Android 序列化原理介绍

无论是android中还是Java中，远程传递数据都是需要序列化的，android中不同组件之间传递数据也是需要序列化的。

1、序列化和反序列化

序列化和反序列化是一个过程，这个过程其实就是我们给数据加一层标识，告诉虚拟机序列化的时候该怎么给数据加壳，反序列化的时候该怎么将数据的壳去掉，取出其中的数据。
实现序列化的方式有两种：Serializable接口和Parcelable接口

2、Serializable接口

Java提供的接口，主要用于文件读写和网络传输数据，也可以用于android组件之间传递数据，只是因为Serializable接口操作数据时候，序列化和反序列化时候，会产生大量的变量，也会有频繁的IO操作，开销大，导致在组件之间传递数据时候效率低，所以不建议在android组件之间传递数据。

/**
 *
 * Classes that require special handling during the serialization and
 * deserialization process must implement special methods with these exact
 * signatures:
 *
 * <PRE>
 * private void writeObject(java.io.ObjectOutputStream out)
 *     throws IOException
 * private void readObject(java.io.ObjectInputStream in)
 *     throws IOException, ClassNotFoundException;
 * private void readObjectNoData()
 *     throws ObjectStreamException;
 * </PRE>
 * <PRE>
 * ANY-ACCESS-MODIFIER static final long serialVersionUID = 42L;
 * </PRE>

 * @author  unascribed
 * @see java.io.ObjectOutputStream
 * @see java.io.ObjectInputStream
 * @see java.io.ObjectOutput
 * @see java.io.ObjectInput
 * @see java.io.Externalizable
 * @since   JDK1.1
 */
public interface Serializable {
}

上面就是Serializable接口在源码中的定义，可以看出来内部就是空的，没有任何方法要去实现，上面注释中提醒我们使用的时候都是要在IO流中使用，并且最好设置serialVersionUID，作为唯一的标识。其实实现过程都是由jdk虚拟机去实现了，我们无需操心具体的实现细节，使用简单方便。

3、Parcelable接口

android提供的接口，主要用于不同组件之间传递数据，因为内部实现靠自定义操作，所以相对于Serializable接口来说，实现比较麻烦，但是给了开发人员高度的自由度，序列化和反序列化时候，不会产生大量的变量，也不会有频繁的IO操作，所以序列化和反序列化时候开销小，效率高。
特别要注意的是：不能用来进行文件操作，否则会报异常。

/**
 * Interface for classes whose instances can be written to
 * and restored from a {@link Parcel}.  Classes implementing the Parcelable
 * interface must also have a non-null static field called <code>CREATOR</code>
 * of a type that implements the {@link Parcelable.Creator} interface.
 * 
 * <p>A typical implementation of Parcelable is:</p>
 * 
 * <pre>
 * public class MyParcelable implements Parcelable {
 *     private int mData;
 *
 *     public int describeContents() {
 *         return 0;
 *     }
 *
 *     public void writeToParcel(Parcel out, int flags) {
 *         out.writeInt(mData);
 *     }
 *
 *     public static final Parcelable.Creator&lt;MyParcelable&gt; CREATOR
 *             = new Parcelable.Creator&lt;MyParcelable&gt;() {
 *         public MyParcelable createFromParcel(Parcel in) {
 *             return new MyParcelable(in);
 *         }
 *
 *         public MyParcelable[] newArray(int size) {
 *             return new MyParcelable[size];
 *         }
 *     };
 *     
 *     private MyParcelable(Parcel in) {
 *         mData = in.readInt();
 *     }
 * }</pre>
 */
public interface Parcelable {
    /** @hide */
    @IntDef(flag = true, prefix = { "PARCELABLE_" }, value = {
            PARCELABLE_WRITE_RETURN_VALUE,
            PARCELABLE_ELIDE_DUPLICATES,
    })
    @Retention(RetentionPolicy.SOURCE)
    public @interface WriteFlags {}

    public static final int PARCELABLE_WRITE_RETURN_VALUE = 0x0001;   
    public static final int PARCELABLE_ELIDE_DUPLICATES = 0x0002;

    /*
     * Bit masks for use with {@link #describeContents}: each bit represents a kind of object considered to have potential special significance when marshalled.
     */

    /** @hide */
    @IntDef(flag = true, prefix = { "CONTENTS_" }, value = {
            CONTENTS_FILE_DESCRIPTOR,
    })
    @Retention(RetentionPolicy.SOURCE)
    public @interface ContentsFlags {}

    /**
     * Descriptor bit used with {@link #describeContents()}: indicates that
     * the Parcelable object's flattened representation includes a file descriptor.
     *
     * @see #describeContents()
     */
    public static final int CONTENTS_FILE_DESCRIPTOR = 0x0001;
    
    /**
     * Describe the kinds of special objects contained in this Parcelable
     * instance's marshaled representation. For example, if the object will
     * include a file descriptor in the output of {@link #writeToParcel(Parcel, int)},
     * the return value of this method must include the
     * {@link #CONTENTS_FILE_DESCRIPTOR} bit.
     *  
     * @return a bitmask indicating the set of special object types marshaled
     * by this Parcelable object instance.
     */
    public @ContentsFlags int describeContents();
    
    /**
     * Flatten this object in to a Parcel.
     * 
     * @param dest The Parcel in which the object should be written.
     * @param flags Additional flags about how the object should be written.
     * May be 0 or {@link #PARCELABLE_WRITE_RETURN_VALUE}.
     */
    public void writeToParcel(Parcel dest, @WriteFlags int flags);

    /**
     * Interface that must be implemented and provided as a public CREATOR
     * field that generates instances of your Parcelable class from a Parcel.
     */
    public interface Creator<T> {
        /**
         * Create a new instance of the Parcelable class, instantiating it
         * from the given Parcel whose data had previously been written by
         * {@link Parcelable#writeToParcel Parcelable.writeToParcel()}.
         * 
         * @param source The Parcel to read the object's data from.
         * @return Returns a new instance of the Parcelable class.
         */
        public T createFromParcel(Parcel source);
        
        /**
         * Create a new array of the Parcelable class.
         * 
         * @param size Size of the array.
         * @return Returns an array of the Parcelable class, with every entry
         * initialized to null.
         */
        public T[] newArray(int size);
    }

    /**
     * Specialization of {@link Creator} that allows you to receive the
     * ClassLoader the object is being created in.
     */
    public interface ClassLoaderCreator<T> extends Creator<T> {
        /**
         * Create a new instance of the Parcelable class, instantiating it
         * from the given Parcel whose data had previously been written by
         * {@link Parcelable#writeToParcel Parcelable.writeToParcel()} and
         * using the given ClassLoader.
         *
         * @param source The Parcel to read the object's data from.
         * @param loader The ClassLoader that this object is being created in.
         * @return Returns a new instance of the Parcelable class.
         */
        public T createFromParcel(Parcel source, ClassLoader loader);
    }
}

由此可以看出，实现Parcelable接口，内部过程都是需要自己实现的，自由度更高，但是也增加开发成本。