Android系统中应用的安装卸载都是通过调用PackageManagerService来完成的，但在PKMS中，对于目录的创建、文件copy、dex优化都是通过调用底层的installd来完成的。至于为什么不直接在PKMS中进行文件的操作，主要是涉及到权限的问题，PKMS只有system的权限，而installd的作用就是处理需要root权限的操作。
1. installd启动
Instlld进程是init.rc中被启动的
service installd /system/bin/installd
class main
socket installd stream 600 system system
class main表示执行installd.cpp的main函数
socket installd stream 600 system system表示在/dev/socket/下创建一个unix domain的socket，并传递创建的文件描述符fd给服务进程.其中type为stream,权限600，用户名和用户组都是system
下面分析installd.cpp的main函数
static int installd_main(const int argc ATTRIBUTE_UNUSED, char *argv[]) {
char buf[BUFFER_MAX];
struct sockaddr addr;
socklen_t alen;
int lsocket, s;
int selinux_enabled = (is_selinux_enabled() > 0);
setenv("ANDROID_LOG_TAGS", "*:v", 1);
android::base::InitLogging(argv);
ALOGI("installd firing up\n");
union selinux_callback cb;
cb.func_log = log_callback;
selinux_set_callback(SELINUX_CB_LOG, cb);
if (!initialize_globals()) {
ALOGE("Could not initialize globals; exiting.\n");
exit(1);
}
if (initialize_directories() < 0) {
ALOGE("Could not create directories; exiting.\n");
exit(1);
}
if (selinux_enabled && selinux_status_open(true) < 0) {
ALOGE("Could not open selinux status; exiting.\n");
exit(1);
}
//从dev/socket下获取名为installd的socket，该socket在init阶段创建
lsocket = android_get_control_socket(SOCKET_PATH);
if (lsocket < 0) {
ALOGE("Failed to get socket from environment: %s\n", strerror(errno));
exit(1);
}
//监听socket，最大连接数为5
if (listen(lsocket, 5)) {
ALOGE("Listen on socket failed: %s\n", strerror(errno));
exit(1);
}
fcntl(lsocket, F_SETFD, FD_CLOEXEC);
for (;;) {
alen = sizeof(addr);
//接受来自java层的连接
s = accept(lsocket, &addr, &alen);
if (s < 0) {
ALOGE("Accept failed: %s\n", strerror(errno));
continue;
}
fcntl(s, F_SETFD, FD_CLOEXEC);
ALOGI("new connection\n");
for (;;) {
unsigned short count;
if (readx(s, &count, sizeof(count))) {
ALOGE("failed to read size\n");
break;
}
if ((count < 1) || (count >= BUFFER_MAX)) {
ALOGE("invalid size %d\n", count);
break;
}
//读取消息
if (readx(s, buf, count)) {
ALOGE("failed to read command\n");
break;
}
buf[count] = 0;
if (selinux_enabled && selinux_status_updated() > 0) {
selinux_android_seapp_context_reload();
}
//执行命令
if (execute(s, buf)) break;
}
ALOGI("closing connection\n");
close(s);
}
return 0;
}Installd作为socket服务端，监听java层的连接并执行java层的命令
2.installer服务启动
installer服务启动
Installer是java层服务，作为client负责和底层installd进行通信，其在SystemServer中被启动。
private void startBootstrapServices() {
// Wait for installd to finish starting up so that it has a chance to
// create critical directories such as /data/user with the appropriate
// permissions. We need this to complete before we initialize other services.
Installer installer = mSystemServiceManager.startService(Installer.class);
......
} public <T extends SystemService> T startService(Class<T> serviceClass) {
try {
final String name = serviceClass.getName();
Slog.i(TAG, "Starting " + name);
Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartService " + name);
// Create the service.
if (!SystemService.class.isAssignableFrom(serviceClass)) {
throw new RuntimeException("Failed to create " + name
+ ": service must extend " + SystemService.class.getName());
}
final T service;
try {
Constructor<T> constructor = serviceClass.getConstructor(Context.class);
service = constructor.newInstance(mContext);
} catch (InstantiationException ex) {
throw new RuntimeException("Failed to create service " + name
+ ": service could not be instantiated", ex);
} catch (IllegalAccessException ex) {
throw new RuntimeException("Failed to create service " + name
+ ": service must have a public constructor with a Context argument", ex);
} catch (NoSuchMethodException ex) {
throw new RuntimeException("Failed to create service " + name
+ ": service must have a public constructor with a Context argument", ex);
} catch (InvocationTargetException ex) {
throw new RuntimeException("Failed to create service " + name
+ ": service constructor threw an exception", ex);
}
// Register it.
mServices.add(service);
// Start it.
try {
service.onStart();
} catch (RuntimeException ex) {
throw new RuntimeException("Failed to start service " + name
+ ": onStart threw an exception", ex);
}
return service;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
}SystemServerManager启动服务，通过反射获取system service的实例，加入内部变量mService中，然后调用service的onstart()方法。
Installer构造函数和onStart()如下：
public Installer(Context context) {
super(context);
mInstaller = new InstallerConnection();
} @Override
public void onStart() {
Slog.i(TAG, "Waiting for installd to be ready.");
mInstaller.waitForConnection();
}在Intaller构造函数中new了一个InstallerConnection对象，并在onStart()中调用InstallerConnection的waitForConnection()方法。
waitForConnection()函数如下：
public void waitForConnection() {
for (;;) {
try {
execute("ping");
return;
} catch (InstallerException ignored) {
}
Slog.w(TAG, "installd not ready");
SystemClock.sleep(1000);
}
}向底层发了ping操作的命令，具体执行流程如下，同时也是其他操作向底层installd通信的流程
首先是execute()函数，如下：
public String[] execute(String cmd, Object... args) throws InstallerException {
final StringBuilder builder = new StringBuilder(cmd);
for (Object arg : args) {
String escaped;
if (arg == null) {
escaped = "";
} else {
escaped = String.valueOf(arg);
}
if (escaped.indexOf('\0') != -1 || escaped.indexOf(' ') != -1 || "!".equals(escaped)) {
throw new InstallerException(
"Invalid argument while executing " + cmd + " " + Arrays.toString(args));
}
if (TextUtils.isEmpty(escaped)) {
escaped = "!";
}
builder.append(' ').append(escaped);
}
final String[] resRaw = transact(builder.toString()).split(" ");
int res = -1;
try {
res = Integer.parseInt(resRaw[0]);
} catch (ArrayIndexOutOfBoundsException | NumberFormatException ignored) {
}
if (res != 0) {
throw new InstallerException(
"Failed to execute " + cmd + " " + Arrays.toString(args) + ": " + res);
}
return resRaw;
}execute()方法首先对参数进行格式处理，然后调用transact(),返回结果保存在resRaw中，若resRaw[0]不为0，则表示执行命令失败。
下面具体看transact()方法
public synchronized String transact(String cmd) {
if (mWarnIfHeld != null && Thread.holdsLock(mWarnIfHeld)) {
Slog.wtf(TAG, "Calling thread " + Thread.currentThread().getName() + " is holding 0x"
+ Integer.toHexString(System.identityHashCode(mWarnIfHeld)), new Throwable());
}
//与底层installd socket建立连接，只需连接一次
if (!connect()) {
Slog.e(TAG, "connection failed");
return "-1";
}
if (!writeCommand(cmd)) {
/*
* If installd died and restarted in the background (unlikely but
* possible) we'll fail on the next write (this one). Try to
* reconnect and write the command one more time before giving up.
*/
Slog.e(TAG, "write command failed? reconnect!");
if (!connect() || !writeCommand(cmd)) {
return "-1";
}
}
if (LOCAL_DEBUG) {
Slog.i(TAG, "send: '" + cmd + "'");
}
//获取返回值
final int replyLength = readReply();
if (replyLength > 0) {
String s = new String(buf, 0, replyLength);
if (LOCAL_DEBUG) {
Slog.i(TAG, "recv: '" + s + "'");
}
return s;
} else {
if (LOCAL_DEBUG) {
Slog.i(TAG, "fail");
}
return "-1";
}
}transact()方法有几个主要的方法：
1.connect(),首先会判断instller是否和底层installd建立了连接，建立连接只需要执行一次
2.writeCommand()，往instlld发送信息
3.readReply()读取instlld返回的信息
先分析connect()
private boolean connect() {
if (mSocket != null) {
return true;
}
Slog.i(TAG, "connecting...");
try {
mSocket = new LocalSocket();
LocalSocketAddress address = new LocalSocketAddress("installd",
LocalSocketAddress.Namespace.RESERVED);
mSocket.connect(address);
mIn = mSocket.getInputStream();
mOut = mSocket.getOutputStream();
} catch (IOException ex) {
disconnect();
return false;
}
return true;
}标准的socket方式的连接请求
writeCommand()如下：
private boolean writeCommand(String cmdString) {
final byte[] cmd = cmdString.getBytes();
final int len = cmd.length;
if ((len < 1) || (len > buf.length)) {
return false;
}
buf[0] = (byte) (len & 0xff);
buf[1] = (byte) ((len >> 8) & 0xff);
try {
mOut.write(buf, 0, 2);
mOut.write(cmd, 0, len);
} catch (IOException ex) {
Slog.e(TAG, "write error");
disconnect();
return false;
}
return true;
}向installd发送消息，先发送连个字节的命令长度，然后在发送命令字符串
readReply()如下：
private int readReply() {
if (!readFully(buf, 2)) {
return -1;
}
final int len = (((int) buf[0]) & 0xff) | ((((int) buf[1]) & 0xff) << 8);
if ((len < 1) || (len > buf.length)) {
Slog.e(TAG, "invalid reply length (" + len + ")");
disconnect();
return -1;
}
if (!readFully(buf, len)) {
return -1;
}
return len;
}同样获取installd发送的信息，前两个字节存储返回字符串长度，若长度匹配，则读取返回信息，保存在buf中，返回字符串长度。
到这，java层已经建立了和installd的连接，请求流程也做了分析。
SystemServer启动PackageManagerService时，会传入Installer对象，当PackageManagerService安装卸载应用时，就会最终调用到InstallerConnection的execute向instlld发送命令。
3.Installd处理流程
前面分析过，在installd.cpp中创建了一个循环，连接到client后，调用用execute执行命令
static int execute(int s, char cmd[BUFFER_MAX])
{
char reply[REPLY_MAX];
char *arg[TOKEN_MAX+1];
unsigned i;
unsigned n = 0;
unsigned short count;
int ret = -1;
// ALOGI("execute('%s')\n", cmd);
/* default reply is "" */
reply[0] = 0;
/* n is number of args (not counting arg[0]) */
arg[0] = cmd;
while (*cmd) {
if (isspace(*cmd)) {
*cmd++ = 0;
n++;
arg[n] = cmd;
if (n == TOKEN_MAX) {
ALOGE("too many arguments\n");
goto done;
}
}
if (*cmd) {
cmd++;
}
}
for (i = 0; i < sizeof(cmds) / sizeof(cmds[0]); i++) {
if (!strcmp(cmds[i].name,arg[0])) {
if (n != cmds[i].numargs) {
ALOGE("%s requires %d arguments (%d given)\n",
cmds[i].name, cmds[i].numargs, n);
} else {
ret = cmds[i].func(arg + 1, reply);
}
goto done;
}
}
ALOGE("unsupported command '%s'\n", arg[0]);
done:
if (reply[0]) {
n = snprintf(cmd, BUFFER_MAX, "%d %s", ret, reply);
} else {
n = snprintf(cmd, BUFFER_MAX, "%d", ret);
}
if (n > BUFFER_MAX) n = BUFFER_MAX;
count = n;
// ALOGI("reply: '%s'\n", cmd);
if (writex(s, &count, sizeof(count))) return -1;
if (writex(s, cmd, count)) return -1;
return 0;
}通过解析传入的信息，调用相应的函数，instlld所有的命令如下。
struct cmdinfo cmds[] = {
{ "ping", 0, do_ping },
{ "create_app_data", 7, do_create_app_data },
{ "restorecon_app_data", 6, do_restorecon_app_data },
{ "migrate_app_data", 4, do_migrate_app_data },
{ "clear_app_data", 5, do_clear_app_data },
{ "destroy_app_data", 5, do_destroy_app_data },
{ "move_complete_app", 7, do_move_complete_app },
{ "get_app_size", 6, do_get_app_size },
{ "get_app_data_inode", 4, do_get_app_data_inode },
{ "create_user_data", 4, do_create_user_data },
{ "destroy_user_data", 3, do_destroy_user_data },
{ "dexopt", 10, do_dexopt },
{ "markbootcomplete", 1, do_mark_boot_complete },
{ "rmdex", 2, do_rm_dex },
{ "freecache", 2, do_free_cache },
{ "linklib", 4, do_linklib },
{ "idmap", 3, do_idmap },
{ "createoatdir", 2, do_create_oat_dir },
{ "rmpackagedir", 1, do_rm_package_dir },
{ "clear_app_profiles", 1, do_clear_app_profiles },
{ "destroy_app_profiles", 1, do_destroy_app_profiles },
{ "linkfile", 3, do_link_file },
{ "move_ab", 3, do_move_ab },
{ "merge_profiles", 2, do_merge_profiles },
{ "dump_profiles", 3, do_dump_profiles },
};