错误日志

NestedServletException

完整错误：org.springframework.web.util.NestedServletException: Handler dispatch failed; nested exception is java.lang.AbstractMethodError: Method com/mchange/v2/c3p0/impl/NewProxyResultSet.isClosed()Z is abstract
解决：
更好C3P0的jar包依赖
将

<dependency>
    <groupId>c3p0</groupId>
    <artifactId>c3p0</artifactId>
    <version>0.9.1.2</version>
</dependency>

更换为：

<!-- https://mvnrepository.com/artifact/c3p0/c3p0 -->
<dependency>
   <groupId>com.mchange</groupId>
   <artifactId>c3p0</artifactId>
   <version>0.9.5.2</version>
</dependency>

虽然不知道这两个jar包的区别，但是更好后就解决这个错误了

HttpMessageNotWritableException

将Java类对象返回时，springMVC会帮我们转为json但是，需要添加依赖，不然会报错

org.springframework.http.converter.HttpMessageNotWritableException: No converter found for return value of type: class com.lyc.crud.entity.Msg

添加如下依赖

        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-core</artifactId>
            <version>2.11.0</version>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.11.0</version>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-annotations</artifactId>
            <version>2.11.0</version>
        </dependency>