When i need to escape Html string?

HTML (nowadays we would better say XML) defines many so called "special" characters, which means that these characters have special meaning for browser in contrast with "normal" characters that just mean themselves. For example, string "Hello, World!" contains only "normal" characters and thus it literally means "Hello, World!" for browser. String "<b>Hello, World!</b>", contains special characters '<', '>' and '/', and for browser it means: typeset string "Hello, World!" in bold instead of just typeset "<b>Hello, World!</b>".

Method escapeHtml (String) probably (I cannot tell for sure because I don't know how it is implemented) converts arbitrary string into HTML code that will instruct browser to literally typeset this string. For example, escapeHtml ("<b>Hello, World!</b>") whill return HTML code that will be interpreted by browser as typeset "<b>Hello, World!</b>" normally instead of typeset string "Hello, World!" in bold. If method escapeHtml (String) is implemented correctly, you should not care how HTML code produced by this method looks like. Just use it where you want to ask browser to typeset some string literally.

From my experience, all of the strings should be escaped from Html before being displayed on the page. Our current project is about managing all the Organization Units from the Active Directory, and these units could contain any special character (including Html Character). When displaying on the page, you could end up with the following code to show a record called User <Marketing>

<a href="viewDetail.do"> <%=request.getAttribute("Name");%> </a>

after the page is rendered, it will become

<a href="viewDetail.do"> User <Marketing> </a>

Which actually appears as User hyperlink on the page.

However, if you escape the Html value before sending to the page

request.setAttribute("Name", StringEscapeUtils.escapeHtml("User <Marketing>"));

after the page is rendered, it will become

 <a href="viewDetail.do"> User &lt;Marketing&gt; </a>

which appear correctly on the JSP page

Shortly, you use escaping Html characters to prevent the special input. If the input contains the Html Character, your page will appear wrong during rendering

you have to escape html or xml when there is a possibility that it might get interpreted along with the page-generated html (read jsp).

this good question also explains it.

I can think of several possibilities to explain why sometimes a string is not escaped:

• perhaps the original programmer was confident that at certain places the string had no special characters (however, in my opinion this would be bad programming practice; it costs very little to escape a string as protection against future changes)
• the string was already escaped at that point in the code. You definitely don't want to escape a string twice; the user will end up seeing the escape sequence instead of the intended text.
• The string was the actual html itself. You don't want to escape the html; you want the browser to process it!

EDIT - The reason for escaping is that special characters like & and < can end up causing the browser to display something other than what you intended. A bare & is technically an error in the html. Most browsers try to deal intelligently with such errors and will display them correctly in most cases. (This will almost certainly happen in your example text if the string were text in a <div>, for instance.) However, because it is bad markup, some browsers will not work well; assistive technologies (e.g., text-to-speech) may fail; and there may be other problems.

There are several cases that will fail despite the best efforts of the browser to recover from bad markup. If your sample string were an attribute value, escaping the quote marks would be absolutely required. There's no way that a browser is going to correctly handle something like:

<img alt=""bread" & "butter"" ... >

The general rule is that any character that is not markup but might be confused as markup need to be escaped.

Note that there are several contexts in which text can appear within an html document, and they have separate requirements for escaping. The following should be escaped:

• all characters that have no representation in the character set of the document (unlikely if you are using UTF-8, but that's not always the case)
• Within attribute values, quote marks (' or ", whichever one matches the delimiters used for the attribute value itself) and the ampersand (&), but not <
• Within text nodes, only & and <
• Within href values, characters that need escaping in a url (and sometimes these need to be doubly escaped so they are still escaped after the browser unescapes them once)
• Within a CDATA block, generally nothing (at the HTML level).

Finally, aside from the hazard of double-escaping, the cost of escaping all text is minimal: a tiny bit of extra processing and a few extra bytes on the network.