What is the recommended way to escape HTML symbols in plain Java?

user_41012 0
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Is there a recommended way to escape <, >, " and & characters when outputting HTML in plain Java code? (Other than manually doing the following, that is).

String source = "The less than sign (<) and ampersand (&) must be escaped before using them in HTML";
String escaped = source.replace("<", "&lt;").replace("&", "&amp;"); // ...
escaping html java
12 Answers

StringEscapeUtils from Apache Commons Lang:

import static org.apache.commons.lang.StringEscapeUtils.escapeHtml;
// ...
String source = "The less than sign (<) and ampersand (&) must be escaped before using them in HTML";
String escaped = escapeHtml(source);

For version 3:

import static org.apache.commons.lang3.StringEscapeUtils.escapeHtml4;
// ...
String escaped = escapeHtml4(source);
user_892660

An alternative to Apache Commons: Use Spring's HtmlUtils.htmlEscape(String input) method.

user_1274790

org.apache.commons.lang3.StringEscapeUtils is now deprecated. You must now use org.apache.commons.text.StringEscapeUtils by 

    <dependency>
        <groupId>org.apache.commons</groupId>
        <artifactId>commons-text</artifactId>
        <version>${commons.text.version}</version>
    </dependency>
user_50248050

The most of libraries offer escaping everything they can including hundreds of symbols and thousands of non-ASCII characters which is not what you want in UTF-8 world.

Also, as Jeff Williams noted, there's no single “escape HTML” option, there are several contexts.

Assuming you never use unquoted attributes, and keeping in mind that different contexts exist, it've written my own version:

private static final long TEXT_ESCAPE =
        1L << '&' | 1L << '<';
private static final long DOUBLE_QUOTED_ATTR_ESCAPE =
        TEXT_ESCAPE | 1L << '"';
private static final long SINGLE_QUOTED_ATTR_ESCAPE =
        TEXT_ESCAPE | 1L << '\'';
private static final long ESCAPES =
        DOUBLE_QUOTED_ATTR_ESCAPE | SINGLE_QUOTED_ATTR_ESCAPE;

// 'quot' and 'apos' are 1 char longer than '#34' and '#39'
// which I've decided to use
private static final String REPLACEMENTS = "&#34;&amp;&#39;&lt;";
private static final int REPL_SLICES = /*  [0,   5,   10,  15, 19) */
        5<<5 | 10<<10 | 15<<15 | 19<<20;
// These 5-bit numbers packed into a single int
// are indices within REPLACEMENTS which is a 'flat' String[]

private static void appendEscaped(
        Appendable builder, CharSequence content, long escapes) {
    try {
        int startIdx = 0, len = content.length();
        for (int i = 0; i < len; i++) {
            char c = content.charAt(i);
            long one;
            if (((c & 63) == c) && ((one = 1L << c) & escapes) != 0) {
            // -^^^^^^^^^^^^^^^   -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            // |                  | take only dangerous characters
            // | java shifts longs by 6 least significant bits,
            // | e. g. << 0b110111111 is same as >> 0b111111.
            // | Filter out bigger characters

                int index = Long.bitCount(ESCAPES & (one - 1));
                builder.append(content, startIdx, i /* exclusive */).append(
                        REPLACEMENTS,
                        REPL_SLICES >>> (5 * index) & 31,
                        REPL_SLICES >>> (5 * (index + 1)) & 31
                );
                startIdx = i + 1;
            }
        }
        builder.append(content, startIdx, len);
    } catch (IOException e) {
        // typically, our Appendable is StringBuilder which does not throw;
        // also, there's no way to declare 'if A#append() throws E,
        // then appendEscaped() throws E, too'
        throw new UncheckedIOException(e);
    }
}

Consider copy-pasting from Gist without line length limit.

UPD: As another answer suggests, > escaping is not necessary; also, " within attr='…' is allowed, too. I've updated the code accordingly.

You may check it out yourself:

<!DOCTYPE html>
<html lang="en">
<head><title>Test</title></head>
<body>

<p title="&lt;&#34;I'm double-quoted!&#34;>">&lt;"Hello!"></p>
<p title='&lt;"I&#39;m single-quoted!">'>&lt;"Goodbye!"></p>

</body>
</html>
user_30502490

Java 8+ Solution:

public static String escapeHTML(String str) {
    return str.chars().mapToObj(c -> c > 127 || "\"'<>&".indexOf(c) != -1 ?
       "&#" + c + ";" : String.valueOf((char) c)).collect(Collectors.joining());
}

String#chars returns an IntStream of the char values from the String. We can then use mapToObj to escape the characters with a character code greater than 127 (non-ASCII characters) as well as the double quote ("), single quote ('), left angle bracket (<), right angle bracket (>), and ampersand (&). Collectors.joining concatenates the Strings back together.

To better handle Unicode characters, String#codePoints can be used instead.

public static String escapeHTML(String str) {
    return str.codePoints().mapToObj(c -> c > 127 || "\"'<>&".indexOf(c) != -1 ?
            "&#" + c + ";" : new String(Character.toChars(c)))
       .collect(Collectors.joining());
}
user_95131840
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