Java序列化示例

た入场券 2023-02-15 01:27 21阅读 0赞

在Java中， `Serialization`意味着将对象转换为字节流，可以将其保存到文件中或通过网络传输， `Deserialization`则相反。

序列化示例。

// Save object into a file.
        public static void writeObject(Object obj, File file) throws IOException {
            try (FileOutputStream fos = new FileOutputStream(file);
                 ObjectOutputStream oos = new ObjectOutputStream(fos)) {
                oos.writeObject(obj);
                oos.flush();
            }
        }

反序列化示例。

// Get object from a file.
        public static Object readObject(File file) throws IOException, ClassNotFoundException {
            Object result = null;
            try (FileInputStream fis = new FileInputStream(file);
                 ObjectInputStream ois = new ObjectInputStream(fis)) {
                result = ois.readObject();
            }
            return result;
        }

## 1. POJO ##

查看一个简单的`Person`类，实现`Serializable` 。

Person.java

package com.mkyong.io.object;
    
    import java.io.Serializable;
    import java.math.BigDecimal;
    
    public class Person implements Serializable {
    
        // optional, if missing, JVM will create it.
        // better declare as a version control.
        private static final long serialVersionUID = 1L;
    
        private String name;
        private int age;
        // dun save this field into file
        private transient BigDecimal salary;
    
        // getters, setters, toString, constructor
    
    }

在Java中，需要序列化的对象必须实现`Serializable`接口，否则会遇到以下错误。

Terminal

Exception in thread "main" java.io.NotSerializableException: com.mkyong.io.object.Person
    
      at java.base/java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1185)
      at java.base/java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:349)
      at com.mkyong.io.object.ObjectUtils.writeObject(ObjectUtils.java:14)
      at com.mkyong.io.object.ObjectUtils.main(ObjectUtils.java:35)

## 2. Java序列化示例 ##

参见注释以进行自我解释。

ObjectUtils.java

package com.mkyong.io.object;
    
    import java.io.*;
    import java.math.BigDecimal;
    
    // serialization and deserialization
    public class ObjectUtils {
    
        // Serialization
        // Save object into a file.
        public static void writeObject(Person obj, File file) throws IOException {
            try (FileOutputStream fos = new FileOutputStream(file);
                 ObjectOutputStream oos = new ObjectOutputStream(fos)) {
                oos.writeObject(obj);
                oos.flush();
            }
        }
    
        // Deserialization
        // Get object from a file.
        public static Person readObject(File file) throws IOException, ClassNotFoundException {
            Person result = null;
            try (FileInputStream fis = new FileInputStream(file);
                 ObjectInputStream ois = new ObjectInputStream(fis)) {
                result = (Person) ois.readObject();
            }
            return result;
        }
    
        public static void main(String[] args) throws IOException, ClassNotFoundException {
    
            Person person = new Person("mkyong", 40, new BigDecimal(900));
    
            // object -> file
            ObjectUtils.writeObject(person, new File("person.obj"));
    
            // file to object
            Person obj = ObjectUtils.readObject(new File("person.obj"));
            System.out.println(obj);
    
        }
    }

输出量

Terminal

Person{name='mkyong', age=40, salary=null}

**什么是瞬态？**  
为什么薪水为零？ 序列化期间，JVM会忽略所有`transient`字段。 如果我们需要在序列化期间排除特定对象的字段，请将其标记为`transient.`

## 3.什么是serialVersionUID？ ##

如果缺少`serialVersionUID` ，则JVM将自动创建它。 `serialVersionUID`类似于版本号。 简而言之，如果我们将对象保存为`1L` ，则需要提供相同的`1L`来读取对象，否则会遇到不兼容的错误。

Person.java

public class Person implements Serializable {
    
        private static final long serialVersionUID = 1L;
        //...
    }

例如，我们将`serialVersionUID = 1L`的对象保存到文件名`person.obj` 。 稍后，我们从对象中添加或删除一些字段，并将`serialVersionUID`更新为`2L` 。 现在，读取`person.obj`文件并尝试将其转换回修改后的对象，因为两个`serialVersionUID`不同，我们将遇到以下不兼容错误：

Terminal

Exception in thread "main" java.io.InvalidClassException: com.mkyong.io.object.Person;
    
      local class incompatible: stream classdesc serialVersionUID = 1, local class serialVersionUID = 2
    
    	at java.base/java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:689)
    	at java.base/java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1903)
    	at java.base/java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1772)
    	at java.base/java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2060)
    	at java.base/java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1594)
    	at java.base/java.io.ObjectInputStream.readObject(ObjectInputStream.java:430)
    	at com.mkyong.io.object.ObjectUtils.readObject(ObjectUtils.java:25)
    	at com.mkyong.io.object.ObjectUtils.main(ObjectUtils.java:38)

## 下载源代码 ##

> $ git clone [https://github.com/mkyong/core-java.git][https_github.com_mkyong_core-java.git]
> 
> $ cd java-io

## 参考文献 ##

*  [Java对象序列化规范][Java]
 *  [可序列化的JavaDoc][JavaDoc]
 *  [ObjectInputStream JavaDoc][]
 *  [ObjectOutputStreamJavaDoc][]
 *  Java –什么是serialVersionUID

标记： [Java][Java 1] [序列化][Link 1]

> 翻译自: [https://mkyong.com/java/java-serialization-examples/][https_mkyong.com_java_java-serialization-examples]

[https_github.com_mkyong_core-java.git]: https://github.com/mkyong/core-java.git
[Java]: https://docs.oracle.com/en/java/javase/11/docs/specs/serialization/input.html
[JavaDoc]: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/io/Serializable.html
[ObjectInputStream JavaDoc]: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/io/ObjectInputStream.html
[ObjectOutputStreamJavaDoc]: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/io/ObjectOutputStream.html
[Java 1]: https://mkyong.com/tag/java/
[Link 1]: https://mkyong.com/tag/serialization/
[https_mkyong.com_java_java-serialization-examples]: https://mkyong.com/java/java-serialization-examples/