How should I escape strings in JSON?

Ideally, find a JSON library in your language that you can feed some appropriate data structure to, and let it worry about how to escape things. It'll keep you much saner. If for whatever reason you don't have a library in your language, you don't want to use one (I wouldn't suggest this¹), or you're writing a JSON library, read on.

Escape it according to the RFC. JSON is pretty liberal: The only characters you must escape are \, ", and control codes (anything less than U+0020).

This structure of escaping is specific to JSON. You'll need a JSON specific function. All of the escapes can be written as \uXXXX where XXXX is the UTF-16 code unit¹ for that character. There are a few shortcuts, such as \\, which work as well. (And they result in a smaller and clearer output.)

For full details, see the RFC.

¹JSON's escaping is built on JS, so it uses \uXXXX, where XXXX is a UTF-16 code unit. For code points outside the BMP, this means encoding surrogate pairs, which can get a bit hairy. (Or, you can just output the character directly, since JSON's encoded for is Unicode text, and allows these particular characters.)

Extract From Jettison:

 public static String quote(String string) {
         if (string == null || string.length() == 0) {
             return "\"\"";
         }

         char         c = 0;
         int          i;
         int          len = string.length();
         StringBuilder sb = new StringBuilder(len + 4);
         String       t;

         sb.append('"');
         for (i = 0; i < len; i += 1) {
             c = string.charAt(i);
             switch (c) {
             case '\\':
             case '"':
                 sb.append('\\');
                 sb.append(c);
                 break;
             case '/':
 //                if (b == '<') {
                     sb.append('\\');
 //                }
                 sb.append(c);
                 break;
             case '\b':
                 sb.append("\\b");
                 break;
             case '\t':
                 sb.append("\\t");
                 break;
             case '\n':
                 sb.append("\\n");
                 break;
             case '\f':
                 sb.append("\\f");
                 break;
             case '\r':
                sb.append("\\r");
                break;
             default:
                 if (c < ' ') {
                     t = "000" + Integer.toHexString(c);
                     sb.append("\\u" + t.substring(t.length() - 4));
                 } else {
                     sb.append(c);
                 }
             }
         }
         sb.append('"');
         return sb.toString();
     }

Try this org.codehaus.jettison.json.JSONObject.quote("your string").

Download it here: http://mvnrepository.com/artifact/org.codehaus.jettison/jettison

org.json.simple.JSONObject.escape() escapes quotes,\, /, \r, \n, \b, \f, \t and other control characters. It can be used to escape JavaScript codes.

import org.json.simple.JSONObject;
String test =  JSONObject.escape("your string");

There is now a StringEscapeUtils#escapeJson(String) method in the Apache Commons Text library.

The methods of interest are as follows:

StringEscapeUtils#escapeJson(String)

StringEscapeUtils#unescapeJson(String)

This functionality was initially released as part of Apache Commons Lang version 3.2 but has since been deprecated and moved to Apache Commons Text. So if the method is marked as deprecated in your IDE, you're importing the implementation from the wrong library (both libraries use the same class name: StringEscapeUtils).

The implementation isn't pure Json. As per the Javadoc:

Escapes the characters in a String using Json String rules. Escapes any values it finds into their Json String form. Deals correctly with quotes and control-chars (tab, backslash, cr, ff, etc.) So a tab becomes the characters '\' and 't'. The only difference between Java strings and Json strings is that in Json, forward-slash (/) is escaped. See http://www.ietf.org/rfc/rfc4627.txt for further details.

org.json.JSONObject quote(String data) method does the job

import org.json.JSONObject;
String jsonEncodedString = JSONObject.quote(data);

Extract from the documentation:

Encodes data as a JSON string. This applies quotes and any necessary character escaping. [...] Null will be interpreted as an empty string

StringEscapeUtils.escapeJavaScript / StringEscapeUtils.escapeEcmaScript should do the trick too.

If you are using fastexml jackson, you can use the following: com.fasterxml.jackson.core.io.JsonStringEncoder.getInstance().quoteAsString(input)

If you are using codehaus jackson, you can use the following: org.codehaus.jackson.io.JsonStringEncoder.getInstance().quoteAsString(input)

Not sure what you mean by "creating json manually", but you can use something like gson (http://code.google.com/p/google-gson/), and that would transform your HashMap, Array, String, etc, to a JSON value. I recommend going with a framework for this.

I have not spent the time to make 100% certain, but it worked for my inputs enough to be accepted by online JSON validators:

org.apache.velocity.tools.generic.EscapeTool.EscapeTool().java("input")

although it does not look any better than org.codehaus.jettison.json.JSONObject.quote("your string")

I simply use velocity tools in my project already - my "manual JSON" building was within a velocity template

For those who came here looking for a command-line solution, like me, cURL's --data-urlencode works fine:

curl -G -v -s --data-urlencode 'query={"type" : "/music/artist"}' 'https://www.googleapis.com/freebase/v1/mqlread'

sends

GET /freebase/v1/mqlread?query=%7B%22type%22%20%3A%20%22%2Fmusic%2Fartist%22%7D HTTP/1.1

, for example. Larger JSON data can be put in a file and you'd use the @ syntax to specify a file to slurp in the to-be-escaped data from. For example, if

$ cat 1.json 
{
  "type": "/music/artist",
  "name": "The Police",
  "album": []
}

you'd use

curl -G -v -s --data-urlencode query@1.json 'https://www.googleapis.com/freebase/v1/mqlread'

And now, this is also a tutorial on how to query Freebase from the command line :-)

Use EscapeUtils class in commons lang API.

EscapeUtils.escapeJavaScript("Your JSON string");

Consider Moshi's JsonWriter class. It has a wonderful API and it reduces copying to a minimum, everything can be nicely streamed to a filed, OutputStream, etc.

OutputStream os = ...;
JsonWriter json = new JsonWriter(Okio.buffer(Okio.sink(os)));
json.beginObject();
json.name("id").value(getId());
json.name("scores");
json.beginArray();
for (Double score : getScores()) {
  json.value(score);
}
json.endArray();
json.endObject();

If you want the string in hand:

Buffer b = new Buffer(); // okio.Buffer
JsonWriter writer = new JsonWriter(b);
//...
String jsonString = b.readUtf8();

If you need to escape JSON inside JSON string, use org.json.JSONObject.quote("your json string that needs to be escaped") seem to work well

Apache commons-text now has a StringEscapeUtils.escapeJson(String).

using the \uXXXX syntax can solve this problem, google UTF-16 with the name of the sign, you can find out XXXX, for example:utf-16 double quote

The methods here that show the actual implementation are all faulty. I don't have Java code, but just for the record, you could easily convert this C#-code:

Courtesy of the mono-project @ https://github.com/mono/mono/blob/master/mcs/class/System.Web/System.Web/HttpUtility.cs

public static string JavaScriptStringEncode(string value, bool addDoubleQuotes)
{
    if (string.IsNullOrEmpty(value))
        return addDoubleQuotes ? "\"\"" : string.Empty;

    int len = value.Length;
    bool needEncode = false;
    char c;
    for (int i = 0; i < len; i++)
    {
        c = value[i];

        if (c >= 0 && c <= 31 || c == 34 || c == 39 || c == 60 || c == 62 || c == 92)
        {
            needEncode = true;
            break;
        }
    }

    if (!needEncode)
        return addDoubleQuotes ? "\"" + value + "\"" : value;

    var sb = new System.Text.StringBuilder();
    if (addDoubleQuotes)
        sb.Append('"');

    for (int i = 0; i < len; i++)
    {
        c = value[i];
        if (c >= 0 && c <= 7 || c == 11 || c >= 14 && c <= 31 || c == 39 || c == 60 || c == 62)
            sb.AppendFormat("\\u{0:x4}", (int)c);
        else switch ((int)c)
            {
                case 8:
                    sb.Append("\\b");
                    break;

                case 9:
                    sb.Append("\\t");
                    break;

                case 10:
                    sb.Append("\\n");
                    break;

                case 12:
                    sb.Append("\\f");
                    break;

                case 13:
                    sb.Append("\\r");
                    break;

                case 34:
                    sb.Append("\\\"");
                    break;

                case 92:
                    sb.Append("\\\\");
                    break;

                default:
                    sb.Append(c);
                    break;
            }
    }

    if (addDoubleQuotes)
        sb.Append('"');

    return sb.ToString();
}

This can be compacted into

    // https://github.com/mono/mono/blob/master/mcs/class/System.Json/System.Json/JsonValue.cs
public class SimpleJSON
{

    private static  bool NeedEscape(string src, int i)
    {
        char c = src[i];
        return c < 32 || c == '"' || c == '\\'
            // Broken lead surrogate
            || (c >= '\uD800' && c <= '\uDBFF' &&
                (i == src.Length - 1 || src[i + 1] < '\uDC00' || src[i + 1] > '\uDFFF'))
            // Broken tail surrogate
            || (c >= '\uDC00' && c <= '\uDFFF' &&
                (i == 0 || src[i - 1] < '\uD800' || src[i - 1] > '\uDBFF'))
            // To produce valid JavaScript
            || c == '\u2028' || c == '\u2029'
            // Escape "</" for <script> tags
            || (c == '/' && i > 0 && src[i - 1] == '<');
    }



    public static string EscapeString(string src)
    {
        System.Text.StringBuilder sb = new System.Text.StringBuilder();

        int start = 0;
        for (int i = 0; i < src.Length; i++)
            if (NeedEscape(src, i))
            {
                sb.Append(src, start, i - start);
                switch (src[i])
                {
                    case '\b': sb.Append("\\b"); break;
                    case '\f': sb.Append("\\f"); break;
                    case '\n': sb.Append("\\n"); break;
                    case '\r': sb.Append("\\r"); break;
                    case '\t': sb.Append("\\t"); break;
                    case '\"': sb.Append("\\\""); break;
                    case '\\': sb.Append("\\\\"); break;
                    case '/': sb.Append("\\/"); break;
                    default:
                        sb.Append("\\u");
                        sb.Append(((int)src[i]).ToString("x04"));
                        break;
                }
                start = i + 1;
            }
        sb.Append(src, start, src.Length - start);
        return sb.ToString();
    }
}

I think the best answer in 2017 is to use the javax.json APIs. Use javax.json.JsonBuilderFactory to create your json objects, then write the objects out using javax.json.JsonWriterFactory. Very nice builder/writer combination.