to set the container's ID to which the pointer points active
CONTAINER_WAIT_FOR_NEW_POS:parameter is a pointer to int;to write in container's ID which need wait for position before calling;to write in the container's new position after calling

The Interface of Container

C++ Interface

C++ layer and Java layer encapsulate the access interface of container device

C++ layer's encapsulation code is in frameworks/native/libs/container/Container.cpp;it generates libcontainer.so from compilation；the definition of interface is as follow:

  int getCurrentContainer(void)                   get container's ID in which current process is
  int registerContainer(int container)            register a container's ID
  int getFrontContainer(void)                     judge the container's ID in front
  int isCurrentContainerTheFront(void)            judge the container's ID if is in front
  int getContainerPosition(int container)         get the container's position in stack
  int setFrontContainer(int container)            set a container in front
  int waitForNewPosition(int container)           make current process sleep until specified container's position changes
  int getAvailableContainers(int containers[])    get all running containers' ID

Java Interface

Java Interface is a kind of android.util.Container class;its source code is in /frameworks/base/core/java/android/util/Container.java;it accesses C++ Interface by JNI（its code is in /frameworks/base/core/jni/android_util_Container.cpp）

int getCurrentContainer():return the ID of container in which current process is
int registerContainer(int container):register a container ID to tell kernel the container has started
int getFrontContainer():return the present active container
int setFrontContainer(int container):set a container active
int getContainerPosition(int container):get the position of a container
int waitForNewPosition(int container):current process stay sleep after calling;until specified container's position changes,it'll return the container's new position
int registerCurrentContainer():register the container in which current process is

Container Switch

IContainerManager

IContainerManager is a interface of inter-process communication,which defines the communication protocol between App and ContainerMana;and the definition is in frameworks/base/core/java/android/util/IContainerManager.aidl

Functions which IContainerManager defines as follow:

  int switchToContainer(int container)    switch to specified container
  int getCurrentContainer()               get container in which current process is
  int getFrontContainer()                 get the front-end container
  boolean isCurrentContainerInFront()     judge the container in which current process is if front-end
  int[] getAvailableContainers()          get all running containers

Modified files

Add file /frameworks/base/core/java/android/util/IContainerManager.aidl
In /frameworks/base/Android.mk,src_files add line /frameworks/base/core/java/android/util/IContainerManager.aidl;if you hope the interface is open for sdk，add it in following file aidl_files

ContainerManagerService

CMS is a service,whose name is "container";it realized all functions IContainerManager defines,AMS's code is in frameworks/base/services/java/com/android/server/ContainerManagerService.java
CMS is started by SystemServer,and consists in every container
Client can make remote calls to CMS by Binder mechanism

Modified files

Add file /frameworks/base/services/java/com/android/server/ContainerManagerService.java
/frameworks/base/servcies/java/com/android/server/SystemServer.java,add instantiation and call to ContainerManager
Modify this catalogue and it will generate system/frameworks/services.jar

ContainerManager SDK

ContainerManager is a client of CMS;its code is in development/contain and it generates ermanager;generated jar package from compilation is in
```
  out/target/common/obj/JAVA_LIBRARIES/containermanager_intermediates/classes.jar
```
ContainerManager realize the check and management by remote calls to CMS's interface function
ContainerManager belongs to Android SDK;it calls relevant functions in App to access interfaces CMS provides

Container Switchover Process

Applications call switching function-switchToContainer(int) of ContainerManager class
ContainerManager class makes remote calls to switchToContainer(int) interface of ContainerManagerService by Binder mechanism
ContainerManagerService calls Java interface of container device;that is setFrontContainer(int) function of android.util.Container class
Java interface calls C++ interface by JNI;that is setFrontContainer(int) function of container in libcontainer.so
C++ interface open /dev/container device,and do ioctl(CONTAINER_SET_FRONT_ID) operation on it
Container device driver updates the sequence in container list after receiving ioctl operation to make the target container top