版本：

一、問題

No converter found for return value of type: class java.util.ArrayList：Json格式轉換問題

No converter found for return value of type: User(自定義的類)：Json格式轉換問題

邏輯、程式碼都正確的情況下，多數是工程中沒有新增json相關的依賴

二、pom.xml中新增json相關依賴

<!-- json -->
<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-databind</artifactId>
  <version>2.10.2</version>
</dependency>
<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-core</artifactId>
  <version>2.10.2</version>
</dependency>
<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-annotations</artifactId>
  <version>2.10.2</version>
</dependency>
<dependency>
  <groupId>com.alibaba</groupId>
  <artifactId>fastjson</artifactId>
  <version>1.2.62</version>
</dependency>