1、总图中的第一步
如图：第一步
~/Android/frameworks/base/core/java/android/app
----ActivityManagerNative.java
class ActivityManagerProxy implements IActivityManager
{
public int startActivity(IApplicationThread caller, Intent intent,
String resolvedType, Uri[] grantedUriPermissions, int grantedMode,
IBinder resultTo, String resultWho,
int requestCode, boolean onlyIfNeeded,
boolean debug) throws RemoteException {
Parcel data = Parcel.obtain();
Parcel reply = Parcel.obtain();
data.writeInterfaceToken(IActivityManager.descriptor);
data.writeStrongBinder(caller != null ? caller.asBinder() : null);
intent.writeToParcel(data, 0);
.........
data.writeStrongBinder(resultTo);
mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0);
} 其中caller为：
final ApplicationThread mAppThread = new ApplicationThread();继承于ApplicationThreadNative，ApplicationThreadNative继承于Binder实现了IApplicationThread。
resultTo如下图所示：
java层的Parcel，writeStrongBinder方法，最后映射到C++层，执行如下：
~/Android/frameworks/base/core/jni
----android_util_Binder.cpp
static void android_os_Parcel_writeStrongBinder(JNIEnv* env, jobject clazz, jobject object)//clazz为Parcel，object指向了在Java层中创建的硬件访问服务FregService
{
Parcel* parcel = parcelForJavaObject(env, clazz);//获取java层Parcel对象data的引用
if (parcel != NULL) {
const status_t err = parcel->writeStrongBinder(ibinderForJavaObject(env, object));
if (err != NO_ERROR) {
jniThrowException(env, "java/lang/OutOfMemoryError", NULL);
}
}
}
ibinderForjavaObject实现如下：
~/Android/frameworks/base/core/jni
----android_util_Binder.cpp
sp<IBinder> ibinderForJavaObject(JNIEnv* env, jobject obj)
{
if (obj == NULL) return NULL;
if (env->IsInstanceOf(obj, gBinderOffsets.mClass)) {
JavaBBinderHolder* jbh = (JavaBBinderHolder*)
env->GetIntField(obj, gBinderOffsets.mObject);//这里把obj对象的mObject成员变量强制转为JavaBBinderHolder对象
return jbh != NULL ? jbh->get(env) : NULL;
}
if (env->IsInstanceOf(obj, gBinderProxyOffsets.mClass)) {
return (IBinder*)
env->GetIntField(obj, gBinderProxyOffsets.mObject);
}
LOGW("ibinderForJavaObject: %p is not a Binder object", obj);
return NULL;
}
（1）如果传入的是caller.asBinder()，那么首先生成一个JavaBBinder本地对象。
（2）如果传入的是resultTo，那么生成一个代理对象。
writeStrongBinder实现如下：
～/Android/frameworks/base/libs/binder
----Parcel.cpp
status_t Parcel::writeStrongBinder(const sp<IBinder>& val)
{
return flatten_binder(ProcessState::self(), val, this);
}
status_t flatten_binder(const sp<ProcessState>& proc,
const sp<IBinder>& binder, Parcel* out)
{
flat_binder_object obj;
obj.flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS;
if (binder != NULL) {
IBinder *local = binder->localBinder();
if (!local) {
BpBinder *proxy = binder->remoteBinder();
if (proxy == NULL) {
LOGE("null proxy");
}
const int32_t handle = proxy ? proxy->handle() : 0;
obj.type = BINDER_TYPE_HANDLE;
obj.handle = handle;
obj.cookie = NULL;
} else {
obj.type = BINDER_TYPE_BINDER;
obj.binder = local->getWeakRefs();
obj.cookie = local;
}
} else {
obj.type = BINDER_TYPE_BINDER;
obj.binder = NULL;
obj.cookie = NULL;
}
return finish_flatten_binder(binder, obj, out);
}
（1）如果是本地对象，obj.cookie为本地对象IBinder地址。
（2）如果是代理对象，obj.handle为代理对象的句柄值。
如图：第二步
Binder Driver：调用binder_transaction:
～/Android//kernel/goldfish/drivers/staging/android
----binder.c
case BINDER_TYPE_BINDER:
case BINDER_TYPE_WEAK_BINDER: {
struct binder_ref *ref;
struct binder_node *node = binder_get_node(proc, fp->binder);
if (node == NULL) {
node = binder_new_node(proc, fp->binder, fp->cookie);
......
}
.......
ref = binder_get_ref_for_node(target_proc, node);
if (ref == NULL) {
return_error = BR_FAILED_REPLY;
goto err_binder_get_ref_for_node_failed;
}
if (fp->type == BINDER_TYPE_BINDER)
fp->type = BINDER_TYPE_HANDLE;
else
fp->type = BINDER_TYPE_WEAK_HANDLE;
fp->handle = ref->desc;
......
} break;
case BINDER_TYPE_HANDLE:
case BINDER_TYPE_WEAK_HANDLE: {
struct binder_ref *ref = binder_get_ref(proc, fp->handle);
......
if (ref->node->proc == target_proc) {
if (fp->type == BINDER_TYPE_HANDLE)
fp->type = BINDER_TYPE_BINDER;
else
fp->type = BINDER_TYPE_WEAK_BINDER;
fp->binder = ref->node->ptr;
fp->cookie = ref->node->cookie;
binder_inc_node(ref->node, fp->type == BINDER_TYPE_BINDER, 0, NULL);
if (binder_debug_mask & BINDER_DEBUG_TRANSACTION)
printk(KERN_INFO " ref %d desc %d -> node %d u%p\n",
ref->debug_id, ref->desc, ref->node->debug_id, ref->node->ptr);
} else {
.......
}
} break; （1）如果是BINDER_TYPE_BINDER，首先创建实体对象，再创建引用对象。handle为引用句柄值。
（2）如果是BINDER_TYPE_HANDLE，首先获取引用对象，再获取实体对象，cookie为本地对象IBinder的地址。
如图：第三步
~/Android/frameworks/base/core/java/android/app
----ActivityManagerNative.java
public abstract class ActivityManagerNative extends Binder implements IActivityManager
{
......
public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
throws RemoteException {
switch (code) {
case START_ACTIVITY_TRANSACTION:
{
data.enforceInterface(IActivityManager.descriptor);
IBinder b = data.readStrongBinder();
<span style="font-size:14px;">IApplicationThread app = ApplicationThreadNative.asInterface(b);</span>
Intent intent = Intent.CREATOR.createFromParcel(data);
String resolvedType = data.readString();
......
IBinder resultTo = data.readStrongBinder();
......
return true;
}
～/Android/frameworks/base/core/jni
----android_util_Binder.cpp
static jobject android_os_Parcel_readStrongBinder(JNIEnv* env, jobject clazz)
{
Parcel* parcel = parcelForJavaObject(env, clazz);//获得Java层reply的引用
if (parcel != NULL) {
return javaObjectForIBinder(env, parcel->readStrongBinder());
}
return NULL;
}
~/Android/frameworks/base/libs/binder
----Parcel.cpp
sp<IBinder> Parcel::readStrongBinder() const
{
sp<IBinder> val;
unflatten_binder(ProcessState::self(), *this, &val);
return val;
}
status_t unflatten_binder(const sp<ProcessState>& proc,
const Parcel& in, sp<IBinder>* out)
{
const flat_binder_object* flat = in.readObject(false);
if (flat) {
switch (flat->type) {
case BINDER_TYPE_BINDER:
*out = static_cast<IBinder*>(flat->cookie);
return finish_unflatten_binder(NULL, *flat, in);
case BINDER_TYPE_HANDLE:
*out = proc->getStrongProxyForHandle(flat->handle);
return finish_unflatten_binder(
static_cast<BpBinder*>(out->get()), *flat, in);
}
}
return BAD_TYPE;
}
（1）如果是BINDER_TYPE_BINDER，返回本地对象。
（2）如果是BINDER_TYPE_HANDLE，根据句柄值，返回代理对象。
然后执行javaObjectForIBinder。
~/Android/frameworks/base/core/jni ----android_util_Binder.cpp
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();
........
return object;
}
// For the rest of the function we will hold this lock, to serialize
// looking/creation of Java proxies for native Binder proxies.
AutoMutex _l(mProxyLock);
// Someone else's... do we know about it?
jobject object = (jobject)val->findObject(&gBinderProxyOffsets);//检查当前进程之前是否已经为它创建过一个BinderProxy对象
if (object != NULL) {//如果有返回来的就是一个指向该BinderProxy对象的WeakReference对象object,即一个弱引用对象
jobject res = env->CallObjectMethod(object, gWeakReferenceOffsets.mGet);//由于弱引用对象object所指向的BinderProxy对象可能已经失效，因此，需要检查它的有效性，方法是调用它的成员函数get来获得一个强引用对象。
if (res != NULL) {//如果不为NULL
......
return res;//直接返回
}
.....
android_atomic_dec(&gNumProxyRefs);//如果为NULL
val->detachObject(&gBinderProxyOffsets);//解除它与一个无效的BinderProxy对象的对应关系
env->DeleteGlobalRef(object);//删除弱引用对象的全局引用
}
object = env->NewObject(gBinderProxyOffsets.mClass, gBinderProxyOffsets.mConstructor);//创建一个BinderProxy对象
if (object != NULL) {
.......
env->SetIntField(object, gBinderProxyOffsets.mObject, (int)val.get());//BinderProxy.mObject成员变量记录了这个BpBinder对象的地址
val->incStrong(object);
// 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));//获取BinderProxy内部的成员变量mSelf(BinderProxy的弱引用对象)，接着再创建一个全局引用对象来引用它
val->attachObject(&gBinderProxyOffsets, refObject,
jnienv_to_javavm(env), proxy_cleanup);//把它放到BpBinder里面去，下次就要使用时，就可以在上一步调用BpBinder::findObj把它找回来了
// Note that a new object reference has been created.
android_atomic_inc(&gNumProxyRefs);
incRefsCreated(env);
}
return object;
}
（1）如果是本地对象，首先向下转型为JavaBBinder，然后取得ActivityRecord对象，它继承了IApplicationToken.Stub。而IApplicationToken.Stub继承Binder，实现了IApplicationToken。所以可以向上转型为IBinder。
IBinder resultTo = data.readStrongBinder();
（2）如果是代理对象，首先生成BinderProxy对象，里面的mObject指向代理对象，向上转型为IBinder。
IBinder b = data.readStrongBinder();
IApplicationThread app = ApplicationThreadNative.asInterface(b); 然后生成ActivityManagerProxy对象，里面mRemote指向BinderProxy对象。
如图：第四步
～/Android/frameworks/base/services/java/com/android/server/am
----ActivityManagerService.java
public final int startActivity(IApplicationThread caller,
Intent intent, String resolvedType, Uri[] grantedUriPermissions,
int grantedMode, IBinder resultTo,
String resultWho, int requestCode, boolean onlyIfNeeded,
boolean debug) {
return mMainStack.startActivityMayWait(caller, intent, resolvedType,
grantedUriPermissions, grantedMode, resultTo, resultWho,
requestCode, onlyIfNeeded, debug, null, null);
}
待续