How to use SpringBoot's EnvironmentPostProcessor

1. Background

The Apollo configuration center was used in the previous project. After docking with the Apollo configuration center, the properties of the configuration center are It can be used in the program, so how is this implemented? When were the properties of the configuration center loaded into the program? So if we find out how this is implemented, can we load configuration properties and encryption and decryption functions of configuration properties from anywhere?

2. Requirements

From the above figure, we know that our requirements are very simple, that is, the attributes we define need to be larger than the configuration file has a higher priority.

3. Analysis

1. When to add our own configuration properties to SpringBoot

When we want to use configuration properties in Bean, then we The configuration properties must be put into Spring Environment before the Bean is instantiated. That is, our interface needs to be called before application context refreshed, and EnvironmentPostProcessor can exactly implement this function.

2. The priority of obtaining configuration properties

We know that there is a priority for obtaining properties in Spring.
For example, we have the following configuration properties username

├─application.properties
│   >> username=huan
├─application-dev.properties
│   >> username=huan.fu

So what is the value of username at this time? Here is a picture from Apollo to explain this problem.

Reference link: https://www.apolloconfig.com/#/zh/design/apollo-design

Spring has been added since version 3.1 ConfigurableEnvironment and PropertySource:

ConfigurableEnvironment

• Spring’s ApplicationContext will contain an Environment (implementing the ConfigurableEnvironment interface)

• ConfigurableEnvironment itself contains many PropertySource

PropertySource

• Property Source

• can be understood as many Key-Value attribute configurations

As can be seen from the schematic diagram above, key appears at the very beginning The priority in PropertySource is higher. In the above example, the value of username in SpringBoot is huan.fu.

3. When to add our own configuration

From the second stepGet the priority of the configuration attribute It can be seen that the higher the PropertySource is, the higher Execute it first, then for our configuration to take effect, it must be placed as early as possible.

As can be seen from the above figure, SpringBoot loads various configurations through EnvironmentPostProcessor, and the specific implementation is ConfigDataEnvironmentPostProcessor to achieve. Then we write an implementation class of EnvironmentPostProcessor ourselves, then execute it after ConfigDataEnvironmentPostProcessor, and add it to the first position in Environment.

4. Implementation

1. Introduce SpringBoot dependency

<?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>2.6.6</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.huan.springcloud</groupId>
    <artifactId>springboot-extension-point</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>springboot-extension-point</name>
    <properties>
        <java.version>1.8</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
    </dependencies>
</project>

2. Configure properties in application.properties

vim application.properties

username=huan

3. Write custom attributes and add them to Spring Environment

Note:
1. If you find that there is no log output in the program, check whether is used slf4jOutput log. At this time, the log cannot be output because the log system has not been initialized. The solution is as follows:

SpringBoot版本
		>= 2.4 可以参考上图中的使用 DeferredLogFactory 来输出日志
		< 2.4
			1、参考如下链接 https://stackoverflow.com/questions/42839798/how-to-log-errors-in-a-environmentpostprocessor-execution
			2、核心代码：
				@Component
				public class MyEnvironmentPostProcessor implements
				        EnvironmentPostProcessor, ApplicationListener<ApplicationEvent> {
				    private static final DeferredLog log = new DeferredLog();
				    @Override
				    public void postProcessEnvironment(
				            ConfigurableEnvironment env, SpringApplication app) {
				        log.error("This should be printed");
				    }
				    @Override
				    public void onApplicationEvent(ApplicationEvent event) {
				        log.replayTo(MyEnvironmentPostProcessor.class);
				    }
				}

4. Make the customized configuration take effect through SPI

1. Create a new# under src/main/resources ##META-INF/spring.factoriesFile

2. Configuration

org.springframework.boot.env.EnvironmentPostProcessor=\
  com.huan.springcloud.extensionpoint.environmentpostprocessor.CustomEnvironmentPostProcessor

5. Write a test class and output the defined username attribute Value

@Component
public class PrintCustomizeEnvironmentProperty implements ApplicationRunner {

    private static final Logger log = LoggerFactory.getLogger(PrintCustomizeEnvironmentProperty.class);
    @Value("${username}")
    private String userName;
    @Override
    public void run(ApplicationArguments args) {
        log.info("获取到的 username 的属性值为: {}", userName);
    }
}

6. Running result

5. Notes

1. The log cannot be output

Refer to the above ’

3. Write custom properties and add the solution provided by in Spring Environment.

2. Check that the configuration does not take effect

• Check the priority of EnvironmentPostProcessor to see if the priority value returned by @Order or Ordered is incorrect.

• 看看别的地方是否实现了 EnvironmentPostProcessor或ApplicationContextInitializer或BeanFactoryPostProcessor或BeanDefinitionRegistryPostProcessor等这些接口，在这个里面修改了 PropertySource的顺序。

• 理解 Spring 获取获取属性的顺序 参考 2、获取配置属性的优先级

3、日志系统如何初始化

如下代码初始化日志系统

org.springframework.boot.context.logging.LoggingApplicationListener

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