轉自：https://boommanpro.blog.csdn.net/article/details/104667530

需求背景
定義了一個@ConfigurationProperties的配置類,然後在其中定義了一些定時任務的配置,如cron表示式,因為專案會有預設配置,遂配置中有預設值,大體如下:

@Data
@Validated
@ConfigurationProperties(value = "task")
public class TaskConfigProperties {
/**
* 任務A在每天的0點5分0秒進行執行
*/
@NotBlank
private String taskA = "0 5 0 * * ? ";

}

定時任務配置:

@Scheduled(cron = "${task.task-a}")
public void finalCaseReportGenerate(){
    log.info("taskA定時任務開始執行");
    //具體的任務
    log.info("taskA定時任務完成執行");
}

但是如上直接使用是有問題的${task.taskA}是沒有值的,必須要在外部化配置中再寫一遍，這樣我們相當於預設值就沒有用了，這怎麼行呢，我們來搞定他。

探究其原理
@ConfigurationProperties、@Value 、SpringEl 他們之間的關係和區別及我認為的正確使用方式。

首先@ConfigurationProperties 是Spring Boot引入的,遂查詢官方文件的講解

Spring Boot -> Externalized Configuration

我們發現外部化配置中沒有值的話,報錯是在
org.springframework.util.PropertyPlaceholderHelper#parseStringValue
其中org.springframework.util.PropertyPlaceholderHelper.PlaceholderResolver是解析的關鍵
我們只要把預設值裝載到系統中,讓org.springframework.util.PropertyPlaceholderHelper.PlaceholderResolver#resolvePlaceholder可以解析到就可以了
遂我們可以把
值裝載到Environment中
/**

• @author wangqimeng

• @date 2020/3/4 0:04
  */
  @Data
  @Slf4j
  @Validated
  @ConfigurationProperties(prefix = "task")
  public class TaskConfigProperties implements InitializingBean , EnvironmentPostProcessor {

  /**

  • 任務A在每天的0點5分0秒進行執行
    */
    @NotBlank
    private String taskA = "0 5 0 * * ? ";

  @Value("${task.task-a}")
  public String taskAValue;

  @Autowired
  private Environment environment;

  @Override
  public void afterPropertiesSet() throws Exception {
  log.info("taskAValue:{}",taskAValue);
  }

  @Override
  public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
  log.info("TaskConfigProperties-> postProcessEnvironment 開始執行");
  //取到當前配置類上的資訊
  MutablePropertySources propertySources = environment.getPropertySources();
  Properties properties = new Properties();
  if (taskA != null) {
  properties.put("task.task-a", this.taskA);
  }
  PropertySource propertySource = new PropertiesPropertySource("task", properties);
  //即優先順序低
  propertySources.addLast(propertySource);
  }
  }

需要在META-INF -> spring.factories中配置

org.springframework.boot.env.EnvironmentPostProcessor=
cn.boommanpro.config.TaskConfigProperties

所以addLast是優先順序最低的，讓我們新加入的配置優先順序最低。

以上就簡單的完成了我們的需求。

最終實現
配置類中的有預設值的不需要在External Configuration中再度配置
通過一個註解@EnableBindEnvironmentProperties，繫結含有@ConfigurationPropertiesClass的預設值到Environment
@EnableBindEnvironmentProperties

/**

• @author wangqimeng

• @date 2020/3/4 1:21
  */
  @Target({ ElementType.TYPE, ElementType.METHOD })
  @Retention(RetentionPolicy.RUNTIME)
  @Documented
  public @interface EnableBindEnvironmentProperties {

  Class<?>[] value() default {};
  }

@EnableBindEnvironmentPropertiesRegister

import java.beans.BeanInfo;
import java.beans.Introspector;
import java.beans.PropertyDescriptor;
import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Properties;

import lombok.extern.slf4j.Slf4j;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.boot.env.EnvironmentPostProcessor;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.MutablePropertySources;
import org.springframework.core.env.PropertiesPropertySource;
import org.springframework.core.env.PropertySource;

/**

• @author wangqimeng

• @date 2020/3/4 15:11
  */
  @Slf4j
  public class EnableBindEnvironmentPropertiesRegister implements EnvironmentPostProcessor {

  @Override
  public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
  MutablePropertySources propertySources = environment.getPropertySources();
  EnableBindEnvironmentProperties annotation = application.getMainApplicationClass().getAnnotation(EnableBindEnvironmentProperties.class);
  Arrays.stream(annotation.value())
  .forEach(aClass -> registerToEnvironment(propertySources, aClass));
  }

  public void registerToEnvironment(MutablePropertySources propertySources, Class<?> clazz) {
  ConfigurationProperties annotation = clazz.getAnnotation(ConfigurationProperties.class);
  if (annotation == null) {
  return;
  }
  String prefix = annotation.prefix();
  String name = String.format("%s-%s", prefix, clazz.getName());
  try {
  Properties properties = toProperties(prefix, clazz.newInstance());
  PropertySource propertySource = new PropertiesPropertySource(name, properties);
  propertySources.addLast(propertySource);
  } catch (Exception e) {
  log.error("Exception:", e);
  throw new RuntimeException();
  }

  }

  public Properties toProperties(String prefix, Object o) throws Exception {
  Properties properties = new Properties();
  Map<String, Object> map = objectToMap(o);
  map.forEach((s, o1) -> {
  properties.put(String.format("%s.%s", prefix, camelToUnderline(s)), o1);
  });

   return properties;
  

  }

  public static String camelToUnderline(String param) {
  if (param == null || "".equals(param.trim())) {
  return "";
  }
  int len = param.length();
  StringBuilder sb = new StringBuilder(len);
  for (int i = 0; i < len; i++) {
  char c = param.charAt(i);
  if (Character.isUpperCase(c)) {
  sb.append("-");
  sb.append(Character.toLowerCase(c));
  } else {
  sb.append(c);
  }
  }
  return sb.toString();
  }

  public static Map<String, Object> objectToMap(Object obj) throws Exception {
  if (obj == null) {
  return null;
  }
  Map<String, Object> map = new HashMap<>(10);
  BeanInfo beanInfo = Introspector.getBeanInfo(obj.getClass());
  PropertyDescriptor[] propertyDescriptors = beanInfo.getPropertyDescriptors();
  for (PropertyDescriptor property : propertyDescriptors) {
  String key = property.getName();
  if (key.compareToIgnoreCase("class") == 0) {
  continue;
  }
  Method getter = property.getReadMethod();
  Object value = getter != null ? getter.invoke(obj) : null;
  if (value == null) {
  continue;
  }
  map.put(key, value);
  }

   return map;
  

  }
  }

配置到META-INF/spring.factories

Application Listeners

org.springframework.boot.env.EnvironmentPostProcessor=
cn.boommanpro.annotation.EnableBindEnvironmentPropertiesRegister