什么时候可以不用注解@Autowired

以前写代码@Autowired满天飞，最近在看Budapest人写的测试代码,发现他们的代码长这样：

public class Gateway {

    private final GatewayProperties gatewayProperties;

    public String url(final String subdomain, final String path) {
        Assert.notNull(path, "the path must not be null");
        Assert.isTrue(StringUtils.isEmpty(path) || path.charAt(0) == '/', "the path must start with a '/'");
        final String host = StringUtils.isEmpty(subdomain) ? gatewayProperties.host : subdomain + "." + gatewayProperties.host;
        return String.format("%s://%s%s", gatewayProperties.scheme, host, path);
    }

}

注意到 private final GatewayProperties gatewayProperties; 这一行没有用到注解@autowired，再去查看GatewayProperties这个类：

@Data
@Configuration
@ConfigurationProperties(prefix = "gateway")
public class GatewayProperties {

    String scheme;
    String host;
    ...

它有个注解@Data成功引起了我的注意。

@Data是lombok的一个注解，它主要用来做什么的呢？

@Data
All together now: A shortcut for @ToString, @EqualsAndHashCode, @Getter on all fields, @Setter on all non-final fields, and @RequiredArgsConstructor!

那么重点是注解@RequiredArgsConstructor又是干嘛的呢？官网解释：

Generates constructor that takes one argument per final / non-null field.
所以它会为final和nonnull的属性作为参数产生一个构造函数。

有个构造函数就这么厉害了？！

然后再去翻spring的官方解释：

Starting with Spring 4.3, if a class, which is configured as a Spring bean, has only one constructor, the Autowired annotation can be omitted and Spring will use that constructor and inject all necessary dependencies.
Regarding the default constructor: You either need the default constructor, a constructor with the Autowired annotation when you have multiple constructors, or only one constructor in your class with or without the Autowired annotation.

结论就是这样的： If a class, which is configured as a Spring bean, has only one constructor, the Autowired annotation can be omitted and Spring will use that constructor and inject all necessary dependencies.

看来以后不用autowired满天飞了。

以上。