从Java事件监听到Spring Cloud事件监听

Dear 丶 2022-03-25 10:28 356阅读 0赞

## 从Java事件监听到Spring Cloud事件监听 ##

## Java 事件监听 ##

可参考上一篇文章：  
**[https://blog.csdn.net/shang\_xs/article/details/86560994][https_blog.csdn.net_shang_xs_article_details_86560994]**

## Spring Cloud事件监听 ##

### 1. `BootstrapApplicationListener` ###

`org.springframework.cloud.bootstrap.BootstrapApplicationListener`第六优先

负责加载bootstrap.properties或者bootstrap.ymal

Spring Cloud “/META-INF/spring.factories”;

# Application Listeners
    org.springframework.context.ApplicationListener=\
    org.springframework.cloud.bootstrap.BootstrapApplicationListener,\
    org.springframework.cloud.bootstrap.LoggingSystemShutdownListener,\
    org.springframework.cloud.context.restart.RestartListener

1.1 负责初始化`Bootstrap Application_ID = "bootstrap"`  
`ConfigurableApplicationContext context = builder.run();`  
1.2 `Bootstrap` 是一个根Spring上下文,parent = null  
联想 `ClassLoader`  
`ExtClassLoader <- AppClassLoader <-System ClassLoader <-Bootstrap ClassLoader`

### 2.优先级分析 ###

`BootstrapApplicationListener`加载的优先级高于`ConfigurableApplicationContext`（第十一优先） ，所以在`application.properties`中定义的信息加载不到  
`ConfigurableApplicationContext -> AnnotationConfigApplicationContext`

### 3.BootStrap 配置属性 ###

BootStrap 配置文件路径
    -spring.cloud.bootstrap.location
    覆盖远程配置属性
    -spring.cloud.config.allowOverride
    自定义Bootstrap配置
    -@BootstrapConfiguration
    自定义Bootstrap配置属性源
    -PropertySourceLocator

### 4.ENV端点 ###

`org.springframework.boot.actuate.env.EnvironmentEndpoint`

`Environment`关联多个带名称的`PropertySource`

`AbstractRefreshableWebApplicationContext`

`org.springframework.web.context.support.AbstractRefreshableWebApplicationContext`

@Override
    protected void initPropertySources() {
        ConfigurableEnvironment env = getEnvironment();
        if (env instanceof ConfigurableWebEnvironment) {
            ((ConfigurableWebEnvironment) env).initPropertySources(this.servletContext, this.servletConfig);
        }
    }

普通类型 `StandardEnvironment`

web类型`StandardServletEnvironment`

*  Environment
    
     *  AbstractEnviroment
        
         *  StandardEnviroment

`Environment`关联着一个`PropertySources` `PropertySources`关联着多个`PropertySource`

比较常用的是：`SystemEnvironmentPropertySource`

Java `System#getProperties`实现：名称"SystemProperties"，对应的内容`System.getProperties()`

Java `System#getEnv`实现：名称"SystemEnviroment"，对应的内容`System.getProperties()`

实现自定义配置：  
实现 `org.springframework.cloud.bootstrap.config.PropertySourceLocator`  
`PropertySourceLocator`

*  实现`PropertySource`  
    `@Configuration`  
    `@Order(Ordered.HIGHEST_PRECEDENCE)`
    
        public class MyPropertySourceLocator implements PropertySourceLocator{
            @Override
            public PropertySource<?> locate(Environment environment) {
                HashMap<String, Object> source = new HashMap<>();
                source.put("server.port","9090");
                MapPropertySource mapPropertySource = new MapPropertySource("my-property-source",source);
                return mapPropertySource;
            }
        }
 *  暴露该实现作为一个Bean 如：注解`@Configuration`
 *  定义并且配置/`META-INF/spring.factories`  
    `org.springframework.cloud.bootstrap.BootstrapConfiguration=com.learn.demo.configuration.MyPropertySourceLocator`  
    注意事项：`Enviroment`允许出现同名配置，优先级高的胜出  
    `MutablePropertySources`
    
    ### 5.具体源码加载实现： ###
    
        private final List<PropertySource<?>> propertySourceList = new CopyOnWriteArrayList<>();
        //由于propertySourceList的FIFO是有顺序的 在如下的代码块中，使用API调用addFirst实现resource的第一优先级
        public void addFirst(PropertySource<?> propertySource) {
            removeIfPresent(propertySource);
            this.propertySourceList.add(0, propertySource);
        }
        /**
        	 * Add the given property source object with lowest precedence.
        	 */
        public void addLast(PropertySource<?> propertySource) {
            removeIfPresent(propertySource);
            this.propertySourceList.add(propertySource);
        }

如何控制顺序：

实现Ordered以及标记Order

在Spring中，数值越小越优先

具体实现参见：[https://github.com/dwyanewede/project-learn][https_github.com_dwyanewede_project-learn]

具体实现：com.learn.demo.configuration.MyPropertySourceLocator

[https_blog.csdn.net_shang_xs_article_details_86560994]: https://blog.csdn.net/shang_xs/article/details/86560994
[https_github.com_dwyanewede_project-learn]: https://github.com/dwyanewede/project-learn