SpringBoot中實現子類的反序列化

目標

在SpringBoot接口中，咱們通常用@RequestBody類註解須要反序列化的對象，可是當存在多個子類的狀況下，常規的反序列化不能知足需求，好比：

咱們有一個類Exam用於表示一張試卷：

@Data
public class Exam {

    private String name;
    private List<Question> questions;
}
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這裏Question比較特殊，Question自己是一個抽象類，提供了一些通用的方法調用，實際子類有單選題、多選題、判斷題多種狀況

實現

SprintBoot內置的序列化是使用的Jackson，查閱文檔後發現Jackson提供了@JsonTypeInfo和@JsonSubTypes這兩個註解，搭配使用，能夠根據指定的字段值來指定實例化中用到的具體的子類類型

這幾個類的實際代碼以下：
抽象基類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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判斷題TrueOrFalseQuestion：

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

    public TrueOrFalseQuestion() {
        this.type = TRUE_OR_FALSE;
    }
}
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選擇題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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單選題SingleChoiceQuestion：

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

    public SingleChoiceQuestion() {
        this.type = SINGLE_CHOICE;
    }
}
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多選題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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測試

接下來測試一下
定義一個接口，咱們能夠使用@RequestBody傳入一個Exam對象，返回解析結果：

@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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項目跑起來，調用接口測試一下：

curl -X POST \
  http://127.0.0.1:8080/exam/ \
  -H 'Content-Type: application/json' \
  -d '{
	"name":"一場考試",
	"questions": [
		{
			"type": "single_choice",
			"content": "單選題",
			"options":  [
			    {
					"code":"A",
					"content": "選項A"
				},{
					"code":"B",
					"content": "選項B"
				}],
			"answer": "A"
		},{
			"type": "multiple_choice",
			"content": "多選題",
			"options":  [
				{
					"code":"A",
					"content": "選項A"
				},{
					"code":"B",
					"content": "選項B"
				}],
			"answer": "AB"
		},{
			"type": "true_or_false",
			"content": "判斷題",
			"answer": "True"
		}]
}'
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接口返回以下：

[
    "Parsed an exam, name = 一場考試",
    "Exam has 3 questions",
    "Questions types: [single_choice, multiple_choice, true_or_false]"
]
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這裏不一樣類型的question，type字段都能正確讀取，代表反序列化過程當中確實是調用了具體子類對應的類來進行實例化的。