使用 readObject 和 writeObject 的 Java 自定义序列化

在许多情况下，您可能需要在 java 中自定义序列化。例如，您有遗留的 Java 类，您出于任何原因都不愿意对其进行修改。也可能存在一些设计限制。或者甚至简单地说，该类预计会在未来的版本中发生变化，这可能会破坏先前序列化对象的反序列化 .

1. Java自定义序列化

在大多数情况下，当您自定义 java 序列化时，您将按顺序一个接一个地编写字段。它是覆盖默认 java 序列化 进程的最常见方式。

比方说，我们有一个 User 对象，我们想要自定义它的序列化过程。

public class User implements Serializable {

	private static final long serialVersionUID = 7829136421241571165L;
	
	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();
	}

	public final String getFirstName() {
		return firstName;
	}

	public final String getLastName() {
		return lastName;
	}

	public final int getAccountNumber() {
		return accountNumber;
	}

	public final Date getDateOpened() {
		return new Date(dateOpened.getTime());
	}

	public final void setFirstName(String aNewFirstName) {
		firstName = aNewFirstName;
	}

	public final void setLastName(String aNewLastName) {
		lastName = aNewLastName;
	}

	public final void setAccountNumber(int aNewAccountNumber) {
		accountNumber = aNewAccountNumber;
	}

	public final void setDateOpened(Date aNewDate) {
		Date newDate = new Date(aNewDate.getTime());
		dateOpened = newDate;
	}
}

1.1. readObject() 和 writeObject() 方法

要自定义序列化和反序列化，请在此类中定义 readObject() 和 writeObject() 方法。

• 在writeObject()方法中，使用ObjectOutputStream提供的writeXXX方法写入类属性。
• 在readObject()方法中，使用ObjectInputStream提供的readXXX方法读取类属性。
• 请注意，类属性在读取和写入方法中的顺序必须相同。

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.Date;

public class User implements Serializable {

	private static final long serialVersionUID = 7829136421241571165L;
	
	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());
	}
}

现在让我们测试代码。

1.2.测试自定义序列化

package com.howtodoinjava.io.example;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.Calendar;
import java.util.Date;

public class TestCustomSerialization 
{
	public static void main(String[] args) 
	{
		// Create new User object
		User myDetails = new User("Lokesh", "Gupta", 102825, new Date(Calendar.getInstance().getTimeInMillis()));

		// Serialization code
		try 
		{
			FileOutputStream fileOut = new FileOutputStream("User.ser");
			ObjectOutputStream out = new ObjectOutputStream(fileOut);
			out.writeObject(myDetails);
			out.close();
			fileOut.close();
		} 
		catch (IOException i) 
		{
			i.printStackTrace();
		}

		// De-serialization code
		User deserializedUser = null;
		try 
		{
			FileInputStream fileIn = new FileInputStream("User.ser");
			ObjectInputStream in = new ObjectInputStream(fileIn);
			deserializedUser = (User) in.readObject();
			in.close();
			fileIn.close();

			// verify the object state
			System.out.println(deserializedUser.getFirstName());
			System.out.println(deserializedUser.getLastName());
			System.out.println(deserializedUser.getAccountNumber());
			System.out.println(deserializedUser.getDateOpened());
		} 
		catch (IOException ioe) 
		{
			ioe.printStackTrace();
		} 
		catch (ClassNotFoundException cnfe) 
		{
			cnfe.printStackTrace();
		}
	}
}

//输出

Lokesh
Gupta
102825
Wed May 24 13:05:25 IST 2017

2.覆盖默认序列化以添加验证

有时您可能需要只执行任何特定的验证，或者在反序列化对象上运行一些业务规则——而不影响默认的 java 序列化机制。当您决定使用 readObject() 和 writeObject() 方法时，这也是可能的。

在此用例中，您可以在 readObject() 和 writeObject() 中使用 defaultReadObject() 和 defaultWriteObject()方法——启用默认序列化和反序列化。然后，您可以在读/写方法中插入自定义验证或业务规则。 这样，JVM 将在默认序列化和反序列化过程发生后立即自动调用您的验证方法。

public class User implements Serializable {

	//class attributes, constructors, setters and getters as shown above

	/**
	 * Always treat de-serialization as a full-blown constructor, by validating the final state of the de-serialized object.
	 */
	private void readObject(ObjectInputStream aInputStream) throws ClassNotFoundException, IOException 
	{
		// perform the default de-serialization first
		aInputStream.defaultReadObject();

		// make defensive copy of the mutable Date field
		dateOpened = new Date(dateOpened.getTime());

		// ensure that object state has not been corrupted or tampered with malicious code
		//validateUserInfo();
	}

	/**
	 * This is the default implementation of writeObject. Customize as necessary.
	 */
	private void writeObject(ObjectOutputStream aOutputStream) throws IOException {
		
		//ensure that object is in desired state. Possibly run any business rules if applicable.
		//checkUserInfo();
		
		// perform the default serialization for all non-transient, non-static fields
		aOutputStream.defaultWriteObject();
	}
}

再次测试代码，您将看到以下输出：

Lokesh
Gupta
102825
Wed May 24 13:10:18 IST 2017

三、总结

正如我们所见，自定义序列化在 Java 中非常容易，它涉及非常简单的设计，即实现 readObject() 和 writeObject() 方法；并添加任何额外的逻辑来支持应用程序业务逻辑。

尽管在大多数情况下默认序列化/反序列化就足够了；仍然需要时，您应在 Java 应用程序中使用自定义序列化。

在评论部分把你的问题告诉我。

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标签2： Java Serialization Java Deserialization Java Serialization

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