Android源码之Parcel

Parcel的意思是“打包”。在Android中，Parcel的作用类似于Java中的serialize，即对象的序列化。Android系统定位为内存受限的设备，因此对性能要求更高，另外系统中采用了新的IPC机制，必然要求使用性能更出色的对象传输方式。在这样的环境下，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。

Parcel的最主要作用是在各个activity之间传递数据。Android开发中，很经常在各activity之间传递数据，而跟据Android的设计架构，即使同一个程序中的Activity都不一定运行在同一个进程中，所以处理数据传递时你不能老假设两个activity都运行于同一进程，那么只能按进程间传递数据来处理，使之具有最广泛的适应性。

Parcel的接口主要有以下几类：

一、Parcel的获取与释放

public static Parcel obtain() {final Parcel[] pool = sOwnedPool;synchronized (pool) {Parcel p;for (int i=0; i<POOL_SIZE; i++) {p = pool[i];if (p != null) {pool[i] = null;if (DEBUG_RECYCLE) {p.mStack = new RuntimeException();}return p;}}}return new Parcel(0);}public final void recycle() {if (DEBUG_RECYCLE) mStack = null;freeBuffer();final Parcel[] pool = mOwnObject != 0 ? sOwnedPool : sHolderPool;synchronized (pool) {for (int i=0; i<POOL_SIZE; i++) {if (pool[i] == null) {pool[i] = this;return;}}}}

获取接口obtain是从一个已分配的Parcel池中取得一个Parcel，即sOwnedPool，其定义如下：

private static final int POOL_SIZE = 6;private static final Parcel[] sOwnedPool = new Parcel[POOL_SIZE];private static final Parcel[] sHolderPool = new Parcel[POOL_SIZE];

当使用完获得的Parcel后，最好使用recycle将其释放回池，以便下次使用，否则导致Parcel池无可分配资源而new新对象。幸运的是，eclipse会给出此类的警告提示。

二、Parcel内部指针的移动

public final native int dataSize();public final native int dataAvail();public final native int dataPosition();public final native int dataCapacity();public final native void setDataSize(int size);public final native void setDataPosition(int pos);public final native void setDataCapacity(int size);

由于Parcel的读写是直接在内存中的，所以这几个接口就是指示当前的读写指针，类似于Linux下的文件读写操作：

dataSize：返回当前内存块中总共的数据

dataPosition：返回当前读操作指针

dataAvail：返回当前可读的数据，即dataSize - dataPosition

dataCapacity：返回当前内存块的容量（在不重新分配内存的前提下），dataCapacity - dataSize即为可写入的数据量

三、数据读写类

1、序列化与反序列化

public final native byte[] marshall();
public final native void unmarshall(byte[] data, int offest, int length);

作用类似于序列化和反序列化。即将当前Parcel的数据序列化为byte数组，或者将byte数组反序列化到当前Parcel中。

注：unmarshall后，如果要读取数据，首先需要将文件指针移动到初始化位置，即setDataPosition(0)。

2、基本数据类型的读写

private native void writeNative(byte[] b, int offset, int len);
public final native void writeInt(int val);
public final native void writeLong(long val);
public final native void writeFloat(float val);
public final native void writeDouble(double val);
public final native void writeString(String val);
public final native void writeStrongBinder(IBinder val);
public final native void writeFileDescriptor(FileDescriptor val);

public final native int readInt();
public final native long readLong();
public final native float readFloat();
public final native double readDouble();
public final native String readString()；
public final native IBinder readStrongBinder();

全部都是native方法，通过JNI来调用底层实现。其余类型的读写都是在基本数据类型的基础上得来的。如：

public final void writeByte(byte val) {writeInt(val);}public final byte readByte() {return (byte)(readInt() & 0xff);}

可见，byte在Parcel中也占用四个字节。

3、Array类型的读写

Array类型的接口按类型来分，如boolean[]/char[]/int[]/float[]/double[]/String[]，而每一种类型都包含三个接口：

public final void writeXXXArray(boolean[] val)；

public final boolean[] createXXXArray()；

public final void readXXXArray(boolean[] val)；

以boolean类型为例：其源码如下：

public final void writeBooleanArray(boolean[] val) {if (val != null) {int N = val.length;writeInt(N);for (int i=0; i<N; i++) {writeInt(val[i] ? 1 : 0);}} else {writeInt(-1);}}public final boolean[] createBooleanArray() {int N = readInt();// >>2 as a fast divide-by-4 works in the create*Array() functions// because dataAvail() will never return a negative number.  4 is// the size of a stored boolean in the stream.if (N >= 0 && N <= (dataAvail() >> 2)) {boolean[] val = new boolean[N];for (int i=0; i<N; i++) {val[i] = readInt() != 0;}return val;} else {return null;}}public final void readBooleanArray(boolean[] val) {int N = readInt();if (N == val.length) {for (int i=0; i<N; i++) {val[i] = readInt() != 0;}} else {throw new RuntimeException("bad array lengths");}}

可见，其write操作的过程是先以整形的方式写入Array的长度，然后依次写入Array的每一个元素。而读操作的过程则正好相反。

注：从以上示例看出，boolean类型在Parcel中也占据四个字节。

4、通用接口

public final void writeValue(Object v) {if (v == null) {writeInt(VAL_NULL);} else if (v instanceof String) {writeInt(VAL_STRING);writeString((String) v);} else if (v instanceof Integer) {writeInt(VAL_INTEGER);writeInt((Integer) v);} else if (v instanceof Map) {writeInt(VAL_MAP);writeMap((Map) v);} else if (v instanceof Bundle) {// Must be before ParcelablewriteInt(VAL_BUNDLE);writeBundle((Bundle) v);} else if (v instanceof Parcelable) {writeInt(VAL_PARCELABLE);writeParcelable((Parcelable) v, 0);} else if (v instanceof Short) {writeInt(VAL_SHORT);writeInt(((Short) v).intValue());} else if (v instanceof Long) {writeInt(VAL_LONG);writeLong((Long) v);} else if (v instanceof Float) {writeInt(VAL_FLOAT);writeFloat((Float) v);} else if (v instanceof Double) {writeInt(VAL_DOUBLE);writeDouble((Double) v);} else if (v instanceof Boolean) {writeInt(VAL_BOOLEAN);writeInt((Boolean) v ? 1 : 0);} else if (v instanceof CharSequence) {// Must be after StringwriteInt(VAL_CHARSEQUENCE);writeCharSequence((CharSequence) v);} else if (v instanceof List) {writeInt(VAL_LIST);writeList((List) v);} else if (v instanceof SparseArray) {writeInt(VAL_SPARSEARRAY);writeSparseArray((SparseArray) v);} else if (v instanceof boolean[]) {writeInt(VAL_BOOLEANARRAY);writeBooleanArray((boolean[]) v);} else if (v instanceof byte[]) {writeInt(VAL_BYTEARRAY);writeByteArray((byte[]) v);} else if (v instanceof String[]) {writeInt(VAL_STRINGARRAY);writeStringArray((String[]) v);} else if (v instanceof CharSequence[]) {// Must be after String[] and before Object[]writeInt(VAL_CHARSEQUENCEARRAY);writeCharSequenceArray((CharSequence[]) v);} else if (v instanceof IBinder) {writeInt(VAL_IBINDER);writeStrongBinder((IBinder) v);} else if (v instanceof Parcelable[]) {writeInt(VAL_PARCELABLEARRAY);writeParcelableArray((Parcelable[]) v, 0);} else if (v instanceof Object[]) {writeInt(VAL_OBJECTARRAY);writeArray((Object[]) v);} else if (v instanceof int[]) {writeInt(VAL_INTARRAY);writeIntArray((int[]) v);} else if (v instanceof long[]) {writeInt(VAL_LONGARRAY);writeLongArray((long[]) v);} else if (v instanceof Byte) {writeInt(VAL_BYTE);writeInt((Byte) v);} else if (v instanceof Serializable) {// Must be lastwriteInt(VAL_SERIALIZABLE);writeSerializable((Serializable) v);} else {throw new RuntimeException("Parcel: unable to marshal value " + v);}}public final Object readValue(ClassLoader loader) {int type = readInt();switch (type) {case VAL_NULL:return null;case VAL_STRING:return readString();case VAL_INTEGER:return readInt();case VAL_MAP:return readHashMap(loader);case VAL_PARCELABLE:return readParcelable(loader);case VAL_SHORT:return (short) readInt();case VAL_LONG:return readLong();case VAL_FLOAT:return readFloat();case VAL_DOUBLE:return readDouble();case VAL_BOOLEAN:return readInt() == 1;case VAL_CHARSEQUENCE:return readCharSequence();case VAL_LIST:return readArrayList(loader);case VAL_BOOLEANARRAY:return createBooleanArray();        case VAL_BYTEARRAY:return createByteArray();case VAL_STRINGARRAY:return readStringArray();case VAL_CHARSEQUENCEARRAY:return readCharSequenceArray();case VAL_IBINDER:return readStrongBinder();case VAL_OBJECTARRAY:return readArray(loader);case VAL_INTARRAY:return createIntArray();case VAL_LONGARRAY:return createLongArray();case VAL_BYTE:return readByte();case VAL_SERIALIZABLE:return readSerializable();case VAL_PARCELABLEARRAY:return readParcelableArray(loader);case VAL_SPARSEARRAY:return readSparseArray(loader);case VAL_SPARSEBOOLEANARRAY:return readSparseBooleanArray();case VAL_BUNDLE:return readBundle(loader); // loading will be deferreddefault:int off = dataPosition() - 4;throw new RuntimeException("Parcel " + this + ": Unmarshalling unknown type code " + type + " at offset " + off);}}

这两个接口依赖于前面基本类型的接口，内部通过instanceof关键字类判断数据类型。各个类型常量定义如下：

private static final int VAL_NULL = -1;private static final int VAL_STRING = 0;private static final int VAL_INTEGER = 1;private static final int VAL_MAP = 2;private static final int VAL_BUNDLE = 3;private static final int VAL_PARCELABLE = 4;private static final int VAL_SHORT = 5;private static final int VAL_LONG = 6;private static final int VAL_FLOAT = 7;private static final int VAL_DOUBLE = 8;private static final int VAL_BOOLEAN = 9;private static final int VAL_CHARSEQUENCE = 10;private static final int VAL_LIST  = 11;private static final int VAL_SPARSEARRAY = 12;private static final int VAL_BYTEARRAY = 13;private static final int VAL_STRINGARRAY = 14;private static final int VAL_IBINDER = 15;private static final int VAL_PARCELABLEARRAY = 16;private static final int VAL_OBJECTARRAY = 17;private static final int VAL_INTARRAY = 18;private static final int VAL_LONGARRAY = 19;private static final int VAL_BYTE = 20;private static final int VAL_SERIALIZABLE = 21;private static final int VAL_SPARSEBOOLEANARRAY = 22;private static final int VAL_BOOLEANARRAY = 23;private static final int VAL_CHARSEQUENCEARRAY = 24;