探索Android中的Parcel机制（上）

2022-09-27 08:42:27

一．先从Serialize说起

我们都知道JAVA中的Serialize机制，译成串行化、序列化……，其作用是能将数据对象存入字节流其中，在须要时又一次生成对象。主要应用是利用外部存储设备保存对象状态，以及通过网络传输对象等。

二．Android中的新的序列化机制

在Android系统中，定位为针对内存受限的设备，因此对性能要求更高，另外系统中採用了新的IPC（进程间通信）机制，必定要求使用性能更出色的对象传输方式。在这种环境下，Parcel被设计出来，其定位就是轻量级的高效的对象序列化和反序列化机制。

三．Parcel类的背后

在Framework中有parcel类，源代码路径是：

Frameworks/base/core/java/android/os/Parcel.java

典型的源代码片断例如以下：

/** * Write an integer value into the parcel at the current dataPosition(), * growing dataCapacity() if needed. */ public final native void writeInt(int val); /** * Write a long integer value into the parcel at the current dataPosition(), * growing dataCapacity() if needed. */ public final native void writeLong(long val);

从中我们看到，从这个源程序文件里我们看不到真正的功能是怎样实现的，必须透过JNI往下走了。于是，Frameworks/base/core/jni/android_util_Binder.cpp中找到了线索

static void android_os_Parcel_writeInt(JNIEnv* env, jobject clazz, jint val) { Parcel* parcel = parcelForJavaObject(env, clazz); if (parcel != NULL) { const status_t err = parcel->writeInt32(val); if (err != NO_ERROR) { jniThrowException(env, "java/lang/OutOfMemoryError", NULL); } } } static void android_os_Parcel_writeLong(JNIEnv* env, jobject clazz, jlong val) { Parcel* parcel = parcelForJavaObject(env, clazz); if (parcel != NULL) { const status_t err = parcel->writeInt64(val); if (err != NO_ERROR) { jniThrowException(env, "java/lang/OutOfMemoryError", NULL); } } }

从这里我们能够得到的信息是函数的实现依赖于Parcel指针，因此还须要找到Parcel的类定义，注意，这里的类已经是用C++语言实现的了。

找到Frameworks/base/include/binder/parcel.h和Frameworks/base/libs/binder/parcel.cpp。最终找到了最终的实现代码了。

有兴趣的朋友能够自己读一下，不难理解，这里把主要的思路总结一下：

1. 整个读写全是在内存中进行，主要是通过malloc()、realloc()、memcpy()等内存操作进行，所以效率比JAVA序列化中使用外部存储器会高非常多；

2. 读写时是4字节对齐的，能够看到#define PAD_SIZE(s) (((s)+3)&~3)这句宏定义就是在做这件事情；

3. 假设预分配的空间不够时newSize = ((mDataSize+len)*3)/2;会一次多分配50%；

4. 对于普通数据，使用的是mData内存地址，对于IBinder类型的数据以及FileDescriptor使用的是mObjects内存地址。后者是通过flatten_binder()和unflatten_binder()实现的，目的是反序列化时读出的对象就是原对象而不用又一次new一个新对象。

好了，这就是Parcel背后的动作，全是在一块内存里进行读写操作，就不啰嗦了，把parcel的代码贴在这供没有源代码的朋友參考吧。接下来我会用一个小DEMO演示一下Parcel类在应用程序中的使用，详见《探索Android中的Parcel机制（下）》。

/* * 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 "Parcel" //#define LOG_NDEBUG 0 #include <binder/Parcel.h> #include <binder/Binder.h> #include <binder/BpBinder.h> #include <utils/Debug.h> #include <binder/ProcessState.h> #include <utils/Log.h> #include <utils/String8.h> #include <utils/String16.h> #include <utils/TextOutput.h> #include <utils/misc.h> #include <utils/Flattenable.h> #include <private/binder/binder_module.h> #include <stdio.h> #include <stdlib.h> #include <stdint.h> #ifndef INT32_MAX #define INT32_MAX ((int32_t)(2147483647)) #endif #define LOG_REFS(...) //#define LOG_REFS(...) LOG(LOG_DEBUG, "Parcel", __VA_ARGS__) // --------------------------------------------------------------------------- #define PAD_SIZE(s) (((s)+3)&~3) // XXX This can be made public if we want to provide // support for typed data. struct small_flat_data { uint32_t type; uint32_t data; }; namespace android { void acquire_object(const sp<ProcessState>& proc, const flat_binder_object& obj, const void* who) { switch (obj.type) { case BINDER_TYPE_BINDER: if (obj.binder) { LOG_REFS("Parcel %p acquiring reference on local %p", who, obj.cookie); static_cast<IBinder*>(obj.cookie)->incStrong(who); } return; case BINDER_TYPE_WEAK_BINDER: if (obj.binder) static_cast<RefBase::weakref_type*>(obj.binder)->incWeak(who); return; case BINDER_TYPE_HANDLE: { const sp<IBinder> b = proc->getStrongProxyForHandle(obj.handle); if (b != NULL) { LOG_REFS("Parcel %p acquiring reference on remote %p", who, b.get()); b->incStrong(who); } return; } case BINDER_TYPE_WEAK_HANDLE: { const wp<IBinder> b = proc->getWeakProxyForHandle(obj.handle); if (b != NULL) b.get_refs()->incWeak(who); return; } case BINDER_TYPE_FD: { // intentionally blank -- nothing to do to acquire this, but we do // recognize it as a legitimate object type. return; } } LOGD("Invalid object type 0x%08lx", obj.type); } void release_object(const sp<ProcessState>& proc, const flat_binder_object& obj, const void* who) { switch (obj.type) { case BINDER_TYPE_BINDER: if (obj.binder) { LOG_REFS("Parcel %p releasing reference on local %p", who, obj.cookie); static_cast<IBinder*>(obj.cookie)->decStrong(who); } return; case BINDER_TYPE_WEAK_BINDER: if (obj.binder) static_cast<RefBase::weakref_type*>(obj.binder)->decWeak(who); return; case BINDER_TYPE_HANDLE: { const sp<IBinder> b = proc->getStrongProxyForHandle(obj.handle); if (b != NULL) { LOG_REFS("Parcel %p releasing reference on remote %p", who, b.get()); b->decStrong(who); } return; } case BINDER_TYPE_WEAK_HANDLE: { const wp<IBinder> b = proc->getWeakProxyForHandle(obj.handle); if (b != NULL) b.get_refs()->decWeak(who); return; } case BINDER_TYPE_FD: { if (obj.cookie != (void*)0) close(obj.handle); return; } } LOGE("Invalid object type 0x%08lx", obj.type); } inline static status_t finish_flatten_binder( const sp<IBinder>& binder, const flat_binder_object& flat, Parcel* out) { return out->writeObject(flat, false); } 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); } status_t flatten_binder(const sp<ProcessState>& proc, const wp<IBinder>& binder, Parcel* out) { flat_binder_object obj; obj.flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS; if (binder != NULL) { sp<IBinder> real = binder.promote(); if (real != NULL) { IBinder *local = real->localBinder(); if (!local) { BpBinder *proxy = real->remoteBinder(); if (proxy == NULL) { LOGE("null proxy"); } const int32_t handle = proxy ? proxy->handle() : 0; obj.type = BINDER_TYPE_WEAK_HANDLE; obj.handle = handle; obj.cookie = NULL; } else { obj.type = BINDER_TYPE_WEAK_BINDER; obj.binder = binder.get_refs(); obj.cookie = binder.unsafe_get(); } return finish_flatten_binder(real, obj, out); } // XXX How to deal? In order to flatten the given binder, // we need to probe it for information, which requires a primary // reference... but we don‘t have one. // // The OpenBinder implementation uses a dynamic_cast<> here, // but we can‘t do that with the different reference counting // implementation we are using. LOGE("Unable to unflatten Binder weak reference!"); obj.type = BINDER_TYPE_BINDER; obj.binder = NULL; obj.cookie = NULL; return finish_flatten_binder(NULL, obj, out); } else { obj.type = BINDER_TYPE_BINDER; obj.binder = NULL; obj.cookie = NULL; return finish_flatten_binder(NULL, obj, out); } } inline static status_t finish_unflatten_binder( BpBinder* proxy, const flat_binder_object& flat, const Parcel& in) { return NO_ERROR; } 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; } status_t unflatten_binder(const sp<ProcessState>& proc, const Parcel& in, wp<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_WEAK_BINDER: if (flat->binder != NULL) { out->set_object_and_refs( static_cast<IBinder*>(flat->cookie), static_cast<RefBase::weakref_type*>(flat->binder)); } else { *out = NULL; } return finish_unflatten_binder(NULL, *flat, in); case BINDER_TYPE_HANDLE: case BINDER_TYPE_WEAK_HANDLE: *out = proc->getWeakProxyForHandle(flat->handle); return finish_unflatten_binder( static_cast<BpBinder*>(out->unsafe_get()), *flat, in); } } return BAD_TYPE; } // --------------------------------------------------------------------------- Parcel::Parcel() { initState(); } Parcel::~Parcel() { freeDataNoInit(); } const uint8_t* Parcel::data() const { return mData; } size_t Parcel::dataSize() const { return (mDataSize > mDataPos ? mDataSize : mDataPos); } size_t Parcel::dataAvail() const { // TODO: decide what to do about the possibility that this can // report an available-data size that exceeds a Java int‘s max // positive value, causing havoc. Fortunately this will only // happen if someone constructs a Parcel containing more than two // gigabytes of data, which on typical phone hardware is simply // not possible. return dataSize() - dataPosition(); } size_t Parcel::dataPosition() const { return mDataPos; } size_t Parcel::dataCapacity() const { return mDataCapacity; } status_t Parcel::setDataSize(size_t size) { status_t err; err = continueWrite(size); if (err == NO_ERROR) { mDataSize = size; LOGV("setDataSize Setting data size of %p to %d/n", this, mDataSize); } return err; } void Parcel::setDataPosition(size_t pos) const { mDataPos = pos; mNextObjectHint = 0; } status_t Parcel::setDataCapacity(size_t size) { if (size > mDataSize) return continueWrite(size); return NO_ERROR; } status_t Parcel::setData(const uint8_t* buffer, size_t len) { status_t err = restartWrite(len); if (err == NO_ERROR) { memcpy(const_cast<uint8_t*>(data()), buffer, len); mDataSize = len; mFdsKnown = false; } return err; } status_t Parcel::appendFrom(Parcel *parcel, size_t offset, size_t len) { const sp<ProcessState> proc(ProcessState::self()); status_t err; uint8_t *data = parcel->mData; size_t *objects = parcel->mObjects; size_t size = parcel->mObjectsSize; int startPos = mDataPos; int firstIndex = -1, lastIndex = -2; if (len == 0) { return NO_ERROR; } // range checks against the source parcel size if ((offset > parcel->mDataSize) || (len > parcel->mDataSize) || (offset + len > parcel->mDataSize)) { return BAD_VALUE; } // Count objects in range for (int i = 0; i < (int) size; i++) { size_t off = objects[i]; if ((off >= offset) && (off < offset + len)) { if (firstIndex == -1) { firstIndex = i; } lastIndex = i; } } int numObjects = lastIndex - firstIndex + 1; // grow data err = growData(len); if (err != NO_ERROR) { return err; } // append data memcpy(mData + mDataPos, data + offset, len); mDataPos += len; mDataSize += len; if (numObjects > 0) { // grow objects if (mObjectsCapacity < mObjectsSize + numObjects) { int newSize = ((mObjectsSize + numObjects)*3)/2; size_t *objects = (size_t*)realloc(mObjects, newSize*sizeof(size_t)); if (objects == (size_t*)0) { return NO_MEMORY; } mObjects = objects; mObjectsCapacity = newSize; } // append and acquire objects int idx = mObjectsSize; for (int i = firstIndex; i <= lastIndex; i++) { size_t off = objects[i] - offset + startPos; mObjects[idx++] = off; mObjectsSize++; flat_binder_object* flat = reinterpret_cast<flat_binder_object*>(mData + off); acquire_object(proc, *flat, this); if (flat->type == BINDER_TYPE_FD) { // If this is a file descriptor, we need to dup it so the // new Parcel now owns its own fd, and can declare that we // officially know we have fds. flat->handle = dup(flat->handle); flat->cookie = (void*)1; mHasFds = mFdsKnown = true; } } } return NO_ERROR; } bool Parcel::hasFileDescriptors() const { if (!mFdsKnown) { scanForFds(); } return mHasFds; } status_t Parcel::writeInterfaceToken(const String16& interface) { // currently the interface identification token is just its name as a string return writeString16(interface); } bool Parcel::checkInterface(IBinder* binder) const { return enforceInterface(binder->getInterfaceDescriptor()); } bool Parcel::enforceInterface(const String16& interface) const { const String16 str(readString16()); if (str == interface) { return true; } else { LOGW("**** enforceInterface() expected ‘%s‘ but read ‘%s‘/n", String8(interface).string(), String8(str).string()); return false; } } const size_t* Parcel::objects() const { return mObjects; } size_t Parcel::objectsCount() const { return mObjectsSize; } status_t Parcel::errorCheck() const { return mError; } void Parcel::setError(status_t err) { mError = err; } status_t Parcel::finishWrite(size_t len) { //printf("Finish write of %d/n", len); mDataPos += len; LOGV("finishWrite Setting data pos of %p to %d/n", this, mDataPos); if (mDataPos > mDataSize) { mDataSize = mDataPos; LOGV("finishWrite Setting data size of %p to %d/n", this, mDataSize); } //printf("New pos=%d, size=%d/n", mDataPos, mDataSize); return NO_ERROR; } status_t Parcel::writeUnpadded(const void* data, size_t len) { size_t end = mDataPos + len; if (end < mDataPos) { // integer overflow return BAD_VALUE; } if (end <= mDataCapacity) { restart_write: memcpy(mData+mDataPos, data, len); return finishWrite(len); } status_t err = growData(len); if (err == NO_ERROR) goto restart_write; return err; } status_t Parcel::write(const void* data, size_t len) { void* const d = writeInplace(len); if (d) { memcpy(d, data, len); return NO_ERROR; } return mError; } void* Parcel::writeInplace(size_t len) { const size_t padded = PAD_SIZE(len); // sanity check for integer overflow if (mDataPos+padded < mDataPos) { return NULL; } if ((mDataPos+padded) <= mDataCapacity) { restart_write: //printf("Writing %ld bytes, padded to %ld/n", len, padded); uint8_t* const data = mData+mDataPos; // Need to pad at end? if (padded != len) { #if BYTE_ORDER == BIG_ENDIAN static const uint32_t mask[4] = { 0x00000000, 0xffffff00, 0xffff0000, 0xff000000 }; #endif #if BYTE_ORDER == LITTLE_ENDIAN static const uint32_t mask[4] = { 0x00000000, 0x00ffffff, 0x0000ffff, 0x000000ff }; #endif //printf("Applying pad mask: %p to %p/n", (void*)mask[padded-len], // *reinterpret_cast<void**>(data+padded-4)); *reinterpret_cast<uint32_t*>(data+padded-4) &= mask[padded-len]; } finishWrite(padded); return data; } status_t err = growData(padded); if (err == NO_ERROR) goto restart_write; return NULL; } status_t Parcel::writeInt32(int32_t val) { return writeAligned(val); } status_t Parcel::writeInt64(int64_t val) { return writeAligned(val); } status_t Parcel::writeFloat(float val) { return writeAligned(val); } status_t Parcel::writeDouble(double val) { return writeAligned(val); } status_t Parcel::writeIntPtr(intptr_t val) { return writeAligned(val); } status_t Parcel::writeCString(const char* str) { return write(str, strlen(str)+1); } status_t Parcel::writeString8(const String8& str) { status_t err = writeInt32(str.bytes()); if (err == NO_ERROR) { err = write(str.string(), str.bytes()+1); } return err; } status_t Parcel::writeString16(const String16& str) { return writeString16(str.string(), str.size()); } status_t Parcel::writeString16(const char16_t* str, size_t len) { if (str == NULL) return writeInt32(-1); status_t err = writeInt32(len); if (err == NO_ERROR) { len *= sizeof(char16_t); uint8_t* data = (uint8_t*)writeInplace(len+sizeof(char16_t)); if (data) { memcpy(data, str, len); *reinterpret_cast<char16_t*>(data+len) = 0; return NO_ERROR; } err = mError; } return err; } status_t Parcel::writeStrongBinder(const sp<IBinder>& val) { return flatten_binder(ProcessState::self(), val, this); } status_t Parcel::writeWeakBinder(const wp<IBinder>& val) { return flatten_binder(ProcessState::self(), val, this); } status_t Parcel::writeNativeHandle(const native_handle* handle) { if (!handle || handle->version != sizeof(native_handle)) return BAD_TYPE; status_t err; err = writeInt32(handle->numFds); if (err != NO_ERROR) return err; err = writeInt32(handle->numInts); if (err != NO_ERROR) return err; for (int i=0 ; err==NO_ERROR && i<handle->numFds ; i++) err = writeDupFileDescriptor(handle->data[i]); if (err != NO_ERROR) { LOGD("write native handle, write dup fd failed"); return err; } err = write(handle->data + handle->numFds, sizeof(int)*handle->numInts); return err; } status_t Parcel::writeFileDescriptor(int fd) { flat_binder_object obj; obj.type = BINDER_TYPE_FD; obj.flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS; obj.handle = fd; obj.cookie = (void*)0; return writeObject(obj, true); } status_t Parcel::writeDupFileDescriptor(int fd) { flat_binder_object obj; obj.type = BINDER_TYPE_FD; obj.flags = 0x7f | FLAT_BINDER_FLAG_ACCEPTS_FDS; obj.handle = dup(fd); obj.cookie = (void*)1; return writeObject(obj, true); } status_t Parcel::write(const Flattenable& val) { status_t err; // size if needed size_t len = val.getFlattenedSize(); size_t fd_count = val.getFdCount(); err = this->writeInt32(len); if (err) return err; err = this->writeInt32(fd_count); if (err) return err; // payload void* buf = this->writeInplace(PAD_SIZE(len)); if (buf == NULL) return BAD_VALUE; int* fds = NULL; if (fd_count) { fds = new int[fd_count]; } err = val.flatten(buf, len, fds, fd_count); for (size_t i=0 ; i<fd_count && err==NO_ERROR ; i++) { err = this->writeDupFileDescriptor( fds[i] ); } if (fd_count) { delete [] fds; } return err; } status_t Parcel::writeObject(const flat_binder_object& val, bool nullMetaData) { const bool enoughData = (mDataPos+sizeof(val)) <= mDataCapacity; const bool enoughObjects = mObjectsSize < mObjectsCapacity; if (enoughData && enoughObjects) { restart_write: *reinterpret_cast<flat_binder_object*>(mData+mDataPos) = val; // Need to write meta-data? if (nullMetaData || val.binder != NULL) { mObjects[mObjectsSize] = mDataPos; acquire_object(ProcessState::self(), val, this); mObjectsSize++; } // remember if it‘s a file descriptor if (val.type == BINDER_TYPE_FD) { mHasFds = mFdsKnown = true; } return finishWrite(sizeof(flat_binder_object)); } if (!enoughData) { const status_t err = growData(sizeof(val)); if (err != NO_ERROR) return err; } if (!enoughObjects) { size_t newSize = ((mObjectsSize+2)*3)/2; size_t* objects = (size_t*)realloc(mObjects, newSize*sizeof(size_t)); if (objects == NULL) return NO_MEMORY; mObjects = objects; mObjectsCapacity = newSize; } goto restart_write; } void Parcel::remove(size_t start, size_t amt) { LOG_ALWAYS_FATAL("Parcel::remove() not yet implemented!"); } status_t Parcel::read(void* outData, size_t len) const { if ((mDataPos+PAD_SIZE(len)) >= mDataPos && (mDataPos+PAD_SIZE(len)) <= mDataSize) { memcpy(outData, mData+mDataPos, len); mDataPos += PAD_SIZE(len); LOGV("read Setting data pos of %p to %d/n", this, mDataPos); return NO_ERROR; } return NOT_ENOUGH_DATA; } const void* Parcel::readInplace(size_t len) const { if ((mDataPos+PAD_SIZE(len)) >= mDataPos && (mDataPos+PAD_SIZE(len)) <= mDataSize) { const void* data = mData+mDataPos; mDataPos += PAD_SIZE(len); LOGV("readInplace Setting data pos of %p to %d/n", this, mDataPos); return data; } return NULL; } template<class T> status_t Parcel::readAligned(T *pArg) const { COMPILE_TIME_ASSERT_FUNCTION_SCOPE(PAD_SIZE(sizeof(T)) == sizeof(T)); if ((mDataPos+sizeof(T)) <= mDataSize) { const void* data = mData+mDataPos; mDataPos += sizeof(T); *pArg = *reinterpret_cast<const T*>(data); return NO_ERROR; } else { return NOT_ENOUGH_DATA; } } template<class T> T Parcel::readAligned() const { T result; if (readAligned(&result) != NO_ERROR) { result = 0; } return result; } template<class T> status_t Parcel::writeAligned(T val) { COMPILE_TIME_ASSERT_FUNCTION_SCOPE(PAD_SIZE(sizeof(T)) == sizeof(T)); if ((mDataPos+sizeof(val)) <= mDataCapacity) { restart_write: *reinterpret_cast<T*>(mData+mDataPos) = val; return finishWrite(sizeof(val)); } status_t err = growData(sizeof(val)); if (err == NO_ERROR) goto restart_write; return err; } status_t Parcel::readInt32(int32_t *pArg) const { return readAligned(pArg); } int32_t Parcel::readInt32() const { return readAligned<int32_t>(); } status_t Parcel::readInt64(int64_t *pArg) const { return readAligned(pArg); } int64_t Parcel::readInt64() const { return readAligned<int64_t>(); } status_t Parcel::readFloat(float *pArg) const { return readAligned(pArg); } float Parcel::readFloat() const { return readAligned<float>(); } status_t Parcel::readDouble(double *pArg) const { return readAligned(pArg); } double Parcel::readDouble() const { return readAligned<double>(); } status_t Parcel::readIntPtr(intptr_t *pArg) const { return readAligned(pArg); } intptr_t Parcel::readIntPtr() const { return readAligned<intptr_t>(); } const char* Parcel::readCString() const { const size_t avail = mDataSize-mDataPos; if (avail > 0) { const char* str = reinterpret_cast<const char*>(mData+mDataPos); // is the string‘s trailing NUL within the parcel‘s valid bounds? const char* eos = reinterpret_cast<const char*>(memchr(str, 0, avail)); if (eos) { const size_t len = eos - str; mDataPos += PAD_SIZE(len+1); LOGV("readCString Setting data pos of %p to %d/n", this, mDataPos); return str; } } return NULL; } String8 Parcel::readString8() const { int32_t size = readInt32(); // watch for potential int overflow adding 1 for trailing NUL if (size > 0 && size < INT32_MAX) { const char* str = (const char*)readInplace(size+1); if (str) return String8(str, size); } return String8(); } String16 Parcel::readString16() const { size_t len; const char16_t* str = readString16Inplace(&len); if (str) return String16(str, len); LOGE("Reading a NULL string not supported here."); return String16(); } const char16_t* Parcel::readString16Inplace(size_t* outLen) const { int32_t size = readInt32(); // watch for potential int overflow from size+1 if (size >= 0 && size < INT32_MAX) { *outLen = size; const char16_t* str = (const char16_t*)readInplace((size+1)*sizeof(char16_t)); if (str != NULL) { return str; } } *outLen = 0; return NULL; } sp<IBinder> Parcel::readStrongBinder() const { sp<IBinder> val; unflatten_binder(ProcessState::self(), *this, &val); return val; } wp<IBinder> Parcel::readWeakBinder() const { wp<IBinder> val; unflatten_binder(ProcessState::self(), *this, &val); return val; } native_handle* Parcel::readNativeHandle() const { int numFds, numInts; status_t err; err = readInt32(&numFds); if (err != NO_ERROR) return 0; err = readInt32(&numInts); if (err != NO_ERROR) return 0; native_handle* h = native_handle_create(numFds, numInts); for (int i=0 ; err==NO_ERROR && i<numFds ; i++) { h->data[i] = dup(readFileDescriptor()); if (h->data[i] < 0) err = BAD_VALUE; } err = read(h->data + numFds, sizeof(int)*numInts); if (err != NO_ERROR) { native_handle_close(h); native_handle_delete(h); h = 0; } return h; } int Parcel::readFileDescriptor() const { const flat_binder_object* flat = readObject(true); if (flat) { switch (flat->type) { case BINDER_TYPE_FD: //LOGI("Returning file descriptor %ld from parcel %p/n", flat->handle, this); return flat->handle; } } return BAD_TYPE; } status_t Parcel::read(Flattenable& val) const { // size const size_t len = this->readInt32(); const size_t fd_count = this->readInt32(); // payload void const* buf = this->readInplace(PAD_SIZE(len)); if (buf == NULL) return BAD_VALUE; int* fds = NULL; if (fd_count) { fds = new int[fd_count]; } status_t err = NO_ERROR; for (size_t i=0 ; i<fd_count && err==NO_ERROR ; i++) { fds[i] = dup(this->readFileDescriptor()); if (fds[i] < 0) err = BAD_VALUE; } if (err == NO_ERROR) { err = val.unflatten(buf, len, fds, fd_count); } if (fd_count) { delete [] fds; } return err; } const flat_binder_object* Parcel::readObject(bool nullMetaData) const { const size_t DPOS = mDataPos; if ((DPOS+sizeof(flat_binder_object)) <= mDataSize) { const flat_binder_object* obj = reinterpret_cast<const flat_binder_object*>(mData+DPOS); mDataPos = DPOS + sizeof(flat_binder_object); if (!nullMetaData && (obj->cookie == NULL && obj->binder == NULL)) { // When transferring a NULL object, we don‘t write it into // the object list, so we don‘t want to check for it when // reading. LOGV("readObject Setting data pos of %p to %d/n", this, mDataPos); return obj; } // Ensure that this object is valid... size_t* const OBJS = mObjects; const size_t N = mObjectsSize; size_t opos = mNextObjectHint; if (N > 0) { LOGV("Parcel %p looking for obj at %d, hint=%d/n", this, DPOS, opos); // Start at the current hint position, looking for an object at // the current data position. if (opos < N) { while (opos < (N-1) && OBJS[opos] < DPOS) { opos++; } } else { opos = N-1; } if (OBJS[opos] == DPOS) { // Found it! LOGV("Parcel found obj %d at index %d with forward search", this, DPOS, opos); mNextObjectHint = opos+1; LOGV("readObject Setting data pos of %p to %d/n", this, mDataPos); return obj; } // Look backwards for it... while (opos > 0 && OBJS[opos] > DPOS) { opos--; } if (OBJS[opos] == DPOS) { // Found it! LOGV("Parcel found obj %d at index %d with backward search", this, DPOS, opos); mNextObjectHint = opos+1; LOGV("readObject Setting data pos of %p to %d/n", this, mDataPos); return obj; } } LOGW("Attempt to read object from Parcel %p at offset %d that is not in the object list", this, DPOS); } return NULL; } void Parcel::closeFileDescriptors() { size_t i = mObjectsSize; if (i > 0) { //LOGI("Closing file descriptors for %d objects...", mObjectsSize); } while (i > 0) { i--; const flat_binder_object* flat = reinterpret_cast<flat_binder_object*>(mData+mObjects[i]); if (flat->type == BINDER_TYPE_FD) { //LOGI("Closing fd: %ld/n", flat->handle); close(flat->handle); } } } const uint8_t* Parcel::ipcData() const { return mData; } size_t Parcel::ipcDataSize() const { return (mDataSize > mDataPos ? mDataSize : mDataPos); } const size_t* Parcel::ipcObjects() const { return mObjects; } size_t Parcel::ipcObjectsCount() const { return mObjectsSize; } void Parcel::ipcSetDataReference(const uint8_t* data, size_t dataSize, const size_t* objects, size_t objectsCount, release_func relFunc, void* relCookie) { freeDataNoInit(); mError = NO_ERROR; mData = const_cast<uint8_t*>(data); mDataSize = mDataCapacity = dataSize; //LOGI("setDataReference Setting data size of %p to %lu (pid=%d)/n", this, mDataSize, getpid()); mDataPos = 0; LOGV("setDataReference Setting data pos of %p to %d/n", this, mDataPos); mObjects = const_cast<size_t*>(objects); mObjectsSize = mObjectsCapacity = objectsCount; mNextObjectHint = 0; mOwner = relFunc; mOwnerCookie = relCookie; scanForFds(); } void Parcel::print(TextOutput& to, uint32_t flags) const { to << "Parcel("; if (errorCheck() != NO_ERROR) { const status_t err = errorCheck(); to << "Error: " << (void*)err << " /"" << strerror(-err) << "/""; } else if (dataSize() > 0) { const uint8_t* DATA = data(); to << indent << HexDump(DATA, dataSize()) << dedent; const size_t* OBJS = objects(); const size_t N = objectsCount(); for (size_t i=0; i<N; i++) { const flat_binder_object* flat = reinterpret_cast<const flat_binder_object*>(DATA+OBJS[i]); to << endl << "Object #" << i << " @ " << (void*)OBJS[i] << ": " << TypeCode(flat->type & 0x7f7f7f00) << " = " << flat->binder; } } else { to << "NULL"; } to << ")"; } void Parcel::releaseObjects() { const sp<ProcessState> proc(ProcessState::self()); size_t i = mObjectsSize; uint8_t* const data = mData; size_t* const objects = mObjects; while (i > 0) { i--; const flat_binder_object* flat = reinterpret_cast<flat_binder_object*>(data+objects[i]); release_object(proc, *flat, this); } } void Parcel::acquireObjects() { const sp<ProcessState> proc(ProcessState::self()); size_t i = mObjectsSize; uint8_t* const data = mData; size_t* const objects = mObjects; while (i > 0) { i--; const flat_binder_object* flat = reinterpret_cast<flat_binder_object*>(data+objects[i]); acquire_object(proc, *flat, this); } } void Parcel::freeData() { freeDataNoInit(); initState(); } void Parcel::freeDataNoInit() { if (mOwner) { //LOGI("Freeing data ref of %p (pid=%d)/n", this, getpid()); mOwner(this, mData, mDataSize, mObjects, mObjectsSize, mOwnerCookie); } else { releaseObjects(); if (mData) free(mData); if (mObjects) free(mObjects); } } status_t Parcel::growData(size_t len) { size_t newSize = ((mDataSize+len)*3)/2; return (newSize <= mDataSize) ? (status_t) NO_MEMORY : continueWrite(newSize); } status_t Parcel::restartWrite(size_t desired) { if (mOwner) { freeData(); return continueWrite(desired); } uint8_t* data = (uint8_t*)realloc(mData, desired); if (!data && desired > mDataCapacity) { mError = NO_MEMORY; return NO_MEMORY; } releaseObjects(); if (data) { mData = data; mDataCapacity = desired; } mDataSize = mDataPos = 0; LOGV("restartWrite Setting data size of %p to %d/n", this, mDataSize); LOGV("restartWrite Setting data pos of %p to %d/n", this, mDataPos); free(mObjects); mObjects = NULL; mObjectsSize = mObjectsCapacity = 0; mNextObjectHint = 0; mHasFds = false; mFdsKnown = true; return NO_ERROR; } status_t Parcel::continueWrite(size_t desired) { // If shrinking, first adjust for any objects that appear // after the new data size. size_t objectsSize = mObjectsSize; if (desired < mDataSize) { if (desired == 0) { objectsSize = 0; } else { while (objectsSize > 0) { if (mObjects[objectsSize-1] < desired) break; objectsSize--; } } } if (mOwner) { // If the size is going to zero, just release the owner‘s data. if (desired == 0) { freeData(); return NO_ERROR; } // If there is a different owner, we need to take // posession. uint8_t* data = (uint8_t*)malloc(desired); if (!data) { mError = NO_MEMORY; return NO_MEMORY; } size_t* objects = NULL; if (objectsSize) { objects = (size_t*)malloc(objectsSize*sizeof(size_t)); if (!objects) { mError = NO_MEMORY; return NO_MEMORY; } // Little hack to only acquire references on objects // we will be keeping. size_t oldObjectsSize = mObjectsSize; mObjectsSize = objectsSize; acquireObjects(); mObjectsSize = oldObjectsSize; } if (mData) { memcpy(data, mData, mDataSize < desired ? mDataSize : desired); } if (objects && mObjects) { memcpy(objects, mObjects, objectsSize*sizeof(size_t)); } //LOGI("Freeing data ref of %p (pid=%d)/n", this, getpid()); mOwner(this, mData, mDataSize, mObjects, mObjectsSize, mOwnerCookie); mOwner = NULL; mData = data; mObjects = objects; mDataSize = (mDataSize < desired) ? mDataSize : desired; LOGV("continueWrite Setting data size of %p to %d/n", this, mDataSize); mDataCapacity = desired; mObjectsSize = mObjectsCapacity = objectsSize; mNextObjectHint = 0; } else if (mData) { if (objectsSize < mObjectsSize) { // Need to release refs on any objects we are dropping. const sp<ProcessState> proc(ProcessState::self()); for (size_t i=objectsSize; i<mObjectsSize; i++) { const flat_binder_object* flat = reinterpret_cast<flat_binder_object*>(mData+mObjects[i]); if (flat->type == BINDER_TYPE_FD) { // will need to rescan because we may have lopped off the only FDs mFdsKnown = false; } release_object(proc, *flat, this); } size_t* objects = (size_t*)realloc(mObjects, objectsSize*sizeof(size_t)); if (objects) { mObjects = objects; } mObjectsSize = objectsSize; mNextObjectHint = 0; } // We own the data, so we can just do a realloc(). if (desired > mDataCapacity) { uint8_t* data = (uint8_t*)realloc(mData, desired); if (data) { mData = data; mDataCapacity = desired; } else if (desired > mDataCapacity) { mError = NO_MEMORY; return NO_MEMORY; } } else { mDataSize = desired; LOGV("continueWrite Setting data size of %p to %d/n", this, mDataSize); if (mDataPos > desired) { mDataPos = desired; LOGV("continueWrite Setting data pos of %p to %d/n", this, mDataPos); } } } else { // This is the first data. Easy! uint8_t* data = (uint8_t*)malloc(desired); if (!data) { mError = NO_MEMORY; return NO_MEMORY; } if(!(mDataCapacity == 0 && mObjects == NULL && mObjectsCapacity == 0)) { LOGE("continueWrite: %d/%p/%d/%d", mDataCapacity, mObjects, mObjectsCapacity, desired); } mData = data; mDataSize = mDataPos = 0; LOGV("continueWrite Setting data size of %p to %d/n", this, mDataSize); LOGV("continueWrite Setting data pos of %p to %d/n", this, mDataPos); mDataCapacity = desired; } return NO_ERROR; } void Parcel::initState() { mError = NO_ERROR; mData = 0; mDataSize = 0; mDataCapacity = 0; mDataPos = 0; LOGV("initState Setting data size of %p to %d/n", this, mDataSize); LOGV("initState Setting data pos of %p to %d/n", this, mDataPos); mObjects = NULL; mObjectsSize = 0; mObjectsCapacity = 0; mNextObjectHint = 0; mHasFds = false; mFdsKnown = true; mOwner = NULL; } void Parcel::scanForFds() const { bool hasFds = false; for (size_t i=0; i<mObjectsSize; i++) { const flat_binder_object* flat = reinterpret_cast<const flat_binder_object*>(mData + mObjects[i]); if (flat->type == BINDER_TYPE_FD) { hasFds = true; break; } } mHasFds = hasFds; mFdsKnown = true; } }; // namespace android

本文的源代码使用的是Android 2.1版本号。

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探索Android中的Parcel机制（上）,布布扣,bubuko.com

探索Android中的Parcel机制（上）

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