Java基础之《jackson转换json、获取泛型Type》 |
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Java基础之《jackson转换json、获取泛型Type》
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1、json字符串转对象，不带实体类 //用法示例 ObjectMapper mapper = new ObjectMapper(); JsonNode rootNode = mapper.readTree(requestStr); //获得根节点 JsonNode txnType = rootNode.get("txnType"); //获得某个json字段 String result = txnType.asText(); //获得它的值2、json字符串转对象，带实体类的转换 //用法示例 ObjectMapper objectMapper = new ObjectMapper(); AccountBean acc = objectMapper.readValue(json, AccountBean.class);3、对象转json字符串 //用法示例 ObjectMapper objectMapper = new ObjectMapper(); String result = objectMapper.writeValueAsString(object);注：writeValueAsString可以转换map、list等集合，实体类 4、json字符串转对象，高级用法一， JavaType UserBean.java package test; public class UserBean { String name; String password; public String getName() { return name; } public void setName(String name) { this.name = name; } public String getPassword() { return password; } public void setPassword(String password) { this.password = password; } }Test.java package test; import java.util.List; import com.fasterxml.jackson.databind.JavaType; import com.fasterxml.jackson.databind.ObjectMapper; public class Test { public static void main(String[] args) { //使用JavaType String constent = "[{\"name\":\"张三\",\"password\":\"123\"},{\"name\":\"李四\",\"password\":\"456\"}]"; ObjectMapper mapper = new ObjectMapper(); JavaType javaType = mapper.getTypeFactory().constructCollectionType(List.class, UserBean.class); try { List lists = mapper.readValue(constent, javaType); System.out.println(lists.get(0).getName()); System.out.println(lists.get(0).getPassword()); } catch (Exception e) { e.printStackTrace(); } } }JavaType的作用：JavaType对象对应着一个java类型，用于寻找对应的反序列化器 参考jackson代码，传入一个上下文，一个工厂，一个JavaType public JsonDeserializer findValueDeserializer(DeserializationContext ctxt, DeserializerFactory factory, JavaType propertyType) throws JsonMappingException { JsonDeserializer deser = _findCachedDeserializer(propertyType); //从缓存中寻找 if (deser != null) { return deser; } deser = _createAndCacheValueDeserializer(ctxt, factory, propertyType); //创建新的 if (deser == null) { deser = _handleUnknownValueDeserializer(propertyType); //处理未知类型 } return deser; }5、json字符串转对象，高级用法二，TypeReference json数组转成list try { String json = "[{\"name\":\"张三\",\"password\":\"123\"},{\"name\":\"李四\",\"password\":\"456\"}]"; List userBeanList = mapper.readValue(json, new TypeReference() {}); String userName = userBeanList.get(0).getName(); System.out.println(userName); } catch (Exception e1) { // TODO Auto-generated catch block e1.printStackTrace(); }参考jackson代码 public abstract class TypeReference implements Comparable { protected final Type _type; protected TypeReference() { Type superClass = getClass().getGenericSuperclass(); if (superClass instanceof Class) { // sanity check, should never happen throw new IllegalArgumentException("Internal error: TypeReference constructed without actual type information"); } /* 22-Dec-2008, tatu: Not sure if this case is safe -- I suspect * it is possible to make it fail? * But let's deal with specific * case when we know an actual use case, and thereby suitable * workarounds for valid case(s) and/or error to throw * on invalid one(s). */ _type = ((ParameterizedType) superClass).getActualTypeArguments()[0]; } public Type getType() { return _type; } /** * The only reason we define this method (and require implementation * of Comparable) is to prevent constructing a * reference without type information. */ @Override public int compareTo(TypeReference o) { return 0; } // just need an implementation, not a good one... hence ^^^ }在TypeReference内部，会获得T的这个泛型类型 写一个例子程序Test.java： try { //生成一个匿名内部类，是TypeReference抽象类的子类 TypeReference ttt = new TypeReference() {}; //获得当前匿名类的类类型 Class clazz = ttt.getClass(); System.out.println("1: " + clazz); //获得此匿名类父类的Type类型，即TypeReference的Type类型 //Type是Java编程语言中所有类型的公共高级接口。它们包括原始类型、参数化类型、数组类型、类型变量和基本类型 //实际上Type接口只是一个标记，Class类实现了Type接口 Type superClass = ttt.getClass().getGenericSuperclass(); System.out.println("2: " + superClass); //获得ParameterizedType参数化类型，即泛型 ParameterizedType p = (ParameterizedType)superClass; System.out.println("3: " + p); //getActualTypeArguments获取参数化类型的数组，泛型可能有多个 Type type = p.getActualTypeArguments()[0]; System.out.println("4: " + type); } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); }执行结果： 1: class test.Test$1 2: com.fasterxml.jackson.core.type.TypeReference 3: com.fasterxml.jackson.core.type.TypeReference 4: java.util.List 加断点可以看到，保存的信息： 参考资料：https://blog.csdn.net/liang5630/article/details/40185591https://www.jianshu.com/p/0851fe6f7d62 |
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