Serializable & Parcelable这两种序列化方法是Android中经常使用的方法，Serializable是Android从Java中继承过来的，Parcelable是Android自己提供的方法，Google是推荐使用Parcelable，至于这两种方法的区别，下面通过对源码的分析来慢慢的了解。

在分析源码之前，首先还是说一下序列化在Android中使用的场景：

1）我们在四大组件之间使用intente来传递数据，intente中所传递的数据是需要序列化的

2）binder，binder是IPC机制中是很重要的，binder中所传递的数据也是需要序列化的

3）还有我们经常要把一些数据持久化到存储设备上，这些数据是需要序列化的

4）在网络中传递对象

好啦，现在我们开始对两种序列化方式的源码进行了解：

Serializable：

因为Serializable是一个标示接口，所以我们主要是对它的注解进行阅读

 * Marks classes that can be serialized by {@link ObjectOutputStream} and
 * deserialized by {@link ObjectInputStream}.

这一句告诉我们，serializable是通过ObjectOutputStream、ObjectInputStream来完成序列化过程的

* <p><strong>Warning:</strong> this interface limits how its implementing
 * classes can change in the future. By implementing {@code Serializable} you
 * expose your flexible in-memory implementation details as a rigid binary
 * representation. Simple code changes--like renaming private fields--are
 * not safe when the changed class is serializable.

这一句告诉我们，当实现Serializable接口的类中有代码变化，可能会导致序列化失败

<p>Every serializable class is assigned a version identifier called a {@code
 * serialVersionUID}. By default, this identifier is computed by hashing the
 * class declaration and its members. This identifier is included in the
 * serialized form so that version conflicts can be detected during
 * deserialization. If the local {@code serialVersionUID} differs from the
 * {@code serialVersionUID} in the serialized data, deserialization will fail
 * with an {@link InvalidClassException}.
 *
 * <p>You can avoid this failure by declaring an explicit {@code
 * serialVersionUID}. Declaring an explicit {@code serialVersionUID} tells the
 * serialization mechanism that the class is forward and backward compatible
 * with all versions that share that {@code serialVersionUID}. Declaring a
 * {@code serialVersionUID} looks like this: <pre>   {@code
 *
 *     private static final long serialVersionUID = 0L;
 * }</pre>
 * If you declare a {@code serialVersionUID}, you should increment it each
 * time your class changes incompatibly with the previous version. Typically
 * this is when you add, change or remove a non-transient field.

序列化和反序列化其实是通过serialVersionUID来进行工作的，默认情况下，serialVersionUID是通过hash计算类的成员变量和类的方法来得到的，serialVersionUID是存储在系列化的文件中的，当要反序列化时，就从序列化文件中能拿到serialVersionUID，如果拿到的serialVersionUiD与通过类中的成员变量和方法hash计算出的serialVersionUID一样的话，就反序列化成功，反序列化成功返回的结果与序列化的结果不是同一个对象，但是值是相同的，如果反序列化失败的话就会抛出一个
IncaildClassException。

当然我们也可以在类中自定义serialVersionUID，比如：

private static final long serialVersionUID = 0L；

这样的好处是，当类中的成员变量或者是方法增加、改变、移除的话，我们的反序列化依旧会成功，但是当类的结构有了翻天覆地的变化的时候，也会反序列化失败的。

上面有一句是：

Typically this is when you add, change or remove a non-transient field.

告诉我们，被transient修饰的成员变量是不会参与序列化的。

* <p>You can take control of your serialized form by implementing these two
 * methods with these exact signatures in your serializable classes:
 * <pre>   {@code
 *
 *   private void writeObject(java.io.ObjectOutputStream out)
 *       throws IOException {
 *     // write 'this' to 'out'...
 *   }
 *
 *   private void readObject(java.io.ObjectInputStream in)
 *       throws IOException, ClassNotFoundException {
 *     // populate the fields of 'this' from the data in 'in'...
 *   }
 * }</pre>

这段注解告诉我们：默认的序列化过程和反序列化过程是可以改变的，只要重写wireteObject、readObject这两个方法就可以啦，但是一般的情况下我们是不会重写这两个方法的，下面就让我们看看默认它们是怎么实现的：

先看writeObject：

它是ObjectOutputStream中的方法：

    public final void writeObject(Object object) throws IOException {
        writeObject(object, false);
    }

private void writeObject(Object object, boolean unshared) throws IOException {
        boolean setOutput = (primitiveTypes == output);
        if (setOutput) {
            primitiveTypes = null;
        }
        // This is the specified behavior in JDK 1.2. Very bizarre way to allow
        // behavior overriding.
        if (subclassOverridingImplementation && !unshared) {
            writeObjectOverride(object);
            return;
        }

        try {
            // First we need to flush primitive types if they were written
            drain();
            // Actual work, and class-based replacement should be computed
            // if needed.
            writeObjectInternal(object, unshared, true, true);
            if (setOutput) {
                primitiveTypes = output;
            }
        } catch (IOException ioEx1) {
            // This will make it pass through until the top caller. Only the top caller writes the
            // exception (where it can).
            if (nestedLevels == 0) {
                try {
                    writeNewException(ioEx1);
                } catch (IOException ioEx2) {
                    // If writing the exception to the output stream causes another exception there
                    // is no need to propagate the second exception or generate a third exception,
                    // both of which might obscure details of the root cause.
                }
            }
            throw ioEx1; // and then we propagate the original exception
        }
    }

这段中会判断是否在子类中实现啦writeObject，如果实现了就直接返回，否则就会调用

writeObjectInternal(object, unshared, true, true);

 private int writeObjectInternal(Object object, boolean unshared,
            boolean computeClassBasedReplacement,
            boolean computeStreamReplacement) throws IOException {
           ...
      f (!(enableReplace && computeStreamReplacement)) {
                // Is it a Class ?
                if (objClass == ObjectStreamClass.CLASSCLASS) {
                    return writeNewClass((Class<?>) object, unshared);
                }
                // Is it an ObjectStreamClass ?
                if (objClass == ObjectStreamClass.OBJECTSTREAMCLASSCLASS) {
                    return writeClassDesc((ObjectStreamClass) object, unshared);
                }
            }
      ''''
}

这里面我只复制了一些代码，像writeNewClass、wirteClassDesc还有好多这种方法，他们的内部都是在output中存储类型，在objectWritten中存储对象。

再看readObject：

readObject是ObjectInoutStream中的方法：

    public final Object readObject() throws OptionalDataException,
            ClassNotFoundException, IOException {
        return readObject(false);
    }

 private Object readObject(boolean unshared) throws OptionalDataException,
            ClassNotFoundException, IOException {
      ...
     Object result;
        try {
            // We need this so we can tell when we are returning to the
            // original/outside caller
            if (++nestedLevels == 1) {
                // Remember the caller's class loader
                callerClassLoader = VMStack.getClosestUserClassLoader(bootstrapLoader, systemLoader);
            }

            result = readNonPrimitiveContent(unshared);
            if (restoreInput) {
                primitiveData = input;
            }
        }
   ...
}

调用readNonPrimitiveContent();

 private Object readNonPrimitiveContent(boolean unshared)
            throws ClassNotFoundException, IOException {
        checkReadPrimitiveTypes();
        if (primitiveData.available() > 0) {
            OptionalDataException e = new OptionalDataException();
            e.length = primitiveData.available();
            throw e;
        }

        do {
            byte tc = nextTC();
            switch (tc) {
                case TC_CLASS:
                    return readNewClass(unshared);
                case TC_CLASSDESC:
                    return readNewClassDesc(unshared);
                case TC_ARRAY:
                    return readNewArray(unshared);
                case TC_OBJECT:
                    return readNewObject(unshared);
                case TC_STRING:
                    return readNewString(unshared);
                case TC_LONGSTRING:
                    return readNewLongString(unshared);
                case TC_ENUM:
                    return readEnum(unshared);
                case TC_REFERENCE:
                    if (unshared) {
                        readNewHandle();
                        throw new InvalidObjectException("Unshared read of back reference");
                    }
                    return readCyclicReference();
                case TC_NULL:
                    return null;
                case TC_EXCEPTION:
                    Exception exc = readException();
                    throw new WriteAbortedException("Read an exception", exc);
                case TC_RESET:
                    resetState();
                    break;
                case TC_ENDBLOCKDATA: // Can occur reading class annotation
                    pushbackTC();
                    OptionalDataException e = new OptionalDataException();
                    e.eof = true;
                    throw e;
                default:
                    throw corruptStream(tc);
            }
            // Only TC_RESET falls through
        } while (true);
    }

这里面就是在一个do—while循环里，通过nextTC()来获取输入流中的字段的类型，然后根据不同的类型得到不同的对象

    private byte nextTC() throws IOException {
        if (hasPushbackTC) {
            hasPushbackTC = false; // We are consuming it
        } else {
            // Just in case a later call decides to really push it back,
            // we don't require the caller to pass it as parameter
            pushbackTC = input.readByte();
        }
        return pushbackTC;
    }

这里input其实就是上面那个output，在do-while循环里就是不断的从input中取，然后获取字段，最终把字段封装成一个对象返回，这就是反序列化的过程。

可以看出在序列化中要进行大量的I\O的操作，而且里面还有用到反射的机制，对资源的消耗也比较大

Parcelable：

老规矩，先看一下注解：

 * Interface for classes whose instances can be written to
 * and restored from a {@link Parcel}.  Classes implementing the Parcelable
 * interface must also have a static field called <code>CREATOR</code>, which
 * is an object implementing the {@link Parcelable.Creator Parcelable.Creator}
 * interface.

这句话意思是，对象能够通过Parcel来进行序列化和反序列化，而且重要的是，在实现Parcelable中必须要有一个成员变量，而且还要被 static field来修饰，类型为Parcelable.Creator，它是Parcelable中的一个内部接口

<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<MyParcelable> CREATOR
 *             = new Parcelable.Creator<MyParcelable>() {
 *         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>

这段注解是Android给我们提供的一个例子，因为Parcelable相比于Serializable实现起来比较复杂，可能处于这种考虑，就给我们一个实例，确实给我们带来啦一些好处，我们就不用那么刻意的去记，当使用的时候看看源码就OK了！

public static final int CONTENTS_FILE_DESCRIPTOR = 0x0001;

    /**
     * Describe the kinds of special objects contained in this Parcelable's
     * marshalled representation.
     *
     * @return a bitmask indicating the set of special object types marshalled
     * by the Parcelable.
     */
    public int describeContents();
    

文件描述符，当对象有文件描述符时就返回1(CONTENTS_FILE_DESCRIPTOP),否则就返回0，大多数情况就返回0

/**
     * 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, int flags);

该方法是将对象写入到序列化结构中，dest：对象应该要被写入的序列化结构的容器

flags：有两种结果，为1(PARCELABLE_WRITE_RETURN_VALUE)从字面意                                                                         思可以看到就是要求要返回对象，大多数情况下是为0的。

 /**
     * 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> {
}

这是Parcelable中的内部接口，它的作用是从一个Parcel中构造出一个实现了Parcelable的类的实例，也就是承担了反序列化的过程。

然后看Creator中的两个接口：

        /**
         * 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);

这个方法就是从Parcel中构造出一个实现了Parcelable的类的实例，一般我们实现是通过类的构造方法中实现的：

return  new T(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);

创建一个指定长度的原始对象的数组，一般我们就直接返回：

return new T[size]；

在Parcelable中实现序列化和反序列化主要是通过Parcel来实现的，对于Parcel，可以去看

Android中的Parcel机制(上)

最后进行一下总结：

至于选择哪种方式序列化，还需酌情选择。

最后，谢谢大家的观看！