Add JsonGenerator.Feature to use single quotation marks instead of double quotes

src/main/java/com/fasterxml/jackson/core/JsonGenerator.java

@@ -210,6 +210,15 @@ public enum Feature {
          * @since 2.5
          */
         IGNORE_UNKNOWN(false),
+
+        /**
+         * Features that determines whether generator will write strings with single quotes
+         * (apostrophe, character '\'') instead of double quotes.
+         * <p>
+         * Since JSON specification requires use of double quotes for field names, this is a
+         * non-standard feature, and as such is disabled by default.
+         */
+        WRITE_SINGLE_QUOTES(false),
         ;
 
         private final boolean _defaultState;

src/main/java/com/fasterxml/jackson/core/base/GeneratorBase.java

@@ -150,6 +150,8 @@ public JsonGenerator enable(Feature f) {
                 if (_writeContext.getDupDetector() == null) { // but only if disabled currently
                     _writeContext = _writeContext.withDupDetector(DupDetector.rootDetector(this));
                 }
+            } else if (f == Feature.WRITE_SINGLE_QUOTES) {
+                setCharacterEscapes(getCharacterEscapes());
             }
         }
         return this;
@@ -166,6 +168,8 @@ public JsonGenerator disable(Feature f) {
                 setHighestNonEscapedChar(0);
             } else if (f == Feature.STRICT_DUPLICATE_DETECTION) {
                 _writeContext = _writeContext.withDupDetector(null);
+            } else if (f == Feature.WRITE_SINGLE_QUOTES) {
+                setCharacterEscapes(getCharacterEscapes());
             }
         }
         return this;

src/main/java/com/fasterxml/jackson/core/io/CharTypes.java

@@ -154,6 +154,7 @@ public final class CharTypes
      * 7-bit ASCII range need to be quoted.
      */
     final static int[] sOutputEscapes128;
+    final static int[] sOutputEscapes128SingleQuoted;
     static {
         int[] table = new int[128];
         // Control chars need generic escape sequence
@@ -173,6 +174,9 @@ public final class CharTypes
         table[0x0A] = 'n';
         table[0x0D] = 'r';
         sOutputEscapes128 = table;
+        sOutputEscapes128SingleQuoted = Arrays.copyOf(table, 128);
+        sOutputEscapes128SingleQuoted['"'] = 0;
+        sOutputEscapes128SingleQuoted['\''] = '\'';
     }
 
     /**
@@ -210,6 +214,15 @@ public final class CharTypes
      */
     public static int[] get7BitOutputEscapes() { return sOutputEscapes128; }
 
+    /**
+     * Accessor for getting a read-only encoding table for first 128 Unicode
+     * code points (single-byte UTF-8 characters) in single quoted strings.
+     * Value of 0 means "no escaping"; other positive values that value is character
+     * to use after backslash; and negative values that generic (backslash - u)
+     * escpaing is to be used.
+     */
+    public static int[] get7BitOutputEscapesSingleQuoted() { return sOutputEscapes128SingleQuoted; }
+
     public static int charToHex(int ch)
     {
         return (ch > 127) ? -1 : sHexValues[ch];

src/main/java/com/fasterxml/jackson/core/json/JsonGeneratorImpl.java

@@ -29,7 +29,9 @@ public abstract class JsonGeneratorImpl extends GeneratorBase
      * (first 128 character codes), used for single-byte UTF-8 characters.
      */
     protected final static int[] sOutputEscapes = CharTypes.get7BitOutputEscapes();
-
+    protected final static int[] sOutputEscapesSingleQuoted
+        = CharTypes.get7BitOutputEscapesSingleQuoted();
+
     /*
     /**********************************************************
     /* Configuration, basic I/O
@@ -50,7 +52,7 @@ public abstract class JsonGeneratorImpl extends GeneratorBase
      * character codes). Defined separately to make potentially
      * customizable
      */
-    protected int[] _outputEscapes = sOutputEscapes;
+    protected int[] _outputEscapes;
 
     /**
      * Value between 128 (0x80) and 65535 (0xFFFF) that indicates highest
@@ -97,6 +99,11 @@ public JsonGeneratorImpl(IOContext ctxt, int features, ObjectCodec codec)
         if (isEnabled(Feature.ESCAPE_NON_ASCII)) {
             setHighestNonEscapedChar(127);
         }
+        if (isEnabled(Feature.WRITE_SINGLE_QUOTES)) {
+            _outputEscapes = sOutputEscapesSingleQuoted;
+        } else {
+            _outputEscapes = sOutputEscapes;
+        }
     }
 
     /*
@@ -121,7 +128,11 @@ public JsonGenerator setCharacterEscapes(CharacterEscapes esc)
     {
         _characterEscapes = esc;
         if (esc == null) { // revert to standard escapes
-            _outputEscapes = sOutputEscapes;
+            if (isEnabled(Feature.WRITE_SINGLE_QUOTES)) {
+                _outputEscapes = sOutputEscapesSingleQuoted;
+            } else {
+                _outputEscapes = sOutputEscapes;
+            }
         } else {
             _outputEscapes = esc.getEscapeCodesForAscii();
         }