Improve toString Performance: Use StringBuilderWriter for toString methods

src/main/java/org/json/JSONArray.java

@@ -5,7 +5,6 @@
  */
 
 import java.io.IOException;
-import java.io.StringWriter;
 import java.io.Writer;
 import java.lang.reflect.Array;
 import java.math.BigDecimal;
@@ -1695,7 +1694,10 @@ public String toString() {
      */
     @SuppressWarnings("resource")
     public String toString(int indentFactor) throws JSONException {
-        StringWriter sw = new StringWriter();
+        // each value requires a comma, so multiply the count by 2
+        // We don't want to oversize the initial capacity
+        int initialSize = myArrayList.size() * 2;
+        Writer sw = new StringBuilderWriter(Math.max(initialSize, 16));
         return this.write(sw, indentFactor, 0).toString();
     }
 

src/main/java/org/json/JSONObject.java

@@ -6,7 +6,6 @@
 
 import java.io.Closeable;
 import java.io.IOException;
-import java.io.StringWriter;
 import java.io.Writer;
 import java.lang.annotation.Annotation;
 import java.lang.reflect.Field;
@@ -2248,7 +2247,10 @@ public Object optQuery(JSONPointer jsonPointer) {
      */
     @SuppressWarnings("resource")
     public static String quote(String string) {
-        StringWriter sw = new StringWriter();
+        if (string == null || string.isEmpty()) {
+            return "\"\"";
+        }
+        Writer sw = new StringBuilderWriter(string.length() + 2);
         try {
             return quote(string, sw).toString();
         } catch (IOException ignored) {
@@ -2647,7 +2649,10 @@ public String toString() {
      */
     @SuppressWarnings("resource")
     public String toString(int indentFactor) throws JSONException {
-        StringWriter w = new StringWriter();
+        // 6 characters are the minimum to serialise a key value pair e.g.: "k":1,
+        // and we don't want to oversize the initial capacity
+        int initialSize = map.size() * 6;
+        Writer w = new StringBuilderWriter(Math.max(initialSize, 16));
         return this.write(w, indentFactor, 0).toString();
     }
 
@@ -2790,13 +2795,18 @@ static final Writer writeValue(Writer writer, Object value,
         if (value == null || value.equals(null)) {
             writer.write("null");
         } else if (value instanceof JSONString) {
+            // JSONString must be checked first, so it can overwrite behaviour of other types below
             Object o;
             try {
                 o = ((JSONString) value).toJSONString();
             } catch (Exception e) {
                 throw new JSONException(e);
             }
             writer.write(o != null ? o.toString() : quote(value.toString()));
+        } else if (value instanceof String) {
+            // assuming most values are Strings, so testing it early
+            quote(value.toString(), writer);
+            return writer;
         } else if (value instanceof Number) {
             // not all Numbers may match actual JSON Numbers. i.e. fractions or Imaginary
             final String numberAsString = numberToString((Number) value);

src/main/java/org/json/StringBuilderWriter.java

@@ -0,0 +1,92 @@
+package org.json;
+
+import java.io.IOException;
+import java.io.Writer;
+
+/**
+ * Performance optimised alternative for {@link java.io.StringWriter}
+ * using internally a {@link StringBuilder} instead of a {@link StringBuffer}.
+ */
+public class StringBuilderWriter extends Writer {
+    private final StringBuilder builder;
+
+    /**
+     * Create a new string builder writer using the default initial string-builder buffer size.
+     */
+    public StringBuilderWriter() {
+        builder = new StringBuilder();
+        lock = builder;
+    }
+
+    /**
+     * Create a new string builder writer using the specified initial string-builder buffer size.
+     *
+     * @param initialSize The number of {@code char} values that will fit into this buffer
+     *                    before it is automatically expanded
+     *
+     * @throws IllegalArgumentException If {@code initialSize} is negative
+     */
+    public StringBuilderWriter(int initialSize) {
+        builder = new StringBuilder(initialSize);
+        lock = builder;
+    }
+
+    @Override
+    public void write(int c) {
+        builder.append((char) c);
+    }
+
+    @Override
+    public void write(char[] cbuf, int offset, int length) {
+        if ((offset < 0) || (offset > cbuf.length) || (length < 0) ||
+                ((offset + length) > cbuf.length) || ((offset + length) < 0)) {
+            throw new IndexOutOfBoundsException();
+        } else if (length == 0) {
+            return;
+        }
+        builder.append(cbuf, offset, length);
+    }
+
+    @Override
+    public void write(String str) {
+        builder.append(str);
+    }
+
+    @Override
+    public void write(String str, int offset, int length) {
+        builder.append(str, offset, offset + length);
+    }
+
+    @Override
+    public StringBuilderWriter append(CharSequence csq) {
+        write(String.valueOf(csq));
+        return this;
+    }
+
+    @Override
+    public StringBuilderWriter append(CharSequence csq, int start, int end) {
+        if (csq == null) {
+            csq = "null";
+        }
+        return append(csq.subSequence(start, end));
+    }
+
+    @Override
+    public StringBuilderWriter append(char c) {
+        write(c);
+        return this;
+    }
+
+    @Override
+    public String toString() {
+        return builder.toString();
+    }
+
+    @Override
+    public void flush() {
+    }
+
+    @Override
+    public void close() throws IOException {
+    }
+}

src/test/java/org/json/junit/JSONStringTest.java

@@ -319,6 +319,22 @@ public void testNullStringValue() throws Exception {
         }
     }
 
+    @Test
+    public void testEnumJSONString() {
+        JSONObject jsonObject = new JSONObject();
+        jsonObject.put("key", MyEnum.MY_ENUM);
+        assertEquals("{\"key\":\"myJsonString\"}", jsonObject.toString());
+    }
+
+    private enum MyEnum implements JSONString {
+        MY_ENUM;
+
+        @Override
+        public String toJSONString() {
+            return "\"myJsonString\"";
+        }
+    }
+
     /**
      * A JSONString that returns a valid JSON string value.
      */

src/test/java/org/json/junit/StringBuilderWriterTest.java

@@ -0,0 +1,60 @@
+package org.json.junit;
+
+import static org.junit.Assert.assertEquals;
+
+import org.json.StringBuilderWriter;
+import org.junit.Before;
+import org.junit.Test;
+
+public class StringBuilderWriterTest {
+    private StringBuilderWriter writer;
+
+    @Before
+    public void setUp() {
+        writer = new StringBuilderWriter();
+    }
+
+    @Test
+    public void testWriteChar() {
+        writer.write('a');
+        assertEquals("a", writer.toString());
+    }
+
+    @Test
+    public void testWriteCharArray() {
+        char[] chars = {'a', 'b', 'c'};
+        writer.write(chars, 0, 3);
+        assertEquals("abc", writer.toString());
+    }
+
+    @Test
+    public void testWriteString() {
+        writer.write("hello");
+        assertEquals("hello", writer.toString());
+    }
+
+    @Test
+    public void testWriteStringWithOffsetAndLength() {
+        writer.write("hello world", 6, 5);
+        assertEquals("world", writer.toString());
+    }
+
+    @Test
+    public void testAppendCharSequence() {
+        writer.append("hello");
+        assertEquals("hello", writer.toString());
+    }
+
+    @Test
+    public void testAppendCharSequenceWithStartAndEnd() {
+        CharSequence csq = "hello world";
+        writer.append(csq, 6, 11);
+        assertEquals("world", writer.toString());
+    }
+
+    @Test
+    public void testAppendChar() {
+        writer.append('a');
+        assertEquals("a", writer.toString());
+    }
+}