Android -- Parcelable 序列化操作数据(上篇)

序列化数据原理：
序列化的过程就是对象写入字节流和从字节流中读取对象。将对象状态转换成字节流之后，可以用java.io包中的各种字节流类将其保存到文件中，管道到另一线程中或通过网络连接将对象数据发送到另一主机。
简单说就是将数据对象存入字节流当中，在需要时重新生成对象。

Android中的序列化机制:
首先android系统利用Binder进行IPC通讯，且定位为针对内存受限的设备，所以则要求使用高效的对象传输方式，因为Parcel应运而生。

代码分析：
frameworks\base\core\java\android\os\Parcel.java
frameworks\base\core\jni\android_util_Binder.cpp JNI函数

以典型代码片段举例:

    /**
     * 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);


    /**
     * Write a floating point value into the parcel at the current
     * dataPosition(), growing dataCapacity() if needed.
     */
    public final native void writeFloat(float val);

JNI层实现：

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);
        }
    }
}


static void android_os_Parcel_writeFloat(JNIEnv* env, jobject clazz, jfloat val)
{
    Parcel* parcel = parcelForJavaObject(env, clazz);
    if (parcel != NULL) {
        const status_t err = parcel->writeFloat(val);
        if (err != NO_ERROR) {
            jniThrowException(env, "java/lang/OutOfMemoryError", NULL);
        }
    }
}

其本质使用 Parcel 对象来完成的，实现代码在：frameworks/base/libs/binder/parcel.cpp

	status_t Parcel::writeInt32(int32_t val)
	{
	    return writeAligned(val);
	}
	--> 直接利用模块实现 
	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; // 直接在此将数据写入到内存中
	        return finishWrite(sizeof(val));
	    }
	
	    status_t err = growData(sizeof(val)); // 数据空间不够的情况下处理
	    if (err == NO_ERROR) goto restart_write;
	    return err;
	}
	
	status_t Parcel::growData(size_t len)
	{
	    size_t newSize = ((mDataSize+len)*3)/2;  // 每次多分配50%的内存空间
	    return (newSize <= mDataSize)
	            ? (status_t) NO_MEMORY
	            : continueWrite(newSize);
	}	

总结：
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
2、读写时是PAD_SIZE=4字节对齐， #define PAD_SIZE(s) (((s)+3)&~3)
3、预分配的空间不够时newSize = ((mDataSize+len)*3)/2;会一次多分配50%
4、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

举个例子说明一下如何在同一个Activity传递复杂数据类型：

package com.test.testparcel;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Parcel;
import android.os.Parcelable;
import android.util.Log;
import android.view.KeyEvent;

public class TestParcelActivity extends Activity {
	ComplexDataStruct complexData = null;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);        
    }

	@Override
	public boolean onKeyUp(int keyCode, KeyEvent event) {
		if(keyCode == KeyEvent.KEYCODE_0){
	        testComplexDS();
		}
		return super.onKeyUp(keyCode, event);
	}
    
	public void testComplexDS(){
		complexData = new ComplexDataStruct();
		Intent intent=new Intent(TestParcelActivity.this,TestParcelActivity.class);

		complexData.a = 10;
		complexData.b = 20;
		complexData.str1 = " hello...";
		complexData.str2 = " world...";
		Log.i("","intent.putExtra before");
		intent.putExtra("testcomplexData", complexData);
		Log.i("","intent.putExtra after");
		
		Log.i("","intent.getParcelableExtra before");
		ComplexDataStruct test = (ComplexDataStruct)intent.getParcelableExtra("testcomplexData");
		Log.i("","intent.getParcelableExtra after");
		Log.i("","a="+test.a);		
		Log.i("","b="+test.b);
		Log.i("","str1="+test.str1);
		Log.i("","str2="+test.str2);
	}
	
	/* 一个复杂的Parcelable对象传递 */
	public class ComplexDataStruct implements Parcelable{
		public int a = 0;
		public int b = 0;
		public String str1 = null;
		public String str2 = null;
		
		ComplexDataStruct(){
			//TODO
		}
		
		public ComplexDataStruct(ComplexDataStruct other) {
			this.a = other.a;
			this.b = other.b;
			this.str1 = other.str1;
			this.str2 = other.str2;
		}
		
		private ComplexDataStruct(Parcel in) {
			Log.e("","readFromParcel is calling...");
			readFromParcel(in);
		}
		
		public void readFromParcel(Parcel in) {
			a = in.readInt();
			b = in.readInt();
			str1 = in.readString();
			str2 = in.readString();
			Log.e("","readFromParcel is calling...");
		}

		public int describeContents() {
			// TODO Auto-generated method stub
			return 0;
		}

		public void writeToParcel(Parcel arg0, int arg1) {
			// TODO Auto-generated method stub
			Log.e("","writeToParcel is calling...");
			arg0.writeInt(a);
			arg0.writeInt(b);
			arg0.writeString(str1);
			arg0.writeString(str2);
			Log.e("","writeToParcel is calling...");
		}
		
		/* 内部实现静态CREATOR类 */
		public final Parcelable.Creator<ComplexDataStruct> CREATOR = 
			new Parcelable.Creator<ComplexDataStruct>() {
			// 从Parcel中读取数据，返回ComplexDataStruct对象
			public ComplexDataStruct createFromParcel(Parcel in) {
				Log.e("","createFromParcel is calling...");
				return new ComplexDataStruct(in);
			}

			public ComplexDataStruct[] newArray(int size) {
				Log.e("","newArray is calling...");
				return new ComplexDataStruct[size]; // 传递数组
			}
		};
	}
}

打印结果：
I/        ( 1324): intent.putExtra before
I/        ( 1324): intent.putExtra after
I/        ( 1324): intent.getParcelableExtra before
I/        ( 1324): intent.getParcelableExtra after
I/        ( 1324): a=10
I/        ( 1324): b=20
I/        ( 1324): str1= hello...
I/        ( 1324): str2= world...


这里利用 intent 发送消息，利用 Bundle 来传递数据,Bundle概念下一篇会讲到。

构建复杂数据类型，需要如下实现：
1、继承自 Parcelable 实现，ex: public class ComplexDataStruct implements Parcelable
2、内部有一个静态的 CREATOR 类 ： public static final Parcelable.Creator<ComplexDataStruct> CREATOR
3、实现 writeToParcel 和 readFromParcel 序列化与反序列化函数

4、传递使用 Intent.java 类的函数：putExtra 及 getParcelableXXX函数