SpringBoot環(huán)境屬性占位符解析和類型轉(zhuǎn)換方式

更新時間：2023年11月27日 11:12:18 作者：Throwable文摘

這篇文章主要介紹了SpringBoot環(huán)境屬性占位符解析和類型轉(zhuǎn)換方式,具有很好的參考價值,希望對大家有所幫助,如有錯誤或未考慮完全的地方,望不吝賜教

前提

前面寫過一篇關(guān)于Environment屬性加載的源碼分析和擴展，里面提到屬性的占位符解析和類型轉(zhuǎn)換是相對復(fù)雜的，這篇文章就是要分析和解讀這兩個復(fù)雜的問題。

關(guān)于這兩個問題，選用一個比較復(fù)雜的參數(shù)處理方法PropertySourcesPropertyResolver#getProperty，解析占位符的時候依賴到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) {
                    //解析帶有占位符的屬性
                    value = resolveNestedPlaceholders((String) value);
                }
                logKeyFound(key, propertySource, value);
                //需要時轉(zhuǎn)換屬性的類型
                return convertValueIfNecessary(value, targetValueType);
            }
        }
    }
    if (logger.isDebugEnabled()) {
        logger.debug("Could not find key '" + key + "' in any property source");
    }
    return null;
}

屬性占位符解析

屬性占位符的解析方法是PropertySourcesPropertyResolver的父類AbstractPropertyResolver#resolveNestedPlaceholders：

protected String resolveNestedPlaceholders(String value) {
    return (this.ignoreUnresolvableNestedPlaceholders ?
        resolvePlaceholders(value) : resolveRequiredPlaceholders(value));
}

ignoreUnresolvableNestedPlaceholders屬性默認(rèn)為false，可以通過AbstractEnvironment#setIgnoreUnresolvableNestedPlaceholders(boolean ignoreUnresolvableNestedPlaceholders)設(shè)置，當(dāng)此屬性被設(shè)置為true，解析屬性占位符失敗的時候(并且沒有為占位符配置默認(rèn)值)不會拋出異常，返回屬性原樣字符串，否則會拋出IllegalArgumentException。

我們這里只需要分析AbstractPropertyResolver#resolveRequiredPlaceholders：

//AbstractPropertyResolver中的屬性：
//ignoreUnresolvableNestedPlaceholders=true情況下創(chuàng)建的PropertyPlaceholderHelper實例
@Nullable
private PropertyPlaceholderHelper nonStrictHelper;

//ignoreUnresolvableNestedPlaceholders=false情況下創(chuàng)建的PropertyPlaceholderHelper實例
@Nullable
private PropertyPlaceholderHelper strictHelper;

//是否忽略無法處理的屬性占位符，這里是false，也就是遇到無法處理的屬性占位符且沒有默認(rèn)值則拋出異常
private boolean ignoreUnresolvableNestedPlaceholders = false;

//屬性占位符前綴，這里是"${"
private String placeholderPrefix = SystemPropertyUtils.PLACEHOLDER_PREFIX;

//屬性占位符后綴，這里是"}"
private String placeholderSuffix = SystemPropertyUtils.PLACEHOLDER_SUFFIX;

//屬性占位符解析失敗的時候配置默認(rèn)值的分隔符，這里是":"
@Nullable
private 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);
}

//創(chuàng)建一個新的PropertyPlaceholderHelper實例，這里ignoreUnresolvablePlaceholders為false
private PropertyPlaceholderHelper createPlaceholderHelper(boolean ignoreUnresolvablePlaceholders) {
    return new PropertyPlaceholderHelper(this.placeholderPrefix, this.placeholderSuffix, this.valueSeparator, ignoreUnresolvablePlaceholders);
}

//這里最終的解析工作委托到PropertyPlaceholderHelper#replacePlaceholders完成
private String doResolvePlaceholders(String text, PropertyPlaceholderHelper helper) {
    return helper.replacePlaceholders(text, this::getPropertyAsRawString);
}

最終只需要分析PropertyPlaceholderHelper#replacePlaceholders，這里需要重點注意：

- 注意到這里的第一個參數(shù)text就是屬性值的源字符串，例如我們需要處理的屬性為myProperties: server.port−server.port− {spring.application.name}，這里的text就是 server\.port−server\.port

− {spring.application.name}。

- replacePlaceholders方法的第二個參數(shù)placeholderResolver，這里比較巧妙，這里的方法引用this::getPropertyAsRawString相當(dāng)于下面的代碼：

//PlaceholderResolver是一個函數(shù)式接口
@FunctionalInterface
public interface PlaceholderResolver {
  @Nullable
  String resolvePlaceholder(String placeholderName);  
}
//this::getPropertyAsRawString相當(dāng)于下面的代碼
return new PlaceholderResolver(){

    @Override
    String resolvePlaceholder(String placeholderName){
        //這里調(diào)用到的是PropertySourcesPropertyResolver#getPropertyAsRawString，有點繞
        return getPropertyAsRawString(placeholderName);
    }
}

接著看PropertyPlaceholderHelper#replacePlaceholders的源碼：

//基礎(chǔ)屬性
//占位符前綴，默認(rèn)是"${"
private final String placeholderPrefix;
//占位符后綴，默認(rèn)是"}"
private final String placeholderSuffix;
//簡單的占位符前綴，默認(rèn)是"{"，主要用于處理嵌套的占位符如${xxxxx.{yyyyy}}
private final String simplePrefix;

//默認(rèn)值分隔符號，默認(rèn)是":"
@Nullable
private final String valueSeparator;
//替換屬性占位符
public String replacePlaceholders(String value, PlaceholderResolver placeholderResolver) {
    Assert.notNull(value, "'value' must not be null");
    return parseStringValue(value, placeholderResolver, new HashSet<>());
}

//遞歸解析帶占位符的屬性為字符串
protected String parseStringValue(
        String value, PlaceholderResolver placeholderResolver, Set<String> visitedPlaceholders) {
    StringBuilder result = new StringBuilder(value);
    int startIndex = value.indexOf(this.placeholderPrefix);
    while (startIndex != -1) {
        //搜索第一個占位符后綴的索引
        int endIndex = findPlaceholderEndIndex(result, startIndex);
        if (endIndex != -1) {
            //提取第一個占位符中的原始字符串，如${server.port}->server.port
            String placeholder = result.substring(startIndex + this.placeholderPrefix.length(), endIndex);
            String originalPlaceholder = placeholder;
            //判重
            if (!visitedPlaceholders.add(originalPlaceholder)) {
                throw new IllegalArgumentException(
                        "Circular placeholder reference '" + originalPlaceholder + "' in property definitions");
            }
            // Recursive invocation, parsing placeholders contained in the placeholder key.
            // 遞歸調(diào)用，實際上就是解析嵌套的占位符，因為提取的原始字符串有可能還有一層或者多層占位符
            placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);
            // Now obtain the value for the fully resolved key...
            // 遞歸調(diào)用完畢后，可以確定得到的字符串一定是不帶占位符，這個時候調(diào)用getPropertyAsRawString獲取key對應(yīng)的字符串值
            String propVal = placeholderResolver.resolvePlaceholder(placeholder);
            // 如果字符串值為null，則進(jìn)行默認(rèn)值的解析，因為默認(rèn)值有可能也使用了占位符，如${server.port:${server.port-2:8080}}
            if (propVal == null && this.valueSeparator != null) {
                int separatorIndex = placeholder.indexOf(this.valueSeparator);
                if (separatorIndex != -1) {
                    String actualPlaceholder = placeholder.substring(0, separatorIndex);
                    // 提取默認(rèn)值的字符串
                    String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length());
                    // 這里是把默認(rèn)值的表達(dá)式做一次解析，解析到null，則直接賦值為defaultValue
                    propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder);
                    if (propVal == null) {
                        propVal = defaultValue;
                    }
                }
            }
            // 上一步解析出來的值不為null，但是它有可能是一個帶占位符的值，所以后面對值進(jìn)行遞歸解析
            if (propVal != null) {
                // Recursive invocation, parsing placeholders contained in the
                // previously resolved placeholder value.
                propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
                // 這一步很重要，替換掉第一個被解析完畢的占位符屬性，例如${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為下一個需要解析的占位符前綴的索引，可能為-1，說明解析結(jié)束
                startIndex = result.indexOf(this.placeholderPrefix, startIndex + propVal.length());
            }
            else if (this.ignoreUnresolvablePlaceholders) {
                // 如果propVal為null并且ignoreUnresolvablePlaceholders設(shè)置為true，直接返回當(dāng)前的占位符之間的原始字符串尾的索引，也就是跳過解析
                // Proceed with unprocessed value.
                startIndex = result.indexOf(this.placeholderPrefix, endIndex + this.placeholderSuffix.length());
            }
            else {
                // 如果propVal為null并且ignoreUnresolvablePlaceholders設(shè)置為false，拋出異常
                throw new IllegalArgumentException("Could not resolve placeholder '" +
                            placeholder + "'" + " in value \"" + value + "\"");
            }
            // 遞歸結(jié)束移除判重集合中的元素
            visitedPlaceholders.remove(originalPlaceholder);
        }
        else {
            // endIndex = -1說明解析結(jié)束
            startIndex = -1;
        }
    }
    return result.toString();
}

//基于傳入的起始索引，搜索第一個占位符后綴的索引，兼容嵌套的占位符
private int findPlaceholderEndIndex(CharSequence buf, int startIndex) {
    //這里index實際上就是實際需要解析的屬性的第一個字符，如${server.port}，這里index指向s
    int index = startIndex + this.placeholderPrefix.length();
    int withinNestedPlaceholder = 0;
    while (index < buf.length()) {
        //index指向"}"，說明有可能到達(dá)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，則返回字符串中占位符后綴的索引值
            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"{"，記錄嵌套占位符個數(shù)withinNestedPlaceholder加1，index更新為嵌套屬性的第一個字符的索引
        else if (StringUtils.substringMatch(buf, index, this.simplePrefix)) {
            withinNestedPlaceholder++;
            index = index + this.simplePrefix.length();
        }
        else {
            //index不是"{"或者"}"，則進(jìn)行自增
            index++;
        }
    }
    //這里說明解析索引已經(jīng)超出了原字符串
    return -1;
}

//StringUtils#substringMatch，此方法會檢查原始字符串str的index位置開始是否和子字符串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;
}

上面的過程相對比較復(fù)雜，因為用到了遞歸，我們舉個實際的例子說明一下整個解析過程，例如我們使用了四個屬性項，我們的目標(biāo)是獲取server.desc的值：

application.name=spring
server.port=9090
spring.application.name=${application.name}
server.desc=${server.port-${spring.application.name}}:${description:"hello"}

屬性類型轉(zhuǎn)換

在上一步解析屬性占位符完畢之后，得到的是屬性字符串值，可以把字符串轉(zhuǎn)換為指定的類型，此功能由AbstractPropertyResolver#convertValueIfNecessary完成：

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...
        // 這里一般只有字符串類型才會命中
        if (ClassUtils.isAssignableValue(targetType, value)) {
            return (T) value;
        }
        conversionServiceToUse = DefaultConversionService.getSharedInstance();
    }
    return conversionServiceToUse.convert(value, targetType);
}

實際上轉(zhuǎn)換的邏輯是委托到DefaultConversionService的父類方法GenericConversionService#convert：

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() + "]");
    }
    // 從緩存中獲取GenericConverter實例，其實這一步相對復(fù)雜，匹配兩個類型的時候，會解析整個類的層次進(jìn)行對比
    GenericConverter converter = getConverter(sourceType, targetType);
    if (converter != null) {
        // 實際上就是調(diào)用轉(zhuǎn)換方法
        Object result = ConversionUtils.invokeConverter(converter, source, sourceType, targetType);
        // 斷言最終結(jié)果和指定類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的實例可以在DefaultConversionService的addDefaultConverters中看到，默認(rèn)添加的轉(zhuǎn)換器實例已經(jīng)超過20個，有些情況下如果無法滿足需求可以添加自定義的轉(zhuǎn)換器，實現(xiàn)GenericConverter接口添加進(jìn)去即可。

總結(jié)

SpringBoot在抽象整個類型轉(zhuǎn)換器方面做的比較好，在SpringMVC應(yīng)用中，采用的是org.springframework.boot.autoconfigure.web.format.WebConversionService，兼容了Converter、Formatter、ConversionService等轉(zhuǎn)換器類型并且對外提供一套統(tǒng)一的轉(zhuǎn)換方法。

以上為個人經(jīng)驗，希望能給大家一個參考，也希望大家多多支持腳本之家。

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