1, Serialization and deserialization of objects

1. Object serialization stream

• Introduction to object serialization

  • Object serialization: saving objects to disk or transferring objects over the network
  • This mechanism uses a byte sequence to represent an object, which contains information such as object type, object data and attributes stored in the object
  • After the byte sequence is written to the file, it is equivalent to persisting the information of an object in the file
  • On the contrary, the byte sequence can also be read back from the file, reconstruct the object and deserialize it

• Object serialization stream: ObjectOutputStream

  • Writes the original data type and graph of the Java object to the OutputStream. You can use ObjectInputStream to read (refactor) objects. Persistent storage of objects can be achieved by using streaming files. If the stream is a network socket stream, you can refactor the object on another host or in another process

• Construction method

  Method name	explain
  ObjectOutputStream(OutputStream out)	Creates an ObjectOutputStream that writes to the specified OutputStream

• Methods for serializing objects

  Method name	explain
  void writeObject(Object obj)	Writes the specified object to ObjectOutputStream

• Sample code

  Student class

  public class Student implements Serializable {
      private String name;
      private int age;
  
      public Student() {
      }
  
      public Student(String name, int age) {
          this.name = name;
          this.age = age;
      }
  
      public String getName() {
          return name;
      }
  
      public void setName(String name) {
          this.name = name;
      }
  
      public int getAge() {
          return age;
      }
  
      public void setAge(int age) {
          this.age = age;
      }
  
      @Override
      public String toString() {
          return "Student{" +
                  "name='" + name + '\'' +
                  ", age=" + age +
                  '}';
      }
  }
  

  Test class

  public class ObjectOutputStreamDemo {
      public static void main(String[] args) throws IOException {
          //ObjectOutputStream(OutputStream out): creates an ObjectOutputStream that writes to the specified OutputStream
          ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("myOtherStream\\oos.txt"));
  
          //create object
          Student s = new Student("Tong Liya",30);
  
          //void writeObject(Object obj): writes the specified object to ObjectOutputStream
          oos.writeObject(s);
  
          //Release resources
          oos.close();
      }
  }
  

• matters needing attention

  • To serialize an object, the class to which the object belongs must implement the Serializable interface
  • Serializable is a tag interface that can be implemented without rewriting any methods

2. Object deserialization stream

• Object deserialization stream: ObjectInputStream

  • ObjectInputStream deserializes raw data and objects previously written using ObjectOutputStream

• Construction method

  Method name	explain
  ObjectInputStream(InputStream in)	Creates an ObjectInputStream read from the specified InputStream

• Method of deserializing object

  Method name	explain
  Object readObject()	Read an object from ObjectInputStream

• Sample code

  public class ObjectInputStreamDemo {
      public static void main(String[] args) throws IOException, ClassNotFoundException {
          //ObjectInputStream(InputStream in): creates an ObjectInputStream read from the specified InputStream
          ObjectInputStream ois = new ObjectInputStream(new FileInputStream("myOtherStream\\oos.txt"));
  
          //Object readObject(): reads an object from ObjectInputStream
          Object obj = ois.readObject();
  
          Student s = (Student) obj;
          System.out.println(s.getName() + "," + s.getAge());
  
          ois.close();
      }
  }
  

3.serialVersionUID&transient

• serialVersionUID

  • After serializing an object with the object serialization stream, if we modify the class file to which the object belongs, will there be a problem reading the data?
    • There will be a problem and an InvalidClassException will be thrown
  • If something goes wrong, how to solve it?
    • Re serialization
    • Add a serialVersionUID to the class to which the object belongs
      • private static final long serialVersionUID = 42L;

• transient

  • If the value of a member variable in an object does not want to be serialized, how to implement it?
    • The member variable is decorated with the transient keyword. The member variable marked with the keyword does not participate in the serialization process

• Sample code

  Student class

  public class Student implements Serializable {
      private static final long serialVersionUID = 42L;
      private String name;
  //    private int age;
      private transient int age;
  
      public Student() {
      }
  
      public Student(String name, int age) {
          this.name = name;
          this.age = age;
      }
  
      public String getName() {
          return name;
      }
  
      public void setName(String name) {
          this.name = name;
      }
  
      public int getAge() {
          return age;
      }
  
      public void setAge(int age) {
          this.age = age;
      }
  
  //    @Override
  //    public String toString() {
  //        return "Student{" +
  //                "name='" + name + '\'' +
  //                ", age=" + age +
  //                '}';
  //    }
  }
  

  Test class

  public class ObjectStreamDemo {
      public static void main(String[] args) throws IOException, ClassNotFoundException {
  //        write();
          read();
      }
  
      //Deserialization
      private static void read() throws IOException, ClassNotFoundException {
          ObjectInputStream ois = new ObjectInputStream(new FileInputStream("myOtherStream\\oos.txt"));
          Object obj = ois.readObject();
          Student s = (Student) obj;
          System.out.println(s.getName() + "," + s.getAge());
          ois.close();
      }
  
      //serialize
      private static void write() throws IOException {
          ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("myOtherStream\\oos.txt"));
          Student s = new Student("Tong Liya", 30);
          oos.writeObject(s);
          oos.close();
      }
  }
  

4. Object operation flow exercise

• Case requirements

  Create multiple student class objects, write them to the file and read them into memory again

• Implementation steps

  • Create serialized stream object
  • Create multiple student objects
  • Add student object to collection
  • Serialize collection objects into a file
  • Create deserialized stream object
  • Read the object data in the file into memory

• code implementation

  Student class

  public class Student implements Serializable{
      
      private static final long serialVersionUID = 2L;
  
      private String name;
      private int age;
  
      public Student() {
      }
  
      public Student(String name, int age) {
          this.name = name;
          this.age = age;
      }
  
      public String getName() {
          return name;
      }
  
      public void setName(String name) {
          this.name = name;
      }
  
      public int getAge() {
          return age;
      }
  
      public void setAge(int age) {
          this.age = age;
      }
  }
  

  Test class

  public class Demo03 {
      /**
       *  read():
       *      Read to the end of the file. The return value is - 1
       *  readLine():
       *      Read to the end of the file and return the value null
       *  readObject():
       *      An exception is thrown directly when reading to the end of the file
       *  If there are multiple objects to be serialized, it is not recommended to directly serialize multiple objects into a file, because exceptions are easy to occur during deserialization
       *      Suggestion: store multiple objects to be serialized into a collection, and then serialize the collection into a file
       */
      public static void main(String[] args) throws Exception {
          /*// serialize
          //1.Create serialized stream object
          ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("myCode\\oos.txt"));
          ArrayList<Student> arrayList = new ArrayList<>();
          //2.Create multiple student objects
          Student s = new Student("Tong Liya ", 30);
          Student s01 = new Student("Tong Liya ", 30);
          //3.Add student object to collection
          arrayList.add(s);
          arrayList.add(s01);
          //4.Serialize collection objects into a file
          oos.writeObject(arrayList);
          oos.close();*/
  
          // Deserialization
        	//5. Create deserialization stream object
          ObjectInputStream ois = new ObjectInputStream(new FileInputStream("myCode\\oos.txt"));
        	//6. Read the object data in the file into memory
          Object obj = ois.readObject();
          ArrayList<Student> arrayList = (ArrayList<Student>)obj;
          ois.close();
          for (Student s : arrayList) {
              System.out.println(s.getName() + "," + s.getAge());
          }
      }
  }