幾天前一個同事問Java層的Binder和Java層的ServiceManager和C++層的ServiceManager是如何對應起來的,竟然一時語噻,一段時間不關注就會感到陌生啊,現在再總結一下,只寫Java層的,C++層的不寫,C++ Binder的知識需掌握,java和native的基本概念及對應關係不講。
Java層Binder的初始化過程
Binder最終是要和它的驅動打交道,Java層要調Linux內核層的東西必然要通過native方法,而對於android的系統服務,native和java的對應,總會有一個註冊過程,先看這個註冊是在哪實現的
JNI Binder的註冊
Zygote在啓動時:
app_main.cpp
int main(int argc, char* const argv[])
{
......
AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));
......
if (zygote) {
runtime.start("com.android.internal.os.ZygoteInit", args, zygote);
}
......
}
AppRuntime是AndroidRuntime的子類,它本身沒有覆蓋start方法,所以要去看AndroidRuntime類中的start方法
void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{
......
/*
* Register android functions.
*/
if (startReg(env) < 0) {
ALOGE("Unable to register all android natives\n");
return;
}
......
}
/*
* Register android native functions with the VM.
*/
/*static*/ int AndroidRuntime::startReg(JNIEnv* env)
{
......
if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {
env->PopLocalFrame(NULL);
return -1;
}
......
}
gRegJNI是個數組:
static const RegJNIRec gRegJNI[] = {
......
REG_JNI(register_android_os_Binder),
......
};
REG_JNI是一個宏定義
#ifdef NDEBUG
#define REG_JNI(name) { name }
struct RegJNIRec {
int (*mProc)(JNIEnv*);
};
#else
#define REG_JNI(name) { name, #name }
struct RegJNIRec {
int (*mProc)(JNIEnv*);
const char* mName;
};
#endif
gRegJNI是一個RegJNIRec的數組,所以gRegJNI的定義就變成了這樣:
static const RegJNIRec gRegJNI[] = {
......
{ register_android_os_Binder },
......
};
register_jni_procs函數如下:
static int register_jni_procs(const RegJNIRec array[], size_t count, JNIEnv* env)
{
for (size_t i = 0; i < count; i++) {
if (array[i].mProc(env) < 0) {
#ifndef NDEBUG
ALOGD("----------!!! %s failed to load\n", array[i].mName);
#endif
return -1;
}
}
return 0;
}
就是循環調用gRegJNI數組中的函數,這時register_android_os_Binder(env)就被調用了。
register_android_os_Binder在AndroidRuntime.cpp中被聲明爲extern:
extern int register_android_os_Binder(JNIEnv* env);
實現它的地方是在android_util_Binder.cpp中:
int register_android_os_Binder(JNIEnv* env)
{
if (int_register_android_os_Binder(env) < 0)
return -1;
if (int_register_android_os_BinderInternal(env) < 0)
return -1;
if (int_register_android_os_BinderProxy(env) < 0)
return -1;
......
return 0;
}
從名字上看,它要關聯3個Java的類,分別是Binder, BinderInternal和BinderProxy,一個一個看來:
const char* const kBinderPathName = "android/os/Binder";
static int int_register_android_os_Binder(JNIEnv* env)
{
jclass clazz = FindClassOrDie(env, kBinderPathName);
gBinderOffsets.mClass = MakeGlobalRefOrDie(env, clazz);
gBinderOffsets.mExecTransact = GetMethodIDOrDie(env, clazz, "execTransact", "(IJJI)Z");
gBinderOffsets.mObject = GetFieldIDOrDie(env, clazz, "mObject", "J");
return RegisterMethodsOrDie(
env, kBinderPathName,
gBinderMethods, NELEM(gBinderMethods));
}
關聯了android.os.Binder類,並會用到該類的mExecTransact域和execTransact方法,將其保存到gBinderOffsets對象中,也就意味着native會將某值保存到mExecTransact中,也會回調execTransact方法。
const char* const kBinderInternalPathName = "com/android/internal/os/BinderInternal";
static int int_register_android_os_BinderInternal(JNIEnv* env)
{
jclass clazz = FindClassOrDie(env, kBinderInternalPathName);
gBinderInternalOffsets.mClass = MakeGlobalRefOrDie(env, clazz);
gBinderInternalOffsets.mForceGc = GetStaticMethodIDOrDie(env, clazz, "forceBinderGc", "()V");
return RegisterMethodsOrDie(
env, kBinderInternalPathName,
gBinderInternalMethods, NELEM(gBinderInternalMethods));
}
關聯了java的com.android.internal.os.BinderInternal類,保存了其forceBinderGc方法。
const char* const kBinderProxyPathName = "android/os/BinderProxy";
static int int_register_android_os_BinderProxy(JNIEnv* env)
{
jclass clazz = FindClassOrDie(env, "java/lang/Error");
gErrorOffsets.mClass = MakeGlobalRefOrDie(env, clazz);
clazz = FindClassOrDie(env, kBinderProxyPathName);
gBinderProxyOffsets.mClass = MakeGlobalRefOrDie(env, clazz);
gBinderProxyOffsets.mConstructor = GetMethodIDOrDie(env, clazz, "<init>", "()V");
gBinderProxyOffsets.mSendDeathNotice = GetStaticMethodIDOrDie(env, clazz, "sendDeathNotice",
"(Landroid/os/IBinder$DeathRecipient;)V");
gBinderProxyOffsets.mObject = GetFieldIDOrDie(env, clazz, "mObject", "J");
gBinderProxyOffsets.mSelf = GetFieldIDOrDie(env, clazz, "mSelf",
"Ljava/lang/ref/WeakReference;");
gBinderProxyOffsets.mOrgue = GetFieldIDOrDie(env, clazz, "mOrgue", "J");
clazz = FindClassOrDie(env, "java/lang/Class");
gClassOffsets.mGetName = GetMethodIDOrDie(env, clazz, "getName", "()Ljava/lang/String;");
return RegisterMethodsOrDie(
env, kBinderProxyPathName,
gBinderProxyMethods, NELEM(gBinderProxyMethods));
}
這個方法比較長,保存的Java的東西也比較多,值得注意的是,它還保存了BinderProxy的構造函數,也就是說它可能會主動new一個BinderProxy實例出來。
看到目前爲止,native的Binder要對應到Java的3個類中,分別是android.os.Binder,com.android.internal.os.BinderInternal和android.os.BinderProxy。爲了方便閱讀,將它們的對應關係以類圖的形式總結下:
將Java層的服務註冊到ServiceManager中的過程
我們知道,native層的Binder有Bn和Bp之說,Bn應該是Binder Native的縮寫,就是本地實現Binder的意思也可以說是C/S中的S,Bp是Binder Proxy的縮寫,相當於C。
以PackageManagerService爲例:
public static PackageManagerService main(Context
......
PackageManagerService m = new PackageManagerService(context, installer,
factoryTest, onlyCore);
......
ServiceManager.addService("package", m);
return m;
}
public static void addService(String name, IBinder service) {
try {
getIServiceManager().addService(name, service, false);
} catch (RemoteException e) {
Log.e(TAG, "error in addService", e);
}
}
getIServiceManager返回的是一個IServiceManager類,且是個單例,最後調用的就是
private static IServiceManager getIServiceManager() {
if (sServiceManager != null) {
return sServiceManager;
}
// Find the service manager
sServiceManager = ServiceManagerNative.asInterface(BinderInternal.getContextObject());
return sServiceManager;
}
以上代碼帶來 三個問題,一個是BinderInternal.getContextObject(),一個是ServiceManagerNative.asInterface,另一個是addService的作用:
BinderInternal.getContextObject()的實質
ServiceManagerNative.asInterface中的參數是BinderInternal.getContextObject(),看它是native方法,對應的native代碼就在剛剛看到的android_util_Binder.cpp文件中:
static jobject android_os_BinderInternal_getContextObject(JNIEnv* env, jobject clazz)
{
// 是一個BpBinder
sp<IBinder> b = ProcessState::self()->getContextObject(NULL);
return javaObjectForIBinder(env, b);
}
jobject javaObjectForIBinder(JNIEnv* env, const sp<IBinder>& val)
{
if (val == NULL) return NULL;
if (val->checkSubclass(&gBinderOffsets)) {
// One of our own!
jobject object = static_cast<JavaBBinder*>(val.get())->object();
LOGDEATH("objectForBinder %p: it's our own %p!\n", val.get(), object);
return object;
}
// For the rest of the function we will hold this lock, to serialize
// looking/creation/destruction of Java proxies for native Binder proxies.
AutoMutex _l(mProxyLock);
// Someone else's... do we know about it?
// 1
jobject object = (jobject)val->findObject(&gBinderProxyOffsets);
if (object != NULL) {
jobject res = jniGetReferent(env, object);
if (res != NULL) {
ALOGV("objectForBinder %p: found existing %p!\n", val.get(), res);
return res;
}
LOGDEATH("Proxy object %p of IBinder %p no longer in working set!!!", object, val.get());
android_atomic_dec(&gNumProxyRefs);
val->detachObject(&gBinderProxyOffsets);
env->DeleteGlobalRef(object);
}
//2
object = env->NewObject(gBinderProxyOffsets.mClass, gBinderProxyOffsets.mConstructor);
if (object != NULL) {
LOGDEATH("objectForBinder %p: created new proxy %p !\n", val.get(), object);
// The proxy holds a reference to the native object.
//3
env->SetLongField(object, gBinderProxyOffsets.mObject, (jlong)val.get());
val->incStrong((void*)javaObjectForIBinder);
// The native object needs to hold a weak reference back to the
// proxy, so we can retrieve the same proxy if it is still active.
jobject refObject = env->NewGlobalRef(
env->GetObjectField(object, gBinderProxyOffsets.mSelf));
......
// Note that a new object reference has been created.
//4
android_atomic_inc(&gNumProxyRefs);
incRefsCreated(env);
}
return object;
}
- 首先判斷下和Java層關聯的Native層的BpBinder是否存在,如果存在,刪除。
- 接着創建一個Java層的BinderProxy實例。
- 將Native層的BpBinder指針保存到java BinderProxy實例的mObject字段中,這樣通過BinderProxy的mObject字段就可以操作Native層的BpBinder了。
- 原子增加gNumProxyRefs和gNumRefsCreated的數量,當gNumRefsCreated的值超過200,將會調用BinderInternal的forceBinderGc方法。
總結下:
- 創建了一個Native層的BpBinder;
- 將BpBinder的指針保存到BinderProxy的mObject,而BpBinder的通信對象就是Native的ServiceManager,這爲Java和Native的ServiceManager的通信埋下了伏筆。
- 可以將BinderProxy說成Java層的Bp端。
ServiceManagerNative.asInterface的作用
static public IServiceManager asInterface(IBinder obj)
{
if (obj == null) {
return null;
}
//1
IServiceManager in =
(IServiceManager)obj.queryLocalInterface(descriptor);
if (in != null) {
return in;
}
//2
return new ServiceManagerProxy(obj);
}
- obj是Native實例化的BinderProxy對象,BinderProxy的代碼在Binder.java中,和Binder位於同一個文件中。queryLocalInterface的返回值爲null。
- 實例化一個ServiceManagerProxy對象,該實例中有一個IBinder類型的mRemote字段,該字段保存BinderProxy實例。
public ServiceManagerProxy(IBinder remote) {
mRemote = remote;
}
總結:
- ServiceManagerNative.asInterface返回的是ServiceManagerProxy的實例,該實例實現IServiceManager的接口。
- ServiceManagerProxy中通過mRemote字段保存了BinderProxy,也就是Java層的Bp端。
addService的作用
有上分析,當調用ServiceManager.addService時,實際上調用的是ServiceManagerProxy類中的addService,ServiceManagerProxy類位於ServiceManagerNative.java文件中。
public void addService(String name, IBinder service, boolean allowIsolated)
throws RemoteException {
Parcel data = Parcel.obtain();
Parcel reply = Parcel.obtain();
data.writeInterfaceToken(IServiceManager.descriptor);
data.writeString(name);
data.writeStrongBinder(service);
data.writeInt(allowIsolated ? 1 : 0);
//1
mRemote.transact(ADD_SERVICE_TRANSACTION, data, reply, 0);
reply.recycle();
data.recycle();
}
- mRemote即爲BinderProxy,它會調用transactNative,transactNative是個native方法,接着看這個方法。
static jboolean android_os_BinderProxy_transact(JNIEnv* env, jobject obj,
//1
IBinder* target = (IBinder*)
env->GetLongField(obj, gBinderProxyOffsets.mObject);
if (target == NULL) {
jniThrowException(env, "java/lang/IllegalStateException", "Binder has been finalized!");
return JNI_FALSE;
}
//2
status_t err = target->transact(code, *data, reply, flags);
......
return JNI_FALSE;
}
- 取出保存在BinderProxy中的BpBinder對象的指針
- 向Native層的ServiceManager發送請求
總結:
- Java層調用ServiceManger.addService後,最終是向Native層的ServiceManager請求增加服務的。
- Native層的ServiceManager是隻有一個的,而java層的ServiceManager相對於Native的ServiceManager也只是個C端,這樣,不管是通過Native add service還是通過java add service,整個Android都可以拿到這個service的Binder。
Java Service端的Bp在哪裏
這時候還有個疑問,當調用ServiceManager.addService時傳入的Service應該對應到Native的BnBinder啊,現在對應關係如何體現呢?這個關係要重看下addService了。
addService第二個參數的類型是IBinder,它是個接口,而它的實現類是Binder,java上層能調用addService添加服務時,所有的實例都是Binder,根據java的規則,只要實例化,其父類的相應的構造函數必被調用:
public Binder() {
init();
......
}
init()是native方法,其實現在android_util_Binder.cpp中:
static void android_os_Binder_init(JNIEnv* env, jobject obj)
{
JavaBBinderHolder* jbh = new JavaBBinderHolder();
...
env->SetLongField(obj, gBinderOffsets.mObject, (jlong)jbh);
}
new一個native層的JavaBBinderHolder,並將其保存到Binder的mObject中。注意哦,雖然native層的gBinderOffsets.mObject是個static的,但java層的Binder的mObject可不是static的哦,就是說每個Binder實例都有一個Native的JavaBBinderHolder對象指針。
JavaBBinderHolder類的聲明如下:
class JavaBBinderHolder : public RefBase
{
}
沒有繼承BBinder,還是沒發現作爲Bn端的蹤影,需要再review下前面的流程,看下ServiceManagerProxy添加服務時發生了什麼:
public void addService(String name, IBinder service, boolean allowIsolated)
throws RemoteException {
Parcel data = Parcel.obtain();
......
data.writeStrongBinder(service);
......
}
writeStrongBinder值得懷疑
writeStrongBinder發生了什麼
Parcel的這個函數如下:
public final void writeStrongBinder(IBinder val) {
nativeWriteStrongBinder(mNativePtr, val);
}
nativeWriteStrongBinder的實現在android_os_Parcel.cpp中:
static void android_os_Parcel_writeStrongBinder(JNIEnv* env, jclass clazz, jlong nativePtr, jobject object)
{
Parcel* parcel = reinterpret_cast<Parcel*>(nativePtr);
if (parcel != NULL) {
const status_t err = parcel->writeStrongBinder(ibinderForJavaObject(env, object));
if (err != NO_ERROR) {
signalExceptionForError(env, clazz, err);
}
}
}
調用了一個ibinderForJavaObject函數,該函數位於android_util_Binder.cpp中
sp<IBinder> ibinderForJavaObject(JNIEnv* env, jobject obj)
{
if (obj == NULL) return NULL;
if (env->IsInstanceOf(obj, gBinderOffsets.mClass)) {
JavaBBinderHolder* jbh = (JavaBBinderHolder*)
env->GetLongField(obj, gBinderOffsets.mObject);
return jbh != NULL ? jbh->get(env, obj) : NULL;
}
if (env->IsInstanceOf(obj, gBinderProxyOffsets.mClass)) {
return (IBinder*)
env->GetLongField(obj, gBinderProxyOffsets.mObject);
}
ALOGW("ibinderForJavaObject: %p is not a Binder object", obj);
return NULL;
}
傳入的obj是Binder實例,所以剛剛創建的JavaBBinderHolder的get函數會被調用:
sp<JavaBBinder> get(JNIEnv* env, jobject obj)
{
AutoMutex _l(mLock);
sp<JavaBBinder> b = mBinder.promote();
if (b == NULL) {
b = new JavaBBinder(env, obj);
mBinder = b;
ALOGV("Creating JavaBinder %p (refs %p) for Object %p, weakCount=%" PRId32 "\n",
b.get(), b->getWeakRefs(), obj, b->getWeakRefs()->getWeakCount());
}
return b;
}
返回的是JavaBBinder實例,
class JavaBBinder : public BBinder
{
}
JavaBBinder繼承了BBinder,所以最終添加到Native ServiceManager中的是這個對象,總結下它們的類圖:
Java層的Binder對象擁有Native JavaBBinderHolder對象的指針,JavaBBinderHolder有JavaBBinder的,而JavaBBinder又有Java層Binder的對象。