java transient反序列化

1.前言

遇到ctf题目里面需要反序列化transient关键字的变量
但是transient关键字修饰的变量是不进入反序列的

下面是部分代码

public class testC implements Serializable {
    private String name;
    private String age;
    private transient String height;

    public testC(String name, String age, String height) {
        this.name = name;
        this.age = age;
        this.height = height;
    }

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getAge() {
        return this.age;
    }

    public void setAge(String age) {
        this.age = age;
    }

    public String getHeight() {
        return this.height;
    }

    public void setHeight(String height) {
        this.height = height;
    }

    private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
        s.defaultReadObject();
        this.height = (String)s.readObject();
    }

    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        } else if (this == obj) {
            return true;
        } else if (obj instanceof testC) {
            testC user = (testC)obj;
            return user.getAge().equals(this.age) && user.getHeight().equals(this.height) && user.getName().equals(this.name);
        } else {
            return false;
        }
    }

    public String toString() {
        return "User{name='" + this.name + '\'' + ", age='" + this.age + '\'' + ", height='" + this.height + '\'' + '}';
    }
}

2.transient关键字的作用

在java中，transient主要用来防止成员变量被反序列化。
使用语法如下

@transient
private String var1;
private transient String var2;

@transient用于hibernate entity class中，使用后就会忽略掉这个字段不会让这些变量写入数据库中

3.让transient变量进入反序列化

用下面的代码来解释一下readObject和writeObject方法

public class User implements Serializable {
  private String firstName;
  private String lastName;
  private int accountNumber;
  private Date dateOpened;
 
  public User(String firstName, String lastName, int accountNumber, Date dateOpened) {
    super();
    this.firstName = firstName;
    this.lastName = lastName;
    this.accountNumber = accountNumber;
    this.dateOpened = dateOpened;
  }
   
  public User() {
    super();
  }
 
  //Setters and Getters
 
  private void readObject(ObjectInputStream aInputStream) throws ClassNotFoundException, IOException 
  {   
    firstName = aInputStream.readUTF();
    lastName = aInputStream.readUTF();
    accountNumber = aInputStream.readInt();
    dateOpened = new Date(aInputStream.readLong());
  }
 
  private void writeObject(ObjectOutputStream aOutputStream) throws IOException 
  {
    aOutputStream.writeUTF(firstName);
    aOutputStream.writeUTF(lastName);
    aOutputStream.writeInt(accountNumber);
    aOutputStream.writeLong(dateOpened.getTime());
  }
}

writeXXX由ObjectOutputStream提供，readXXX同理
读取和写入的顺序必须是一样的
序列化的时候会调用writeXXX把其中的字段写入
反序列化的时候就会调用readXXX，按写入顺序读取（类型要相同）
writeUTF是写入字符串，Int是整数，等等
writeObject可以写入任意的类型

那么在看回最开始的题目，显而易见，只需要在testC中这个类加入writeObject，在进行序列化就可以了

public class testC implements Serializable {
    private String name;
    private String age;
    private transient String height;

    public testC(String name, String age, String height) {
        this.name = name;
        this.age = age;
        this.height = height;
    }

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getAge() {
        return this.age;
    }

    public void setAge(String age) {
        this.age = age;
    }

    public String getHeight() {
        return this.height;
    }

    public void setHeight(String height) {
        this.height = height;
    }

    private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
        s.defaultReadObject();
        this.height = (String)s.readObject();
    }

    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        } else if (this == obj) {
            return true;
        } else if (obj instanceof testC) {
            testC user = (testC)obj;
            return user.getAge().equals(this.age) && user.getHeight().equals(this.height) && user.getName().equals(this.name);
        } else {
            return false;
        }
    }

    private void writeObject(ObjectOutputStream s) throws IOException{
        s.defaultWriteObject();
        s.writeObject(this.height);
    }

    public String toString() {
        return "User{name='" + this.name + '\'' + ", age='" + this.age + '\'' + ", height='" + this.height + '\'' + '}';
    }
}

参考链接

https://www.delftstack.com/howto/java/transient-in-java/

https://howtodoinjava.com/java/serialization/custom-serialization-readobject-writeobject/