Hướng dẫn và ví dụ Java ObjectInputStream

Tóm Tắt

1- ObjectInputStream

ObjectInputStream là một lớp con của lớp InputStream. Nó quản lý một đối tượng InputStream và cung cấp các phương thức để đọc dữ liệu nguyên thuỷ (primitive data) hoặc đối tượng từ InputStream mà nó quản lý.


public class ObjectInputStream
          extends InputStream implements ObjectInput, ObjectStreamConstants

ObjectInputStream được sử dụng để đọc các nguồn dữ liệu được ghi bởi ObjectOutputStream.

Bạn đang đọc: Hướng dẫn và ví dụ Java ObjectInputStream

ObjectInputStream methods


public boolean readBoolean() throws IOException  
public byte readByte() throws IOException  
public int readUnsignedByte() throws IOException  
public char readChar() throws IOException  
public short readShort() throws IOException  
public int readUnsignedShort() throws IOException  
public int readInt() throws IOException  
public long readLong() throws IOException  
public float readFloat() throws IOException  
public double readDouble() throws IOException  
public void readFully(byte[] buf) throws IOException  
public void readFully(byte[] buf, int off, int len) throws IOException  
public int skipBytes(int len) throws IOException  
public String readUTF() throws IOException  
public final ObjectInputFilter getObjectInputFilter()  
public final void setObjectInputFilter(ObjectInputFilter filter)  

public final Object readObject() throws IOException, ClassNotFoundException  
public Object readUnshared() throws IOException, ClassNotFoundException  
public void defaultReadObject() throws IOException, ClassNotFoundException  
public ObjectInputStream.GetField readFields() throws IOException, ClassNotFoundException  
public void registerValidation(ObjectInputValidation obj, int prio) throws NotActiveException, InvalidObjectException

protected Class resolveClass(ObjectStreamClass desc) throws IOException, ClassNotFoundException  
protected Object readObjectOverride() throws IOException, ClassNotFoundException  
protected Class resolveProxyClass(String[] interfaces) throws IOException, ClassNotFoundException  
protected Object resolveObject(Object obj) throws IOException  
protected boolean enableResolveObject(boolean enable) throws SecurityException  
protected void readStreamHeader() throws IOException, StreamCorruptedException  
protected ObjectStreamClass readClassDescriptor() throws IOException, ClassNotFoundException  

String readTypeString() throws IOException

// Methods inherited from InputStream:

public int read() throws IOException  
public int read(byte[] buf, int off, int len) throws IOException  
public int available() throws IOException  
public void close() throws IOException

ObjectInputStream constructors


public ObjectInputStream(InputStream in)

2- Example 1

Trong ví dụ này chúng ta sẽ ghi các đối tượng Employee vào một file, sau đó sử dụng ObjectInputStream để đọc file đó.

Lớp Employee cần phải thi hành (implement) interface Serializable, điều này là cần thiết để nó có thể được ghi vào một ObjectOutputStream.

Employee. java


package org.o7planning.beans;
 
import java.io.Serializable;
 
public class Employee implements Serializable {
 
    private static final long serialVersionUID = 1L;
 
    private String fullName;
    private float salary;
 
    public Employee(String fullName, float salary) {
        this.fullName = fullName;
        this.salary = salary;
    }
 
    public String getFullName() {
        return fullName;
    }
 
    public void setFullName(String firstName) {
        this.fullName = firstName;
    }
 
    public float getSalary() {
        return salary;
    }
 
    public void setSalary(float lastName) {
        this.salary = lastName;
    }
}

Tiếp theo, sử dụng ObjectOutputStream để ghi các đối tượng Employee vào file.

WriteEmployeeDataEx.java

Xem thêm: Các Cấu trúc Dữ liệu Array, Linked list, Stack năm 2022


package org.o7planning.objectinputstream.ex;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;
import java.io.OutputStream;
import java.util.Date;

import org.o7planning.beans.Employee;

public class WriteEmployeeDataEx {

    // Windows: C:/Data/test/employees.data
    private static String file_path = "/Volumes/Data/test/employees.data";

    public static void main(String[] args) throws IOException {
        File outFile = new File(file_path);

        outFile.getParentFile().mkdirs();

        Employee e1 = new Employee("Tom", 1000f);
        Employee e2 = new Employee("Jerry", 2000f);
        Employee e3 = new Employee("Donald", 1200f);

        Employee[] employees = new Employee[] { e1, e2, e3 };

        OutputStream os = new FileOutputStream(outFile);
        ObjectOutputStream oos = new ObjectOutputStream(os);

        System.out.println("Writing file: " + outFile.getAbsolutePath());

        oos.writeObject(new Date());
        oos.writeUTF("Employee data"); // Some informations.

        oos.writeInt(employees.length); // Number of Employees

        for (Employee e : employees) {
            oos.writeObject(e);
        }
        oos.close();
        System.out.println("Finished!");
    }
}

Sau khi chạy lớp WriteEmployeeDataEx chúng ta nhận được một file với nội dung khá khó hiểu

Cuối cùng, sử dụng ObjectInputStream để đọc file vừa được ghi ra ở bước trên.

ReadEmployeeDataEx. java


package org.o7planning.objectinputstream.ex;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.util.Date;

import org.o7planning.beans.Employee;

public class ReadEmployeeDataEx {

    // Windows: C:/Data/test/employees.data
    private static String file_path = "/Volumes/Data/test/employees.data";

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        File inFile = new File(file_path);
 

        InputStream is = new FileInputStream(inFile);
        ObjectInputStream ois = new ObjectInputStream(is);

        System.out.println("Reading file: " + inFile.getAbsolutePath());
        System.out.println();

        Date date = (Date) ois.readObject();
        String info = ois.readUTF();
        
        System.out.println(date);
        System.out.println(info);
        System.out.println();
        
        int employeeCount = ois.readInt();
        
        for(int i=0; i< employeeCount; i++) {
            Employee e = (Employee) ois.readObject();
            System.out.println("Employee Name: " + e.getFullName() +" / Salary: " + e.getSalary());
        }
        ois.close();
    }
}

Output :


Reading file: /Volumes/Data/test/employees.data

Sat Mar 20 18:54:24 KGT 2021
Employee data

Employee Name: Tom / Salary: 1000.0
Employee Name: Jerry / Salary: 2000.0
Employee Name: Donald / Salary: 1200.0

Giả sử bạn sử dụng ObjectInputStream để đọc một đối tượng GameSetting từ một file. Đồng thời, trong quá trình đọc được đối tượng GameSetting bạn muốn sửa đổi giá trị của một vài trường (field).

GameSetting. java


package org.o7planning.beans;

import java.io.IOException;
import java.io.ObjectInputStream;

public class GameSetting implements java.io.Serializable {

    private static final long serialVersionUID = 1L;

    private int sound;
    private int bightness;
    private String difficultyLevel;

    private String userNote;

    public GameSetting(int sound, int bightness, String difficultyLevel, String userNote) {
        this.sound = sound;
        this.bightness = bightness;
        this.difficultyLevel = difficultyLevel;
        this.userNote = userNote;
    }

    public int getSound() {
        return sound;
    }

    public int getBightness() {
        return bightness;
    }

    public String getDifficultyLevel() {
        return difficultyLevel;
    }

    public String getUserNote() {
        return userNote;
    }

    // Do not change name and parameter of this method.
    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        ObjectInputStream.GetField fields = in.readFields();

        this.sound = fields.get("sound", 50);
        this.bightness = fields.get("bightness", 50);
        
        // Edit fields
        this.difficultyLevel = (String) fields.get("difficultyLevel", "Easy"); // Default
        if (this.difficultyLevel == null) {
            this.difficultyLevel = "Easy";
        }

        this.userNote = (String) fields.get("userNote", "Have fun!"); // Default
        if (this.userNote == null) {
            this.userNote = "Have fun!";
        }
    }
}

ObjectInputStream_readFields. java


package org.o7planning.objectinputstream.ex;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.OutputStream;
import java.util.Date;

import org.o7planning.beans.GameSetting;

public class ObjectInputStream_readFields {

    // Windows: C:/Data/test/game_setting.data
    private static String file_path = "/Volumes/Data/test/game_setting.data";

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        
        GameSetting setting = new GameSetting(10, 80, null, null);

        writeGameSetting(setting);
        readGameSetting();
    }

    private static void writeGameSetting(GameSetting setting) throws IOException {
        File file = new File(file_path);
        file.getParentFile().mkdirs();

        OutputStream os = new FileOutputStream(file);
        ObjectOutputStream oos = new ObjectOutputStream(os);

        // Write a String
        oos.writeUTF("Game Settings, Save at " + new Date());
        // Write Object
        oos.writeObject(setting);

        oos.close();
    }

    private static void readGameSetting() throws IOException, ClassNotFoundException {
        File file = new File(file_path);
        file.getParentFile().mkdirs();

        InputStream is = new FileInputStream(file);
        ObjectInputStream ois = new ObjectInputStream(is);

        // Read a String
        String info = ois.readUTF();

        // Read fields
        GameSetting setting = (GameSetting) ois.readObject();

        System.out.println("sound: " + setting.getSound());
        System.out.println("bightness: " + setting.getBightness());
        System.out.println("difficultyLevel: " + setting.getDifficultyLevel());
        System.out.println("userNote: " + setting.getUserNote());  // null.

        ois.close();
    }
}

Output :


sound: 10
bightness: 80
difficultyLevel: Easy
userNote: Have fun!

Phương thức ObjectInputStream.readUnshared() được sử dụng để đọc một đối tượng đã được ghi bởi phương thức ObjectOutputStream.writeUnshared(Object).

Xem thêm: Interface là gì? Mục đích sử dụng của Interface

ObjectInputStream_readUnshared . java


package org.o7planning.objectinputstream.ex;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.OutputStream;
import java.util.ArrayList;

public class ObjectInputStream_readUnshared {

    // Windows: C:/Data/test/test1.data
    private static String file_path = "/Volumes/Data/test/test.data";

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        writeUnsharedTest();
        
        readUnsharedTest();
    }
    
    private static void writeUnsharedTest() throws IOException  {
        File file = new File(file_path);
        file.getParentFile().mkdirs();
        
        ArrayList list = new ArrayList();
        list.add("One");
        list.add("Two");
        
        OutputStream os = new FileOutputStream(file);
        ObjectOutputStream oos = new ObjectOutputStream(os);
        
        oos.writeUnshared(list); // Write the first time
        oos.writeUnshared(list); // Write the second time
        oos.close();
    }

    @SuppressWarnings({ "unchecked" })
    private static void readUnsharedTest() throws IOException, ClassNotFoundException  {
        File file = new File(file_path);
        
        ArrayList list = new ArrayList();
        list.add("One");
        list.add("Two");
        
        InputStream is = new FileInputStream(file);
        ObjectInputStream ois = new ObjectInputStream(is);
        
        ArrayList list1 = (ArrayList) ois.readUnshared();  
        ArrayList list2 = (ArrayList) ois.readUnshared();  
        
        System.out.println("list1 == list2? " + (list1 == list2));
        
        ois.close();
    }
}

Output :


list1 == list2? false

Xem thêm giải thích về phương thức ObjectOutputStream.writeUnshared(Object):

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