一、结构类图
①、PropertyResolver : Environment的顶层接口，主要提供属性检索和解析带占位符的文本。bean.xml配置中的所有占位符例如${}都由它解析
②、ConfigurablePropertyResolver : 该接口定义了如何对组件本身进行配置。如：刚刚提到获取value时可以指定任意类型，这依赖于ConversionService进行类型转换，当前接口就提供了对ConversionService的设置和获取。另外，可以配置属性占位符的格式，包括：占位符前缀（默认为"${"）、占位符后缀（默认为"}"）、占位符值分隔符（默认为":"，用于分隔propertyName和defaultValue）。组件还可以设置哪些属性是必须存在的，还可以校验必须存在的属性是否真的存在（不存在的话会抛出异常）
③、AbstractPropertyResolver : 实现了ConfigurablePropertyResolver接口的所有方法
④、PropertySourcesPropertyResolver : 以PropertySources属性源集合（内部持有属性源列表List<PropertySource>）为属性值的来源，按序遍历每个PropertySource，获取到一个非null的属性值则返回
二、demo示例
public static void main(String[] args) {
Properties properties = System.getProperties();
properties.setProperty("prefixName", "read-code");
ApplicationContext ac = new ClassPathXmlApplicationContext("classpath:${prefixName}-spring.xml");
ReadCodeService readCodeService = (ReadCodeService) ac.getBean("readCodeService");
readCodeService.say();
}
三、源码剖析
1、入口 :
ClassPathXmlApplicationContext 构造函数setConfigLocations
public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
throws BeansException { super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh();
}
}
2、AbstractRefreshableConfigApplicationContext
①、ClassPathXmlApplicationContext构造函数调用它的基类AbstractRefreshableConfigApplicationContext.setConfigLocations
/**
* Set the config locations for this application context.
* <p>If not set, the implementation may use a default as appropriate.
*/
public void setConfigLocations(String... locations) {
if (locations != null) {
Assert.noNullElements(locations, "Config locations must not be null");
this.configLocations = new String[locations.length];
for (int i = 0; i < locations.length; i++) {
this.configLocations[i] = resolvePath(locations[i]).trim(); // 解析路劲
}
}
else {
this.configLocations = null;
}
}
②、解析路劲
/**
* Resolve the given path, replacing placeholders with corresponding
* environment property values if necessary. Applied to config locations.
* @param path the original file path
* @return the resolved file path
* @see org.springframework.core.env.Environment#resolveRequiredPlaceholders(String)
*/
protected String resolvePath(String path) {
return getEnvironment().resolveRequiredPlaceholders(path);
}
3、AbstractPropertyResolver
public String resolveRequiredPlaceholders(String text) throws IllegalArgumentException {
if (this.strictHelper == null) {
this.strictHelper = createPlaceholderHelper(false);
}
return doResolvePlaceholders(text, this.strictHelper);
}
上述方法主要做了两件事 :
①、初始化占位符解析器
createPlaceholderHelper : 主要是初始化占位符的常量,eg : 前缀 ${ 后缀} and so on
②、调用私有方法---替换占位符具体值
private String doResolvePlaceholders(String text, PropertyPlaceholderHelper helper) {
return helper.replacePlaceholders(text, new PropertyPlaceholderHelper.PlaceholderResolver() {
@Override
public String resolvePlaceholder(String placeholderName) {
return getPropertyAsRawString(placeholderName);
}
});
}
4、占位符 key - > value ,
实现PropertyPlaceholderHelper内部接口PlaceholderResolver方法resolvePlaceholder。找到占位符key对应的value,为下文替换key埋下伏笔
protected String getPropertyAsRawString(String key) {
return getProperty(key, String.class, false);
}
代码太多了，这里只给出重点
protected <T> T getProperty(String key, Class<T> targetValueType, boolean resolveNestedPlaceholders) {
boolean debugEnabled = logger.isDebugEnabled();
if (logger.isTraceEnabled()) {
logger.trace(String.format("getProperty(\"%s\", %s)", key, targetValueType.getSimpleName()));
}
if (this.propertySources != null) {
for (PropertySource<?> propertySource : this.propertySources) {
if (debugEnabled) {
logger.debug(String.format("Searching for key '%s' in [%s]", key, propertySource.getName()));
}
Object value;
if ((value = propertySource.getProperty(key)) != null) {
Class<?> valueType = value.getClass();
if (resolveNestedPlaceholders && value instanceof String) {
value = resolveNestedPlaceholders((String) value);
}
if (debugEnabled) {
logger.debug(String.format("Found key '%s' in [%s] with type [%s] and value '%s'",
key, propertySource.getName(), valueType.getSimpleName(), value));
}
if (!this.conversionService.canConvert(valueType, targetValueType)) {
throw new IllegalArgumentException(String.format(
"Cannot convert value [%s] from source type [%s] to target type [%s]",
value, valueType.getSimpleName(), targetValueType.getSimpleName()));
}
return this.conversionService.convert(value, targetValueType);
}
}
}
if (debugEnabled) {
logger.debug(String.format("Could not find key '%s' in any property source. Returning [null]", key));
}
return null;
}
5、占位符解析器, 解析并替换具体值得逻辑在这里
public String replacePlaceholders(String value, PlaceholderResolver placeholderResolver) {
Assert.notNull(value, "'value' must not be null");
return parseStringValue(value, placeholderResolver, new HashSet<String>());
}
递归查找占位符
protected String parseStringValue(
String strVal, PlaceholderResolver placeholderResolver, Set<String> visitedPlaceholders) { StringBuilder result = new StringBuilder(strVal); int startIndex = strVal.indexOf(this.placeholderPrefix);
while (startIndex != -1) {
int endIndex = findPlaceholderEndIndex(result, startIndex);
if (endIndex != -1) {
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");
}
// Recursive invocation, parsing placeholders contained in the placeholder key.
placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders); // 递归查找
// Now obtain the value for the fully resolved key...
String propVal = placeholderResolver.resolvePlaceholder(placeholder);
if (propVal == null && this.valueSeparator != null) {
int separatorIndex = placeholder.indexOf(this.valueSeparator);
if (separatorIndex != -1) {
String actualPlaceholder = placeholder.substring(0, separatorIndex);
String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length());
propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder); // 这里是调用第四步骤的实现PropertyPlaceholderHelper内部接口PlaceholderResolver方法resolvePlaceholder ：占位符 key -> value
if (propVal == null) {
propVal = defaultValue;
}
}
}
if (propVal != null) {
// Recursive invocation, parsing placeholders contained in the
// previously resolved placeholder value.
propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal); // 替换占位符具体值
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 string value \"" + strVal + "\"");
}
visitedPlaceholders.remove(originalPlaceholder);
}
else {
startIndex = -1;
}
} return result.toString();
}
findPlaceholderEndIndex 查找占位符在所在字符串后缀的位置
private int findPlaceholderEndIndex(CharSequence buf, int startIndex) {
int index = startIndex + this.placeholderPrefix.length();
int withinNestedPlaceholder = 0;
while (index < buf.length()) {
if (StringUtils.substringMatch(buf, index, this.placeholderSuffix)) {
if (withinNestedPlaceholder > 0) {
withinNestedPlaceholder--;
index = index + this.placeholderSuffix.length();
}
else {
return index;
}
}
else if (StringUtils.substringMatch(buf, index, this.simplePrefix)) {
withinNestedPlaceholder++;
index = index + this.simplePrefix.length();
}
else {
index++;
}
}
return -1;
}
StringUtis.substringMatch 匹配当前位置的字符是否为占位符后缀
public static boolean substringMatch(CharSequence str, int index, CharSequence substring) {
for (int j = 0; j < substring.length(); j++) {
int i = index + j;
if (i >= str.length() || str.charAt(i) != substring.charAt(j)) {
return false;
}
}
return true;
}