@Conditional：判斷@Conditional指定的條件是否成立，如果成立才會給容器中添加組件，配置類里面的內容才會生效

我們發現有很多的自動配置類，但是這些自動配置類都有指定的條件，必須滿足這些條件，自動配置才會生效，比如

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那我們怎么查看哪些自動配置類生效了呢
可以通過debug模式
1.application.properties配置以下內容

2.啟動運行我們就可以查看到哪些自動配置類生效了
Positive matches：代表生效的自動配置類
Negative matches：沒有生效的自動配置類

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然后通過實例進行測試 1.編寫Admin類

/*** @author:抱著魚睡覺的喵喵* @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;}@Overridepublic String toString() {return "admin{" +"name='" + name + '\'' +", img='" + img + '\'' +'}';}public String getName() {return name;}public void setName(String name) {this.name = name;} }

2.編寫配置類

@Configuration(proxyBeanMethods = true)//proxyBeanMethods = true為默認值 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);}}

結果：false
說明容器中沒有這個組件

-將配置類中的組件導入

@Configuration(proxyBeanMethods = true)//proxyBeanMethods = true為默認值 public class MyConfig {@Bean("admin")public Admin admin(){return new Admin("jerry","@the");} }

結果：true

那么真正的測試就開始了，使用衍生注解@ConditionOnBean

編寫User類

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;}@Overridepublic String toString() {return "User{" +"username='" + username + '\'' +", password='" + password + '\'' +", age=" + age +", admin=" + admin +'}';} }

配置類中增加組件導入，使用@ConditionOnBean注解進行判斷
下面的意思是如果容器中不存在admin組件，則也不導入hao組件

@Configuration(proxyBeanMethods = true) public class MyConfig {@ConditionalOnBean(name = "admin")@Bean("hao")public User user(){User tom = new User("tom", "123", 20);//hao組件依賴了admin組件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);}}

運行結果：
false
true
為什么會這樣呢?分析流程可得，把配置類中的admin組件注冊放在user組件注冊之后，當程序運行時，執行到user()方法時，由于加了@ConditionOnBean注解，所以會進行判斷，看容器中是否存在admin組件，由于admin()方法還未執行，所以容器中沒有admin組件；所以此時@ConditionOnBean注解返回false，則不再執行user（）方法；即user組件不再導入

我們將改變組件的導入順序

@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組件依賴了admin組件tom.setAdmin(admin());return tom;}}

再次運行
結果：
true
true

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將@ConditionalOnBean注解添加到類的上面

@Configuration(proxyBeanMethods = false) @ConditionalOnMissingBean (name = "admin") public class MyConfig {@Bean("user")public User user(){//hao組件依賴了admin組件return new User("tom", "123", 20);}@Bean("admin")public Admin admin(){return new Admin("jerry","@the");} }

運行結果：
false
false

這就很容易了，由于事先容器中沒有admin組件，所以此時ConditionalOnBean返回false，則這個配置類中的內容不在執行

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