什么是序列化和反序列化
序列化:可以将对象转化成一个字节序列,便于存储。
反序列化:将序列化的字节序列还原。
优点:可以实现对象的”持久性”, 所谓持久性就是指对象的生命周期不取决于程序。
序列化方式1: 实现Serializable接口(隐式序列化)
通过实现Serializable接口,这种是隐式序列化(不需要手动),这种是最简单的序列化方式,会自动序列化所有非static和 transient关键字修饰的成员变量。
代码示例:
class Student implements Serializable{
private String name;
private int age;
public static int QQ = 1234;
private transient String address = "CHINA";
Student(String name, int age ){
this.name = name;
this.age = age;
}
public String toString() {
return "name: " + name + "\n"
+"age: " + age + "\n"
+"QQ: " + QQ + "\n"
+ "address: " + address;
}
public void SetAge(int age) {
this.age = age;
}
}
public class SerializableDemo {
public static void main(String[] args) throws IOException,
ClassNotFoundException {
//创建可序列化对象
System.out.println("原来的对象:");
Student stu = new Student("Ming", 16);
System.out.println(stu);
//创建序列化输出流
ByteArrayOutputStream buff = new ByteArrayOutputStream();
ObjectOutputStream out = new ObjectOutputStream(buff);
//将序列化对象存入缓冲区
out.writeObject(stu);
//修改相关值
Student.QQ = 6666; // 发现打印结果QQ的值被改变
stu.SetAge(18); //发现值没有被改变
//从缓冲区取回被序列化的对象
ObjectInputStream in = new ObjectInputStream(new
ByteArrayInputStream(buff.toByteArray()));
Student newStu = (Student) in.readObject();
System.out.println("序列化后取出的对象:");
System.out.println(newStu);
}
}
打印的结果是:
原来的对象:name: Ming age: 16 QQ: 1234
address: CHINA
序列化后取出的对象:
name: Ming age: 16 QQ: 6666 address: null
发现address(被transient)和QQ(被static)也没有被序列化,中途修改QQ的值是为了以防误会QQ被序列化了。因为序列化可以保存对象的状态,但是QQ的值被改变了,说明没有被序列化。static成员不属于对象实例,可能被别的对象修改没办法序列化,序列化是序列对象。对于address被反序列化后由于没有对应的引用,所以为null。而且Serializable不会调用构造方法。
PS:细心的可能发现序列化很诱人,可以保存对象的初始信息,在以后可以回到这个初始状态
序列化方式2:实现Externalizable接口(显式序列化)
实现Externalizable接口继承自Serializable, 我们在实现该接口时,必须实现writeExternal()和readExternal()方法,而且只能通过手动进行序列化,并且两个方法是自动调用的,因此,这个序列化过程是可控的,可以自己选择哪些部分序列化
代码示例:
public class Blip implements Externalizable{
private int i ;
private String s;
public Blip() {}
public Blip(String x, int a) {
System.out.println("Blip (String x, int a)");
s = x;
i = a;
}
public String toString() {
return s+i;
}
@Override
public void writeExternal(ObjectOutput out) throws IOException {
// TODO Auto-generated method stub
System.out.println("Blip.writeExternal");
out.writeObject(s);
out.writeInt(i);
}
@Override
public void readExternal(ObjectInput in) throws IOException,
ClassNotFoundException {
// TODO Auto-generated method stub
System.out.println("Blip.readExternal");
s = (String)in.readObject();
i = in.readInt();
}
public static void main(String[] args) throws FileNotFoundException,IOException, ClassNotFoundException {
System.out.println("Constructing objects");
Blip b = new Blip("A Stirng", 47);
System.out.println(b);
ObjectOutputStream o = new ObjectOutputStream(new
FileOutputStream("F://Demo//file1.txt"));
System.out.println("保存对象");
o.writeObject(b);
o.close();
//获得对象
System.out.println("获取对象");
ObjectInputStream in = new ObjectInputStream(new
FileInputStream("F://Demo//file1.txt"));
System.out.println("Recovering b");
b = (Blip)in.readObject();
System.out.println(b);
}
}
打印的结果是:
Constructing objects
Blip (String x, int a)
A Stirng47
保存对象:
Blip.writeExternal
获取对象:
Recovering b
Blip.readExternal
A Stirng47
当注释掉writeExternal和readExternal方法后打印结果为:
Constructing objects Blip (String x, int a) A Stirng47 保存对象
Blip.writeExternal 获取对象 Recovering b Blip.readExternal null0
这说明:Externalizable类会调用public的构造函数先初始化对象,在调用所保存的内容将对象还原。假如构造方法不是public则会出现运行时错误
序列化方式3:实现Serializable接口+添加writeObject()和readObject()方法。(显+隐序列化)
如果想将方式1和方式2的优点都用到的话,可以采用方式3, 先实现Serializable接口,并且添加writeObject()和readObject()方法。注意这里是添加,不是重写或者覆盖。但是添加的这两个方法必须有相应的格式。
1. 方法必须要被private修饰 —–>才能被调用
2. 第一行调用默认的defaultRead/WriteObject() —–>隐式序列化非static和transient
3. 调用read/writeObject()将获得的值赋给相应的值 —–>显式序列化
代码示例:
public class SerDemo implements Serializable{
public transient int age = 23;
public String name ;
public SerDemo(){
System.out.println("默认构造器。。。");
}
public SerDemo(String name) {
this.name = name;
}
private void writeObject(ObjectOutputStream stream) throws IOException {
stream.defaultWriteObject();
stream.writeInt(age);
}
private void readObject(ObjectInputStream stream) throws
ClassNotFoundException, IOException {
stream.defaultReadObject();
age = stream.readInt();
}
public String toString() {
return "年龄" + age + " " + name;
}
public static void main(String[] args) throws IOException,
ClassNotFoundException {
SerDemo stu = new SerDemo("Ming");
ByteArrayOutputStream bout = new ByteArrayOutputStream();
ObjectOutputStream out = new ObjectOutputStream(bout);
out.writeObject(stu);
ObjectInputStream in = new ObjectInputStream(new
ByteArrayInputStream(bout.toByteArray()));
SerDemo stu1 = (SerDemo) in.readObject();
System.out.println(stu1);
}
}
打印结果为:年龄23 Ming
注释掉stream.writeInt(age)和age= stream.readInt()后:年龄0 Ming
方式3结合了显式和隐式序列化,Ming被正常序列化,由于age被trancient修饰,所以需要显式序列化
最后再说一下 Json序列化
Json序列化一般会使用jackson包,通过ObjectMapper类来进行一些操作,比如将对象转化为byte数组或者将json串转化为对象。现在的大多数公司都将json作为服务器端返回的数据格式。比如调用一个服务器接口,通常的请求为:xxx.json?a=xxx&b=xxx的形式
接着上个例子:
package serialize;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import com.fasterxml.jackson.databind.ObjectMapper;
public class JsonSerialize {
public static void main(String[] args) throws IOException {
new JsonSerialize().start();
}
public void start() throws IOException {
User u = new User();
List<User> friends = new ArrayList<>();
u.setUserName("张三");
u.setPassWord("123456");
u.setUserInfo("张三是一个很牛逼的人");
u.setFriends(friends);
User f1 = new User();
f1.setUserName("李四");
f1.setPassWord("123456");
f1.setUserInfo("李四是一个很牛逼的人");
User f2 = new User();
f2.setUserName("王五");
f2.setPassWord("123456");
f2.setUserInfo("王五是一个很牛逼的人");
friends.add(f1);
friends.add(f2);
ObjectMapper mapper = new ObjectMapper();
Long t1 = System.currentTimeMillis();
byte[] writeValueAsBytes = null;
for (int i = 0; i < 10; i++) {
writeValueAsBytes = mapper.writeValueAsBytes(u);
}
System.out.println("json serialize: " + (System.currentTimeMillis() -
t1) + "ms; 总大小:" + writeValueAsBytes.length);
Long t2 = System.currentTimeMillis();
User user = mapper.readValue(writeValueAsBytes, User.class);
System.out.println("json deserialize: " + (System.currentTimeMillis() -
t2) + "ms; User: " + user);
}
}
运行结果:
json serialize: 55ms; 总大小:341 json deserialize: 35ms;
User: User [userId=null, userName=张三, passWord=123456, userInfo=张三是一个很牛逼的人,
friends=[User [userId=null, userName=李四, passWord=123456, userInfo=李四是一个很牛逼的人, friends=null], User [userId=null, userName=王五, passWord=123456,
userInfo=王五是一个很牛逼的人, friends=null]]]