Spring源码解析之PropertyPlaceholderHelper(占位符解析器)

PropertyPlaceholderHelper作用：
将字符串里的占位符内容，用我们配置的properties里的替换。这个是一个单纯的类，没有继承没有实现，而且也没简单，没有依赖Spring框架其他的任何类。个人感觉自己项目中可以拿来模仿用。

我们解析的过程：
1、这里先贴一下Spring框架里的全部代码
2、分块解析
3、跑测试看结果，加深理解

看不懂的话，建议debug跟着跑，这样对效果有直观表达，更好理解。

1、请看全部代码

/*
 * Copyright 2002-2014 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.util;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Properties;
import java.util.Set;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * Utility class for working with Strings that have placeholder values in them. A placeholder takes the form
 * {@code ${name}}. Using {@code PropertyPlaceholderHelper} these placeholders can be substituted for
 * user-supplied values. <p> Values for substitution can be supplied using a {@link Properties} instance or
 * using a {@link PlaceholderResolver}.
 *
 * @author Juergen Hoeller
 * @author Rob Harrop
 * @since 3.0
 */
public class PropertyPlaceholderHelper {

    private static final Log logger = LogFactory.getLog(PropertyPlaceholderHelper.class);

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

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


    private final String placeholderPrefix;

    private final String placeholderSuffix;

    private final String simplePrefix;

    private final String valueSeparator;

    private final boolean ignoreUnresolvablePlaceholders;


    /**
     * Creates a new {@code PropertyPlaceholderHelper} that uses the supplied prefix and suffix.
     * Unresolvable placeholders are ignored.
     * @param placeholderPrefix the prefix that denotes the start of a placeholder
     * @param placeholderSuffix the suffix that denotes the end of a placeholder
     */
    public PropertyPlaceholderHelper(String placeholderPrefix, String placeholderSuffix) {
        this(placeholderPrefix, placeholderSuffix, null, true);
    }

    /**
     * Creates a new {@code PropertyPlaceholderHelper} that uses the supplied prefix and suffix.
     * @param placeholderPrefix the prefix that denotes the start of a placeholder
     * @param placeholderSuffix the suffix that denotes the end of a placeholder
     * @param valueSeparator the separating character between the placeholder variable
     * and the associated default value, if any
     * @param ignoreUnresolvablePlaceholders indicates whether unresolvable placeholders should
     * be ignored ({@code true}) or cause an exception ({@code false})
     */
    public PropertyPlaceholderHelper(String placeholderPrefix, String placeholderSuffix,
            String valueSeparator, boolean ignoreUnresolvablePlaceholders) {

        Assert.notNull(placeholderPrefix, "'placeholderPrefix' must not be null");
        Assert.notNull(placeholderSuffix, "'placeholderSuffix' must not be null");
        this.placeholderPrefix = placeholderPrefix;
        this.placeholderSuffix = placeholderSuffix;
        String simplePrefixForSuffix = wellKnownSimplePrefixes.get(this.placeholderSuffix);
        if (simplePrefixForSuffix != null && this.placeholderPrefix.endsWith(simplePrefixForSuffix)) {
            this.simplePrefix = simplePrefixForSuffix;
        }
        else {
            this.simplePrefix = this.placeholderPrefix;
        }
        this.valueSeparator = valueSeparator;
        this.ignoreUnresolvablePlaceholders = ignoreUnresolvablePlaceholders;
    }


    /**
     * Replaces all placeholders of format {@code ${name}} with the corresponding
     * property from the supplied {@link Properties}.
     * @param value the value containing the placeholders to be replaced
     * @param properties the {@code Properties} to use for replacement
     * @return the supplied value with placeholders replaced inline
     */
    public String replacePlaceholders(String value, final Properties properties) {
        Assert.notNull(properties, "'properties' must not be null");
        return replacePlaceholders(value, new PlaceholderResolver() {
            public String resolvePlaceholder(String placeholderName) {
                return properties.getProperty(placeholderName);
            }
        });
    }

    /**
     * Replaces all placeholders of format {@code ${name}} with the value returned
     * from the supplied {@link PlaceholderResolver}.
     * @param value the value containing the placeholders to be replaced
     * @param placeholderResolver the {@code PlaceholderResolver} to use for replacement
     * @return the supplied value with placeholders replaced inline
     */
    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);

        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);
                // 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);
                        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);
                    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();
    }

    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;
    }


    /**
     * Strategy interface used to resolve replacement values for placeholders contained in Strings.
     */
    public static interface PlaceholderResolver {

        /**
         * Resolve the supplied placeholder name to the replacement value.
         * @param placeholderName the name of the placeholder to resolve
         * @return the replacement value, or {@code null} if no replacement is to be made
         */
        String resolvePlaceholder(String placeholderName);
    }

}

2、分块解析
下面这个是PropertyPlaceholderHelper占位解析的主要入口。其大致的过程是将占位的内容一块一块地取下来，通过递归将最内层的占位符先替换掉，然后跳出来替换外面的占位符。

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

        StringBuilder result = new StringBuilder(strVal);
        //获取路径中占位符前缀的索引
        int startIndex = strVal.indexOf(this.placeholderPrefix);
        //匹配到占位符前缀,进入循环体
        while (startIndex != -1) {
            int endIndex = findPlaceholderEndIndex(result, startIndex);
            if (endIndex != -1) {
                //截取前缀占位符和后缀占位符之间的字符串placeholder
                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");
                }
                // 递归调用,继续解析placeholder
                placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);
                // 获取placeholder的值
                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);
                        if (propVal == null) {
                            propVal = defaultValue;
                        }
                    }
                }
                if (propVal != null) {
                    //对替换完成的value进行解析,防止properties的value值里也有占位符
                    propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
                    result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal);
                    if (logger.isTraceEnabled()) {
                        logger.trace("Resolved placeholder '" + placeholder + "'");
                    }
                    //重新定位开始索引
                    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;
            }
        }
        System.out.println("enter..." + result);
        return result.toString();
    }

下面这个方法是用来寻找占位符后缀索引的，需要注意的是withinNestedPlaceholder这个参数控制当我们获取到占位符后缀的时候是选择直接返回还是继续去获取占位符后缀。

private int findPlaceholderEndIndex(CharSequence buf, int startIndex) {
        //获取前缀后面一个字符的索引
        int index = startIndex + this.placeholderPrefix.length();
        int withinNestedPlaceholder = 0;
        //如果前缀后面还有字符的话
        while (index < buf.length()) {
            //判断源字符串在index处是否与后缀匹配
            if (substringMatch(buf, index, this.placeholderSuffix)) {
                //如果匹配到后缀,但此时前缀数量>后缀,则继续匹配后缀
                if (withinNestedPlaceholder > 0) {
                    withinNestedPlaceholder--;
                    index = index + this.placeholderSuffix.length();
                } else {
                    return index;
                }
            } else if (substringMatch(buf, index, this.simplePrefix)) {
                //判断源字符串在index处是否与前缀匹配,若匹配,说明前缀后面还是前缀,则把前缀长度累加到index上,继续循环寻找后缀
                //withinNestedPlaceholder确保前缀和后缀成对出现后
                withinNestedPlaceholder++;
                index = index + this.simplePrefix.length();
            } else {
                //如果index出既不能和suffix又不能和simplePrefix匹配,则自增,继续循环
                index++;
            }
        }
        return -1;
    }

下面这个方法是用来判断str在index索引位置是否和substring匹配。

private 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;
    }

3、测试

test01.properties：

name=wangzha
age=18
sex=man
name18man=love

main方法：

public static void main(String[] args) throws Exception {
        String a = "{name}{age}{sex}";
        String b = "{name{age}{sex}}";
        PropertyPlaceholderHelper propertyPlaceholderHelper = new PropertyPlaceholderHelper("{", "}");
        InputStream in = new BufferedInputStream(new FileInputStream(new File("F:\\ormosia\\openapitestservice\\src\\main\\resources\\test01.properties")));
        Properties properties = new Properties();
        properties.load(in);

        System.out.println("替换前:" + a);
        System.out.println("替换后:" + propertyPlaceholderHelper.parseStringValue(a, new PlaceholderResolver() {
            @Override
            public String resolvePlaceholder(String placeholderName) {
                String value = properties.getProperty(placeholderName);
                return value;
            }
        }, new HashSet<>()));
        System.out.println("====================================================");
        System.out.println("替换前:" + b);
        System.out.println("替换后:" + propertyPlaceholderHelper.parseStringValue(b, new PlaceholderResolver() {
            @Override
            public String resolvePlaceholder(String placeholderName) {
                String value = properties.getProperty(placeholderName);
                return value;
            }
        }, new HashSet<>()));
    }

测试结果：