【踩坑】XML转JSON中如何把单个元素转成数组

在某些业务场景下，你可能需要把一个XML 转换成一个 JSON，其实这个转换并不难，网上有很多现成的工具类。但这里都有一个问题：比如这个节点设计者想表达的是一个是数组，但由于XML语法在设计上的缺陷，当只有一个子节点或者没有子节点的情况下，如果你尝试把它转成JSON时，你会发现他默认转成了一个JSON对象了，而不是数组。

当前XML中books节点下有多个book，设计者想表达的是books下有多个元素（多本书），这种场景下转换是没有问题的。

1、转换前的XML数据

<?xml version="1.0" encoding="utf-8"?>
<library><books><book><name>Java</name></book><book><name>C语言</name></book></books>
</library>

2、转换后的JSON数据

{"library": {"books": {"book": [{"name": "Java"},{"name": "C语言"}]}}
}

结合上面XML中books节点，我们删除其中一个子节点，其实设计者依然想表达的是books下有多个元素（多本书），当由于XML不能像JSON对象一样，能清晰的表达对象和数组，这种场景下转换成JSON对象就有问题了。

1、转换前的XML数据

<?xml version="1.0" encoding="utf-8"?>
<library><books><book><name>Java</name></book></books>
</library>

2、转换后的JSON数据

{"library": {"books": {"book": {"name": "Java"}}}
}

我们发现经过工具类转换后，book 被转换成一个JSON对象 {} 了，而不是一个JSON数组 []，因为此处XML的语义无法清晰的表达是对象还是数组，就导致转换的结果是一个对象。其实你期待输出一个标准固定的格式给用户，不管几个节点，都是输出为JSON数组，而不是因为节点变化，导致数据格式发生变化，导致客户端无法用统一的格式解析数据。

网上有很多办法，提供了很多思路，比如（Convert XML to JSON and force array），它是一个.net语言的框架，但作为Java开发者真正解决这个问题的代码并不多。你需要自己写代码来解决这个问题。我在遇到这个问题后，也参考了部分开源代码的实现思路，你通过下面的代码可以轻松的解决这个问题。

package com.demo;
import java.util.ArrayList;
import java.util.HashMap;/*** @author youyun.xu* @Description: Tag* @date 2022/1/17 14:15*/
public class Tag {private String mPath;private String mName;private ArrayList<Tag> mChildren = new ArrayList<>();private String mContent;Tag(String path, String name) {mPath = path;mName = name;}void addChild(Tag tag) {mChildren.add(tag);}void setContent(String content) {// checks that there is a relevant content (not only spaces or \n)boolean hasContent = false;if (content != null) {for(int i=0; i<content.length(); ++i) {char c = content.charAt(i);if ((c != ' ') && (c != '\n')) {hasContent = true;break;}}}if (hasContent) {mContent = content;}}String getName() {return mName;}String getContent() {return mContent;}ArrayList<Tag> getChildren() {return mChildren;}boolean hasChildren() {return (mChildren.size() > 0);}int getChildrenCount() {return mChildren.size();}Tag getChild(int index) {if ((index >= 0) && (index < mChildren.size())) {return mChildren.get(index);}return null;}HashMap<String, ArrayList<Tag>> getGroupedElements() {HashMap<String, ArrayList<Tag>> groups = new HashMap<>();for(Tag child : mChildren) {String key = child.getName();ArrayList<Tag> group = groups.get(key);if (group == null) {group = new ArrayList<>();groups.put(key, group);}group.add(child);}return groups;}String getPath() {return mPath;}@Overridepublic String toString() {return "Tag: " + mName + ", " + mChildren.size() + " children, Content: " + mContent;}
}

package com.demo;import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import org.xmlpull.v1.XmlPullParser;
import org.xmlpull.v1.XmlPullParserException;
import org.xmlpull.v1.XmlPullParserFactory;import java.io.IOException;
import java.io.InputStream;
import java.io.StringReader;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.regex.Matcher;
import java.util.regex.Pattern;import static java.util.regex.Pattern.DOTALL;/*** @author youyun.xu* @Description: Xml转Json支持单个转数组(建造者模式)* @date 2022/1/17 14:15*/
public class XmlToJson {private static final String TAG = "XmlToJson";private static final String DEFAULT_CONTENT_NAME = "content";private static final String DEFAULT_ENCODING = "utf-8";private static final String DEFAULT_INDENTATION = "   ";private String mIndentationPattern = DEFAULT_INDENTATION;// default values when a Tag is emptyprivate static final String DEFAULT_EMPTY_STRING = "";private static final int DEFAULT_EMPTY_INTEGER = 0;private static final long DEFAULT_EMPTY_LONG = 0;private static final double DEFAULT_EMPTY_DOUBLE = 0;private static final boolean DEFAULT_EMPTY_BOOLEAN = false;/*** Builder class to create a XmlToJson object*/public static class Builder {private StringReader mStringSource;private InputStream mInputStreamSource;private String mInputEncoding = DEFAULT_ENCODING;private HashSet<String> mForceListPaths = new HashSet<>();private HashSet<Pattern> mForceListPatterns = new HashSet<>();private HashMap<String, String> mAttributeNameReplacements = new HashMap<>();private HashMap<String, String> mContentNameReplacements = new HashMap<>();private HashMap<String, Class> mForceClassForPath = new HashMap<>();    // Integer, Long, Double, Booleanprivate HashSet<String> mSkippedAttributes = new HashSet<>();private HashSet<String> mSkippedTags = new HashSet<>();/*** Constructor** @param xmlSource XML source*/public Builder( String xmlSource) {mStringSource = new StringReader(xmlSource);}/*** Constructor** @param inputStreamSource XML source* @param inputEncoding     XML encoding format, can be null (uses UTF-8 if null).*/public Builder( InputStream inputStreamSource,  String inputEncoding) {mInputStreamSource = inputStreamSource;mInputEncoding = (inputEncoding != null) ? inputEncoding : DEFAULT_ENCODING;}/*** Force a XML Tag to be interpreted as a list** @param path Path for the tag, with format like "/parentTag/childTag/tagAsAList"* @return the Builder*/public Builder forceList( String path) {mForceListPaths.add(path);return this;}/*** Force a XML Tag to be interpreted as a list, using a RegEx pattern for the path** @param pattern Path for the tag using RegEx, like "*childTag/tagAsAList"* @return the Builder*/public Builder forceListPattern( String pattern) {Pattern pat = Pattern.compile(pattern, DOTALL);mForceListPatterns.add(pat);return this;}/*** Change the name of an attribute** @param attributePath   Path for the attribute, using format like "/parentTag/childTag/childTagAttribute"* @param replacementName Name used for replacement (childTagAttribute becomes replacementName)* @return the Builder*/public Builder setAttributeName( String attributePath,  String replacementName) {mAttributeNameReplacements.put(attributePath, replacementName);return this;}/*** Change the name of the key for a XML content* In XML there is no extra key name for a tag content. So a default name "content" is used.* This "content" name can be replaced with a custom name.** @param contentPath     Path for the Tag that holds the content, using format like "/parentTag/childTag"* @param replacementName Name used in place of the default "content" key* @return the Builder*/public Builder setContentName( String contentPath,  String replacementName) {mContentNameReplacements.put(contentPath, replacementName);return this;}/*** Force an attribute or content value to be a INTEGER. A default value is used if the content is missing.* @param path Path for the Tag content or Attribute, using format like "/parentTag/childTag"* @return the Builder*/public Builder forceIntegerForPath( String path) {mForceClassForPath.put(path, Integer.class);return this;}/*** Force an attribute or content value to be a LONG. A default value is used if the content is missing.* @param path Path for the Tag content or Attribute, using format like "/parentTag/childTag"* @return the Builder*/public Builder forceLongForPath( String path) {mForceClassForPath.put(path, Long.class);return this;}/*** Force an attribute or content value to be a DOUBLE. A default value is used if the content is missing.* @param path Path for the Tag content or Attribute, using format like "/parentTag/childTag"* @return the Builder*/public Builder forceDoubleForPath( String path) {mForceClassForPath.put(path, Double.class);return this;}/*** Force an attribute or content value to be a BOOLEAN. A default value is used if the content is missing.* @param path Path for the Tag content or Attribute, using format like "/parentTag/childTag"* @return the Builder*/public Builder forceBooleanForPath( String path) {mForceClassForPath.put(path, Boolean.class);return this;}/*** Skips a Tag (will not be present in the JSON)** @param path Path for the Tag, using format like "/parentTag/childTag"* @return the Builder*/public Builder skipTag( String path) {mSkippedTags.add(path);return this;}/*** Skips an attribute (will not be present in the JSON)** @param path Path for the Attribute, using format like "/parentTag/childTag/ChildTagAttribute"* @return the Builder*/public Builder skipAttribute( String path) {mSkippedAttributes.add(path);return this;}/*** Creates the XmlToJson object** @return a XmlToJson instance*/public XmlToJson build() {return new XmlToJson(this);}}private StringReader mStringSource;private InputStream mInputStreamSource;private String mInputEncoding;private HashSet<String> mForceListPaths;private HashSet<Pattern> mForceListPatterns = new HashSet<>();private HashMap<String, String> mAttributeNameReplacements;private HashMap<String, String> mContentNameReplacements;private HashMap<String, Class> mForceClassForPath;private HashSet<String> mSkippedAttributes = new HashSet<>();private HashSet<String> mSkippedTags = new HashSet<>();private JSONObject mJsonObject; // Used for caching the resultprivate XmlToJson(Builder builder) {mStringSource = builder.mStringSource;mInputStreamSource = builder.mInputStreamSource;mInputEncoding = builder.mInputEncoding;mForceListPaths = builder.mForceListPaths;mForceListPatterns = builder.mForceListPatterns;mAttributeNameReplacements = builder.mAttributeNameReplacements;mContentNameReplacements = builder.mContentNameReplacements;mForceClassForPath = builder.mForceClassForPath;mSkippedAttributes = builder.mSkippedAttributes;mSkippedTags = builder.mSkippedTags;mJsonObject = convertToJSONObject(); // Build now so that the InputStream can be closed just after}/*** @return the JSONObject built from the XML*/publicJSONObject toJson() {return mJsonObject;}privateJSONObject convertToJSONObject() {try {Tag parentTag = new Tag("", "xml");XmlPullParserFactory factory = XmlPullParserFactory.newInstance();factory.setNamespaceAware(true);   // tags with namespace are taken as-is ("namespace:tagname")XmlPullParser xpp = factory.newPullParser();setInput(xpp);int eventType = xpp.getEventType();while (eventType != XmlPullParser.START_DOCUMENT) {eventType = xpp.next();}readTags(parentTag, xpp);unsetInput();return convertTagToJson(parentTag, false);} catch (XmlPullParserException | IOException e) {e.printStackTrace();return null;}}private void setInput(XmlPullParser xpp) {if (mStringSource != null) {try {xpp.setInput(mStringSource);} catch (XmlPullParserException e) {e.printStackTrace();}} else {try {xpp.setInput(mInputStreamSource, mInputEncoding);} catch (XmlPullParserException e) {e.printStackTrace();}}}private void unsetInput() {if (mStringSource != null) {mStringSource.close();}// else the InputStream has been given by the user, it is not our role to close it}private void readTags(Tag parent, XmlPullParser xpp) {try {int eventType;do {eventType = xpp.next();if (eventType == XmlPullParser.START_TAG) {String tagName = xpp.getName();String path = parent.getPath() + "/" + tagName;boolean skipTag = mSkippedTags.contains(path);Tag child = new Tag(path, tagName);if (!skipTag) {parent.addChild(child);}// Attributes are taken into account as key/values in the childint attrCount = xpp.getAttributeCount();for (int i = 0; i < attrCount; ++i) {String attrName = xpp.getAttributeName(i);String attrValue = xpp.getAttributeValue(i);String attrPath = parent.getPath() + "/" + child.getName() + "/" + attrName;// Skip Attributesif (mSkippedAttributes.contains(attrPath)) {continue;}attrName = getAttributeNameReplacement(attrPath, attrName);Tag attribute = new Tag(attrPath, attrName);attribute.setContent(attrValue);child.addChild(attribute);}readTags(child, xpp);} else if (eventType == XmlPullParser.TEXT) {String text = xpp.getText();parent.setContent(text);} else if (eventType == XmlPullParser.END_TAG) {return;} else if (eventType == XmlPullParser.END_DOCUMENT) {return;} else {//Log.i(TAG, "unknown xml eventType " + eventType);}} while (eventType != XmlPullParser.END_DOCUMENT);} catch (XmlPullParserException | IOException | NullPointerException e) {e.printStackTrace();}}private JSONObject convertTagToJson(Tag tag, boolean isListElement) {JSONObject json = new JSONObject();// Content is injected as a key/valueif (tag.getContent() != null) {String path = tag.getPath();String name = getContentNameReplacement(path, DEFAULT_CONTENT_NAME);putContent(path, json, name, tag.getContent());}try {HashMap<String, ArrayList<Tag>> groups = tag.getGroupedElements(); // groups by tag names so that we can detect lists or single elementsfor (ArrayList<Tag> group : groups.values()) {if (group.size() == 1) {    // element, or list of 1Tag child = group.get(0);if (isForcedList(child)) {  // list of 1JSONArray list = new JSONArray();list.put(convertTagToJson(child, true));String childrenNames = child.getName();json.put(childrenNames, list);} else {    // stand alone elementif (child.hasChildren()) {JSONObject jsonChild = convertTagToJson(child, false);json.put(child.getName(), jsonChild);} else {String path = child.getPath();putContent(path, json, child.getName(), child.getContent());}}} else {    // listJSONArray list = new JSONArray();for (Tag child : group) {list.put(convertTagToJson(child, true));}String childrenNames = group.get(0).getName();json.put(childrenNames, list);}}return json;} catch (JSONException e) {e.printStackTrace();}return null;}private void putContent(String path, JSONObject json, String tag, String content) {try {// checks if the user wants to force a class (Int, Double... for a given path)Class forcedClass = mForceClassForPath.get(path);if (forcedClass == null) {  // default behaviour, put it as a Stringif (content == null) {content = DEFAULT_EMPTY_STRING;}json.put(tag, content);} else {if (forcedClass == Integer.class) {try {Integer number = Integer.parseInt(content);json.put(tag, number);} catch (NumberFormatException exception) {json.put(tag, DEFAULT_EMPTY_INTEGER);}} else if (forcedClass == Long.class) {try {Long number = Long.parseLong(content);json.put(tag, number);} catch (NumberFormatException exception) {json.put(tag, DEFAULT_EMPTY_LONG);}} else if (forcedClass == Double.class) {try {Double number = Double.parseDouble(content);json.put(tag, number);} catch (NumberFormatException exception) {json.put(tag, DEFAULT_EMPTY_DOUBLE);}} else if (forcedClass == Boolean.class) {if (content == null) {json.put(tag, DEFAULT_EMPTY_BOOLEAN);} else if (content.equalsIgnoreCase("true")) {json.put(tag, true);} else if (content.equalsIgnoreCase("false")) {json.put(tag, false);} else {json.put(tag, DEFAULT_EMPTY_BOOLEAN);}}}} catch (JSONException exception) {// keep continue in case of error}}private boolean isForcedList(Tag tag) {String path = tag.getPath();if (mForceListPaths.contains(path)) {return true;}for(Pattern pattern : mForceListPatterns) {Matcher matcher = pattern.matcher(path);if (matcher.find()) {return true;}}return false;}private String getAttributeNameReplacement(String path, String defaultValue) {String result = mAttributeNameReplacements.get(path);if (result != null) {return result;}return defaultValue;}private String getContentNameReplacement(String path, String defaultValue) {String result = mContentNameReplacements.get(path);/* if (result != null) {return result;}*/return result;}@Overridepublic String toString() {if (mJsonObject != null) {return mJsonObject.toString();}return null;}/*** Format the Json with indentation and line breaks** @param indentationPattern indentation to use, for example " " or "\t".*                           if null, use the default 3 spaces indentation* @return the formatted Json*/public String toFormattedString( String indentationPattern) {if (indentationPattern == null) {mIndentationPattern = DEFAULT_INDENTATION;} else {mIndentationPattern = indentationPattern;}return toFormattedString();}/*** Format the Json with indentation and line breaks.* Uses the last intendation pattern used, or the default one (3 spaces)** @return the Builder*/public String toFormattedString() {if (mJsonObject != null) {String indent = "";StringBuilder builder = new StringBuilder();builder.append("{\n");format(mJsonObject, builder, indent);builder.append("}\n");return builder.toString();}return null;}private void format(JSONObject jsonObject, StringBuilder builder, String indent) {Iterator<String> keys = jsonObject.keys();while (keys.hasNext()) {String key = keys.next();builder.append(indent);builder.append(mIndentationPattern);builder.append("\"");builder.append(key);builder.append("\": ");Object value = jsonObject.opt(key);if (value instanceof JSONObject) {JSONObject child = (JSONObject) value;builder.append(indent);builder.append("{\n");format(child, builder, indent + mIndentationPattern);builder.append(indent);builder.append(mIndentationPattern);builder.append("}");} else if (value instanceof JSONArray) {JSONArray array = (JSONArray) value;formatArray(array, builder, indent + mIndentationPattern);} else {formatValue(value, builder);}if (keys.hasNext()) {builder.append(",\n");} else {builder.append("\n");}}}private void formatArray(JSONArray array, StringBuilder builder, String indent) {builder.append("[\n");for (int i = 0; i < array.length(); ++i) {Object element = array.opt(i);if (element instanceof JSONObject) {JSONObject child = (JSONObject) element;builder.append(indent);builder.append(mIndentationPattern);builder.append("{\n");format(child, builder, indent + mIndentationPattern);builder.append(indent);builder.append(mIndentationPattern);builder.append("}");} else if (element instanceof JSONArray) {JSONArray child = (JSONArray) element;formatArray(child, builder, indent + mIndentationPattern);} else {formatValue(element, builder);}if (i < array.length() - 1) {builder.append(",");}builder.append("\n");}builder.append(indent);builder.append("]");}private void formatValue(Object value, StringBuilder builder) {if (value instanceof String) {String string = (String) value;// Escape special charactersstring = string.replaceAll("\\\\", "\\\\\\\\");                     // escape backslashstring = string.replaceAll("\"", Matcher.quoteReplacement("\\\"")); // escape double quotesstring = string.replaceAll("/", "\\\\/");                           // escape slashstring = string.replaceAll("\n", "\\\\n").replaceAll("\t", "\\\\t");  // escape \n and \tstring = string.replaceAll("\r", "\\\\r");  // escape \rbuilder.append("\"");builder.append(string);builder.append("\"");} else if (value instanceof Long) {Long longValue = (Long) value;builder.append(longValue);} else if (value instanceof Integer) {Integer intValue = (Integer) value;builder.append(intValue);} else if (value instanceof Boolean) {Boolean bool = (Boolean) value;builder.append(bool);} else if (value instanceof Double) {Double db = (Double) value;builder.append(db);} else {builder.append(value.toString());}}}

实现逻辑都写好了，集成到自己的项目中还是非常容易的，你只需要在你的项目中引入对应的Maven依赖

    <dependencies><dependency><groupId>com.vaadin.external.google</groupId><artifactId>android-json</artifactId><version>RELEASE</version><scope>compile</scope></dependency><dependency><groupId>xmlpull</groupId><artifactId>xmlpull</artifactId><version>1.1.3.1</version></dependency><dependency><groupId>kxml2</groupId><artifactId>kxml2</artifactId><version>2.3.0</version></dependency></dependencies>

通过下面的工具类你就可以把 Single element XML to JSON Array。其中forceList（）函数就是你需要设置强转成数组的节点绝对路径(xPath)

public class Test {public static void main(String [] args){String xml = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n" +"<library>" +"    <books>" +"<book>" +"<name>Java</name>" +"</book>" +"</books>" +"</library>";XmlToJson xmlToJson2 = new XmlToJson.Builder(xml).forceList("/library/books/book").build();System.out.println(xmlToJson2);}
}

{"library": {"books": {"book": [{"name": "Java"}]}}
}

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