ObjectMapper的高级使用

<think>我们正在讨论ObjectMapper的高级使用，ObjectMapper是Jackson库中的核心类，用于JSON的序列化和反序列化。高级使用包括：配置选项、处理多态类型、自定义序列化/反序列化、JSON树模型等。下面将介绍几个高级特性，并给出代码示例。1.配置选项：可以设置ObjectMapper的各种特性，例如是否美化输出，是否允许未知属性等。2.处理多态类型：使用@JsonTypeInfo和@JsonSubTypes注解来处理多态类型。3.自定义序列化和反序列化：通过继承StdSerializer和StdDeserializer来实现。4.使用JsonNode（树模型）来动态处理JSON。我们使用Java语言进行演示。首先，我们来看一些配置选项的示例：</think> ### ObjectMapper 的高级使用详解 `ObjectMapper` 是 Jackson 库的核心类，用于处理 JSON 数据的序列化和反序列化。以下是其高级使用方法和示例代码： --- ### 1. **自定义序列化/反序列化** 通过 `SimpleModule` 注册自定义转换器处理特殊对象： ```java import com.fasterxml.jackson.databind.*; import com.fasterxml.jackson.databind.module.SimpleModule; import java.io.IOException; import java.time.LocalDate; import java.time.format.DateTimeFormatter; // 自定义序列化器 class LocalDateSerializer extends JsonSerializer<LocalDate> { @Override public void serialize(LocalDate value, JsonGenerator gen, SerializerProvider provider) throws IOException { gen.writeString(value.format(DateTimeFormatter.ISO_LOCAL_DATE)); } } // 自定义反序列化器 class LocalDateDeserializer extends JsonDeserializer<LocalDate> { @Override public LocalDate deserialize(JsonParser p, DeserializationContext ctx) throws IOException { return LocalDate.parse(p.getText(), DateTimeFormatter.ISO_LOCAL_DATE); } } public class AdvancedMapper { public static void main(String[] args) throws Exception { ObjectMapper mapper = new ObjectMapper(); SimpleModule module = new SimpleModule(); module.addSerializer(LocalDate.class, new LocalDateSerializer()); module.addDeserializer(LocalDate.class, new LocalDateDeserializer()); mapper.registerModule(module); // 注册模块 // 测试对象 class User { public String name; public LocalDate birthDate; } User user = new User(); user.name = "Alice"; user.birthDate = LocalDate.now(); String json = mapper.writeValueAsString(user); System.out.println(json); // {"name":"Alice","birthDate":"2023-10-05"} User parsed = mapper.readValue(json, User.class); System.out.println(parsed.birthDate); // 2023-10-05 } } ``` --- ### 2. **动态忽略属性** 使用 `@JsonView` 实现动态字段过滤： ```java import com.fasterxml.jackson.annotation.JsonView; import com.fasterxml.jackson.databind.ObjectMapper; import com.fasterxml.jackson.databind.SerializationFeature; public class ViewExample { // 定义视图接口 public static class PublicView {} public static class PrivateView extends PublicView {} static class User { @JsonView(PublicView.class) public String name; @JsonView(PrivateView.class) public String password; } public static void main(String[] args) throws Exception { ObjectMapper mapper = new ObjectMapper(); mapper.disable(SerializationFeature.FAIL_ON_EMPTY_BEANS); User user = new User(); user.name = "Bob"; user.password = "secret"; // 序列化时选择视图 String publicJson = mapper .writerWithView(ViewExample.PublicView.class) .writeValueAsString(user); System.out.println(publicJson); // {"name":"Bob"} String privateJson = mapper .writerWithView(ViewExample.PrivateView.class) .writeValueAsString(user); System.out.println(privateJson); // {"name":"Bob","password":"secret"} } } ``` --- ### 3. **处理多态类型** 通过 `@JsonTypeInfo` 支持继承结构的序列化： ```java import com.fasterxml.jackson.annotation.*; import com.fasterxml.jackson.databind.ObjectMapper; @JsonTypeInfo( use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type" ) @JsonSubTypes({ @JsonSubTypes.Type(value = Cat.class, name = "cat"), @JsonSubTypes.Type(value = Dog.class, name = "dog") }) abstract class Animal { public String name; } class Cat extends Animal { public int lives = 9; } class Dog extends Animal { public boolean goodBoy = true; } public class PolymorphicExample { public static void main(String[] args) throws Exception { ObjectMapper mapper = new ObjectMapper(); Animal cat = new Cat(); cat.name = "Whiskers"; String json = mapper.writeValueAsString(cat); System.out.println(json); // {"type":"cat","name":"Whiskers","lives":9} Animal parsed = mapper.readValue(json, Animal.class); System.out.println(parsed.getClass().getSimpleName()); // Cat } } ``` --- ### 4. **JSON 树模型** 使用 `JsonNode` 动态操作 JSON 结构： ```java import com.fasterxml.jackson.databind.*; import com.fasterxml.jackson.databind.node.ObjectNode; import com.fasterxml.jackson.databind.node.ArrayNode; public class TreeModelExample { public static void main(String[] args) throws Exception { ObjectMapper mapper = new ObjectMapper(); // 创建新JSON ObjectNode root = mapper.createObjectNode(); root.put("name", "Charlie"); ArrayNode hobbies = root.putArray("hobbies"); hobbies.add("Reading"); hobbies.add("Hiking"); System.out.println(root.toString()); // {"name":"Charlie","hobbies":["Reading","Hiking"]} // 修改已有JSON String inputJson = "{\"city\":\"Paris\",\"population\":2161000}"; ObjectNode node = (ObjectNode) mapper.readTree(inputJson); node.put("country", "France"); node.put("population", node.get("population").asInt() + 100000); System.out.println(node.toPrettyString()); /* { "city" : "Paris", "population" : 2261000, "country" : "France" } */ } } ``` --- ### **关键配置选项** ```java ObjectMapper mapper = new ObjectMapper() // 美化输出 .enable(SerializationFeature.INDENT_OUTPUT) // 允许单引号 .enable(JsonParser.Feature.ALLOW_SINGLE_QUOTES) // 允许注释 .enable(JsonParser.Feature.ALLOW_COMMENTS) // 未知属性不报错 .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES) // 空值处理 .setSerializationInclusion(JsonInclude.Include.NON_NULL); ``` --- ### **注意事项** 1. 处理循环引用：使用 `@JsonIdentityInfo` 2. 自定义日期格式：`mapper.setDateFormat()` 3. 字段命名策略：`mapper.setPropertyNamingStrategy()` 4. 线程安全：`ObjectMapper` 实例可全局共享