SpringBoot自動裝配Condition的實現(xiàn)方式

更新時間：2021年08月02日 16:17:01 作者：張鐵牛

這篇文章主要介紹了SpringBoot自動裝配Condition的實現(xiàn)方式,本文通過實例代碼給大家介紹的非常詳細，對大家的學(xué)習(xí)或工作具有一定的參考借鑒價值，需要的朋友可以參考下

1. 簡介

@Conditional注解在Spring4.0中引入，其主要作用就是判斷條件是否滿足，從而決定是否初始化并向容器注冊Bean。

2. 定義

2.1 @Conditional

@Conditional注解定義如下：其內(nèi)部只有一個參數(shù)為Class對象數(shù)組，且必須繼承自Condition接口，通過重寫Condition接口的matches方法來判斷是否需要加載Bean

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {
  Class<? extends Condition>[] value();
}

2.2 Condition

Condition接口定義如下：該接口為一個函數(shù)式接口，只有一個matches接口，形參為ConditionContext context, AnnotatedTypeMetadata metadata。ConditionContext定義如2.2.1,AnnotatedTypeMetadata見名知意，就是用來獲取注解的元信息的

@FunctionalInterface
public interface Condition {
  boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);
}

2.2.1 ConditionContext

ConditionContext接口定義如下：通過查看源碼可以知道，從這個類中可以獲取很多有用的信息

public interface ConditionContext {
  /**
   * 返回Bean定義信息
   * Return the {@link BeanDefinitionRegistry} that will hold the bean definition
   * should the condition match.
   * @throws IllegalStateException if no registry is available (which is unusual:
   * only the case with a plain {@link ClassPathScanningCandidateComponentProvider})
   */
  BeanDefinitionRegistry getRegistry();

  /**
   * 返回Bean工廠
   * Return the {@link ConfigurableListableBeanFactory} that will hold the bean
   * definition should the condition match, or {@code null} if the bean factory is
   * not available (or not downcastable to {@code ConfigurableListableBeanFactory}).
   */
  @Nullable
  ConfigurableListableBeanFactory getBeanFactory();

  /**
   * 返回環(huán)境變量 比如在application.yaml中定義的信息
   * Return the {@link Environment} for which the current application is running.
   */
  Environment getEnvironment();

  /**
   * 返回資源加載器
   * Return the {@link ResourceLoader} currently being used.
   */
  ResourceLoader getResourceLoader();

  /**
   * 返回類加載器
   * Return the {@link ClassLoader} that should be used to load additional classes
   * (only {@code null} if even the system ClassLoader isn't accessible).
   * @see org.springframework.util.ClassUtils#forName(String, ClassLoader)
   */
  @Nullable
  ClassLoader getClassLoader();
}

3. 使用說明

通過一個簡單的小例子測試一下@Conditional是不是真的能實現(xiàn)Bean的條件化注入。

3.1 創(chuàng)建項目

首先我們創(chuàng)建一個SpringBoot項目

3.1.1 導(dǎo)入依賴

這里我們除了springboot依賴，再添加個lombok依賴

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.5.3</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.ldx</groupId>
    <artifactId>condition</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>condition</name>
    <description>Demo project for Spring Boot</description>
    <properties>
        <java.version>1.8</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.12</version>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

3.1.2 添加配置信息

在application.yaml 中加入配置信息

user:
  enable: false

3.1.3 創(chuàng)建User類

package com.ldx.condition;

import lombok.AllArgsConstructor;
import lombok.Data;

/**
 * 用戶信息
 * @author ludangxin
 * @date 2021/8/1
 */
@Data
@AllArgsConstructor
public class User {
   private String name;
   private Integer age;
}

3.1.4 創(chuàng)建條件實現(xiàn)類

package com.ldx.condition;

import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.env.Environment;
import org.springframework.core.type.AnnotatedTypeMetadata;

/**
 * 用戶bean條件判斷
 * @author ludangxin
 * @date 2021/8/1
 */
public class UserCondition implements Condition {
   @Override
   public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {
      Environment environment = conditionContext.getEnvironment();
      // 獲取property user.enable
      String property = environment.getProperty("user.enable");
      // 如果user.enable的值等于true 那么返回值為true，反之為false
      return "true".equals(property);
   }
}

3.1.5 修改啟動類

package com.ldx.condition;

import lombok.extern.slf4j.Slf4j;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;

@Slf4j
@SpringBootApplication
public class ConditionApplication {

   public static void main(String[] args) {
      ConfigurableApplicationContext applicationContext = SpringApplication.run(ConditionApplication.class, args);
      // 獲取類型為User類的Bean
      User user = applicationContext.getBean(User.class);
      log.info("user bean === {}", user);
   }

  /**
   * 注入User類型的Bean
   */
   @Bean
   @Conditional(UserCondition.class)
   public User getUser(){
      return new User("張三",18);
   }

}

3.2 測試

3.2.1 當user.enable=false

報錯找不到可用的User類型的Bean

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::                (v2.5.3)

2021-08-01 17:07:51.994  INFO 47036 --- [           main] com.ldx.condition.ConditionApplication   : Starting ConditionApplication using Java 1.8.0_261 on ludangxindeMacBook-Pro.local with PID 47036 (/Users/ludangxin/workspace/idea/condition/target/classes started by ludangxin in /Users/ludangxin/workspace/idea/condition)
2021-08-01 17:07:51.997  INFO 47036 --- [           main] com.ldx.condition.ConditionApplication   : No active profile set, falling back to default profiles: default
2021-08-01 17:07:52.461  INFO 47036 --- [           main] com.ldx.condition.ConditionApplication   : Started ConditionApplication in 0.791 seconds (JVM running for 1.371)
Exception in thread "main" org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.ldx.condition.User' available
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:351)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:342)
	at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1172)
	at com.ldx.condition.ConditionApplication.main(ConditionApplication.java:16)

Process finished with exit code 1

3.2.2 當user.enable=true

正常輸出UserBean實例信息

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::                (v2.5.3)

2021-08-01 17:13:38.022  INFO 47129 --- [           main] com.ldx.condition.ConditionApplication   : Starting ConditionApplication using Java 1.8.0_261 on ludangxindeMacBook-Pro.local with PID 47129 (/Users/ludangxin/workspace/idea/condition/target/classes started by ludangxin in /Users/ludangxin/workspace/idea/condition)
2021-08-01 17:13:38.024  INFO 47129 --- [           main] com.ldx.condition.ConditionApplication   : No active profile set, falling back to default profiles: default
2021-08-01 17:13:38.434  INFO 47129 --- [           main] com.ldx.condition.ConditionApplication   : Started ConditionApplication in 0.711 seconds (JVM running for 1.166)
2021-08-01 17:13:38.438  INFO 47129 --- [           main] com.ldx.condition.ConditionApplication   : user bean === User(name=張三, age=18)

3.3 小結(jié)

上面的例子通過使用@Conditional和Condition接口，實現(xiàn)了spring bean的條件化注入。

好處：

可以實現(xiàn)某些配置的開關(guān)功能，如上面的例子，我們可以將UserBean換成開啟緩存的配置，當property的值為true時，我們才開啟緩存的配置。
當有多個同名的bean時，如何抉擇的問題。
實現(xiàn)自動化的裝載。如判斷當前classpath中有mysql的驅(qū)動類時（說明我們當前的系統(tǒng)需要使用mysql），我們就自動的讀取application.yaml中的mysql配置，實現(xiàn)自動裝載；當沒有驅(qū)動時，就不加載。

4. 改進

從上面的使用說明中我們了解到了條件注解的大概使用方法，但是代碼中還是有很多硬編碼的問題。比如：UserCondition中的property的key包括value都是硬編碼，其實我們可以通過再擴展一個注解來實現(xiàn)動態(tài)的判斷和綁定。

4.1 創(chuàng)建注解

import org.springframework.context.annotation.Conditional;
import java.lang.annotation.*;

/**
 * 自定義條件屬性注解
 * <p>
 *   當配置的property name對應(yīng)的值 與設(shè)置的 value值相等時，則注入bean
 * @author ludangxin
 * @date 2021/8/1
 */
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
// 指定condition的實現(xiàn)類
@Conditional({UserCondition.class})
public @interface MyConditionOnProperty {
   // 配置信息的key
   String name();
   // 配置信息key對應(yīng)的值
   String value();
}

4.2 修改UserCondition

package com.ldx.condition;

import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.env.Environment;
import org.springframework.core.type.AnnotatedTypeMetadata;

import java.util.Map;

/**
 * 用戶bean條件判斷
 * @author ludangxin
 * @date 2021/8/1
 */
public class UserCondition implements Condition {
   @Override
   public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {
      Environment environment = conditionContext.getEnvironment();
      // 獲取自定義的注解
      Map<String, Object> annotationAttributes = annotatedTypeMetadata.getAnnotationAttributes("com.ldx.condition.MyConditionOnProperty");
      // 獲取在注解中指定的name的property的值 如：user.enable的值
      String property = environment.getProperty(annotationAttributes.get("name").toString());
      // 獲取預(yù)期的值
      String value = annotationAttributes.get("value").toString();
      return value.equals(property);
   }
}

測試后，結(jié)果符合預(yù)期。

其實在spring中已經(jīng)內(nèi)置了許多常用的條件注解，其中我們剛實現(xiàn)的就在內(nèi)置的注解中已經(jīng)實現(xiàn)了，如下。

5. Spring內(nèi)置條件注解

注解	說明
@ConditionalOnSingleCandidate	當給定類型的bean存在并且指定為Primary的給定類型存在時,返回true
@ConditionalOnMissingBean	當給定的類型、類名、注解、昵稱在beanFactory中不存在時返回true.各類型間是or的關(guān)系
@ConditionalOnBean	與上面相反，要求bean存在
@ConditionalOnMissingClass	當給定的類名在類路徑上不存在時返回true,各類型間是and的關(guān)系
@ConditionalOnClass	與上面相反，要求類存在
@ConditionalOnCloudPlatform	當所配置的CloudPlatform為激活時返回true
@ConditionalOnExpression	spel表達式執(zhí)行為true
@ConditionalOnJava	運行時的java版本號是否包含給定的版本號.如果包含,返回匹配,否則,返回不匹配
@ConditionalOnProperty	要求配置屬性匹配條件
@ConditionalOnJndi	給定的jndi的Location 必須存在一個.否則,返回不匹配
@ConditionalOnNotWebApplication	web環(huán)境不存在時
@ConditionalOnWebApplication	web環(huán)境存在時
@ConditionalOnResource	要求制定的資源存在