springboot2.2.6.RELEASE chapter4

您可以自由使用任何标准的Spring框架技术来定义bean及其注入的依赖项。为了简单起见，我们经常发现使用@ComponentScan（来查找bean）和使用@Autowired（来进行构造函数注入）可以很好地工作。

如果按照上面的建议构造代码（在根包中定位应用程序类），则可以添加@ComponentScan而无需任何参数。所有应用程序组件（@Component，@Service，@Repository，@Controller等）都自动注册为springbean。

以下示例显示了一个@Service Bean，它使用构造函数注入来获取所需的RiskAssessor Bean：

The following example shows a @Service Bean that uses constructor injection to obtain a required RiskAssessor bean:

package com.example.service;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class DatabaseAccountService implements AccountService {

    private final RiskAssessor riskAssessor;

    @Autowired
    public DatabaseAccountService(RiskAssessor riskAssessor) {
        this.riskAssessor = riskAssessor;
    }

    // ...

}

If a bean has one constructor, you can omit the @Autowired, as shown in the following example:

@Service
public class DatabaseAccountService implements AccountService {

    private final RiskAssessor riskAssessor;

    public DatabaseAccountService(RiskAssessor riskAssessor) {
        this.riskAssessor = riskAssessor;
    }

    // ...

}

Notice how using constructor injection lets the riskAssessor field be marked as final, indicating that it cannot be subsequently changed.