Spring解析@Value

1、初始化PropertyPlaceholderHelper对象

    protected String placeholderPrefix = "${";

	protected String placeholderSuffix = "}";
	@Nullable
	protected String valueSeparator = ":";

private static final Map<String, String> wellKnownSimplePrefixes = new HashMap<>(4);

	static {
		wellKnownSimplePrefixes.put("}", "{");
		wellKnownSimplePrefixes.put("]", "[");
		wellKnownSimplePrefixes.put(")", "(");
	}

public PropertyPlaceholderHelper(String placeholderPrefix, String placeholderSuffix,
			@Nullable String valueSeparator, boolean ignoreUnresolvablePlaceholders) {

		Assert.notNull(placeholderPrefix, "'placeholderPrefix' must not be null");
		Assert.notNull(placeholderSuffix, "'placeholderSuffix' must not be null");
        //默认值${
		this.placeholderPrefix = placeholderPrefix;
        //默认值}
		this.placeholderSuffix = placeholderSuffix;
		String simplePrefixForSuffix = wellKnownSimplePrefixes.get(this.placeholderSuffix);
        //当前缀为空或跟定义的不匹配，取传入的前缀
		if (simplePrefixForSuffix != null && this.placeholderPrefix.endsWith(simplePrefixForSuffix)) {
			this.simplePrefix = simplePrefixForSuffix;
		}
		else {
			this.simplePrefix = this.placeholderPrefix;
		}
        //默认值:
		this.valueSeparator = valueSeparator;
		this.ignoreUnresolvablePlaceholders = ignoreUnresolvablePlaceholders;
	}

2、解析@Value 

protected String parseStringValue(
			String value, PlaceholderResolver placeholderResolver, Set<String> visitedPlaceholders) {

		StringBuilder result = new StringBuilder(value);
        //是否包含前缀，返回第一个前缀的开始index
		int startIndex = value.indexOf(this.placeholderPrefix);
		while (startIndex != -1) {
            //找到最后一个后缀的index
			int endIndex = findPlaceholderEndIndex(result, startIndex);
			if (endIndex != -1) {
                //去掉前缀后缀，取出里面的字符串
				String placeholder = result.substring(startIndex + this.placeholderPrefix.length(), endIndex);
				String originalPlaceholder = placeholder;
				if (!visitedPlaceholders.add(originalPlaceholder)) {
					throw new IllegalArgumentException(
							"Circular placeholder reference '" + originalPlaceholder + "' in property definitions");
				}
				// 递归判断是否存在占位符，可以这样写${acm.endpoint:${address.server.domain:}}
				placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);
				// 根据key获取对应的值
				String propVal = placeholderResolver.resolvePlaceholder(placeholder);
                // 值不存在，但存在默认值的分隔符
				if (propVal == null && this.valueSeparator != null) {
                    // 获取默认值的索引
					int separatorIndex = placeholder.indexOf(this.valueSeparator);
					if (separatorIndex != -1) {
                        // 切掉默认值的字符串
						String actualPlaceholder = placeholder.substring(0, separatorIndex);
                        // 切出默认值
						String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length());
                        // 根据新的key获取对应的值
						propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder);
                        // 如果值不存在，则把默认值赋值给当前值
						if (propVal == null) {
							propVal = defaultValue;
						}
					}
				}
                // 如果当前值不为NULL
				if (propVal != null) {
					// 递归获取存在占位符的值信息
					propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
                    // 替换占位符
					result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal);
					if (logger.isTraceEnabled()) {
						logger.trace("Resolved placeholder '" + placeholder + "'");
					}
					startIndex = result.indexOf(this.placeholderPrefix, startIndex + propVal.length());
				}
				else if (this.ignoreUnresolvablePlaceholders) {
					// Proceed with unprocessed value.
					startIndex = result.indexOf(this.placeholderPrefix, endIndex + this.placeholderSuffix.length());
				}
				else {
					throw new IllegalArgumentException("Could not resolve placeholder '" +
							placeholder + "'" + " in value \"" + value + "\"");
				}
				visitedPlaceholders.remove(originalPlaceholder);
			}
			else {
				startIndex = -1;
			}
		}

		return result.toString();
	}