聊聊springboot的EnvironmentPostProcessor

序

本文主要研究一下springboot的EnvironmentPostProcessor

EnvironmentPostProcessor

org/springframework/boot/env/EnvironmentPostProcessor.java

@FunctionalInterface
public interface EnvironmentPostProcessor {

    /**
     * Post-process the given {@code environment}.
     * @param environment the environment to post-process
     * @param application the application to which the environment belongs
     */
    void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application);

}

springboot提供了EnvironmentPostProcessor接口，该接口有postProcessEnvironment方法，其中envrionment参数类型为ConfigurableEnvironment，即应用可以通过实现这个接口进行env环境变量的操作

EnvironmentPostProcessorApplicationListener

org/springframework/boot/env/EnvironmentPostProcessorApplicationListener.java

/**
 * {@link SmartApplicationListener} used to trigger {@link EnvironmentPostProcessor
 * EnvironmentPostProcessors} registered in the {@code spring.factories} file.
 *
 * @author Phillip Webb
 * @since 2.4.0
 */
public class EnvironmentPostProcessorApplicationListener implements SmartApplicationListener, Ordered {

    /**
     * The default order for the processor.
     */
    public static final int DEFAULT_ORDER = Ordered.HIGHEST_PRECEDENCE + 10;

    private final DeferredLogs deferredLogs;

    private int order = DEFAULT_ORDER;

    private final EnvironmentPostProcessorsFactory postProcessorsFactory;

    /**
     * Create a new {@link EnvironmentPostProcessorApplicationListener} with
     * {@link EnvironmentPostProcessor} classes loaded via {@code spring.factories}.
     */
    public EnvironmentPostProcessorApplicationListener() {
        this(EnvironmentPostProcessorsFactory
                .fromSpringFactories(EnvironmentPostProcessorApplicationListener.class.getClassLoader()));
    }

    /**
     * Create a new {@link EnvironmentPostProcessorApplicationListener} with post
     * processors created by the given factory.
     * @param postProcessorsFactory the post processors factory
     */
    public EnvironmentPostProcessorApplicationListener(EnvironmentPostProcessorsFactory postProcessorsFactory) {
        this(postProcessorsFactory, new DeferredLogs());
    }

    EnvironmentPostProcessorApplicationListener(EnvironmentPostProcessorsFactory postProcessorsFactory,
            DeferredLogs deferredLogs) {
        this.postProcessorsFactory = postProcessorsFactory;
        this.deferredLogs = deferredLogs;
    }

    @Override
    public boolean supportsEventType(Class<? extends ApplicationEvent> eventType) {
        return ApplicationEnvironmentPreparedEvent.class.isAssignableFrom(eventType)
                || ApplicationPreparedEvent.class.isAssignableFrom(eventType)
                || ApplicationFailedEvent.class.isAssignableFrom(eventType);
    }

    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        if (event instanceof ApplicationEnvironmentPreparedEvent) {
            onApplicationEnvironmentPreparedEvent((ApplicationEnvironmentPreparedEvent) event);
        }
        if (event instanceof ApplicationPreparedEvent) {
            onApplicationPreparedEvent((ApplicationPreparedEvent) event);
        }
        if (event instanceof ApplicationFailedEvent) {
            onApplicationFailedEvent((ApplicationFailedEvent) event);
        }
    }

    private void onApplicationEnvironmentPreparedEvent(ApplicationEnvironmentPreparedEvent event) {
        ConfigurableEnvironment environment = event.getEnvironment();
        SpringApplication application = event.getSpringApplication();
        for (EnvironmentPostProcessor postProcessor : getEnvironmentPostProcessors(event.getBootstrapContext())) {
            postProcessor.postProcessEnvironment(environment, application);
        }
    }

    private void onApplicationPreparedEvent(ApplicationPreparedEvent event) {
        finish();
    }

    private void onApplicationFailedEvent(ApplicationFailedEvent event) {
        finish();
    }

    private void finish() {
        this.deferredLogs.switchOverAll();
    }

    List<EnvironmentPostProcessor> getEnvironmentPostProcessors(ConfigurableBootstrapContext bootstrapContext) {
        return this.postProcessorsFactory.getEnvironmentPostProcessors(this.deferredLogs, bootstrapContext);
    }

    @Override
    public int getOrder() {
        return this.order;
    }

    public void setOrder(int order) {
        this.order = order;
    }

}

EnvironmentPostProcessorApplicationListener用于在接收到ApplicationEnvironmentPreparedEvent事件时触发执行EnvironmentPostProcessor的postProcessEnvironment方法

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.env.EnvironmentPostProcessorApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

示例

public class EnvironmentPostProcessorExample implements EnvironmentPostProcessor {

    private final YamlPropertySourceLoader loader = new YamlPropertySourceLoader();

    @Override
    public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
        Resource path = new ClassPathResource("com/example/myapp/config.yml");
        PropertySource<?> propertySource = loadYaml(path);
        environment.getPropertySources().addLast(propertySource);
    }

    private PropertySource<?> loadYaml(Resource path) {
        if (!path.exists()) {
            throw new IllegalArgumentException("Resource " + path + " does not exist");
        }
        try {
            return this.loader.load("custom-resource", path).get(0);
        }
        catch (IOException ex) {
            throw new IllegalStateException("Failed to load yaml configuration from " + path, ex);
        }
    }

}

EnvironmentPostProcessorExample实现了postProcessEnvironment方法，它额外加载com/example/myapp/config.yml里头的配置最为最后的propertySource

小结

springboot的EnvironmentPostProcessor提供了一个environment的扩展接口，方便应用去做environment的扩展，比如扩展propertySource等