一，serviceManger时序图

本文涉及到的源码文件：

/frameworks/native/cmds/servicemanager/main.cpp
/frameworks/native/libs/binder/ProcessState.cpp
/frameworks/native/cmds/servicemanager/ServiceManager.cpp
/frameworks/native/libs/binder/IPCThreadState.cpp
/system/core/libutils/Looper.cpp
 ServiceManager是一个由C/C++编写的系统服务，源码位于/framework/native/cmds/servicemanager中，存在如下文件结构

java层SM

public final class ServiceManager {
    private static final String TAG = "ServiceManager";

    private static IServiceManager sServiceManager;
    private static HashMap<String, IBinder> sCache = new HashMap<String, IBinder>();

    private static IServiceManager getIServiceManager() {
        if (sServiceManager != null) {
            return sServiceManager;
        }

        // Find the service manager
        sServiceManager = ServiceManagerNative
                .asInterface(Binder.allowBlocking(BinderInternal.getContextObject()));
        return sServiceManager;
    }

    /**
     * Returns a reference to a service with the given name.
     * 
     * @param name the name of the service to get
     * @return a reference to the service, or <code>null</code> if the service doesn't exist
     */
    public static IBinder getService(String name) {
        try {
            IBinder service = sCache.get(name);
            if (service != null) {
                return service;
            } else {
                return Binder.allowBlocking(getIServiceManager().getService(name));
            }
        } catch (RemoteException e) {
            Log.e(TAG, "error in getService", e);
        }
        return null;
    }

    /**
     * Returns a reference to a service with the given name, or throws
     * {@link NullPointerException} if none is found.
     *
     * @hide
     */
    public static IBinder getServiceOrThrow(String name) throws ServiceNotFoundException {
        final IBinder binder = getService(name);
        if (binder != null) {
            return binder;
        } else {
            throw new ServiceNotFoundException(name);
        }
    }

    /**
     * Place a new @a service called @a name into the service
     * manager.
     * 
     * @param name the name of the new service
     * @param service the service object
     */
    public static void addService(String name, IBinder service) {
        try {
            getIServiceManager().addService(name, service, false);
        } catch (RemoteException e) {
            Log.e(TAG, "error in addService", e);
        }
    }

    /**
     * Place a new @a service called @a name into the service
     * manager.
     * 
     * @param name the name of the new service
     * @param service the service object
     * @param allowIsolated set to true to allow isolated sandboxed processes
     * to access this service
     */
    public static void addService(String name, IBinder service, boolean allowIsolated) {
        try {
            getIServiceManager().addService(name, service, allowIsolated);
        } catch (RemoteException e) {
            Log.e(TAG, "error in addService", e);
        }
    }
    
    /**
     * Retrieve an existing service called @a name from the
     * service manager.  Non-blocking.
     */
    public static IBinder checkService(String name) {
        try {
            IBinder service = sCache.get(name);
            if (service != null) {
                return service;
            } else {
                return Binder.allowBlocking(getIServiceManager().checkService(name));
            }
        } catch (RemoteException e) {
            Log.e(TAG, "error in checkService", e);
            return null;
        }
    }

    /**
     * Return a list of all currently running services.
     * @return an array of all currently running services, or <code>null</code> in
     * case of an exception
     */
    public static String[] listServices() {
        try {
            return getIServiceManager().listServices();
        } catch (RemoteException e) {
            Log.e(TAG, "error in listServices", e);
            return null;
        }
    }

    /**
     * This is only intended to be called when the process is first being brought
     * up and bound by the activity manager. There is only one thread in the process
     * at that time, so no locking is done.
     * 
     * @param cache the cache of service references
     * @hide
     */
    public static void initServiceCache(Map<String, IBinder> cache) {
        if (sCache.size() != 0) {
            throw new IllegalStateException("setServiceCache may only be called once");
        }
        sCache.putAll(cache);
    }

    /**
     * Exception thrown when no service published for given name. This might be
     * thrown early during boot before certain services have published
     * themselves.
     *
     * @hide
     */
    public static class ServiceNotFoundException extends Exception {
        public ServiceNotFoundException(String name) {
            super("No service published for: " + name);
        }
    }
}

native层的SM

/*
 * Copyright (C) 2005 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#define LOG_TAG "ServiceManager"

#include <binder/IServiceManager.h>

#include <utils/Log.h>
#include <binder/IPCThreadState.h>
#include <binder/Parcel.h>
#include <utils/String8.h>
#include <utils/SystemClock.h>
#include <utils/CallStack.h>

#include <private/binder/Static.h>

#include <unistd.h>

namespace android {

sp<IServiceManager> defaultServiceManager()
{
    if (gDefaultServiceManager != NULL) return gDefaultServiceManager;

    {
        AutoMutex _l(gDefaultServiceManagerLock);
        while (gDefaultServiceManager == NULL) {
            gDefaultServiceManager = interface_cast<IServiceManager>(
                ProcessState::self()->getContextObject(NULL));
            if (gDefaultServiceManager == NULL)
                sleep(1);
        }
    }

    return gDefaultServiceManager;
}

bool checkCallingPermission(const String16& permission)
{
    return checkCallingPermission(permission, NULL, NULL);
}

static String16 _permission("permission");


bool checkCallingPermission(const String16& permission, int32_t* outPid, int32_t* outUid)
{
    IPCThreadState* ipcState = IPCThreadState::self();
    pid_t pid = ipcState->getCallingPid();
    uid_t uid = ipcState->getCallingUid();
    if (outPid) *outPid = pid;
    if (outUid) *outUid = uid;
    return checkPermission(permission, pid, uid);
}

bool checkPermission(const String16& permission, pid_t pid, uid_t uid)
{
    sp<IPermissionController> pc;
    gDefaultServiceManagerLock.lock();
    pc = gPermissionController;
    gDefaultServiceManagerLock.unlock();

    int64_t startTime = 0;

    while (true) {
        if (pc != NULL) {
            bool res = pc->checkPermission(permission, pid, uid);
            if (res) {
                if (startTime != 0) {
                    ALOGI("Check passed after %d seconds for %s from uid=%d pid=%d",
                            (int)((uptimeMillis()-startTime)/1000),
                            String8(permission).string(), uid, pid);
                }
                return res;
            }

            // Is this a permission failure, or did the controller go away?
            if (IInterface::asBinder(pc)->isBinderAlive()) {
                ALOGW("Permission failure: %s from uid=%d pid=%d",
                        String8(permission).string(), uid, pid);
                return false;
            }

            // Object is dead!
            gDefaultServiceManagerLock.lock();
            if (gPermissionController == pc) {
                gPermissionController = NULL;
            }
            gDefaultServiceManagerLock.unlock();
        }

        // Need to retrieve the permission controller.
        sp<IBinder> binder = defaultServiceManager()->checkService(_permission);
        if (binder == NULL) {
            // Wait for the permission controller to come back...
            if (startTime == 0) {
                startTime = uptimeMillis();
                ALOGI("Waiting to check permission %s from uid=%d pid=%d",
                        String8(permission).string(), uid, pid);
            }
            sleep(1);
        } else {
            pc = interface_cast<IPermissionController>(binder);
            // Install the new permission controller, and try again.
            gDefaultServiceManagerLock.lock();
            gPermissionController = pc;
            gDefaultServiceManagerLock.unlock();
        }
    }
}

// ----------------------------------------------------------------------

class BpServiceManager : public BpInterface<IServiceManager>
{
public:
    explicit BpServiceManager(const sp<IBinder>& impl)
        : BpInterface<IServiceManager>(impl)
    {
    }

    virtual sp<IBinder> getService(const String16& name) const
    {
        unsigned n;
        for (n = 0; n < 5; n++){
            if (n > 0) {
                if (!strcmp(ProcessState::self()->getDriverName().c_str(), "/dev/vndbinder")) {
                    ALOGI("Waiting for vendor service %s...", String8(name).string());
                    CallStack stack(LOG_TAG);
                } else {
                    ALOGI("Waiting for service %s...", String8(name).string());
                }
                sleep(1);
            }
            sp<IBinder> svc = checkService(name);
            if (svc != NULL) return svc;
        }
        return NULL;
    }

    virtual sp<IBinder> checkService( const String16& name) const
    {
        Parcel data, reply;
        data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
        data.writeString16(name);
        remote()->transact(CHECK_SERVICE_TRANSACTION, data, &reply);
        return reply.readStrongBinder();
    }

    virtual status_t addService(const String16& name, const sp<IBinder>& service,
            bool allowIsolated)
    {
        Parcel data, reply;
        data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
        data.writeString16(name);
        data.writeStrongBinder(service);
        data.writeInt32(allowIsolated ? 1 : 0);
        status_t err = remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply);
        return err == NO_ERROR ? reply.readExceptionCode() : err;
    }

    virtual Vector<String16> listServices()
    {
        Vector<String16> res;
        int n = 0;

        for (;;) {
            Parcel data, reply;
            data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());
            data.writeInt32(n++);
            status_t err = remote()->transact(LIST_SERVICES_TRANSACTION, data, &reply);
            if (err != NO_ERROR)
                break;
            res.add(reply.readString16());
        }
        return res;
    }
};

IMPLEMENT_META_INTERFACE(ServiceManager, "android.os.IServiceManager");

}; // namespace android

真正的SM服务

/* Copyright 2008 The Android Open Source Project
 */

#include <errno.h>
#include <fcntl.h>
#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <cutils/android_filesystem_config.h>
#include <cutils/multiuser.h>

#include <selinux/android.h>
#include <selinux/avc.h>

#include "binder.h"

#ifdef VENDORSERVICEMANAGER
#define LOG_TAG "VendorServiceManager"
#else
#define LOG_TAG "ServiceManager"
#endif
#include <log/log.h>

struct audit_data {
    pid_t pid;
    uid_t uid;
    const char *name;
};

const char *str8(const uint16_t *x, size_t x_len)
{
    static char buf[128];
    size_t max = 127;
    char *p = buf;

    if (x_len < max) {
        max = x_len;
    }

    if (x) {
        while ((max > 0) && (*x != '\0')) {
            *p++ = *x++;
            max--;
        }
    }
    *p++ = 0;
    return buf;
}

int str16eq(const uint16_t *a, const char *b)
{
    while (*a && *b)
        if (*a++ != *b++) return 0;
    if (*a || *b)
        return 0;
    return 1;
}

static char *service_manager_context;
static struct selabel_handle* sehandle;

static bool check_mac_perms(pid_t spid, uid_t uid, const char *tctx, const char *perm, const char *name)
{
    char *sctx = NULL;
    const char *class = "service_manager";
    bool allowed;
    struct audit_data ad;

    if (getpidcon(spid, &sctx) < 0) {
        ALOGE("SELinux: getpidcon(pid=%d) failed to retrieve pid context.\n", spid);
        return false;
    }

    ad.pid = spid;
    ad.uid = uid;
    ad.name = name;

    int result = selinux_check_access(sctx, tctx, class, perm, (void *) &ad);
    allowed = (result == 0);

    freecon(sctx);
    return allowed;
}

static bool check_mac_perms_from_getcon(pid_t spid, uid_t uid, const char *perm)
{
    return check_mac_perms(spid, uid, service_manager_context, perm, NULL);
}

static bool check_mac_perms_from_lookup(pid_t spid, uid_t uid, const char *perm, const char *name)
{
    bool allowed;
    char *tctx = NULL;

    if (!sehandle) {
        ALOGE("SELinux: Failed to find sehandle. Aborting service_manager.\n");
        abort();
    }

    if (selabel_lookup(sehandle, &tctx, name, 0) != 0) {
        ALOGE("SELinux: No match for %s in service_contexts.\n", name);
        return false;
    }

    allowed = check_mac_perms(spid, uid, tctx, perm, name);
    freecon(tctx);
    return allowed;
}

static int svc_can_register(const uint16_t *name, size_t name_len, pid_t spid, uid_t uid)
{
    const char *perm = "add";

    if (multiuser_get_app_id(uid) >= AID_APP) {
        return 0; /* Don't allow apps to register services */
    }

    return check_mac_perms_from_lookup(spid, uid, perm, str8(name, name_len)) ? 1 : 0;
}

static int svc_can_list(pid_t spid, uid_t uid)
{
    const char *perm = "list";
    return check_mac_perms_from_getcon(spid, uid, perm) ? 1 : 0;
}

static int svc_can_find(const uint16_t *name, size_t name_len, pid_t spid, uid_t uid)
{
    const char *perm = "find";
    return check_mac_perms_from_lookup(spid, uid, perm, str8(name, name_len)) ? 1 : 0;
}

struct svcinfo
{
    struct svcinfo *next;
    uint32_t handle;
    struct binder_death death;
    int allow_isolated;
    size_t len;
    uint16_t name[0];
};

struct svcinfo *svclist = NULL;

struct svcinfo *find_svc(const uint16_t *s16, size_t len)
{
    struct svcinfo *si;

    for (si = svclist; si; si = si->next) {
        if ((len == si->len) &&
            !memcmp(s16, si->name, len * sizeof(uint16_t))) {
            return si;
        }
    }
    return NULL;
}

void svcinfo_death(struct binder_state *bs, void *ptr)
{
    struct svcinfo *si = (struct svcinfo* ) ptr;

    ALOGI("service '%s' died\n", str8(si->name, si->len));
    if (si->handle) {
        binder_release(bs, si->handle);
        si->handle = 0;
    }
}

uint16_t svcmgr_id[] = {
    'a','n','d','r','o','i','d','.','o','s','.',
    'I','S','e','r','v','i','c','e','M','a','n','a','g','e','r'
};


uint32_t do_find_service(const uint16_t *s, size_t len, uid_t uid, pid_t spid)
{
    struct svcinfo *si = find_svc(s, len);

    if (!si || !si->handle) {
        return 0;
    }

    if (!si->allow_isolated) {
        // If this service doesn't allow access from isolated processes,
        // then check the uid to see if it is isolated.
        uid_t appid = uid % AID_USER;
        if (appid >= AID_ISOLATED_START && appid <= AID_ISOLATED_END) {
            return 0;
        }
    }

    if (!svc_can_find(s, len, spid, uid)) {
        return 0;
    }

    return si->handle;
}

int do_add_service(struct binder_state *bs,
                   const uint16_t *s, size_t len,
                   uint32_t handle, uid_t uid, int allow_isolated,
                   pid_t spid)
{
    struct svcinfo *si;

    //ALOGI("add_service('%s',%x,%s) uid=%d\n", str8(s, len), handle,
    //        allow_isolated ? "allow_isolated" : "!allow_isolated", uid);

    if (!handle || (len == 0) || (len > 127))
        return -1;

    if (!svc_can_register(s, len, spid, uid)) {
        ALOGE("add_service('%s',%x) uid=%d - PERMISSION DENIED\n",
             str8(s, len), handle, uid);
        return -1;
    }

    si = find_svc(s, len);
    if (si) {
        if (si->handle) {
            ALOGE("add_service('%s',%x) uid=%d - ALREADY REGISTERED, OVERRIDE\n",
                 str8(s, len), handle, uid);
            svcinfo_death(bs, si);
        }
        si->handle = handle;
    } else {
        si = malloc(sizeof(*si) + (len + 1) * sizeof(uint16_t));
        if (!si) {
            ALOGE("add_service('%s',%x) uid=%d - OUT OF MEMORY\n",
                 str8(s, len), handle, uid);
            return -1;
        }
        si->handle = handle;
        si->len = len;
        memcpy(si->name, s, (len + 1) * sizeof(uint16_t));
        si->name[len] = '\0';
        si->death.func = (void*) svcinfo_death;
        si->death.ptr = si;
        si->allow_isolated = allow_isolated;
        si->next = svclist;
        svclist = si;
    }

    binder_acquire(bs, handle);
    binder_link_to_death(bs, handle, &si->death);
    return 0;
}

int svcmgr_handler(struct binder_state *bs,
                   struct binder_transaction_data *txn,
                   struct binder_io *msg,
                   struct binder_io *reply)
{
    struct svcinfo *si;
    uint16_t *s;
    size_t len;
    uint32_t handle;
    uint32_t strict_policy;
    int allow_isolated;

    //ALOGI("target=%p code=%d pid=%d uid=%d\n",
    //      (void*) txn->target.ptr, txn->code, txn->sender_pid, txn->sender_euid);

    if (txn->target.ptr != BINDER_SERVICE_MANAGER)
        return -1;

    if (txn->code == PING_TRANSACTION)
        return 0;

    // Equivalent to Parcel::enforceInterface(), reading the RPC
    // header with the strict mode policy mask and the interface name.
    // Note that we ignore the strict_policy and don't propagate it
    // further (since we do no outbound RPCs anyway).
    strict_policy = bio_get_uint32(msg);
    s = bio_get_string16(msg, &len);
    if (s == NULL) {
        return -1;
    }

    if ((len != (sizeof(svcmgr_id) / 2)) ||
        memcmp(svcmgr_id, s, sizeof(svcmgr_id))) {
        fprintf(stderr,"invalid id %s\n", str8(s, len));
        return -1;
    }

    if (sehandle && selinux_status_updated() > 0) {
#ifdef VENDORSERVICEMANAGER
        struct selabel_handle *tmp_sehandle = selinux_android_vendor_service_context_handle();
#else
        struct selabel_handle *tmp_sehandle = selinux_android_service_context_handle();
#endif
        if (tmp_sehandle) {
            selabel_close(sehandle);
            sehandle = tmp_sehandle;
        }
    }

    switch(txn->code) {
    case SVC_MGR_GET_SERVICE:
    case SVC_MGR_CHECK_SERVICE:
        s = bio_get_string16(msg, &len);
        if (s == NULL) {
            return -1;
        }
        handle = do_find_service(s, len, txn->sender_euid, txn->sender_pid);
        if (!handle)
            break;
        bio_put_ref(reply, handle);
        return 0;

    case SVC_MGR_ADD_SERVICE:
        s = bio_get_string16(msg, &len);
        if (s == NULL) {
            return -1;
        }
        handle = bio_get_ref(msg);
        allow_isolated = bio_get_uint32(msg) ? 1 : 0;
        if (do_add_service(bs, s, len, handle, txn->sender_euid,
            allow_isolated, txn->sender_pid))
            return -1;
        break;

    case SVC_MGR_LIST_SERVICES: {
        uint32_t n = bio_get_uint32(msg);

        if (!svc_can_list(txn->sender_pid, txn->sender_euid)) {
            ALOGE("list_service() uid=%d - PERMISSION DENIED\n",
                    txn->sender_euid);
            return -1;
        }
        si = svclist;
        while ((n-- > 0) && si)
            si = si->next;
        if (si) {
            bio_put_string16(reply, si->name);
            return 0;
        }
        return -1;
    }
    default:
        ALOGE("unknown code %d\n", txn->code);
        return -1;
    }

    bio_put_uint32(reply, 0);
    return 0;
}


static int audit_callback(void *data, __unused security_class_t cls, char *buf, size_t len)
{
    struct audit_data *ad = (struct audit_data *)data;

    if (!ad || !ad->name) {
        ALOGE("No service manager audit data");
        return 0;
    }

    snprintf(buf, len, "service=%s pid=%d uid=%d", ad->name, ad->pid, ad->uid);
    return 0;
}

int main(int argc, char** argv)
{
    struct binder_state *bs;
    union selinux_callback cb;
    char *driver;

    if (argc > 1) {
        driver = argv[1];
    } else {
        driver = "/dev/binder";
    }

    bs = binder_open(driver, 128*1024);
    if (!bs) {
#ifdef VENDORSERVICEMANAGER
        ALOGW("failed to open binder driver %s\n", driver);
        while (true) {
            sleep(UINT_MAX);
        }
#else
        ALOGE("failed to open binder driver %s\n", driver);
#endif
        return -1;
    }

    if (binder_become_context_manager(bs)) {
        ALOGE("cannot become context manager (%s)\n", strerror(errno));
        return -1;
    }

    cb.func_audit = audit_callback;
    selinux_set_callback(SELINUX_CB_AUDIT, cb);
    cb.func_log = selinux_log_callback;
    selinux_set_callback(SELINUX_CB_LOG, cb);

#ifdef VENDORSERVICEMANAGER
    sehandle = selinux_android_vendor_service_context_handle();
#else
    sehandle = selinux_android_service_context_handle();
#endif
    selinux_status_open(true);

    if (sehandle == NULL) {
        ALOGE("SELinux: Failed to acquire sehandle. Aborting.\n");
        abort();
    }

    if (getcon(&service_manager_context) != 0) {
        ALOGE("SELinux: Failed to acquire service_manager context. Aborting.\n");
        abort();
    }


    binder_loop(bs, svcmgr_handler);

    return 0;
}

二，serviceManger运行时时序图

从上述代码可以明确知道：ServiceManager程序一旦启动运行，则不会停止（系统不宕机的情况下），将会一直looper->pollAll(-1)。除此之外，还知道ServiceManager默认情况依托于/dev/binder这个设备节点，当然可以通过向ServiceManager传递参数指定设备节点。ServiceManager作为binder机制的核心组件之一，在实现进程间通信中占据着不可获取的地位。

从Android.bp可以知道，ServiceManager对应的程序名称是servicemanager。从Android.bp文件中，还存在一个名为vndservicemanager的程序，它们的源码都是一样的，只是rc文件存在出入，将传入/dev/vndbinder作为binder驱动。在Android启动启动过程中，vndservicemanager和vndservicemanager都会被init拉起，他俩的区别总结如下：

1、对于servicemanager:

servicemanager是 Android 系统中的核心服务管理器，用于管理系统级的服务。
servicemanager管理的服务包括系统级服务（如ActivityManager、PackageManager、WindowManager等）和由应用程序注册的系统服务。
servicemanager的访问权限较高，一般只有系统级应用或者具有系统权限的应用程序才能够使用servicemanager进行服务的注册和查询。
2、对于vndservicemanager：

vndservicemanager是 servicemanager的一个扩展，用于管理供应商特定的服务。
vndservicemanager管理的服务通常是供应商（vendor）特定的、定制化的服务，例如硬件厂商提供的驱动程序或服务。
vndservicemanager的访问权限一般较低，通常只有供应商特定的应用程序或系统组件才能够使用vndservicemanager进行服务的注册和查询。
二、ServiceManager的启动
第一小节简要介绍了ServiceManager，我们知道ServiceManager是一个具体程序，如果是个程序，那么该程序是如何启动的呢？又在什么时候触发运行的呢？

我们知道Android系统的第一个进程是init，这是由内核决定的。那么在init中将通过解析init.rc来启动系统的一些关键服务进程。其中ServiceManager就在这个时候启动运行