Spring中的PropertyResolver

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PropertyResolver在Spring中的作用是负责属性解析，例如在XML中替换占位符(${})属性，@Value注解等

PropertyResolver

PropertySourcesPropertyResolver

PropertySource & PropertySources

在介绍PropertyResolver之前，先介绍下PropertySource和PropertySources。PropertySource可以看做是包含了name/value的对象，而PropertySources则是由多个PropertySource组成的对象

PropertySource

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public abstract class PropertySource<T> {

    protected final String name;

    // 存储name/value的对象，可以是任意对象，只要覆盖`getProperty`即可
    protected final T source;

    public PropertySource(String name, T source) {
        Assert.hasText(name, "Property source name must contain at least one character");
        Assert.notNull(source, "Property source must not be null");
        this.name = name;
        this.source = source;
    }

    @SuppressWarnings("unchecked")
    public PropertySource(String name) {
        this(name, (T) new Object());
    }

    public String getName() {
        return this.name;
    }

    public T getSource() {
        return this.source;
    }

    // 判断`source`中是否包含指定属性(`name`)
    public boolean containsProperty(String name) {
        return (getProperty(name) != null);
    }

    // 从`source`中根据指定属性名称(`name`)获取属性值(`value`)，由子类实现
    public abstract Object getProperty(String name);


    // 省略`toString`，`hashCode`，`equals`方法


    // 返回一个`PropertySource`
    // 主要供内部使用，也可以用于集合比较，可以看方法注释中的示例代码
    public static PropertySource<?> named(String name) {
        return new ComparisonPropertySource(name);
    }


    // 默认实现，用于当`ApplicationContext`创建时无法及时初始化`PropertySource`时使用。
    // 例如，`ServletContext`相关的属性必须等到`ServletContext`托付给`ApplicationContext`之后才能使用，可以在上下文刷新时替换
    public static class StubPropertySource extends PropertySource<Object> {

        public StubPropertySource(String name) {
            super(name, new Object());
        }

        @Override
        public String getProperty(String name) {
            return null;
        }
    }

    static class ComparisonPropertySource extends StubPropertySource {

        private static final String USAGE_ERROR =
                "ComparisonPropertySource instances are for use with collection comparison only";

        public ComparisonPropertySource(String name) {
            super(name);
        }

        @Override
        public Object getSource() {
            throw new UnsupportedOperationException(USAGE_ERROR);
        }

        @Override
        public boolean containsProperty(String name) {
            throw new UnsupportedOperationException(USAGE_ERROR);
        }

        @Override
        public String getProperty(String name) {
            throw new UnsupportedOperationException(USAGE_ERROR);
        }

        @Override
        public String toString() {
            return String.format("%s [name='%s']", getClass().getSimpleName(), this.name);
        }
    }

}

PropertySources

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public interface PropertySources extends Iterable<PropertySource<?>> {

    // 是否包含指定名称的`PropertySources`
    boolean contains(String name);

    // 根据名称获取`PropertySources`
    PropertySource<?> get(String name);
}

MutablePropertySources是PropertySources的默认实现，保存了List<PropertySource<?>>(虽然是list但是实际上通过PropertySources.name保证了每个PropertySources唯一性)，并且还添加了添加/删除等方法，例如addFirst(PropertySource<?> propertySource)，remove(String name)等

PropertySourcesPropertyResolver

PropertySourcesPropertyResolver做的事，就是循环PropertySources，根据属性名称从PropertySource中获取属性值，并且对属性值做占位符解析替换

写个例子：

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public static void main(String[] args) {

    Map<String, Object> mapProperties = new HashMap<>();
    mapProperties.put("name", "Allen");
    mapProperties.put("age", 12);
    mapProperties.put("nickname", "${name}|${age}");

    MutablePropertySources propertySources = new MutablePropertySources();
    // 此处使用MapPropertySource，也可以使用其他的PropertySource实现类
    propertySources.addLast(new MapPropertySource("user", mapProperties));
    
    PropertySourcesPropertyResolver propertySourcesPropertyResolver = new PropertySourcesPropertyResolver(propertySources);
    
    // 是否忽略不能替换的占位符，默认为`false`。如果设置为`false`，当占位符无法替换时，抛出异常
    propertySourcesPropertyResolver.setIgnoreUnresolvableNestedPlaceholders(false);
    // 设置分隔符，默认为`:`。例如${a:1234}，如果属性`a`存在，返回属性`a`的值，否则返回`1234`；
    propertySourcesPropertyResolver.setValueSeparator(":");
    // 设置占位符格式，默认为`${}`
    propertySourcesPropertyResolver.setPlaceholderSuffix("}");
    propertySourcesPropertyResolver.setPlaceholderPrefix("${");

    System.out.println("name -> " + propertySourcesPropertyResolver.getProperty("name")); // name -> Allen
    System.out.println("age -> " + propertySourcesPropertyResolver.getProperty("age")); // age -> 12
    System.out.println("nickname -> " + propertySourcesPropertyResolver.getProperty("nickname")); // nickname -> Allen|12

    System.out.println("introduction -> " + propertySourcesPropertyResolver.resolvePlaceholders("My name is ${name}, I'm ${age}.")); // introduction -> My name is Allen, I'm 12.
}

getProperty()方法实现

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// PropertySourcesPropertyResolver.java
// 参数`key`为属性名称
// 参数`targetValueType`为属性值类型
// 参数`resolveNestedPlaceholders`是否需要占位符替换
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) {
        // 循环所有的`propertySource`，查找属性名称(`key`)对应的属性值
        for (PropertySource<?> propertySource : this.propertySources) {
            if (debugEnabled) {
                logger.debug(String.format("Searching for key '%s' in [%s]", key, propertySource.getName()));
            }
            Object value = propertySource.getProperty(key);
            if (value != null) {
                Class<?> valueType = value.getClass();
                // 如果需要占位符解析 && 属性值类型为`String`时做占位符替换
                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()));
                }
                // 将属性值转为指定类型返回
                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;
}

resolveNestedPlaceholders()调用的是AbstractPropertyResolver.resolveNestedPlaceholders()：

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// AbstractPropertyResolver.java部分代码

protected String resolveNestedPlaceholders(String value) {
    // 根据`是否忽略不能识别的占位符名称`调用不同的解析方法
    // 区别在于如果使用`${a}`时，属性`a`没有对应的属性值，`resolvePlaceholders`会输出`${a}`，而`resolveRequiredPlaceholders`则抛出异常
    return (this.ignoreUnresolvableNestedPlaceholders ?
            resolvePlaceholders(value) : resolveRequiredPlaceholders(value));
}

@Override
public String resolvePlaceholders(String text) {
    if (this.nonStrictHelper == null) {
        this.nonStrictHelper = createPlaceholderHelper(true);
    }
    return doResolvePlaceholders(text, this.nonStrictHelper);
}

@Override
public String resolveRequiredPlaceholders(String text) throws IllegalArgumentException {
    if (this.strictHelper == null) {
        this.strictHelper = createPlaceholderHelper(false);
    }
    return doResolvePlaceholders(text, this.strictHelper);
}

// 创建`PropertyPlaceholderHelper`占位符解析器
private PropertyPlaceholderHelper createPlaceholderHelper(boolean ignoreUnresolvablePlaceholders) {
    return new PropertyPlaceholderHelper(this.placeholderPrefix, this.placeholderSuffix,
            this.valueSeparator, ignoreUnresolvablePlaceholders);
}

// 处理占位符
private String doResolvePlaceholders(String text, PropertyPlaceholderHelper helper) {
    return helper.replacePlaceholders(text, new PropertyPlaceholderHelper.PlaceholderResolver() {
        @Override
        public String resolvePlaceholder(String placeholderName) {
            // 此处实际调用的是`PropertySourcesPropertyResolver`中的`getPropertyAsRawString()`方法，最终回到调用`getProperty()`
            return getPropertyAsRawString(placeholderName);
        }
    });
}

// 子类实现
protected abstract String getPropertyAsRawString(String key);

可以看出，占位符处理工作是在PropertyPlaceholderHelper.replacePlaceholders()中实现：

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// PropertyPlaceholderHelper.java
public String replacePlaceholders(String value, PlaceholderResolver placeholderResolver) {
    Assert.notNull(value, "'value' must not be null");
    return parseStringValue(value, placeholderResolver, new HashSet<String>());
}

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

    StringBuilder result = new StringBuilder(strVal);

    // 占位符可以自定义，默认为`${}`
    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);

            // 以${name}来说，此处结果为name
            String originalPlaceholder = placeholder;

            // 1.此处防止循环依赖
            if (!visitedPlaceholders.add(originalPlaceholder)) {
                throw new IllegalArgumentException(
                        "Circular placeholder reference '" + originalPlaceholder + "' in property definitions");
            }

            // 2.此处对结果递归解析，处理形如${name${age}}的形式，由内而外做处理，
            // 如果age值为1，会先处理${age}，解析为${name1}，再处理
            placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);

            // 处理占位符，调用PropertySourcesPropertyResolver.getPropertyAsRawString() -> getProperty()上述方法
            String propVal = placeholderResolver.resolvePlaceholder(placeholder);

            // 3.此处解释了分隔符的作用，分隔符可以自定义，默认为`:`
            // 可以处理形如${name:123}，解析为${name}，如果name值不存在，则会使用默认值123
            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);
                    if (propVal == null) {
                        propVal = defaultValue;
                    }
                }
            }
            if (propVal != null) {
                // 2.此处对结果递归解析，如age->12，name->${age}，p->${name}，即对解析结果再做解析
                propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
                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 string value \"" + strVal + "\"");
            }
            visitedPlaceholders.remove(originalPlaceholder);
        }
        else {
            startIndex = -1;
        }
    }

    return result.toString();
}

1.循环依赖

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mapProperties.put("def", "${abc}");
mapProperties.put("abc", "${def}");
// 或者如下形式
mapProperties.put("a", "${a}"); // IllegalArgumentException

2.占位符递归

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mapProperties.put("a", "a");
mapProperties.put("b", "b");
mapProperties.put("ab", "${a}");
mapProperties.put("abc", "${a${b}}"); // getProperty("abc") -> 最终输出`a`

3.分隔符使用

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mapProperties.put("a", "${abc:123}"); // getProperty("a") -> 如果`abc`存在，输出`abc`的值；否则输出`123`