SpringBoot中实现子类的反序列化

目标

在SpringBoot接口中，咱们通常用@RequestBody类注解须要反序列化的对象，可是当存在多个子类的状况下，常规的反序列化不能知足需求，好比：

咱们有一个类Exam用于表示一张试卷：

@Data
public class Exam {

    private String name;
    private List<Question> questions;
}
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这里Question比较特殊，Question自己是一个抽象类，提供了一些通用的方法调用，实际子类有单选题、多选题、判断题多种状况

实现

SprintBoot内置的序列化是使用的Jackson，查阅文档后发现Jackson提供了@JsonTypeInfo和@JsonSubTypes这两个注解，搭配使用，能够根据指定的字段值来指定实例化中用到的具体的子类类型

这几个类的实际代码以下：
抽象基类Question：

@Data
@JsonTypeInfo(
        use = JsonTypeInfo.Id.NAME,
        include = JsonTypeInfo.As.EXISTING_PROPERTY,
        property = "type",
        visible = true)
@JsonSubTypes({
        @JsonSubTypes.Type(value = SingleChoiceQuestion.class, name = Question.SINGLE_CHOICE),
        @JsonSubTypes.Type(value = MultipleChoiceQuestion.class, name = Question.MULTIPLE_CHOICE),
        @JsonSubTypes.Type(value = TrueOrFalseQuestion.class, name = Question.TRUE_OR_FALSE),
})
public abstract class Question {

    protected static final String SINGLE_CHOICE = "single_choice";
    protected static final String MULTIPLE_CHOICE = "multiple_choice";
    protected static final String TRUE_OR_FALSE = "true_or_false";

    protected String type;
    protected String content;
    protected String answer;

    protected boolean isCorrect(String answer) {
        return this.answer.equals(answer);
    }
}
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判断题TrueOrFalseQuestion：

@Data
@EqualsAndHashCode(callSuper = true)
public class TrueOrFalseQuestion extends Question {

    public TrueOrFalseQuestion() {
        this.type = TRUE_OR_FALSE;
    }
}
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选择题ChoiceQuestion：

@Data
@EqualsAndHashCode(callSuper = true)
public abstract class ChoiceQuestion extends Question {

    private List<Option> options;

    @Data
    public static class Option {
        private String code;
        private String content;
    }
}
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单选题SingleChoiceQuestion：

@Data
@EqualsAndHashCode(callSuper = true)
public class SingleChoiceQuestion extends ChoiceQuestion {

    public SingleChoiceQuestion() {
        this.type = SINGLE_CHOICE;
    }
}
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多选题MultipleChoiceQuestion：

@Data
@EqualsAndHashCode(callSuper = true)
public class MultipleChoiceQuestion extends ChoiceQuestion {

    public MultipleChoiceQuestion() {
        this.type = MULTIPLE_CHOICE;
    }

    @Override
    public void setAnswer(String answer) {
        this.answer = sortString(answer);
    }

    @Override
    public boolean isCorrect(String answer) {
        return this.answer.equals(sortString(answer));
    }

    private String sortString(String str) {
        char[] chars = str.toCharArray();
        Arrays.sort(chars);
        return String.valueOf(chars);
    }
}
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测试

接下来测试一下
定义一个接口，咱们能够使用@RequestBody传入一个Exam对象，返回解析结果：

@RequestMapping(value = "/exam", method = RequestMethod.POST)
public List<String> parseExam(@RequestBody Exam exam) {
    List<String> results = new ArrayList<>();
    results.add(String.format("Parsed an exam, name = %s", exam.getName()));
    results.add(String.format("Exam has %s questions", exam.getQuestions().size())) 
    
    List<String> types = new ArrayList<>();
    for (Question question : exam.getQuestions()) {
        types.add(question.getType());
    }
    results.add(String.format("Questions types: %s", types.toString()));
    return results;
}
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项目跑起来，调用接口测试一下：

curl -X POST \
  http://127.0.0.1:8080/exam/ \
  -H 'Content-Type: application/json' \
  -d '{
	"name":"一场考试",
	"questions": [
		{
			"type": "single_choice",
			"content": "单选题",
			"options":  [
			    {
					"code":"A",
					"content": "选项A"
				},{
					"code":"B",
					"content": "选项B"
				}],
			"answer": "A"
		},{
			"type": "multiple_choice",
			"content": "多选题",
			"options":  [
				{
					"code":"A",
					"content": "选项A"
				},{
					"code":"B",
					"content": "选项B"
				}],
			"answer": "AB"
		},{
			"type": "true_or_false",
			"content": "判断题",
			"answer": "True"
		}]
}'
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接口返回以下：

[
    "Parsed an exam, name = 一场考试",
    "Exam has 3 questions",
    "Questions types: [single_choice, multiple_choice, true_or_false]"
]
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这里不一样类型的question，type字段都能正确读取，代表反序列化过程当中确实是调用了具体子类对应的类来进行实例化的。