Spring推断构造方法（创建Bean多个构造方法时，到底用哪一个）

Spring在基于某个类生成Bean的过程中，需要通过构造方法来实例化对象，但是如果一个类存在多个构造方法时，Spring会使用哪一个？

在回答这个问题之前，先看几个例子：

例子一

配置类

package mytest.spring;

import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.EnableAspectJAutoProxy;
import org.springframework.transaction.annotation.EnableTransactionManagement;

@ComponentScan("mytest.spring")
@Configuration
public class AppConfig {

}

用户服务

package mytest.spring;

import org.springframework.stereotype.Component;

@Component
public class UserService {

    private String name;

    private Integer age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }
}

定单服务，无参构造

package mytest.spring;

import org.springframework.stereotype.Component;

@Component
public class OrderService {

    private UserService userService;

    public void showResult() {
        if (userService == null) {
            System.out.println("用户服务未注入");
        } else {
            System.out.println("用户服务已注入");
        }
    }

}

//main方法测试
public static void main(String[] args) {
    AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
    OrderService orderService = context.getBean("orderService", OrderService.class);
    orderService.showResult();
}

输出结果为：“用户服务未注入”。

例子二

订单服务，一个有参构造

@Component
public class OrderService {

    private UserService userService;

    public OrderService(UserService userService) {
        this.userService = userService;
    }

    public void showResult() {
        if (userService == null) {
            System.out.println("用户服务未注入");
        } else {
            System.out.println("用户服务已注入");
        }
    }
}

输出结果为：“用户服务已注入”。

例子三

订单服务，两个有参+无参构造

@Component
public class OrderService {

    private UserService userService;

    public OrderService() {
    }

    public OrderService(UserService userService) {
        this.userService = userService;
    }

    public OrderService(UserService userService, UserService userService1) {
        this.userService = userService;
    }

    public void showResult() {
        if (userService == null) {
            System.out.println("用户服务未注入");
        } else {
            System.out.println("用户服务已注入");
        }
    }
}

输出结果为：“用户服务未注入”。此时走的是无参构造。

例子四

订单服务，两个有参构造

@Component
public class OrderService {

    private UserService userService;

    public OrderService(UserService userService) {
        this.userService = userService;
    }

    public OrderService(UserService userService, UserService userService1) {
        this.userService = userService;
    }

    public void showResult() {
        if (userService == null) {
            System.out.println("用户服务未注入");
        } else {
            System.out.println("用户服务已注入");
        }
    }
}

输出结果：报错 No default constructor found

例子五

订单服务，两个有参构造+@Autowired

@Component
public class OrderService {

    private UserService userService;

    public OrderService(UserService userService) {
        this.userService = userService;
        System.out.println("2");
    }

    @Autowired
    public OrderService(UserService userService, UserService userService1) {
        this.userService = userService;
        System.out.println("3");
    }

    public void showResult() {
        if (userService == null) {
            System.out.println("用户服务未注入");
        } else {
            System.out.println("用户服务已注入");
        }
    }
}

输出结果为：

“3”

“用户服务已注入”

总结：Spring推断构造逻辑如下

1. 如果一个类只有一个构造方法，那没的选择，只能用这个构造方法
2. 如果一个类有多个构造方法，首先会判断是否有@Autowired指定构造方法，再看是否有无参构造方法。如果都没有就会报错 No default constructor found