Make ByteSourceJsonBootstrapper use StringReader for < 8KiB byte[] inputs (#1081)

Author: schlosna

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

@@ -22,6 +22,9 @@ public final class ByteSourceJsonBootstrapper
     public final static byte UTF8_BOM_2 = (byte) 0xBB;
     public final static byte UTF8_BOM_3 = (byte) 0xBF;
 
+    // [jackson-core#488] Limit in bytes for input byte array length to use StringReader instead of InputStreamReader
+    private static final int STRING_READER_BYTE_ARRAY_LENGTH_LIMIT = 8192;
+
     /*
     /**********************************************************
     /* Configuration
@@ -230,6 +233,12 @@ public Reader constructReader() throws IOException
                 InputStream in = _in;
 
                 if (in == null) {
+                    int length = _inputEnd - _inputPtr;
+                    if (length <= STRING_READER_BYTE_ARRAY_LENGTH_LIMIT) {
+                        // [jackson-core#488] Avoid overhead of heap ByteBuffer allocated by InputStreamReader
+                        // when processing small inputs up to 8KiB.
+                        return new StringReader(new String(_inputBuffer, _inputPtr, length, enc.getJavaName()));
+                    }
                     in = new ByteArrayInputStream(_inputBuffer, _inputPtr, _inputEnd);
                 } else {
                     /* Also, if we have any read but unused input (usually true),