SpringBoot自动装配Condition的实现方式
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2022-06-25 10:04:34
目录2.1 @conditional2.2 condition4.2 修改usercondition5. spring内置条件注解1. 简介@conditional注解在spring4.0中引入，其主...
1. 简介
@conditional注解在spring4.0中引入，其主要作用就是判断条件是否满足，从而决定是否初始化并向容器注册bean。
2. 定义
2.1 @conditional
@conditional注解定义如下：其内部只有一个参数为class对象数组，且必须继承自condition接口，通过重写condition接口的matches方法来判断是否需要加载bean
@target({elementtype.type, elementtype.method})
@retention(retentionpolicy.runtime)
@documented
public @interface conditional {
class<? extends condition>[] value();
}
2.2 condition
condition接口定义如下：该接口为一个函数式接口，只有一个matches接口，形参为conditioncontext context, annotatedtypemetadata metadata。conditioncontext定义如2.2.1,annotatedtypemetadata见名知意，就是用来获取注解的元信息的
@functionalinterface
public interface condition {
boolean matches(conditioncontext context, annotatedtypemetadata metadata);
}
2.2.1 conditioncontext
conditioncontext接口定义如下：通过查看源码可以知道，从这个类中可以获取很多有用的信息
public interface conditioncontext {
/**
* 返回bean定义信息
* 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工厂
* 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();
/**
* 返回环境变量 比如在application.yaml中定义的信息
* return the {@link environment} for which the current application is running.
*/
environment getenvironment();
/**
* 返回资源加载器
* return the {@link resourceloader} currently being used.
*/
resourceloader getresourceloader();
/**
* 返回类加载器
* 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. 使用说明
通过一个简单的小例子测试一下@conditional是不是真的能实现bean的条件化注入。
3.1 创建项目
首先我们创建一个springboot项目
3.1.1 导入依赖
这里我们除了springboot依赖，再添加个lombok依赖
<?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 创建user类
package com.ldx.condition;
import lombok.allargsconstructor;
import lombok.data;
/**
* 用户信息
* @author ludangxin
* @date 2021/8/1
*/
@data
@allargsconstructor
public class user {
private string name;
private integer age;
}
3.1.4 创建条件实现类
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;
/**
* 用户bean条件判断
* @author ludangxin
* @date 2021/8/1
*/
public class usercondition implements condition {
@override
public boolean matches(conditioncontext conditioncontext, annotatedtypemetadata annotatedtypemetadata) {
environment environment = conditioncontext.getenvironment();
// 获取property user.enable
string property = environment.getproperty("user.enable");
// 如果user.enable的值等于true 那么返回值为true，反之为false
return "true".equals(property);
}
}
3.1.5 修改启动类
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);
// 获取类型为user类的bean
user user = applicationcontext.getbean(user.class);
log.info("user bean === {}", user);
}
/**
* 注入user类型的bean
*/
@bean
@conditional(usercondition.class)
public user getuser(){
return new user("张三",18);
}
}
3.2 测试
3.2.1 当user.enable=false
报错找不到可用的user类型的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 当user.enable=true
正常输出userbean实例信息
. ____ _ __ _ _ /\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \ ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \ \\/ ___)| |_)| | | | | || (_| | ) ) ) ) ' |____| .__|_| |_|_| |_\__, | / / / / =========|_|==============|___/=/_/_/_/ :: 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=张三, age=18)
3.3 小结
上面的例子通过使用@conditional和condition接口，实现了spring bean的条件化注入。
好处：
- 可以实现某些配置的开关功能，如上面的例子，我们可以将userbean换成开启缓存的配置，当property的值为true时，我们才开启缓存的配置。
- 当有多个同名的bean时，如何抉择的问题。
- 实现自动化的装载。如判断当前classpath中有mysql的驱动类时（说明我们当前的系统需要使用mysql），我们就自动的读取application.yaml中的mysql配置，实现自动装载；当没有驱动时，就不加载。
4. 改进
从上面的使用说明中我们了解到了条件注解的大概使用方法，但是代码中还是有很多硬编码的问题。比如：usercondition中的property的key包括value都是硬编码，其实我们可以通过再扩展一个注解来实现动态的判断和绑定。
4.1 创建注解
import org.springframework.context.annotation.conditional;
import java.lang.annotation.*;
/**
* 自定义条件属性注解
* <p>
* 当配置的property name对应的值 与设置的 value值相等时，则注入bean
* @author ludangxin
* @date 2021/8/1
*/
@target({elementtype.type, elementtype.method})
@retention(retentionpolicy.runtime)
@documented
// 指定condition的实现类
@conditional({usercondition.class})
public @interface myconditiononproperty {
// 配置信息的key
string name();
// 配置信息key对应的值
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;
/**
* 用户bean条件判断
* @author ludangxin
* @date 2021/8/1
*/
public class usercondition implements condition {
@override
public boolean matches(conditioncontext conditioncontext, annotatedtypemetadata annotatedtypemetadata) {
environment environment = conditioncontext.getenvironment();
// 获取自定义的注解
map<string, object> annotationattributes = annotatedtypemetadata.getannotationattributes("com.ldx.condition.myconditiononproperty");
// 获取在注解中指定的name的property的值 如：user.enable的值
string property = environment.getproperty(annotationattributes.get("name").tostring());
// 获取预期的值
string value = annotationattributes.get("value").tostring();
return value.equals(property);
}
}
测试后，结果符合预期。
其实在spring中已经内置了许多常用的条件注解，其中我们刚实现的就在内置的注解中已经实现了，如下。
5. spring内置条件注解
| 注解 | 说明 |
|---|---|
| @conditionalonsinglecandidate | 当给定类型的bean存在并且指定为primary的给定类型存在时,返回true |
| @conditionalonmissingbean | 当给定的类型、类名、注解、昵称在beanfactory中不存在时返回true.各类型间是or的关系 |
| @conditionalonbean | 与上面相反，要求bean存在 |
| @conditionalonmissingclass | 当给定的类名在类路径上不存在时返回true,各类型间是and的关系 |
| @conditionalonclass | 与上面相反，要求类存在 |
| @conditionaloncloudplatform | 当所配置的cloudplatform为激活时返回true |
| @conditionalonexpression | spel表达式执行为true |
| @conditionalonjava | 运行时的java版本号是否包含给定的版本号.如果包含,返回匹配,否则,返回不匹配 |
| @conditionalonproperty | 要求配置属性匹配条件 |
| @conditionalonjndi | 给定的jndi的location 必须存在一个.否则,返回不匹配 |
| @conditionalonnotwebapplication | web环境不存在时 |
| @conditionalonwebapplication | web环境存在时 |
| @conditionalonresource | 要求制定的资源存在 |
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