Kryo序列化器（快速上手）

1. 简介

将Java对象进行缓存或者在网络间传输时，Kryo用于将Java对象序列化成为字节数组，或将字节数组反序列化成Java对象。

2. 优点

序列化后的结果小；功能完善；支持用户自定序列化器，注册类的序列化器非常方便。

3. 示例与对比

定义类：学生

public class Student implements Serializable {
    private String name;
    private int age;
    private boolean isMale;
  
    public Student(){}
 
    public Student(String name, int age, boolean isMale) {
        this.name = name;
        this.age = age;
        this.isMale = isMale;
    }
}

方法一：用JDK自带的序列化功能

Student student = new Student("张三", 18, true);
 
//serialize
ByteArrayOutputStream bos = new ByteArrayOutputStream();
ObjectOutputStream out = new ObjectOutputStream(bos);
out.writeObject(student);
out.close();
byte[] bytes = bos.toByteArray();
 
//deserialize
ByteArrayInputStream bis = new ByteArrayInputStream(bytes);
ObjectInputStream in = new ObjectInputStream(bis);
Student newStudent = (Student) in.readObject();
in.close();
 
assert newStudent.equals(student);

方法二：用Kryo原生的序列化反序列化功能

Student student = new Student("张三", 18, true);
Kryo kryo = new Kryo();
 
//serialize
ByteArrayOutputStream bos = new ByteArrayOutputStream();
Output output = new Output(bos);
kryo.writeObject(output, student);
output.flush();
byte[] bytes = bos.toByteArray();
 
//deserialize
Input input = new Input(bytes);
Student newStudent = kryo.readObject(input, Student.class);
 
assert student.equals(newStudent);

方法三：自定义Student的序列化器，并注册到Kryo中

public class StudentSerializer extends Serializer<Student> {
    @Override
    public void write(Kryo kryo, Output output, Student object) {
        output.writeString(object.getName());
        output.writeVarInt(object.getAge(), true);
        output.writeBoolean(object.isMale());
    }
 
    @Override
    public Student read(Kryo kryo, Input input, Class<Student> type) {
        String name = input.readString();
        int age = input.readVarInt(true);
        boolean isMale = input.readBoolean();
        return new Student(name, age, isMale);
    }
}

向Kryo注册序列化器

Kryo kryo = new Kryo();
kryo.register(Student.class, new StudentSerializer());

序列化和反序列化与方法二一致。

4. 序列化效果对比

5. Kryo核心类图与基本用法

Kryo核心类图

Kryo对Class的序列化：

kryo.writeClass(Output output,  Class type)
kryo.readClass(Input input)
//原理：初次序列化类名，后续序列化Registeration的ID

Kryo对Object的序列化：

kryo.writeObject(Output output, Object object)
kryo.readObject<T>(Input input, Class<T> type)
//原理：先根据Class信息获取Registeration，并获取对应的Serializer
// 然后调用Serializer的read和write方法
kryo.writeObjectOrNull(Output output, Object object，Class type)
kryo.readObjectOrNull<T>(Input input, Class<T> type)
//原理：写入每个对象的序列化结果之前，先往buffer中写入一个boolean用来表示当前对象是否为null，序列化结果会更大
kryo.writeClassAndObject(Output output, Object object)
kryo.readClassAndObject<T>(Input input)
//写入对象和对象的类信息，用于在反序列化时不知道对象Class的情况

6. Kryo对多引用对象的优化：kryo.setReferences(true)

优点：当被序列化对象（grade）中存在对另外一个对象(student)的多次引用时，只序列化引用对象一次，后面只是序列化该对象的ID
缺点：需要为每个Object绑定自增ID，并维护其Mapping。

//初始化参数和注册序列化器
Kryo kryo = new Kryo();
kryo.setReferences(true);
kryo.register(Student.class, new StudentSerializer());
kryo.register(Grade.class, new GradeSerializer());
 
//创建对象
Student student = new Student("张三", 18, true);
List<Student>  students = new ArrayList<Student>(10);
int size = 10;
while(size -- >0){
    students.add(student);
}
Grade grade = new Grade("Grade1",students);
//序列化
...
kryo.writeObject(output, grade);
...
//反序列化
...
Grade newGrade = kryo.readObject(input, Grade.class);
...

Grade类

public class Grade {
    private String gradeName;
    private List<Student> students;
}

reference=false

reference=true