前提
前面写过一篇关于environment属性加载的源码分析和扩展，里面提到属性的占位符解析和类型转换是相对复杂的，这篇文章就是要分析和解读这两个复杂的问题。关于这两个问题，选用一个比较复杂的参数处理方法propertysourcespropertyresolver#getproperty，解析占位符的时候依赖到
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propertysourcespropertyresolver#getpropertyasrawstring：protected string getpropertyasrawstring(string key) { return getproperty(key, string.class, false);}protected <t> t getproperty(string key, class<t> targetvaluetype, boolean resolvenestedplaceholders) { if (this.propertysources != null) { for (propertysource<?> propertysource : this.propertysources) { if (logger.istraceenabled()) { logger.trace("searching for key '" + key + "' in propertysource '" + propertysource.getname() + "'"); } object value = propertysource.getproperty(key); if (value != null) { if (resolvenestedplaceholders && value instanceof string) { //解析带有占位符的属性 value = resolvenestedplaceholders((string) value); } logkeyfound(key, propertysource, value); //需要时转换属性的类型 return convertvalueifnecessary(value, targetvaluetype); } } } if (logger.isdebugenabled()) { logger.debug("could not find key '" + key + "' in any property source"); } return null;} |
属性占位符解析
属性占位符的解析方法是propertysourcespropertyresolver的父类abstractpropertyresolver#resolvenestedplaceholders：
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protected string resolvenestedplaceholders(string value) { return (this.ignoreunresolvablenestedplaceholders ? resolveplaceholders(value) : resolverequiredplaceholders(value));} |
ignoreunresolvablenestedplaceholders属性默认为false，可以通过abstractenvironment#setignoreunresolvablenestedplaceholders(boolean ignoreunresolvablenestedplaceholders)设置，当此属性被设置为true，解析属性占位符失败的时候(并且没有为占位符配置默认值)不会抛出异常，返回属性原样字符串，否则会抛出illegalargumentexception。我们这里只需要分析abstractpropertyresolver#resolverequiredplaceholders：
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//abstractpropertyresolver中的属性：//ignoreunresolvablenestedplaceholders=true情况下创建的propertyplaceholderhelper实例@nullableprivate propertyplaceholderhelper nonstricthelper;//ignoreunresolvablenestedplaceholders=false情况下创建的propertyplaceholderhelper实例@nullableprivate propertyplaceholderhelper stricthelper;//是否忽略无法处理的属性占位符，这里是false，也就是遇到无法处理的属性占位符且没有默认值则抛出异常private boolean ignoreunresolvablenestedplaceholders = false;//属性占位符前缀，这里是"${"private string placeholderprefix = systempropertyutils.placeholder_prefix;//属性占位符后缀，这里是"}"private string placeholdersuffix = systempropertyutils.placeholder_suffix;//属性占位符解析失败的时候配置默认值的分隔符，这里是":"@nullableprivate string valueseparator = systempropertyutils.value_separator;public string resolverequiredplaceholders(string text) throws illegalargumentexception { if (this.stricthelper == null) { this.stricthelper = createplaceholderhelper(false); } return doresolveplaceholders(text, this.stricthelper);}//创建一个新的propertyplaceholderhelper实例，这里ignoreunresolvableplaceholders为falseprivate propertyplaceholderhelper createplaceholderhelper(boolean ignoreunresolvableplaceholders) { return new propertyplaceholderhelper(this.placeholderprefix, this.placeholdersuffix, this.valueseparator, ignoreunresolvableplaceholders);}//这里最终的解析工作委托到propertyplaceholderhelper#replaceplaceholders完成private string doresolveplaceholders(string text, propertyplaceholderhelper helper) { return helper.replaceplaceholders(text, this::getpropertyasrawstring);} |
最终只需要分析propertyplaceholderhelper#replaceplaceholders，这里需要重点注意：
注意到这里的第一个参数text就是属性值的源字符串，例如我们需要处理的属性为myproperties: ${server.port}-${spring.application.name}，这里的text就是${server.port}-${spring.application.name}。
replaceplaceholders方法的第二个参数placeholderresolver，这里比较巧妙，这里的方法引用this::getpropertyasrawstring相当于下面的代码：
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//placeholderresolver是一个函数式接口@functionalinterfacepublic interface placeholderresolver { @nullable string resolveplaceholder(string placeholdername); }//this::getpropertyasrawstring相当于下面的代码return new placeholderresolver(){ @override string resolveplaceholder(string placeholdername){ //这里调用到的是propertysourcespropertyresolver#getpropertyasrawstring，有点绕 return getpropertyasrawstring(placeholdername); }} |
接着看propertyplaceholderhelper#replaceplaceholders的源码：
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//基础属性//占位符前缀，默认是"${"private final string placeholderprefix;//占位符后缀，默认是"}"private final string placeholdersuffix;//简单的占位符前缀，默认是"{"，主要用于处理嵌套的占位符如${xxxxx.{yyyyy}}private final string simpleprefix;//默认值分隔符号，默认是":"@nullableprivate final string valueseparator;//替换属性占位符public string replaceplaceholders(string value, placeholderresolver placeholderresolver) { assert.notnull(value, "'value' must not be null"); return parsestringvalue(value, placeholderresolver, new hashset<>());}//递归解析带占位符的属性为字符串protected string parsestringvalue( string value, placeholderresolver placeholderresolver, set<string> visitedplaceholders) { stringbuilder result = new stringbuilder(value); int startindex = value.indexof(this.placeholderprefix); while (startindex != -1) { //搜索第一个占位符后缀的索引 int endindex = findplaceholderendindex(result, startindex); if (endindex != -1) { //提取第一个占位符中的原始字符串，如${server.port}->server.port string placeholder = result.substring(startindex + this.placeholderprefix.length(), endindex); string originalplaceholder = placeholder; //判重 if (!visitedplaceholders.add(originalplaceholder)) { throw new illegalargumentexception( "circular placeholder reference '" + originalplaceholder + "' in property definitions"); } // recursive invocation, parsing placeholders contained in the placeholder key. // 递归调用，实际上就是解析嵌套的占位符，因为提取的原始字符串有可能还有一层或者多层占位符 placeholder = parsestringvalue(placeholder, placeholderresolver, visitedplaceholders); // now obtain the value for the fully resolved key... // 递归调用完毕后，可以确定得到的字符串一定是不带占位符，这个时候调用getpropertyasrawstring获取key对应的字符串值 string propval = placeholderresolver.resolveplaceholder(placeholder); // 如果字符串值为null，则进行默认值的解析，因为默认值有可能也使用了占位符，如${server.port:${server.port-2:8080}} if (propval == null && this.valueseparator != null) { int separatorindex = placeholder.indexof(this.valueseparator); if (separatorindex != -1) { string actualplaceholder = placeholder.substring(0, separatorindex); // 提取默认值的字符串 string defaultvalue = placeholder.substring(separatorindex + this.valueseparator.length()); // 这里是把默认值的表达式做一次解析，解析到null，则直接赋值为defaultvalue propval = placeholderresolver.resolveplaceholder(actualplaceholder); if (propval == null) { propval = defaultvalue; } } } // 上一步解析出来的值不为null，但是它有可能是一个带占位符的值，所以后面对值进行递归解析 if (propval != null) { // recursive invocation, parsing placeholders contained in the // previously resolved placeholder value. propval = parsestringvalue(propval, placeholderresolver, visitedplaceholders); // 这一步很重要，替换掉第一个被解析完毕的占位符属性，例如${server.port}-${spring.application.name} -> 9090--${spring.application.name} result.replace(startindex, endindex + this.placeholdersuffix.length(), propval); if (logger.istraceenabled()) { logger.trace("resolved placeholder '" + placeholder + "'"); } // 重置startindex为下一个需要解析的占位符前缀的索引，可能为-1，说明解析结束 startindex = result.indexof(this.placeholderprefix, startindex + propval.length()); } else if (this.ignoreunresolvableplaceholders) { // 如果propval为null并且ignoreunresolvableplaceholders设置为true，直接返回当前的占位符之间的原始字符串尾的索引，也就是跳过解析 // proceed with unprocessed value. startindex = result.indexof(this.placeholderprefix, endindex + this.placeholdersuffix.length()); } else { // 如果propval为null并且ignoreunresolvableplaceholders设置为false，抛出异常 throw new illegalargumentexception("could not resolve placeholder '" + placeholder + "'" + " in value \"" + value + "\""); } // 递归结束移除判重集合中的元素 visitedplaceholders.remove(originalplaceholder); } else { // endindex = -1说明解析结束 startindex = -1; } } return result.tostring();}//基于传入的起始索引，搜索第一个占位符后缀的索引，兼容嵌套的占位符private int findplaceholderendindex(charsequence buf, int startindex) { //这里index实际上就是实际需要解析的属性的第一个字符，如${server.port}，这里index指向s int index = startindex + this.placeholderprefix.length(); int withinnestedplaceholder = 0; while (index < buf.length()) { //index指向"}"，说明有可能到达占位符尾部或者嵌套占位符尾部 if (stringutils.substringmatch(buf, index, this.placeholdersuffix)) { //存在嵌套占位符，则返回字符串中占位符后缀的索引值 if (withinnestedplaceholder > 0) { withinnestedplaceholder--; index = index + this.placeholdersuffix.length(); } else { //不存在嵌套占位符，直接返回占位符尾部索引 return index; } } //index指向"{"，记录嵌套占位符个数withinnestedplaceholder加1，index更新为嵌套属性的第一个字符的索引 else if (stringutils.substringmatch(buf, index, this.simpleprefix)) { withinnestedplaceholder++; index = index + this.simpleprefix.length(); } else { //index不是"{"或者"}"，则进行自增 index++; } } //这里说明解析索引已经超出了原字符串 return -1;}//stringutils#substringmatch，此方法会检查原始字符串str的index位置开始是否和子字符串substring完全匹配public static boolean substringmatch(charsequence str, int index, charsequence substring) { if (index + substring.length() > str.length()) { return false; } for (int i = 0; i < substring.length(); i++) { if (str.charat(index + i) != substring.charat(i)) { return false; } } return true;} |
上面的过程相对比较复杂，因为用到了递归，我们举个实际的例子说明一下整个解析过程，例如我们使用了四个属性项，我们的目标是获取server.desc的值：
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application.name=springserver.port=9090spring.application.name=${application.name}server.desc=${server.port-${spring.application.name}}:${description:"hello"} |
属性类型转换
在上一步解析属性占位符完毕之后，得到的是属性字符串值，可以把字符串转换为指定的类型，此功能由abstractpropertyresolver#convertvalueifnecessary完成：
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protected <t> t convertvalueifnecessary(object value, @nullable class<t> targettype) { if (targettype == null) { return (t) value; } conversionservice conversionservicetouse = this.conversionservice; if (conversionservicetouse == null) { // avoid initialization of shared defaultconversionservice if // no standard type conversion is needed in the first place... // 这里一般只有字符串类型才会命中 if (classutils.isassignablevalue(targettype, value)) { return (t) value; } conversionservicetouse = defaultconversionservice.getsharedinstance(); } return conversionservicetouse.convert(value, targettype);} |
实际上转换的逻辑是委托到defaultconversionservice的父类方法genericconversionservice#convert：
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public <t> t convert(@nullable object source, class<t> targettype) { assert.notnull(targettype, "target type to convert to cannot be null"); return (t) convert(source, typedescriptor.forobject(source), typedescriptor.valueof(targettype));}public object convert(@nullable object source, @nullable typedescriptor sourcetype, typedescriptor targettype) { assert.notnull(targettype, "target type to convert to cannot be null"); if (sourcetype == null) { assert.istrue(source == null, "source must be [null] if source type == [null]"); return handleresult(null, targettype, convertnullsource(null, targettype)); } if (source != null && !sourcetype.getobjecttype().isinstance(source)) { throw new illegalargumentexception("source to convert from must be an instance of [" + sourcetype + "]; instead it was a [" + source.getclass().getname() + "]"); } // 从缓存中获取genericconverter实例，其实这一步相对复杂，匹配两个类型的时候，会解析整个类的层次进行对比 genericconverter converter = getconverter(sourcetype, targettype); if (converter != null) { // 实际上就是调用转换方法 object result = conversionutils.invokeconverter(converter, source, sourcetype, targettype); // 断言最终结果和指定类型是否匹配并且返回 return handleresult(sourcetype, targettype, result); } return handleconverternotfound(source, sourcetype, targettype);} |
上面所有的可用的genericconverter的实例可以在defaultconversionservice的adddefaultconverters中看到，默认添加的转换器实例已经超过20个，有些情况下如果无法满足需求可以添加自定义的转换器，实现genericconverter接口添加进去即可。
小结
springboot在抽象整个类型转换器方面做的比较好，在springmvc应用中，采用的是org.springframework.boot.autoconfigure.web.format.webconversionservice，兼容了converter、formatter、conversionservice等转换器类型并且对外提供一套统一的转换方法。
总结
以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，如果有疑问大家可以留言交流，谢谢大家对服务器之家的支持。
原文链接：https://www.cnblogs.com/throwable/p/9417827.html
