第一百六十二节 Java IO教程 - Java对象序列化

Java IO教程 - Java对象序列化

ObjectOutputStream类的一个对象用于序列化一个对象。

ObjectInputStream类的一个对象用于反序列化一个对象。

ObjectOutputStream继承自OutputStream。 ObjectInputStream继承自InputStream。

类必须实现Serializable或Externalizable接口以便序列化或反序列化。

Serializable接口是一个标记接口。

如果我们想要一个Person类的对象被序列化，我们需要声明Person类如下：

public class Person   implements Serializable  {
    
}

Java负责处理从/向流读取/写入Serializable对象的细节。我们只需要将对象写入/读取流到流类中的一个方法。

实现Externalizable接口使我们能够更好地控制从流中读取和写入对象。

它继承Serializable接口。它声明如下：

public interface  Externalizable extends Serializable  {
    void  readExternal(ObjectInput in)  throws   IOException,  ClassNotFoundException;
    void  writeExternal(ObjectOutput out) throws   IOException;
}

当我们从流中读取一个对象时，Java调用readExternal()方法。当我们向一个流写一个对象时，它调用writeExternal()方法。

我们必须编写逻辑来分别读取和写入readExternal()和writeExternal()方法中的对象的字段。

实现Externalizable接口的类如下所示：

public class Person  implements Externalizable  {
    public void  readExternal(ObjectInput in)  throws   IOException,  ClassNotFoundException {
        // Write the logic to read the Person object fields  from  the   stream
    }    
    public void  writeExternal(ObjectOutput out) throws   IOException  {
        // Write  the   logic to write Person   object fields  to the   stream
    }
}

序列化对象

以下代码创建ObjectOutputStream类的对象，并将对象保存到person.ser文件。

ObjectOutputStream oos  = new ObjectOutputStream(new FileOutputStream("person.ser"));

要将对象保存到ByteArrayOutputStream，我们构造一个对象输出流如下：

ByteArrayOutputStream baos  = new ByteArrayOutputStream();

// Creates an  object output stream to write objects to the   byte   array  output stream
ObjectOutputStream oos  = new ObjectOutputStream(baos);

使用ObjectOutputStream类的writeObject()方法通过将对象引用作为参数传递来序列化对象，如下所示：

oos.writeObject(p1);

最后，当我们完成将所有对象写入时，使用close()方法关闭对象输出流：

oos.close();

以下代码显示如何序列化实现可序列化接口的Person类。

import java.io.Serializable;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;

class Person implements Serializable {
  private String name = "Unknown";
  private String gender = "Unknown";
  private double height = Double.NaN;

  public Person(String name, String gender, double height) {
    this.name = name;
    this.gender = gender;
    this.height = height;
  }

  @Override
  public String toString() {
    return "Name: " + this.name + ", Gender:   " + this.gender + ",  Height: "
        + this.height;
  }
}

public class Main {
  public static void main(String[] args) {
    Person p1 = new Person("John", "Male", 1.7);
    Person p2 = new Person("Wally", "Male", 1.7);
    Person p3 = new Person("Katrina", "Female", 1.4);

    File fileObject = new File("person.ser");

    try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(
        fileObject))) {

      oos.writeObject(p1);
      oos.writeObject(p2);
      oos.writeObject(p3);

      // Display the serialized objects on the standard output
      System.out.println(p1);
      System.out.println(p2);
      System.out.println(p3);
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
}

上面的代码生成以下结果。

反序列化对象

以下代码显示如何创建ObjectInputStream类的对象，并从person.ser文件读取对象。

ObjectInputStream ois  = new ObjectInputStream(new FileInputStream("person.ser"));

要从ByteArrayInputStream读取对象，请按如下所示创建对象输出流：

ObjectInputStream ois  = new ObjectInputStream(Byte-Array-Input-Stream-Reference);

使用ObjectInputStream类的readObject()方法来反序列化对象。

Object obj  = oos.readObject();

最后，关闭对象输入流如下:

ois.close();

以下代码显示如何从文件读取对象。

import java.io.File;
import java.io.FileInputStream;
import java.io.ObjectInputStream;

public class Main {
  public static void main(String[] args) {
    File fileObject = new File("person.ser");

    try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream(
        fileObject))) {

      Person p1 = (Person) ois.readObject();
      Person p2 = (Person) ois.readObject();
      Person p3 = (Person) ois.readObject();

      System.out.println(p1);
      System.out.println(p2);
      System.out.println(p3);

    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}

可外化对象序列化

要序列化和反序列化可外部化对象，请实现Externalizable接口。

import java.io.Externalizable;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInput;
import java.io.ObjectInputStream;
import java.io.ObjectOutput;
import java.io.ObjectOutputStream;

class PersonExt implements Externalizable {
  private String name = "Unknown";
  private String gender = "Unknown";
  private double height = Double.NaN;

  public PersonExt() {
  }

  public PersonExt(String name, String gender, double height) {
    this.name = name;
    this.gender = gender;
    this.height = height;
  }
  public String toString() {
    return "Name: " + this.name + ", Gender:   " + this.gender + ",  Height: "
        + this.height;
  }

  public void readExternal(ObjectInput in) throws IOException,
      ClassNotFoundException {
    this.name = in.readUTF();
    this.gender = in.readUTF();
  }

  public void writeExternal(ObjectOutput out) throws IOException {
    out.writeUTF(this.name);
    out.writeUTF(this.gender);
  }
}



public class Main {
  public static void main(String[] args) {
    PersonExt p1 = new PersonExt("John", "Male", 6.7);
    PersonExt p2 = new PersonExt("Wally", "Male", 5.7);
    PersonExt p3 = new PersonExt("Katrina", "Female", 5.4);

    File fileObject = new File("personext.ser");

    try (ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(
        fileObject))) {
      oos.writeObject(p1);
      oos.writeObject(p2);
      oos.writeObject(p3);

      System.out.println(p1);
      System.out.println(p2);
      System.out.println(p3);
    } catch (IOException e1) {
      e1.printStackTrace();
    }

    fileObject = new File("personext.ser");

    try (ObjectInputStream ois = new ObjectInputStream(new FileInputStream(
        fileObject))) {

      p1 = (PersonExt) ois.readObject();
      p2 = (PersonExt) ois.readObject();
      p3 = (PersonExt) ois.readObject();

      // Let"s display the objects that are read
      System.out.println(p1);
      System.out.println(p2);
      System.out.println(p3);

      // Print the input path
      System.out.println("Objects were  read   from  "
          + fileObject.getAbsolutePath());
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}

上面的代码生成以下结果。