Spring Boot 启动（四） EnvironmentPostProcessor

Spring 系列目录(https://www.cnblogs.com/binarylei/p/10198698.html)

1. Spring Boot 配置使用
2. Spring Boot 配置文件加载流程分析 - ConfigFileApplicationListener
3. Spring Boot 配置文件加载 - EnvironmentPostProcessor

一、EnvironmentPostProcessor

ConfigFileApplicationListener 是 Spring Boot 中处理配置文件的监听器。

// ConfigFileApplicationListener
private void onApplicationEnvironmentPreparedEvent(
        ApplicationEnvironmentPreparedEvent event) {
    // 1. 委托给 EnvironmentPostProcessor 处理，也是通过 spring.factories 配置
    List<EnvironmentPostProcessor> postProcessors = loadPostProcessors();
    // 2. ConfigFileApplicationListener 本身也实现了 EnvironmentPostProcessor 接口
    postProcessors.add(this);
    // 3. spring 都都通过 AnnotationAwareOrderComparator 控制执行顺序
    AnnotationAwareOrderComparator.sort(postProcessors);
    // 4. 执行 EnvironmentPostProcessor
    for (EnvironmentPostProcessor postProcessor : postProcessors) {
        postProcessor.postProcessEnvironment(event.getEnvironment(), event.getSpringApplication());
    }
}

在 spring.factories 配置文件中默认定义了三个 EnvironmentPostProcessor 的实现类：

# Environment Post Processors
org.springframework.boot.env.EnvironmentPostProcessor=org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor

优先级 SystemEnvironmentPropertySourceEnvironmentPostProcessor > SpringApplicationJsonEnvironmentPostProcessor

• SystemEnvironmentPropertySourceEnvironmentPostProcessor 对 systemEnvironment 属性进行了包装。
• SpringApplicationJsonEnvironmentPostProcessor 解析 spring.application.json 或 SPRING_APPLICATION_JSON 配置的 json 字符串。

本节专门介绍一下 SystemEnvironmentPropertySourceEnvironmentPostProcessor 如何解析 JSON 格式的。

二、spring.application.json 使用

spring.application.json 或 SPRING_APPLICATION_JSON 定义的 json 字符串。命令行配置如下：

java -jar xxx.jar --spring.application.json='{"foo":"bar"}'

编程方式如下：

@Test
public void list() {
    assertThat(this.environment.resolvePlaceholders("${foo[1]:}")).isEmpty();
    TestPropertySourceUtils.addInlinedPropertiesToEnvironment(this.environment,
            "SPRING_APPLICATION_JSON={\"foo\":[\"bar\",\"spam\"]}");
    this.processor.postProcessEnvironment(this.environment, null);
    assertThat(this.environment.resolvePlaceholders("${foo[1]:}")).isEqualTo("spam");
}

三、源码分析

@Override
public void postProcessEnvironment(ConfigurableEnvironment environment,
        SpringApplication application) {
    MutablePropertySources propertySources = environment.getPropertySources();
    // environment 中定义的 spring.application.json 或 SPRING_APPLICATION_JSON 解析成 JsonPropertySource
    // 默认只会解析第一个配置的 json 字符串
    propertySources.stream().map(JsonPropertyValue::get).filter(Objects::nonNull)
            .findFirst().ifPresent((v) -> processJson(environment, v));
}

private void processJson(ConfigurableEnvironment environment,
        JsonPropertyValue propertyValue) {
    JsonParser parser = JsonParserFactory.getJsonParser();
    Map<String, Object> map = parser.parseMap(propertyValue.getJson());
    if (!map.isEmpty()) {
        addJsonPropertySource(environment,
                new JsonPropertySource(propertyValue, flatten(map)));
    }
}

这里我们还需要注意 JsonPropertySource 的读取顺序。spring.application.json 的级别非常高，只低于命令行配置。

private void addJsonPropertySource(ConfigurableEnvironment environment,
        PropertySource<?> source) {
    MutablePropertySources sources = environment.getPropertySources();
    String name = findPropertySource(sources);
    if (sources.contains(name)) {
        sources.addBefore(name, source);
    } else {
        sources.addFirst(source);
    }
}
// 默认会放到 jndiProperties 或 systemProperties 之前
private String findPropertySource(MutablePropertySources sources) {
    if (ClassUtils.isPresent(SERVLET_ENVIRONMENT_CLASS, null) && sources
            .contains(StandardServletEnvironment.JNDI_PROPERTY_SOURCE_NAME)) {
        return StandardServletEnvironment.JNDI_PROPERTY_SOURCE_NAME;

    }
    return StandardEnvironment.SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME;
}

3.1 map 解析成 properties 分析

这里可以看到 Spring 将 map 转为 properties 的代码十分简洁。

// "SPRING_APPLICATION_JSON={\"foo\":[\"bar\",\"spam\"]}" -> ${foo[1]:}=spam
// "SPRING_APPLICATION_JSON={\"foo.bar\":\"spam\"}" -> ${foo.bar:}=spam
// "SPRING_APPLICATION_JSON={\"foo\":{\"bar\":\"spam\",\"rab\":\"maps\"}}" -> ${foo.bar:}=spam
private Map<String, Object> flatten(Map<String, Object> map) {
    Map<String, Object> result = new LinkedHashMap<>();
    flatten(null, result, map);
    return result;
}

private void flatten(String prefix, Map<String, Object> result,
        Map<String, Object> map) {
    String namePrefix = (prefix != null) ? prefix + "." : "";
    map.forEach((key, value) -> extract(namePrefix + key, result, value));
}

private void extract(String name, Map<String, Object> result, Object value) {
    if (value instanceof Map) {
        flatten(name, result, (Map<String, Object>) value);
    } else if (value instanceof Collection) {
        int index = 0;
        for (Object object : (Collection<Object>) value) {
            extract(name + "[" + index + "]", result, object);
            index++;
        }
    } else {
        result.put(name, value);
    }
}

原文地址：https://www.cnblogs.com/binarylei/p/10645912.html