首先，我们建立一个实体类，这里简单定义了 name、sex、age 三个属性，以及 get、set、tostring方法。

public class Student {private String name;private String sex;private Integer age;public String getName() {return name;}public String getSex() {return sex;}public Integer getAge() {return age;}public void setName(String name) {this.name = name;}public void setSex(String sex) {this.sex = sex;}public void setAge(Integer age) {this.age = age;}@Overridepublic String toString() {return "Student [name=" + name + ", sex=" + sex + ", age=" + age + "]";}
}

使用阿里巴巴的 JSON

依赖：

<dependency><groupId>com.alibaba</groupId><artifactId>fastjson</artifactId><version>1.2.75</version>
</dependency>

JSON串的字段少于实体类的字段，实体类对象中没有匹配到的属性被赋予默认值：

@Testpublic void testFastjson01() {String jsonStr = "{\"age\":18,\"name\":\"zhangsan\"}";Student stu = JSON.parseObject(jsonStr, Student.class);System.out.println(stu); // Student [name=zhangsan, sex=null, age=18]}

JSON串的字段多于实体类的字段：

 @Testpublic void testFastjson02() {String jsonStr = "{\"age\":20,\"name\":\"lisi\",\"sex\":\"男\",\"hobby\":\"basketball\"}";Student stu = JSON.parseObject(jsonStr, Student.class);System.out.println(stu); // Student [name=lisi, sex=男, age=20]}

上面 JSON 串中的 \" 可以替换成单引号：

    String jsonStr = "{'id':001,'model':'A09','picUrl':'x001.jpe','description':'非常便宜','price':20}";

使用 ObjectMapper

依赖：

<dependency><groupId>com.konghq</groupId><artifactId>unirest-objectmapper-jackson</artifactId><version>4.0.0-RC4</version>
</dependency>

JSON串的字段少于实体类的字段，实体类对象中没有匹配到的属性被赋予默认值：

 @Testpublic void testJackson01() throws Exception{String jsonStr = "{\"age\":18,\"name\":\"zhangsan\"}";ObjectMapper om = new ObjectMapper();Student stu = om.readValue(jsonStr, Student.class);System.out.println(stu); // Student [name=zhangsan, sex=null, age=18]}

JSON串的字段多于实体类的字段，会报错：

 @Testpublic void testJackson02() throws Exception{String jsonStr = "{\"age\":20,\"name\":\"lisi\",\"sex\":\"男\",\"hobby\":\"basketball\"}";ObjectMapper om = new ObjectMapper();Student stu = om.readValue(jsonStr, Student.class);System.out.println(stu);}

错误信息：

com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "hobby" (class com.cyj.demo03.Student), not marked as ignorable (3 known properties: "name", "sex", "age"])at [Source: {"age":20,"name":"lisi","sex":"男","hobby":"basketball"}; line: 1, column: 44] (through reference chain: com.cyj.demo03.Student["hobby"])at com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException.from(UnrecognizedPropertyException.java:51)at com.fasterxml.jackson.databind.DeserializationContext.reportUnknownProperty(DeserializationContext.java:817)at com.fasterxml.jackson.databind.deser.std.StdDeserializer.handleUnknownProperty(StdDeserializer.java:958)at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownProperty(BeanDeserializerBase.java:1324)at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownVanilla(BeanDeserializerBase.java:1302)at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:249)at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:136)at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3564)at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2580)at com.cyj.demo03.TestJson.testJackson02(TestJson.java:40)at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:497)at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)at org.junit.runners.ParentRunner.run(ParentRunner.java:363)at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:538)at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:760)at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:460)at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:206)

由此可见，在默认情况下，ObjectMapper是不支持json串的字段多于实体类字段的，但是，ObjectMapper也提供了解决方案，我们在实体类上，添加注解 @JsonIgnoreProperties，这个注解有一个ignoreUnknown属性，默认值为false，我们可以将它改为true，这样jackson在反序列化的时候，就会忽略掉不存在的属性了。

@JsonIgnoreProperties(ignoreUnknown = true)
public class Student {

JSON字段与实体类属性不一致

项目开发过程中，有时候可能要对接第三方平台，可能就会存在字段名不一样的问题，而双方的字段名不一致， 而代码开发已经有一段时间勒，如果就因为字段名不一样的问题，双方改成属性名一致，这样改动量太大，也不保险，下面我来接手一下，针对这种场景，如果解决。

假设： 系统通过http对接，传的是json。有一个参数"名称"

第三方平台：name

开发环境：userName

直接通过实体类映射，不用说肯定有问题。

在实体类的属性上增加一个注解就可以解决这种问题：

@JsonProperty("name")
private String userName;