Java基础--序列化Serializable

对Java对象序列化的目的是持久化对象或者为RMI（远程方法调用）传递参数和返回值。

下面是一个序列化对象写入文件的例子：

----------------------------

 package utils;

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

 //同样需要实现Serializable
 class Data implements Serializable {
     private static final long serialVersionUID = 3502676600783498362L;
     private String name;
     public Data(String name){this.name=name;}
     public String toString(){return "d:"+name;}
 }

 //Serializable 为序列化标记接口，不包括任何方法
 class User implements Serializable {
     //序列化标识，用于标记类是否发生变化
     private static final long serialVersionUID = -6950896768312602572L;
     private transient String id;        //transient关键字可以阻止该字段被序列化
     private String name;
     private int age;
     private Data data;        //包含另一个对象
     public static String kind="human";        //static 字段不可序列化
     public User(String id, String name, int age){
         this.id=id;
         this.name=name;
         this.age=age;
         this.data=new Data(name);
     }
     @Override
     public String toString(){
         return "["+id+"-"+name+"-"+age+"-"+kind+"-"+data+"]";
     }
 }

 public class SerialUtil {
     //序列化会暴露对象所有信息(包括private)，所以要对敏感信息加密后再序列化
     public static void main(String[] args) throws IOException, ClassNotFoundException {
         serialize();        //序列化
         deSerialize();        //反序列化
     }

     public static void serialize() throws IOException {
         User bean1 = new User("1", "lg", 123);
         User bean2 = new User("2", "ss", 23);
         //ObjectOutputStream为对象输出流，输出到users文件中，名称后缀随意
         ObjectOutputStream oo = new ObjectOutputStream(new FileOutputStream(new File("users")));
         oo.writeObject(bean1);        //将对象序列化到输出流中
         oo.writeObject(bean2);        //写入另一个
         oo.close();
     }

     //如果远程jvm中没有User.class，将抛出ClassNotFoundException
     public static void deSerialize() throws IOException, ClassNotFoundException {
         //使用ObjectInputStream对象输入流读取文件，反序列化对象
         ObjectInputStream ois = new ObjectInputStream(new FileInputStream(new File("users")));
         User bean1 = (User) ois.readObject();        //通过二进制构造对象，并不是通过构造函数
         User bean2 = (User) ois.readObject();        //读取下一个
         ois.close();
         System.out.println(bean1);
         System.out.println(bean2);
     }

 }

----------------------------

输出结果：

[null-lg-123-human-d:lg]

[null-ss-23-human-d:ss]

如果需要进一步定制序列化反序列化过程，可通过实现接口Externalizable，实现writeExternal和readExternal方法，下面是一个例子：

---------------------------------

 package demos;

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

 public class SerializeBean implements Externalizable{
     private int i;
     private String s;
     private String ss;

     //从运行结果可见反序列化需要调用该默认构造器，如果为private则会抛出InvalidClassException
     public SerializeBean(){
         System.out.println("SerializeBean default constractor.");
     }

     public SerializeBean(int i, String s, String ss){
         this.i=i;
         this.s=s;
         this.ss=ss;
     }

     @Override
     public String toString(){
         return "["+i+"-"+s+"-"+ss+"]";
     }

     //序列化过程中自动调用
     @Override
     public void writeExternal(ObjectOutput out) throws IOException {
         System.out.println("writeExternal here");
         out.writeObject(s);        //此处添加需要序列化的属性
         out.writeInt(i);
     }

     //反序列化过程中自动调用
     @Override
     public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
         System.out.println("readExternal here");
         s=(String)in.readObject();        //此处添加自定义处理逻辑
         i=in.readInt();
     }

     public static void main(String[] args) throws FileNotFoundException, IOException, ClassNotFoundException {
         SerializeBean bean1 = new SerializeBean(99, "iamstring","ss");
         SerializeBean bean2 = new SerializeBean(-44, "string too", "ss");

         //序列化到文件
         ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("out"));
         oos.writeObject(bean1);
         oos.writeObject(bean2);
         oos.close();
         bean1=null;
         bean2=null;

         //从文件反序列化
         ObjectInputStream ois = new ObjectInputStream(new FileInputStream("out"));
         bean1 = (SerializeBean)ois.readObject();
         bean2 = (SerializeBean)ois.readObject();
         System.out.println(bean1);
         System.out.println(bean2);
     }

 }

---------------------------------

输出：

writeExternal here

writeExternal here

SerializeBean default constractor.

readExternal here

SerializeBean default constractor.

readExternal here

[99-iamstring-null]

[-44-string too-null]

ss并未序列化，可见起值为null