【源码分析-Spring Boot】-6.Spring Boot Banner 是如何打印的

Spring Boot Banner打印：【从零开始学Spring Boot】-6.Spring Boot Banner自定义

1.banner 是如何打印的？

banner 是如何打印的呢？

在 spring boot 项目启动时，调用 run 方法，最终会调用到 printBanner 方法

public ConfigurableApplicationContext run(String... args) {
    // ...
    try {
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
        configureIgnoreBeanInfo(environment);
        // 打印 banner
        Banner printedBanner = printBanner(environment);
        // ...
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, listeners);
        throw new IllegalStateException(ex);
    }

    //...
    return context;
}

它的实现如下

private Banner printBanner(ConfigurableEnvironment environment) {
    // 如果关闭 banner，直接返回
    if (this.bannerMode == Banner.Mode.OFF) {
        return null;
    }
    // 默认 resourceLoader=null，使用 DefaultResourceLoader
    ResourceLoader resourceLoader = (this.resourceLoader != null) ? this.resourceLoader
        : new DefaultResourceLoader(getClassLoader());
    SpringApplicationBannerPrinter bannerPrinter = new SpringApplicationBannerPrinter(resourceLoader, this.banner);
    // bannerMode 有3中取值，OFF、CONSOLE、LOG，这里默认为 CONSOLE
    if (this.bannerMode == Mode.LOG) {
        return bannerPrinter.print(environment, this.mainApplicationClass, logger);
    }
    // 打印 banner
    return bannerPrinter.print(environment, this.mainApplicationClass, System.out);
}

bannerPrinter.print

Banner print(Environment environment, Class<?> sourceClass, PrintStream out) {
    // 获取 banner，先获获取图片类型，再获取文本类型，都没有则答应 Spring Boot
    Banner banner = getBanner(environment);
    // 如果有多种类型，循环打印；按照添加顺序，先图片再文本
    banner.printBanner(environment, sourceClass, out);
    return new PrintedBanner(banner, sourceClass);
}

banner.printBanner

@Override
public void printBanner(Environment environment, Class<?> sourceClass, PrintStream out) {
    for (Banner banner : this.banners) {
        banner.printBanner(environment, sourceClass, out);
    }
}

这里进行文本打印，ResourceBanner.printBanner

@Override
public void printBanner(Environment environment, Class<?> sourceClass, PrintStream out) {
    try {
        // 获取编码格式
        String banner = StreamUtils.copyToString(this.resource.getInputStream(),
                                                 environment.getProperty("spring.banner.charset", Charset.class, StandardCharsets.UTF_8));
        // 处理 banner 文件中属性值
        for (PropertyResolver resolver : getPropertyResolvers(environment, sourceClass)) {
            banner = resolver.resolvePlaceholders(banner);
        }
        // 使用 java bio 进行打印
        out.println(banner);
    }
    catch (Exception ex) {
        logger.warn(
            "Banner not printable: " + this.resource + " (" + ex.getClass() + ": '" + ex.getMessage() + "')",
            ex);
    }
}