# 6.5. Spring Bean 和依赖注入

您可以自由地使用任何标准 Spring 框架技术来定义您的 bean 及其注入的依赖项。我们通常建议使用构造函数注入来连接依赖项并通过`@ComponentScan`查找 bean。

如果您按照上面的建议构建代码（将应用程序类定位在顶级包中），则可以在`@ComponentScan`不添加任何参数的情况下添加或使用`@SpringBootApplication`隐式包含`@ComponentScan`注释。您的所有应用程序组件（`@Component`、`@Service`、`@Repository`、`@Controller`和其他）都会自动注册为 Spring Bean。

以下示例显示了使用构造函数注入来获取所需 `@Service`bean 的`RiskAssessor` Bean：

```
@Service
public class MyAccountService implements AccountService {
​
    private final RiskAssessor riskAssessor;
​
    public MyAccountService(RiskAssessor riskAssessor) {
        this.riskAssessor = riskAssessor;
    }
​
    // ...
​
}
```

如果一个 bean 有多个构造函数，您需要通过`@Autowired`标记您希望 Spring 使用的构造函数：

```
@Service
public class MyAccountService implements AccountService {
​
    private final RiskAssessor riskAssessor;
​
    private final PrintStream out;
​
    @Autowired
    public MyAccountService(RiskAssessor riskAssessor) {
        this.riskAssessor = riskAssessor;
        this.out = System.out;
    }
​
    public MyAccountService(RiskAssessor riskAssessor, PrintStream out) {
        this.riskAssessor = riskAssessor;
        this.out = out;
    }
​
    // ...
​
}
```

> 请注意如何使用构造函数注入将`riskAssessor`字段标记为`final`，表示随后无法对其进行更改。


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