1.检查是否加了jackson包

 <dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>2.0.1</version>
</dependency>

<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind -->
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.0.1</version>
</dependency>

2.检查配置文件是否缺少

  <mvc:annotation-driven/>
    <mvc:default-servlet-handler/>