Jsoup官方说明里，一个重要的功能就是output tidy HTML。这里我们看看Jsoup是如何输出HTML的。
HTML相关知识
分析代码前，我们不妨先想想，“tidy HTML"到底包括哪些东西：
- 换行，块级标签习惯上都会独占一行
- 缩进，根据HTML标签嵌套层数，行首缩进会不同
- 严格的标签闭合，如果是可以自闭合的标签并且没有内容，则进行自闭合
- HTML实体的转义
这里要补充一下HTML标签的知识。HTML Tag可以分为block和inline两类。关于Tag的inline和block的定义可以参考http://www.w3schools.com/html/html_blocks.asp，而Jsoup的Tag类则是对Java开发者非常好的学习资料。
// internal static initialisers:
// prepped from http://www.w3.org/TR/REC-html40/sgml/dtd.html and other sources
//block tags，需要换行
private static final String[] blockTags = {
"html", "head", "body", "frameset", "script", "noscript", "style", "meta", "link", "title", "frame",
"noframes", "section", "nav", "aside", "hgroup", "header", "footer", "p", "h1", "h2", "h3", "h4", "h5", "h6",
"ul", "ol", "pre", "div", "blockquote", "hr", "address", "figure", "figcaption", "form", "fieldset", "ins",
"del", "s", "dl", "dt", "dd", "li", "table", "caption", "thead", "tfoot", "tbody", "colgroup", "col", "tr", "th",
"td", "video", "audio", "canvas", "details", "menu", "plaintext"
};
//inline tags，无需换行
private static final String[] inlineTags = {
"object", "base", "font", "tt", "i", "b", "u", "big", "small", "em", "strong", "dfn", "code", "samp", "kbd",
"var", "cite", "abbr", "time", "acronym", "mark", "ruby", "rt", "rp", "a", "img", "br", "wbr", "map", "q",
"sub", "sup", "bdo", "iframe", "embed", "span", "input", "select", "textarea", "label", "button", "optgroup",
"option", "legend", "datalist", "keygen", "output", "progress", "meter", "area", "param", "source", "track",
"summary", "command", "device"
};
//emptyTags是不能有内容的标签，这类标签都是可以自闭合的
private static final String[] emptyTags = {
"meta", "link", "base", "frame", "img", "br", "wbr", "embed", "hr", "input", "keygen", "col", "command",
"device"
};
private static final String[] formatAsInlineTags = {
"title", "a", "p", "h1", "h2", "h3", "h4", "h5", "h6", "pre", "address", "li", "th", "td", "script", "style",
"ins", "del", "s"
};
//在这些标签里，需要保留空格
private static final String[] preserveWhitespaceTags = {
"pre", "plaintext", "title", "textarea"
};
另外，Jsoup的Entities类里包含了一些HTML实体转义的东西。这些转义的对应数据保存在entities-full.properties和entities-base.properties里。
Jsoup的格式化实现
在Jsoup里，直接调用Document.toString()(继承自Element)，即可对文档进行输出。另外OutputSettings可以控制输出格式，主要是prettyPrint(是否重新格式化)、outline(是否强制所有标签换行)、indentAmount(缩进长度)等。
里面的继承和互相调用关系略微复杂，大概是这样子：
Document.toString()=>Document.outerHtml()=>Element.html()，最终Element.html()又会循环调用所有子元素的outerHtml()，拼接起来作为输出。
private void html(StringBuilder accum) {
for (Node node : childNodes)
node.outerHtml(accum);
}
而outerHtml()会使用一个OuterHtmlVisitor对所以子节点做遍历，并拼装起来作为结果。
protected void outerHtml(StringBuilder accum) {
new NodeTraversor(new OuterHtmlVisitor(accum, getOutputSettings())).traverse(this);
}
OuterHtmlVisitor会对所有子节点做遍历，并调用node.outerHtmlHead()和node.outerHtmlTail两个方法。
private static class OuterHtmlVisitor implements NodeVisitor {
private StringBuilder accum;
private Document.OutputSettings out;
public void head(Node node, int depth) {
node.outerHtmlHead(accum, depth, out);
}
public void tail(Node node, int depth) {
if (!node.nodeName().equals("#text")) // saves a void hit.
node.outerHtmlTail(accum, depth, out);
}
}
我们终于找到了真正工作的代码，node.outerHtmlHead()和node.outerHtmlTail。Jsoup里每种Node的输出方式都不太一样，这里只讲讲两种主要节点：Element和TextNode。Element是格式化的主要对象，它的两个方法代码如下：
void outerHtmlHead(StringBuilder accum, int depth, Document.OutputSettings out) {
if (accum.length() > 0 && out.prettyPrint()
&& (tag.formatAsBlock() || (parent() != null && parent().tag().formatAsBlock()) || out.outline()) )
//换行并调整缩进
indent(accum, depth, out);
accum
.append("<")
.append(tagName());
attributes.html(accum, out);
if (childNodes.isEmpty() && tag.isSelfClosing())
accum.append(" />");
else
accum.append(">");
}
void outerHtmlTail(StringBuilder accum, int depth, Document.OutputSettings out) {
if (!(childNodes.isEmpty() && tag.isSelfClosing())) {
if (out.prettyPrint() && (!childNodes.isEmpty() && (
tag.formatAsBlock() || (out.outline() && (childNodes.size()>1 || (childNodes.size()==1 && !(childNodes.get(0) instanceof TextNode))))
)))
//换行并调整缩进
indent(accum, depth, out);
accum.append("</").append(tagName()).append(">");
}
}
而ident方法的代码只有一行：
protected void indent(StringBuilder accum, int depth, Document.OutputSettings out) {
//out.indentAmount()是缩进长度，默认是1
accum.append("\n").append(StringUtil.padding(depth * out.indentAmount()));
}
代码简单明了，就没什么好说的了。值得一提的是，StringUtil.padding()方法为了减少字符串生成，把常用的缩进保存到了一个数组中。
好了，水了一篇文章，下一篇将比较有技术含量的parser部分。
另外，通过本节的学习，我们学到了要把StringBuilder命名为accum，而不是sb。
