概念
- json 序列化：把 javabean 转换成 json 字符串
- json 反序列化：把 json 字符串转换成对应的 javabean
问题
- 序列化时，是调用 getXxx 方法来获取 xxx 属性的值吗？
- 反序列化时，是调用 setXxx 方法来设置 xxx 属性的值吗？
试验
用一个 People 类来做试验，注意 43 行的代码
public class People {
/** 姓名 **/
private String name;
/** 身高 m **/
private double height;
/** 体重 kg **/
private double weight;
/** bmi 指数 **/
private double bmi;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public double getHeight() {
return height;
}
public void setHeight(double height) {
this.height = height;
}
public double getWeight() {
return weight;
}
public void setWeight(double weight) {
this.weight = weight;
}
public double getBmi() {
bmi = weight / (height * height);
return bmi;
}
public void setBmi(double bmi) {
this.bmi = bmi;
}
}fastjson
@Test
public void fastjson() {
People people = new People();
people.setName("八戒");
people.setWeight(100.00);
people.setHeight(1.70);
people.setBmi(30);
log.info(JSON.toJSONString(people));
}输出结果为
{"bmi":34.602076124567475,"height":1.7,"name":"八戒","weight":100.0}可见，fastjosn 是调用了 getXxx 方法来获取属性值
gson
@Test
public void gson() {
People people = new People();
people.setName("八戒");
people.setWeight(100.00);
people.setHeight(1.70);
people.setBmi(30);
log.info(new Gson().toJson((people)));
}输出结果为
{"name":"八戒","height":1.7,"weight":100.0,"bmi":30.0}可见，gson 是直接获取 xxx 属性的值
试验二
修改一下 People 类，删除 bmi 属性，仅保留 getBmi() 方法，如下
public class People {
/** 姓名 **/
private String name;
/** 身高 m **/
private double height;
/** 体重 kg **/
private double weight;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public double getHeight() {
return height;
}
public void setHeight(double height) {
this.height = height;
}
public double getWeight() {
return weight;
}
public void setWeight(double weight) {
this.weight = weight;
}
public double getBmi() {
return weight / (height * height);
}
}fastjson
@Test
public void fastjson() {
People people = new People();
people.setName("八戒");
people.setWeight(100.00);
people.setHeight(1.70);
// people.setBmi(30);
log.info(JSON.toJSONString(people));
}输出结果为
{"bmi":34.602076124567475,"height":1.7,"name":"八戒","weight":100.0}可见，即使不存在属性，但只要有 get 方法，fastjosn 会视同有这个属性
gson
@Test
public void gson() {
People people = new People();
people.setName("八戒");
people.setWeight(100.00);
people.setHeight(1.70);
// people.setBmi(30);
log.info(new Gson().toJson((people)));
}输出结果为
{"name":"八戒","height":1.7,"weight":100.0}可见，gson 不会根据 get 方法来推断属性
fastjson 序列化和反序列化的风险
由于 fastjson 是根据 get 方法来推断属性并取值，就存在这样的风险：一个对象，经过 fastjson 序列化再反序列化以后，会得到一个不相等(equals)的对象，看下面代码
public class Car {
private String brand;
private long speed;
public String getBrand() {
return brand;
}
public void setBrand(String brand) {
this.brand = brand;
}
public long getSpeed() {
if (speed < 100) {
speed = 100;
} else {
speed = 200;
}
return speed;
}
}测试用例如下
@Test
public void test() {
Car car = new Car();
car.setBrand("bmw");
String json = JSON.toJSONString(car);
log.info(json);
Car car1 = JSON.parseObject(json, Car.class);
Assert.assertNotEquals(car.getSpeed(), car1.getSpeed());
}运行测试，发现两个对象并不相等
结论
虽然上面仅对 get 方法进行了试验，没有试验 set 方法，但应该不影响结论——无论是用 fastjson 还是 gson 对一个 java 对象进行序列化/反序列化，都要确保以下 2 点
get/set方法对应的属性存在- 属性值和通过 get 方法得到的值相同：即在
get/set方法里不要做额外的操作
如果上面 2 点满足不了，可能会出现莫名其妙的问题
思考
什么是 javabean
到底什么是 javabean？wikipedia 的 定义 如下
In computing based on the Java Platform, JavaBeans are classes that encapsulate many objects into a single object (the bean). They are serializable, have a zero-argument constructor, and allow access to properties using getter and setter methods
关于属性值和 get/set 方法是这样的
access to properties using getter and setter methods
根据这个说法，我认为 get/set 方法是用来访问属性值的，这意味着
- 如果没有这个属性，就不该有 get/set 方法
- get/set 方法应该是获取/设置属性值，而不包含其他功能
gson 和 fastjson 的处理对象
先看 gson
A Java serialization/deserialization library to convert Java Objects into JSON and back
Gson is a Java library that can be used to convert Java Objects into their JSON representation. It can also be used to convert a JSON string to an equivalent Java object. Gson can work with arbitrary Java objects including pre-existing objects that you do not have source-code of.
gson 是对 java 对象进行序列化和反序列化的库，并没有限定在 javabean 上，我的想法是（未验证）
- gson 默认是对 javabean 进行操作并遵循 javabean 的规范
- gson 也可以操作非 javabean 的对象，但该对象要通过特殊的注解来标记需要处理的属性或方法
再看 fastjson
A fast JSON parser/generator for Java
Fastjson is a Java library that can be used to convert Java Objects into their JSON representation. It can also be used to convert a JSON string to an equivalent Java object. Fastjson can work with arbitrary Java objects including pre-existing objects that you do not have source-code of.
居然和 gson 一毛一样，也不知道谁抄谁的
