Spring MessageSource详解

protected void initMessageSource() {
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();
        // 判断是否含有 messageSource
        if (beanFactory.containsLocalBean(MESSAGE_SOURCE_BEAN_NAME)) {
            // 读取xml配置文件中 id="messageSource"的数据
            this.messageSource = beanFactory.getBean(MESSAGE_SOURCE_BEAN_NAME, MessageSource.class);
            // Make MessageSource aware of parent MessageSource.
            if (this.parent != null && this.messageSource instanceof HierarchicalMessageSource) {
                HierarchicalMessageSource hms = (HierarchicalMessageSource) this.messageSource;
                if (hms.getParentMessageSource() == null) {
                    // Only set parent context as parent MessageSource if no parent MessageSource
                    // registered already.
                    hms.setParentMessageSource(getInternalParentMessageSource());
                }
            }
            if (logger.isTraceEnabled()) {
                logger.trace("Using MessageSource [" + this.messageSource + "]");
            }
        }
        else {
            // Use empty MessageSource to be able to accept getMessage calls.
            // 没有使用默认的 DelegatingMessageSource
            DelegatingMessageSource dms = new DelegatingMessageSource();
            dms.setParentMessageSource(getInternalParentMessageSource());
            this.messageSource = dms;
            // 注册单例对象
            beanFactory.registerSingleton(MESSAGE_SOURCE_BEAN_NAME, this.messageSource);
            if (logger.isTraceEnabled()) {
                logger.trace("No '" + MESSAGE_SOURCE_BEAN_NAME + "' bean, using [" + this.messageSource + "]");
            }
        }
    }
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@Override
    public String getMessage(String code, @Nullable Object[] args, Locale locale) throws NoSuchMessageException {
        return getMessageSource().getMessage(code, args, locale);
    }
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@Override
    public final String getMessage(String code, @Nullable Object[] args, Locale locale) throws NoSuchMessageException {
        // 获取对应的信息
        String msg = getMessageInternal(code, args, locale);
        if (msg != null) {
            return msg;
        }
        // 默认信息 null
        String fallback = getDefaultMessage(code);
        if (fallback != null) {
            return fallback;
        }
        throw new NoSuchMessageException(code, locale);
    }
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@Nullable
    protected String getDefaultMessage(String code) {
        // 判断是否使用默认值
        if (isUseCodeAsDefaultMessage()) {
            return code;
        }
        return null;
    }
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@Nullable
    protected String getMessageInternal(@Nullable String code, @Nullable Object[] args, @Nullable Locale locale) {
        if (code == null) {
            return null;
        }
        if (locale == null) {
            // 获取语言默认值
            locale = Locale.getDefault();
        }
        Object[] argsToUse = args;
        if (!isAlwaysUseMessageFormat() && ObjectUtils.isEmpty(args)) {
            // Optimized resolution: no arguments to apply,
            // therefore no MessageFormat needs to be involved.
            // Note that the default implementation still uses MessageFormat;
            // this can be overridden in specific subclasses.
            String message = resolveCodeWithoutArguments(code, locale);
            if (message != null) {
                return message;
            }
        }
        else {
            // Resolve arguments eagerly, for the case where the message
            // is defined in a parent MessageSource but resolvable arguments
            // are defined in the child MessageSource.
            argsToUse = resolveArguments(args, locale);
            MessageFormat messageFormat = resolveCode(code, locale);
            if (messageFormat != null) {
                synchronized (messageFormat) {
                    return messageFormat.format(argsToUse);
                }
            }
        }
        // Check locale-independent common messages for the given message code.
        Properties commonMessages = getCommonMessages();
        if (commonMessages != null) {
            String commonMessage = commonMessages.getProperty(code);
            if (commonMessage != null) {
                return formatMessage(commonMessage, args, locale);
            }
        }
        // Not found -> check parent, if any.
        return getMessageFromParent(code, argsToUse, locale);
    }
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@Override
    protected String resolveCodeWithoutArguments(String code, Locale locale) {
        Set<String> basenames = getBasenameSet();
        for (String basename : basenames) {
            // 加载 basename
            ResourceBundle bundle = getResourceBundle(basename, locale);
            if (bundle != null) {
                // 从basename对应的文件中获取对应的值
                String result = getStringOrNull(bundle, code);
                if (result != null) {
                    return result;
                }
            }
        }
        return null;
    }
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