每日一面 - Spring Boot 中的 ApplicationContext 的分层是什么意思？

﹏ヽ暗。殇╰゛Y 2022-11-19 09:53 126阅读 0赞

ApplicationContext 是 spring 用来容纳管理 beans 以及其生命周期的容器。ApplicationContext 的分层规定了bean的界限以及可以复用的 bean。关于 ApplicationContext 层级可以参考官方文档(http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/\#howto-build-an-application-context-hierarchy)，这里我们通过一个简单的例子来说明下 ApplicationContext 层级以及其中的bean界限，例如某些 bean 可以被多个 ApplicationContext 共享，同时某些 bean 只在某个 ApplicationContext 生效，不同 ApplicationContext 可以声明同名或者同类型的bean这样。我们将实现一个下图所示的 ApplicationContext 结构：

![image][]

我们会实现，一个 parent context 与三个对应 child context 的结构。

首先定义Parent context：

**Bean类：**

package com.test.spring.context.bean;
    
    import lombok.Data;
    import lombok.NoArgsConstructor;
    
    @Data
    @NoArgsConstructor
    public class RootBean {
        private Stirng name;
    }

**Context类：**

import com.hopegaming.scaffold.spring.context.bean.RootBean;
    import org.springframework.context.annotation.Bean;
    import org.springframework.context.annotation.Configuration;
    
    @Configuration
    @PropertySource(value = "classpath:/root.yaml", factory = YamlPropertyLoaderFactory.class)
    public class RootContext {
        @Bean
        public RootBean getFatherBean() {
            RootBean rootBean = new RootBean();
            rootBean.setName("root");
            return rootBean;
        }
    }

**root.yml：**

# 配置这些主要是将actuator相关接口暴露出来。
    management:
      endpoint:
        health:
          show-details: always
      endpoints:
        jmx:
          exposure:
            exclude: '*'
        web:
          exposure:
            include: '*'

由于我们使用了yml，这里需要我们自定义一个`YamlPropertyLoaderFactory`用于加载yml配置：

package com.test.spring.context.config;
    
    import org.springframework.boot.env.YamlPropertySourceLoader;
    import org.springframework.core.env.PropertySource;
    import org.springframework.core.io.support.DefaultPropertySourceFactory;
    import org.springframework.core.io.support.EncodedResource;
    
    import java.io.IOException;
    
    public class YamlPropertyLoaderFactory extends DefaultPropertySourceFactory {
        @Override
        public PropertySource<?> createPropertySource(String name, EncodedResource resource) throws IOException {
            if (resource == null){
                return super.createPropertySource(name, resource);
            }
    
            return new YamlPropertySourceLoader().load(resource.getResource().getFilename(), resource.getResource()).get(0);
        }
    }

定义child context的公共Bean类：

package com.test.spring.context.bean;
    
    import lombok.Data;
    import lombok.NoArgsConstructor;
    
    @Data
    @NoArgsConstructor
    public class ChildBean {
        private RootBean fatherBean;
        private String name;
    }

定义ChildContext1：

package com.test.spring.context.config.child1;
    
    import com.hopegaming.scaffold.spring.context.bean.ChildBean;
    import com.hopegaming.scaffold.spring.context.bean.RootBean;
    import com.hopegaming.scaffold.spring.context.config.YamlPropertyLoaderFactory;
    import org.springframework.beans.factory.annotation.Value;
    import org.springframework.boot.autoconfigure.SpringBootApplication;
    import org.springframework.context.annotation.Bean;
    import org.springframework.context.annotation.PropertySource;
    
    @SpringBootApplication(scanBasePackages = {"com.test.spring.context.controller"})
    @PropertySource(value = "classpath:/bean-config-1.yaml", factory = YamlPropertyLoaderFactory.class)
    public class ChildContext1 {
        @Bean
        public ChildBean getChildBean(@Value("${spring.application.name}") String name, RootBean fatherBean) {
            ChildBean childBean = new ChildBean();
            childBean.setFatherBean(fatherBean);
            childBean.setName(name);
            return childBean;
        }
    }

**bean-config-1.yaml**：

server:
      port: 8080
    spring:
      application:
        name: child1

接下来分别是ChildContext2，ChildContext3的：

package com.test.spring.context.config.child2;
    
    import com.hopegaming.scaffold.spring.context.bean.ChildBean;
    import com.hopegaming.scaffold.spring.context.bean.RootBean;
    import com.hopegaming.scaffold.spring.context.config.YamlPropertyLoaderFactory;
    import org.springframework.beans.factory.annotation.Value;
    import org.springframework.boot.autoconfigure.SpringBootApplication;
    import org.springframework.context.annotation.Bean;
    import org.springframework.context.annotation.PropertySource;
    
    @SpringBootApplication(scanBasePackages = {"com.test.spring.context.controller"})
    @PropertySource(value = "classpath:/bean-config-2.yaml", factory = YamlPropertyLoaderFactory.class)
    public class ChildContext2 {
        @Bean
        public ChildBean getChildBean(@Value("${spring.application.name}") String name, RootBean fatherBean) {
            ChildBean childBean = new ChildBean();
            childBean.setFatherBean(fatherBean);
            childBean.setName(name);
            return childBean;
        }
    }

server:
      port: 8081
    spring:
      application:
        name: child2
    
    management:
      endpoint:
        health:
          show-details: always
      endpoints:
        jmx:
          exposure:
            exclude: '*'
        web:
          exposure:
            include: '*'

package com.test.spring.context.config.child3;
    
    import com.hopegaming.scaffold.spring.context.bean.ChildBean;
    import com.hopegaming.scaffold.spring.context.bean.RootBean;
    import com.hopegaming.scaffold.spring.context.config.YamlPropertyLoaderFactory;
    import org.springframework.beans.factory.annotation.Value;
    import org.springframework.boot.autoconfigure.SpringBootApplication;
    import org.springframework.context.annotation.Bean;
    import org.springframework.context.annotation.PropertySource;
    
    @SpringBootApplication(scanBasePackages = {"com.test.spring.context.controller"})
    @PropertySource(value = "classpath:/bean-config-3.yaml", factory = YamlPropertyLoaderFactory.class)
    public class ChildContext3 {
        @Bean
        public ChildBean getChildBean(@Value("${spring.application.name}") String name, RootBean fatherBean) {
            ChildBean childBean = new ChildBean();
            childBean.setFatherBean(fatherBean);
            childBean.setName(name);
            return childBean;
        }
    }

server:
      port: 8082
    spring:
      application:
        name: child3
    
    management:
      endpoint:
        health:
          show-details: always
      endpoints:
        jmx:
          exposure:
            exclude: '*'
        web:
          exposure:
            include: '*'

测试接口`TestController`：

package com.test.spring.context.controller;
    
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.beans.factory.annotation.Value;
    import org.springframework.web.bind.annotation.RequestMapping;
    import org.springframework.web.bind.annotation.RestController;
    
    import java.util.Locale;
    
    @RestController
    public class TestController {
        @Autowired
        private ChildBean childBean;
    
        @RequestMapping("/test")
        public ChildBean getChildBean() {
            return childBean;
        }
    }

启动类：

package com.test.spring.context;
    
    import com.hopegaming.scaffold.spring.context.config.child1.ChildContext1;
    import com.hopegaming.scaffold.spring.context.config.child2.ChildContext2;
    import com.hopegaming.scaffold.spring.context.config.child3.ChildContext3;
    import com.hopegaming.scaffold.spring.context.config.root.RootContext;
    import org.springframework.boot.builder.SpringApplicationBuilder;
    import org.springframework.context.ConfigurableApplicationContext;
    
    public class ContextMain {
        public static void main(String[] args) {
            SpringApplicationBuilder appBuilder =
                    new SpringApplicationBuilder()
                            .sources(RootContext.class)
                            //第一个子context用child，剩下的都用sibling
                            .child(ChildContext1.class)
                            .sibling(ChildContext2.class)
                            .sibling(ChildContext3.class);
            ConfigurableApplicationContext applicationContext = appBuilder.run();
        }
    }

启动后，访问`http://127.0.0.1:8080/test`返回：

{"fatherBean":{"name":"root"},"name":"child1"}

访问`http://127.0.0.1:8081/test`返回：

{"fatherBean":{"name":"root"},"name":"child2"}

访问`http://127.0.0.1:8082/test`返回：

{"fatherBean":{"name":"root"},"name":"child3"}

访问`http://127.0.0.1:8080/actuator/beans`会有类似于下面的返回(省略了不关心的bean)：

{
    	"contexts": {
    		"application-1": {
    			"beans": {
    				"getChildBean": {
    					"aliases": [],
    					"scope": "singleton",
    					"type": "com.hopegaming.scaffold.spring.context.bean.ChildBean",
    					"resource": "com.hopegaming.scaffold.spring.context.config.child2.ChildContext2",
    					"dependencies": [
    						"getFatherBean"
    					]
    				},
    				"childContext2": {
    					"aliases": [],
    					"scope": "singleton",
    					"type": "com.hopegaming.scaffold.spring.context.config.child2.ChildContext2$$EnhancerBySpringCGLIB$$26f80b15",
    					"resource": null,
    					"dependencies": []
    				}
    				.......
    			},
    			"parentId": "application"
    		},
    		"application": {
    			"beans": {
    				"getFatherBean": {
    					"aliases": [],
    					"scope": "singleton",
    					"type": "com.hopegaming.scaffold.spring.context.bean.RootBean",
    					"resource": "com.hopegaming.scaffold.spring.context.config.root.RootContext",
    					"dependencies": []
    				},
    				"rootContext": {
    					"aliases": [],
    					"scope": "singleton",
    					"type": "com.hopegaming.scaffold.spring.context.config.root.RootContext$$EnhancerBySpringCGLIB$$18d9c26f",
    					"resource": null,
    					"dependencies": []
    				}
    				.......
    			},
    			"parentId": null
    		}
    	}
    }

通过这个例子，想必大家对于 ApplicationContext 层级有了一定的理解

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![image][image 1]

[image]: /images/20221119/6b0ebfafcf374191b3a6067cce956aa4.png
[image 1]: https://img-blog.csdnimg.cn/img_convert/024685d8d89cbb693f3c55d552e32018.png