Jackson is "the Java JSON library" or "the best JSON parser for Java". Or simply as "JSON for Java".

Jackson Core Source Code files are provided in the source packge (jackson-core-2.14.0-sources.jar). You can download it at Jackson Maven Website.

You can also browse Jackson Core Source Code below:

⏎ com/fasterxml/jackson/core/json/ByteSourceJsonBootstrapper.java

package com.fasterxml.jackson.core.json;

import java.io.*;

import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.core.format.InputAccessor;
import com.fasterxml.jackson.core.format.MatchStrength;
import com.fasterxml.jackson.core.io.*;
import com.fasterxml.jackson.core.sym.ByteQuadsCanonicalizer;
import com.fasterxml.jackson.core.sym.CharsToNameCanonicalizer;

/**
 * This class is used to determine the encoding of byte stream
 * that is to contain JSON content. Rules are fairly simple, and
 * defined in JSON specification (RFC-4627 or newer), except
 * for BOM handling, which is a property of underlying
 * streams.
 */
public final class ByteSourceJsonBootstrapper
{
    public final static byte UTF8_BOM_1 = (byte) 0xEF;
    public final static byte UTF8_BOM_2 = (byte) 0xBB;
    public final static byte UTF8_BOM_3 = (byte) 0xBF;
    
    /*
    /**********************************************************
    /* Configuration
    /**********************************************************
     */

    private final IOContext _context;

    private final InputStream _in;

    /*
    /**********************************************************
    /* Input buffering
    /**********************************************************
     */

    private final byte[] _inputBuffer;

    private int _inputPtr;

    private int _inputEnd;

    /**
     * Flag that indicates whether buffer above is to be recycled
     * after being used or not.
     */
    private final boolean _bufferRecyclable;

    /*
    /**********************************************************
    /* Input location
    /**********************************************************
     */

    /**
     * Current number of input units (bytes or chars) that were processed in
     * previous blocks,
     * before contents of current input buffer.
     *<p>
     * Note: includes possible BOMs, if those were part of the input.
     */
//    private int _inputProcessed;

    /*
    /**********************************************************
    /* Data gathered
    /**********************************************************
     */

    /**
     * Whether input has been detected to be in Big-Endian encoding or not.
     */
    private boolean _bigEndian = true;

    private int _bytesPerChar; // 0 means "dunno yet"

    /*
    /**********************************************************
    /* Life-cycle
    /**********************************************************
     */

    public ByteSourceJsonBootstrapper(IOContext ctxt, InputStream in) {
        _context = ctxt;
        _in = in;
        _inputBuffer = ctxt.allocReadIOBuffer();
        _inputEnd = _inputPtr = 0;
//        _inputProcessed = 0;
        _bufferRecyclable = true;
    }

    public ByteSourceJsonBootstrapper(IOContext ctxt, byte[] inputBuffer, int inputStart, int inputLen) {
        _context = ctxt;
        _in = null;
        _inputBuffer = inputBuffer;
        _inputPtr = inputStart;
        _inputEnd = (inputStart + inputLen);
        // Need to offset this for correct location info
//        _inputProcessed = -inputStart;
        _bufferRecyclable = false;
    }

    /*
    /**********************************************************
    /*  Encoding detection during bootstrapping
    /**********************************************************
     */
    
    /**
     * Method that should be called after constructing an instace.
     * It will figure out encoding that content uses, to allow
     * for instantiating a proper scanner object.
     *
     * @return {@link JsonEncoding} detected, if any; {@code JsonEncoding.UTF8} otherwise
     *
     * @throws IOException If read from underlying input source fails
     */
    public JsonEncoding detectEncoding() throws IOException
    {
        boolean foundEncoding = false;

        // First things first: BOM handling
        /* Note: we can require 4 bytes to be read, since no
         * combination of BOM + valid JSON content can have
         * shorter length (shortest valid JSON content is single
         * digit char, but BOMs are chosen such that combination
         * is always at least 4 chars long)
         */
        if (ensureLoaded(4)) {
            int quad =  (_inputBuffer[_inputPtr] << 24)
                | ((_inputBuffer[_inputPtr+1] & 0xFF) << 16)
                | ((_inputBuffer[_inputPtr+2] & 0xFF) << 8)
                | (_inputBuffer[_inputPtr+3] & 0xFF);
            
            if (handleBOM(quad)) {
                foundEncoding = true;
            } else {
                /* If no BOM, need to auto-detect based on first char;
                 * this works since it must be 7-bit ascii (wrt. unicode
                 * compatible encodings, only ones JSON can be transferred
                 * over)
                 */
                // UTF-32?
                if (checkUTF32(quad)) {
                    foundEncoding = true;
                } else if (checkUTF16(quad >>> 16)) {
                    foundEncoding = true;
                }
            }
        } else if (ensureLoaded(2)) {
            int i16 = ((_inputBuffer[_inputPtr] & 0xFF) << 8)
                | (_inputBuffer[_inputPtr+1] & 0xFF);
            if (checkUTF16(i16)) {
                foundEncoding = true;
            }
        }

        JsonEncoding enc;

        /* Not found yet? As per specs, this means it must be UTF-8. */
        if (!foundEncoding) {
            enc = JsonEncoding.UTF8;
        } else {
            switch (_bytesPerChar) {
            case 1: enc = JsonEncoding.UTF8;
                break;
            case 2: enc = _bigEndian ? JsonEncoding.UTF16_BE : JsonEncoding.UTF16_LE;
                break;
            case 4: enc = _bigEndian ? JsonEncoding.UTF32_BE : JsonEncoding.UTF32_LE;
                break;
            default: throw new RuntimeException("Internal error"); // should never get here
            }
        }
        _context.setEncoding(enc);
        return enc;
    }

    /**
     * Helper method that may be called to see if given {@link DataInput}
     * has BOM marker, and if so, to skip it.
     *
     * @param input DataInput to read content from
     *
     * @return Byte (as unsigned {@code int}) read after possible UTF-8 BOM
     *
     * @throws IOException If read from underlying input source fails
     *
     * @since 2.8
     */
    public static int skipUTF8BOM(DataInput input) throws IOException
    {
        int b = input.readUnsignedByte();
        if (b != 0xEF) {
            return b;
        }
        // since this is not legal byte in JSON otherwise, except
        // that we do get BOM; if not, report error
        b = input.readUnsignedByte();
        if (b != 0xBB) {
            throw new IOException("Unexpected byte 0x"+Integer.toHexString(b)
                +" following 0xEF; should get 0xBB as part of UTF-8 BOM");
        }
        b = input.readUnsignedByte();
        if (b != 0xBF) {
            throw new IOException("Unexpected byte 0x"+Integer.toHexString(b)
                +" following 0xEF 0xBB; should get 0xBF as part of UTF-8 BOM");
        }
        return input.readUnsignedByte();
    }

    /*
    /**********************************************************
    /* Constructing a Reader
    /**********************************************************
     */
    
    @SuppressWarnings("resource")
    public Reader constructReader() throws IOException
    {
        JsonEncoding enc = _context.getEncoding();
        switch (enc.bits()) {
        case 8: // only in non-common case where we don't want to do direct mapping
        case 16:
            {
                // First: do we have a Stream? If not, need to create one:
                InputStream in = _in;

                if (in == null) {
                    in = new ByteArrayInputStream(_inputBuffer, _inputPtr, _inputEnd);
                } else {
                    /* Also, if we have any read but unused input (usually true),
                     * need to merge that input in:
                     */
                    if (_inputPtr < _inputEnd) {
                        in = new MergedStream(_context, in, _inputBuffer, _inputPtr, _inputEnd);
                    }
                }
                return new InputStreamReader(in, enc.getJavaName());
            }
        case 32:
            return new UTF32Reader(_context, _in, _inputBuffer, _inputPtr, _inputEnd,
                    _context.getEncoding().isBigEndian());
        }
        throw new RuntimeException("Internal error"); // should never get here
    }

    public JsonParser constructParser(int parserFeatures, ObjectCodec codec,
            ByteQuadsCanonicalizer rootByteSymbols, CharsToNameCanonicalizer rootCharSymbols,
            int factoryFeatures) throws IOException
    {
        int prevInputPtr = _inputPtr;
        JsonEncoding enc = detectEncoding();
        int bytesProcessed = _inputPtr - prevInputPtr;

        if (enc == JsonEncoding.UTF8) {
            /* and without canonicalization, byte-based approach is not performant; just use std UTF-8 reader
             * (which is ok for larger input; not so hot for smaller; but this is not a common case)
             */
            if (JsonFactory.Feature.CANONICALIZE_FIELD_NAMES.enabledIn(factoryFeatures)) {
                ByteQuadsCanonicalizer can = rootByteSymbols.makeChild(factoryFeatures);
                return new UTF8StreamJsonParser(_context, parserFeatures, _in, codec, can,
                        _inputBuffer, _inputPtr, _inputEnd, bytesProcessed, _bufferRecyclable);
            }
        }
        return new ReaderBasedJsonParser(_context, parserFeatures, constructReader(), codec,
                rootCharSymbols.makeChild(factoryFeatures));
    }

    /*
    /**********************************************************
    /*  Encoding detection for data format auto-detection
    /**********************************************************
     */

    /**
     * Current implementation is not as thorough as other functionality
     * ({@link com.fasterxml.jackson.core.json.ByteSourceJsonBootstrapper}); 
     * supports UTF-8, for example. But it should work, for now, and can
     * be improved as necessary.
     *
     * @param acc InputAccessor to use for accessing content to check
     *
     * @return Strength of match (never {@code null})
     *
     * @throws IOException if input access fails due to read problem
     */
    public static MatchStrength hasJSONFormat(InputAccessor acc) throws IOException
    {
        // Ideally we should see "[" or "{"; but if not, we'll accept double-quote (String)
        // in future could also consider accepting non-standard matches?
        
        if (!acc.hasMoreBytes()) {
            return MatchStrength.INCONCLUSIVE;
        }
        byte b = acc.nextByte();
        // Very first thing, a UTF-8 BOM?
        if (b == UTF8_BOM_1) { // yes, looks like UTF-8 BOM
            if (!acc.hasMoreBytes()) {
                return MatchStrength.INCONCLUSIVE;
            }
            if (acc.nextByte() != UTF8_BOM_2) {
                return MatchStrength.NO_MATCH;
            }
            if (!acc.hasMoreBytes()) {
                return MatchStrength.INCONCLUSIVE;
            }
            if (acc.nextByte() != UTF8_BOM_3) {
                return MatchStrength.NO_MATCH;
            }
            if (!acc.hasMoreBytes()) {
                return MatchStrength.INCONCLUSIVE;
            }
            b = acc.nextByte();
        }
        // Then possible leading space
        int ch = skipSpace(acc, b);
        if (ch < 0) {
            return MatchStrength.INCONCLUSIVE;
        }
        // First, let's see if it looks like a structured type:
        if (ch == '{') { // JSON object?
            // Ideally we need to find either double-quote or closing bracket
            ch = skipSpace(acc);
            if (ch < 0) {
                return MatchStrength.INCONCLUSIVE;
            }
            if (ch == '"' || ch == '}') {
                return MatchStrength.SOLID_MATCH;
            }
            // ... should we allow non-standard? Let's not yet... can add if need be
            return MatchStrength.NO_MATCH;
        }
        MatchStrength strength;
        
        if (ch == '[') {
            ch = skipSpace(acc);
            if (ch < 0) {
                return MatchStrength.INCONCLUSIVE;
            }
            // closing brackets is easy; but for now, let's also accept opening...
            if (ch == ']' || ch == '[') {
                return MatchStrength.SOLID_MATCH;
            }
            return MatchStrength.SOLID_MATCH;
        } else {
            // plain old value is not very convincing...
            strength = MatchStrength.WEAK_MATCH;
        }

        if (ch == '"') { // string value
            return strength;
        }
        if (ch <= '9' && ch >= '0') { // number
            return strength;
        }
        if (ch == '-') { // negative number
            ch = skipSpace(acc);
            if (ch < 0) {
                return MatchStrength.INCONCLUSIVE;
            }
            return (ch <= '9' && ch >= '0') ? strength : MatchStrength.NO_MATCH;
        }
        // or one of literals
        if (ch == 'n') { // null
            return tryMatch(acc, "ull", strength);
        }
        if (ch == 't') { // true
            return tryMatch(acc, "rue", strength);
        }
        if (ch == 'f') { // false
            return tryMatch(acc, "alse", strength);
        }
        return MatchStrength.NO_MATCH;
    }

    private static MatchStrength tryMatch(InputAccessor acc, String matchStr, MatchStrength fullMatchStrength)
        throws IOException
    {
        for (int i = 0, len = matchStr.length(); i < len; ++i) {
            if (!acc.hasMoreBytes()) {
                return MatchStrength.INCONCLUSIVE;
            }
            if (acc.nextByte() != matchStr.charAt(i)) {
                return MatchStrength.NO_MATCH;
            }
        }
        return fullMatchStrength;
    }

    private static int skipSpace(InputAccessor acc) throws IOException
    {
        if (!acc.hasMoreBytes()) {
            return -1;
        }
        return skipSpace(acc, acc.nextByte());
    }

    private static int skipSpace(InputAccessor acc, byte b) throws IOException
    {
        while (true) {
            int ch = (int) b & 0xFF;
            if (!(ch == ' ' || ch == '\r' || ch == '\n' || ch == '\t')) {
                return ch;
            }
            if (!acc.hasMoreBytes()) {
                return -1;
            }
            b = acc.nextByte();
        }
    }

    /*
    /**********************************************************
    /* Internal methods, parsing
    /**********************************************************
     */

    /**
     * @return True if a BOM was succesfully found, and encoding
     *   thereby recognized.
     */
    private boolean handleBOM(int quad) throws IOException
    {
        /* Handling of (usually) optional BOM (required for
         * multi-byte formats); first 32-bit charsets:
         */
        switch (quad) {
        case 0x0000FEFF:
            _bigEndian = true;
            _inputPtr += 4;
            _bytesPerChar = 4;
            return true;
        case 0xFFFE0000: // UCS-4, LE?
            _inputPtr += 4;
            _bytesPerChar = 4;
            _bigEndian = false;
            return true;
        case 0x0000FFFE: // UCS-4, in-order...
            reportWeirdUCS4("2143"); // throws exception
            break; // never gets here
        case 0xFEFF0000: // UCS-4, in-order...
            reportWeirdUCS4("3412"); // throws exception
            break; // never gets here
        default:
        }
        // Ok, if not, how about 16-bit encoding BOMs?
        int msw = quad >>> 16;
        if (msw == 0xFEFF) { // UTF-16, BE
            _inputPtr += 2;
            _bytesPerChar = 2;
            _bigEndian = true;
            return true;
        }
        if (msw == 0xFFFE) { // UTF-16, LE
            _inputPtr += 2;
            _bytesPerChar = 2;
            _bigEndian = false;
            return true;
        }
        // And if not, then UTF-8 BOM?
        if ((quad >>> 8) == 0xEFBBBF) { // UTF-8
            _inputPtr += 3;
            _bytesPerChar = 1;
            _bigEndian = true; // doesn't really matter
            return true;
        }
        return false;
    }

    private boolean checkUTF32(int quad) throws IOException
    {
        /* Handling of (usually) optional BOM (required for
         * multi-byte formats); first 32-bit charsets:
         */
        if ((quad >> 8) == 0) { // 0x000000?? -> UTF32-BE
            _bigEndian = true;
        } else if ((quad & 0x00FFFFFF) == 0) { // 0x??000000 -> UTF32-LE
            _bigEndian = false;
        } else if ((quad & ~0x00FF0000) == 0) { // 0x00??0000 -> UTF32-in-order
            reportWeirdUCS4("3412");
        } else if ((quad & ~0x0000FF00) == 0) { // 0x0000??00 -> UTF32-in-order
            reportWeirdUCS4("2143");
        } else {
            // Can not be valid UTF-32 encoded JSON...
            return false;
        }
        // Not BOM (just regular content), nothing to skip past:
        //_inputPtr += 4;
        _bytesPerChar = 4;
        return true;
    }

    private boolean checkUTF16(int i16)
    {
        if ((i16 & 0xFF00) == 0) { // UTF-16BE
            _bigEndian = true;
        } else if ((i16 & 0x00FF) == 0) { // UTF-16LE
            _bigEndian = false;
        } else { // nope, not  UTF-16
            return false;
        }
        // Not BOM (just regular content), nothing to skip past:
        //_inputPtr += 2;
        _bytesPerChar = 2;
        return true;
    }

    /*
    /**********************************************************
    /* Internal methods, problem reporting
    /**********************************************************
     */

    private void reportWeirdUCS4(String type) throws IOException {
        throw new CharConversionException("Unsupported UCS-4 endianness ("+type+") detected");
    }

    /*
    /**********************************************************
    /* Internal methods, raw input access
    /**********************************************************
     */

    protected boolean ensureLoaded(int minimum) throws IOException {
        /* Let's assume here buffer has enough room -- this will always
         * be true for the limited used this method gets
         */
        int gotten = (_inputEnd - _inputPtr);
        while (gotten < minimum) {
            int count;

            if (_in == null) { // block source
                count = -1;
            } else {
                count = _in.read(_inputBuffer, _inputEnd, _inputBuffer.length - _inputEnd);
            }
            if (count < 1) {
                return false;
            }
            _inputEnd += count;
            gotten += count;
        }
        return true;
    }
}

⏎ com/fasterxml/jackson/core/json/ByteSourceJsonBootstrapper.java

Or download all of them as a single archive file:

File name: jackson-core-2.14.0-sources.jar
File size: 497693 bytes
Release date: 2022-11-05
Download 

⇒ Download and Install Jackson Binary Package

⇐ What Is Jackson

⇑ Downloading and Reviewing jackson-*.jar

⇑⇑ Jackson - Java JSON library