Serialization and deserialization of objects

1) Object serialization is to convert the Object object into byte sequence, otherwise it is called deserialization of the object.

2) Serialization stream (ObjectOutputStream) is a filtered stream of bytes - writeObject() method

Deserialization stream (ObjectInputStream) - readObject() method

3) Serializable interface (Serializable)

The object must implement the serialization interface before it can be serialized, otherwise an exception will occur.

Note: This interface does not have any methods, it is just a [standard]

1. The most basic sequence The process of serialization and deserialization

Serialization and deserialization are all operated on Object objects. Here is a simple case to demonstrate the object serialization and deserialization. process.

1. Create a new Student class (test class)

Note: A class that implements the serialization interface is required to perform serialization operations! !

@SuppressWarnings("serial")
public?class?Student?implements?Serializable{
????private?String?stuno;//id
????private?String?stuna;//姓名
????private?int?stuage;//年齡
????public?String?getStuno()?{
????????return?stuno;
????}
????public?void?setStuno(String?stuno)?{
????????this.stuno?=?stuno;
????}
????public?String?getStuna()?{
????????return?stuna;
????}
????public?void?setStuna(String?stuna)?{
????????this.stuna?=?stuna;
????}
????public?Student()?{
????????super();
????????//?TODO?Auto-generated?constructor?stub
????}
????public?Student(String?stuno,?String?stuna,?int?stuage)?{
????????super();
????????this.stuno?=?stuno;
????????this.stuna?=?stuna;
????????this.stuage?=?stuage;
????}
????@Override
????public?String?toString()?{
????????return?"Student?[stuno="?+?stuno?+?",?stuna="?+?stuna?+?",?stuage="?+?stuage?+?"]";
????}
????public?int?getStuage()?{
????????return?stuage;
????}
????public?void?setStuage(int?stuage)?{
????????this.stuage?=?stuage;
????}
}

2. Serialize instances of the Student class into files

The basic operation steps are as follows:

1), specify serialization and save file

2), construct the ObjectOutputStream class

3), construct a Student class

4), use the writeObject method to serialize

5), Use the close() method to close the stream

String?file="demo/obj.dat";
????????//對象的序列化
????????ObjectOutputStream?oos=new?ObjectOutputStream(
????????????????new?FileOutputStream(file));
????????//把Student對象保存起來，就是對象的序列化
????????Student?stu=new?Student("01","mike",18);
????????//使用writeObject方法序列化
????????oos.writeObject(stu);
????????oos.close();

Running results: You can see that the serialized file obj.dat was generated in the demo directory

3. Deserialize the file and read out the Student class object

The basic operation steps are as follows:

1), specify the file to be deserialized

2), Construct the ObjectInputStream class

3), use the readObject method to deserialize

1), and use the close method to close the stream

String?file="demo/obj.dat";
????????ObjectInputStream?ois?=new?ObjectInputStream(
????????????????new?FileInputStream(file));
????????//使用readObject()方法序列化
????????Student?stu=(Student)ois.readObject();//強制類型轉換
????????System.out.println(stu);
????????ois.close();

Running results:

Note: When deserializing a file, the objects taken out by the readObject method are of type Object by default and must be forced to the corresponding type.

2. Transient and ArrayList source code analysis

In the daily programming process, we sometimes do not want all the elements of a class to be Serialized by the compiler, what should I do at this time?

Java provides a transient keyword to modify elements that we do not want to be automatically serialized by the jvm. Let’s briefly explain this keyword.

transient keyword: The element modified by transient will not be serialized by jvm by default, but you can complete the serialization of this element by yourself.

Note:

1) In future network programming, if there are certain elements that do not need to be transmitted, they can be modified with transient to save traffic; yes Efficient element serialization to improve performance.

2) You can use writeObject to complete the serialization of this element yourself.

ArrayList is optimized using this method. ArrayList's core container Object[] elementData uses transient modification, but writeObject implements serialization of the elementData array itself. Only valid elements in the array are serialized. readObject is similar.

--------------My own way of serialization- --------------

Add two methods to the class to be serialized (These two methods are derived from the ArrayList source code Two special methods extracted from JVM will automatically use these two methods to help us complete this action.

There is another question here. Why do we still need to complete serialization and deserialization manually? What is the meaning?

This problem needs to be analyzed from the source code of ArrayList:

It can be seen that the source code of ArrayList The purpose of self-serialization: The bottom layer of ArrayList is an array. Self-serialization can filter invalid elements in the array and only serialize valid elements in the array, thereby improving performance

Therefore, during the actual programming process, we can complete the serialization ourselves as needed to improve performance.

三、序列化中子父類構造函數(shù)問題

在類的序列化和反序列化中，如果存在子類和父類的關系時，序列化和反序列化的過程又是怎么樣的呢？

這里我寫一個測試類來測試子類和父類實現(xiàn)序列化和反序列化時構造函數(shù)的實現(xiàn)變化。

public?static?void?main(String[]?args)?throws?IOException?{
????????//?TODO?Auto-generated?method?stub
????????String?file="demo/foo.dat";
????????ObjectOutputStream?oos=new?ObjectOutputStream(
????????????????new?FileOutputStream(file));
????????Foo2?foo2?=new?Foo2();
????????oos.writeObject(foo2);
????????oos.flush();
????????oos.close();
????}

}
class?Foo?implements?Serializable{
????public?Foo(){
????????System.out.println("foo");
????}
}
class?Foo1?extends?Foo{
????public?Foo1(){
????????System.out.println("foo1");
????}
????
}
class?Foo2?extends?Foo1{
????public?Foo2(){
????????System.out.println("foo2");
????}
}

運行結果：這是序列化時遞歸調(diào)用了父類的構造函數(shù)

接來下看看反序列化時，是否遞歸調(diào)用父類的構造函數(shù)。

ObjectInputStream?ois=new?ObjectInputStream(
new?FileInputStream(file));
Foo2?foo2=(Foo2)ois.readObject();
ois.close();

運行結果：控制臺沒有任何輸出。

那么這個結果是否證明反序列化過程中父類的構造函數(shù)就是始終不調(diào)用的呢？

然而不能證明??！

因為再看下面這個不同的測試例子：

class?Bar?{
????public?Bar(){
????????System.out.println("bar");
????}
}
class?Bar1?extends?Bar?implements?Serializable{
????public?Bar1(){
????????System.out.println("bar1");
????}
}
class?Bar2?extends?Bar1{
????public?Bar2(){
????????System.out.println("bar2");
????}
}

我們用這個例子來測試序列化和反序列化。

序列化結果：

反序列化結果：沒實現(xiàn)序列化接口的父類被顯示調(diào)用構造函數(shù)

【反序列化時】，向上遞歸調(diào)用構造函數(shù)會從【可序列化的一級父類結束】。即誰實現(xiàn)了可序列化（包括繼承實現(xiàn)的），誰的構造函數(shù)就不會調(diào)用。

總結：

1）父類實現(xiàn)了serializable接口，子類繼承就可序列化。

子類在反序列化時，父類實現(xiàn)了序列化接口，則不會遞歸調(diào)用其構造函數(shù)。

2）父類未實現(xiàn)serializable接口，子類自行實現(xiàn)可序列化

子類在反序列化時，父類沒有實現(xiàn)序列化接口，則會遞歸調(diào)用其構造函數(shù)。

本文來自?java入門?欄目，歡迎學習！

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