Java&Android 基礎知識梳理(2) 序列化

1、Parcelable和Serializable

對象的序列化是把Java對象轉化爲字節序列並存儲至一個存儲媒介（硬盤或者內存）的過程，反序列化則是把字節序列恢復爲Java對象的過程，但它們僅處理Java變量而不處理方法。

序列化的緣由：

• 永久性保存對象，保存對象的字節序列到本地文件中。Serializable
• 經過序列化對象在網絡中傳遞對象。Serializable
• 經過序列化在進程間傳遞對象。Parcelable

兩種序列化的區別：

• Serializable只須要對某個類以及它的屬性實現Serializable接口便可，它的缺點是使用了反射，序列化的過程比較慢，這種機制會在序列化的時候建立許多的臨時對象，容易引起頻繁的gc。
• 而Parcelable是Android平臺特有的，在使用內存的時候性能更好，但Parcelable不能使用在要將數據存儲在磁盤的狀況下，由於Parcelable不能很好的保證數據的持續性在外界有變化的狀況。

2、序列化在Android平臺上的應用

2.1 經過intent傳遞複雜對象

intent支持傳遞的數據類型包括：

• 基本類型的數據、及其數組。
• String/CharSequence類型的數據、及其數組。
• Parcelable/Serializable，及其數組/列表數據。

2.2 SharePreference存儲複雜對象

3、Serializable和Parcelable

3.1 使用Serializable的讀寫操做

首先定義咱們要序列化的對象。

public class SBook implements Serializable {            
     public int id;    
     public String name;
}
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進行讀寫操做：

private void readSerializable() {
        ObjectInputStream object = null;
        try {
            FileInputStream out = new FileInputStream(Environment.getExternalStorageDirectory() + "/sbook.txt");
            object = new ObjectInputStream(out);
            SBook book = (SBook) object.readObject();
            if (book != null) {
                Log.d(TAG, "book, id=" + book.id + ",name=" + book.name);
            } else {
                Log.d(TAG, "book is null");
            }
        } catch (Exception e) {
            Log.d(TAG, "readSerializable:" + e);
        } finally {
            try {
                if (object != null) {
                    object.close();
                }
            } catch (Exception e) {
                Log.d(TAG, "readSerializable:" + e);
            }
        }
    }

    private void writeSerializable() {
        SBook book = new SBook();
        book.id = 1;
        book.name = "SBook";
        ObjectOutputStream object = null;
        try {
            FileOutputStream out = new FileOutputStream(Environment.getExternalStorageDirectory()  + "/sbook.txt");
            object = new ObjectOutputStream(out);
            object.writeObject(book);
            object.flush();
        } catch (Exception e) {
            Log.d(TAG, "writeSerializable:" + e);
        } finally {
            try {
                if (object != null) {
                    object.close();
                }
            } catch (Exception e) {
                Log.d(TAG, "writeSerializable:" + e);
            }
        }
    }
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3.2 使用Parcelable的讀寫操做

定義序列化對象：

public class PBook implements Parcelable {

    public int id;
    public String name;

    public PBook(int id, String name) {
        this.id = id;
        this.name = name;
    }

    private PBook(Parcel in) {
        id = in.readInt();
        name = in.readString();
    }

    public static final Parcelable.Creator<PBook> CREATOR = new Parcelable.Creator<PBook>() {

        @Override
        public PBook[] newArray(int size) {
            return new PBook[size];
        }

        @Override
        public PBook createFromParcel(Parcel source) {
            return new PBook(source);
        }
    };

    @Override
    public int describeContents() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeInt(id);
        dest.writeString(name);
    }
}
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寫入和讀取：

private Intent writeParcelable() {
        PBook tony = new PBook(1, "tony");
        PBook king = new PBook(2, "king");
        ArrayList<PBook> list = new ArrayList<>();
        list.add(tony);
        list.add(king);
        Intent intent = new Intent();
        intent.putParcelableArrayListExtra("PBook", list);
        return intent;
    }

    private void readParcelable(Intent intent) {
        if (intent != null) {
            ArrayList<PBook> list = intent.getParcelableArrayListExtra("PBook");
            if (list != null) {
                for (PBook book : list) {
                    Log.d(TAG, "readParcelable, id=" + book.id + ", name=" + book.name);
                }
            }
        }
    }
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4、SharePreference存儲複雜對象

//obejct -> ObjectOutputStream(ByteArrayOutputStream) -> ByteArrayOutputStream() -> byte[] -> String -> sp
    private void writeSP() {
        SBook book = new SBook();
        book.id = 2;
        book.name = "sp";
        SharedPreferences sp = getSharedPreferences("SBookSP", MODE_PRIVATE);
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        try {
            ObjectOutputStream object = new ObjectOutputStream(os);
            object.writeObject(book);
            String base64 = new String(Base64.encode(os.toByteArray(), Base64.DEFAULT));
            SharedPreferences.Editor editor = sp.edit();
            editor.putString("SBook", base64);
            editor.apply();
        } catch (Exception e) {}
    }
    
    //sp -> string -> byte[] -> ByteArrayInputStream(byte[]) -> ObjectInputStream(ByteArrayInputStream) -> object
    private void readSP() {
        SharedPreferences sp = getSharedPreferences("SBookSP", MODE_PRIVATE);
        String sbook = sp.getString("SBook", "");
        if (sbook.length() > 0) {
            byte[] base64 = Base64.decode(sbook.getBytes(), Base64.DEFAULT);
            ByteArrayInputStream is = new ByteArrayInputStream(base64);
            try {
                ObjectInputStream object = new ObjectInputStream(is);
                SBook book = (SBook) object.readObject();
                if (book != null) {
                    Log.d(TAG, "readSP, id=" + book.id + ", name=" + book.name);
                }
            } catch (Exception e) {

            }
        }
    }
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