@Conditional：判断@Conditional指定的条件是否成立，如果成立才会给容器中添加组件，配置类里面的内容才会生效
我们发现有很多的自动配置类，但是这些自动配置类都有指定的条件，必须满足这些条件，自动配置才会生效，比如
那我们怎么查看哪些自动配置类生效了呢
可以通过debug模式
1.application.properties配置以下内容
2.启动运行我们就可以查看到哪些自动配置类生效了
Positive matches：代表生效的自动配置类
Negative matches：没有生效的自动配置类
然后通过实例进行测试 1.编写Admin类
/**
* @author:抱着鱼睡觉的喵喵
* @date:2020/12/22
* @description:
*/
public class Admin {
private String name;
private String img;
public Admin() {
}
public Admin(String name, String img) {
this.name = name;
this.img = img;
}
@Override
public String toString() {
return "admin{" +
"name='" + name + '\'' +
", img='" + img + '\'' +
'}';
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
2.编写配置类
@Configuration(proxyBeanMethods = true)//proxyBeanMethods = true为默认值
public class MyConfig {
// @Bean("admin")
public Admin admin(){
return new Admin("jerry","@the");
}
}
3.主配置文件
@SpringBootApplication
public class SpringBootOnApplication {
public static void main(String[] args) {
ConfigurableApplicationContext run=SpringApplication.run(SpringBootOnApplication.class, args);
boolean admin = run.containsBean("admin");
System.out.println(admin);
}
}
结果：false
说明容器中没有这个组件
-将配置类中的组件导入
@Configuration(proxyBeanMethods = true)//proxyBeanMethods = true为默认值
public class MyConfig {
@Bean("admin")
public Admin admin(){
return new Admin("jerry","@the");
}
}
结果：true
那么真正的测试就开始了，使用衍生注解@ConditionOnBean
编写User类
public class User {
private String username;
private String password;
private int age;
private Admin admin;
public Admin getAdmin() {
return admin;
}
public void setAdmin(Admin admin) {
this.admin = admin;
}
public User() {
}
public User(String username, String password, int age) {
this.username = username;
this.password = password;
this.age = age;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "User{" +
"username='" + username + '\'' +
", password='" + password + '\'' +
", age=" + age +
", admin=" + admin +
'}';
}
}
配置类中增加组件导入，使用@ConditionOnBean注解进行判断
下面的意思是如果容器中不存在admin组件，则也不导入hao组件
@Configuration(proxyBeanMethods = true)
public class MyConfig {
@ConditionalOnBean(name = "admin")
@Bean("hao")
public User user(){
User tom = new User("tom", "123", 20);
//hao组件依赖了admin组件
tom.setAdmin(admin());
return tom;
}
@Bean("admin")
public Admin admin(){
return new Admin("jerry","@the");
}
}
主配置文件
@SpringBootApplication
public class SpringBootOnApplication {
public static void main(String[] args) {
ConfigurableApplicationContext run=SpringApplication.run(SpringBootOnApplication.class, args);
boolean user = run.containsBean("hao");
System.out.println(user);
boolean admin = run.containsBean("admin");
System.out.println(admin);
}
}
运行结果：
false
true
为什么会这样呢?分析流程可得，把配置类中的admin组件注册放在user组件注册之后，当程序运行时，执行到user()方法时，由于加了@ConditionOnBean注解，所以会进行判断，看容器中是否存在admin组件，由于admin()方法还未执行，所以容器中没有admin组件；所以此时@ConditionOnBean注解返回false，则不再执行user（）方法；即user组件不再导入
我们将改变组件的导入顺序
@Configuration(proxyBeanMethods = true)
public class MyConfig {
@Bean("admin")
public Admin admin(){
return new Admin("jerry","@the");
}
@ConditionalOnBean(name = "admin")
@Bean("hao")
public User user(){
User tom = new User("tom", "123", 20);
//hao组件依赖了admin组件
tom.setAdmin(admin());
return tom;
}
}
再次运行
结果：
true
true
将@ConditionalOnBean注解添加到类的上面
@Configuration(proxyBeanMethods = false)
@ConditionalOnMissingBean (name = "admin")
public class MyConfig {
@Bean("user")
public User user(){
//hao组件依赖了admin组件
return new User("tom", "123", 20);
}
@Bean("admin")
public Admin admin(){
return new Admin("jerry","@the");
}
}
运行结果：
false
false
这就很容易了，由于事先容器中没有admin组件，所以此时ConditionalOnBean返回false，则这个配置类中的内容不在执行
