TemplateEngine.process抛空指针：工具类抽离后异常，控制器内正常

问题描述 #

将HTML渲染逻辑抽离到ProcessHtmlUtil工具类后，调用htmlTemplateEngine.process()时抛出空指针异常，但把相同逻辑直接写在控制器方法内却能正常运行。

核心原因 #

Spring的依赖注入（如@Autowired）仅对Spring容器管理的Bean生效：

• 你在控制器中手动实例化ProcessHtmlUtil：ProcessHtmlUtil processHtmlUtil=new ProcessHtmlUtil();，这个实例不在Spring容器的管控范围内，因此Spring不会自动注入htmlTemplateEngine属性，导致它为null，调用方法时触发空指针。
• 控制器本身是Spring管理的Bean（带有@Controller/@RestController注解），当逻辑写在控制器内时，TemplateEngine会被Spring正常注入，所以无异常。

解决方案 #

方案1：将ProcessHtmlUtil声明为Spring Bean

1. 给ProcessHtmlUtil添加@Component注解，让Spring容器管理该类：

@Component
public class ProcessHtmlUtil {

    @Autowired
    TemplateEngine htmlTemplateEngine;

    public String processHTML(){
        try {
            final Context ctx=new Context();
            return htmlTemplateEngine.process("Redoc-Integrate.html",ctx);
        }
        catch(RuntimeException e){
            log.error("Runtime exception occurred: {}", e.getMessage());
            throw e;
        } catch(Exception e){
            log.error("Internal processing error occurred: {}", e.getMessage());
            throw new InternalServerException("Internal processing error");
        }
    }
}

2. 在控制器中通过@Autowired注入ProcessHtmlUtil，替代手动new：

@RestController
public class YourController {

    @Autowired
    private ProcessHtmlUtil processHtmlUtil;

    @GetMapping(value = "/v1/redoc")
    public ResponseEntity<?> gotoRedocUi(){
        HttpHeaders responseHeaders = new HttpHeaders();
        responseHeaders.add("Content-Type", "text/html; charset=UTF-8");
        return new ResponseEntity<>(processHtmlUtil.processHTML(),responseHeaders, HttpStatus.OK);
    }
}

方案2：手动传入TemplateEngine到工具类

如果不想将工具类交给Spring管理，可在调用时主动传入依赖：

1. 修改ProcessHtmlUtil，通过构造方法接收TemplateEngine：

public class ProcessHtmlUtil {

    private TemplateEngine htmlTemplateEngine;

    public ProcessHtmlUtil(TemplateEngine htmlTemplateEngine) {
        this.htmlTemplateEngine = htmlTemplateEngine;
    }

    public String processHTML(){
        try {
            final Context ctx=new Context();
            return htmlTemplateEngine.process("Redoc-Integrate.html",ctx);
        }
        catch(RuntimeException e){
            log.error("Runtime exception occurred: {}", e.getMessage());
            throw e;
        } catch(Exception e){
            log.error("Internal processing error occurred: {}", e.getMessage());
            throw new InternalServerException("Internal processing error");
        }
    }
}

2. 在控制器中注入TemplateEngine，并传给工具类实例：

@RestController
public class YourController {

    @Autowired
    private TemplateEngine htmlTemplateEngine;

    @GetMapping(value = "/v1/redoc")
    public ResponseEntity<?> gotoRedocUi(){
        ProcessHtmlUtil processHtmlUtil=new ProcessHtmlUtil(htmlTemplateEngine);
        HttpHeaders responseHeaders = new HttpHeaders();
        responseHeaders.add("Content-Type", "text/html; charset=UTF-8");
        return new ResponseEntity<>(processHtmlUtil.processHTML(),responseHeaders, HttpStatus.OK);
    }
}

补充提示 #

推荐使用方案1，符合Spring依赖注入的设计规范，便于后续代码的扩展和维护。

内容的提问来源于stack exchange，提问作者naruto