Base64Decoder源码

import java.io.*;

/*******************************************************************************
 * A class to decode Base64 streams and strings. See RFC 1521 section 5.2 for
 * details of the Base64 algorithm.
 * <p>
 * This class can be used for decoding strings: <blockquote>
 * 
 * <pre>
 * String encoded = &quot;d2VibWFzdGVyOnRyeTJndWVTUw&quot;;
 * String decoded = Base64Decoder.decode(encoded);
 * </pre>
 * 
 * </blockquote> or for decoding streams: <blockquote>
 * 
 * <pre>
 * InputStream in = new Base64Decoder(System.in);
 * </pre>
 * 
 * </blockquote>
 * 
 * @author <b>Jason Hunter</b>, Copyright &#169; 2000
 * @version 1.1, 2002/11/01, added decodeToBytes() to better handle binary data
 *          (thanks to Sean Graham)
 * @version 1.0, 2000/06/11
 */
public class Base64Decoder extends FilterInputStream {

	private static final char[] chars = { 'A', 'B', 'C', 'D', 'E', 'F', 'G',
			'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T',
			'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g',
			'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't',
			'u', 'v', 'w', 'x', 'y', 'z', '0', '1', '2', '3', '4', '5', '6',
			'7', '8', '9', '+', '/' };

	// A mapping between char values and six-bit integers
	private static final int[] ints = new int[128];
	static {
		for (int i = 0; i < 64; i++) {
			ints[chars[i]] = i;
		}
	}

	private int charCount;
	private int carryOver;

	/***************************************************************************
	 * Constructs a new Base64 decoder that reads input from the given
	 * InputStream.
	 * 
	 * @param in
	 *            the input stream
	 */
	public Base64Decoder(InputStream in) {
		super(in);
	}

	/***************************************************************************
	 * Returns the next decoded character from the stream, or -1 if end of
	 * stream was reached.
	 * 
	 * @return the decoded character, or -1 if the end of the input stream is
	 *         reached
	 * @exception IOException
	 *                if an I/O error occurs
	 */
	public int read() throws IOException {
		// Read the next non-whitespace character
		int x;
		do {
			x = in.read();
			if (x == -1) {
				return -1;
			}
		} while (Character.isWhitespace((char) x));
		charCount++;

		// The '=' sign is just padding
		if (x == '=') {
			return -1; // effective end of stream
		}

		// Convert from raw form to 6-bit form
		x = ints[x];

		// Calculate which character we're decoding now
		int mode = (charCount - 1) % 4;

		// First char save all six bits, go for another
		if (mode == 0) {
			carryOver = x & 63;
			return read();
		}
		// Second char use previous six bits and first two new bits,
		// save last four bits
		else if (mode == 1) {
			int decoded = ((carryOver << 2) + (x >> 4)) & 255;
			carryOver = x & 15;
			return decoded;
		}
		// Third char use previous four bits and first four new bits,
		// save last two bits
		else if (mode == 2) {
			int decoded = ((carryOver << 4) + (x >> 2)) & 255;
			carryOver = x & 3;
			return decoded;
		}
		// Fourth char use previous two bits and all six new bits
		else if (mode == 3) {
			int decoded = ((carryOver << 6) + x) & 255;
			return decoded;
		}
		return -1; // can't actually reach this line
	}

	/***************************************************************************
	 * Reads decoded data into an array of bytes and returns the actual number
	 * of bytes read, or -1 if end of stream was reached.
	 * 
	 * @param buf
	 *            the buffer into which the data is read
	 * @param off
	 *            the start offset of the data
	 * @param len
	 *            the maximum number of bytes to read
	 * @return the actual number of bytes read, or -1 if the end of the input
	 *         stream is reached
	 * @exception IOException
	 *                if an I/O error occurs
	 */
	public int read(byte[] buf, int off, int len) throws IOException {
		if (buf.length < (len + off - 1)) {
			throw new IOException("The input buffer is too small: " + len
					+ " bytes requested starting at offset " + off
					+ " while the buffer " + " is only " + buf.length
					+ " bytes long.");
		}

		// This could of course be optimized
		int i;
		for (i = 0; i < len; i++) {
			int x = read();
			if (x == -1 && i == 0) { // an immediate -1 returns -1
				return -1;
			} else if (x == -1) { // a later -1 returns the chars read so far
				break;
			}
			buf[off + i] = (byte) x;
		}
		return i;
	}

	/***************************************************************************
	 * Returns the decoded form of the given encoded string, as a String. Note
	 * that not all binary data can be represented as a String, so this method
	 * should only be used for encoded String data. Use decodeToBytes()
	 * otherwise.
	 * 
	 * @param encoded
	 *            the string to decode
	 * @return the decoded form of the encoded string
	 */
	public static String decode(String encoded) {
		return new String(decodeToBytes(encoded));
	}
	
	public static byte[] decodeReturnByte(String encodedStr) {
		return decodeToBytes(encodedStr);
	}

	/***************************************************************************
	 * Returns the decoded form of the given encoded string, as bytes.
	 * 
	 * @param encoded
	 *            the string to decode
	 * @return the decoded form of the encoded string
	 */
	public static byte[] decodeToBytes(String encoded) {
		byte[] bytes = null;
		try {
			bytes = encoded.getBytes("8859_1");
		} catch (UnsupportedEncodingException ignored) {
		}

		Base64Decoder in = new Base64Decoder(new ByteArrayInputStream(bytes));

		ByteArrayOutputStream out = new ByteArrayOutputStream(
				(int) (bytes.length * 0.67));

		try {
			byte[] buf = new byte[4 * 1024]; // 4K buffer
			int bytesRead;
			while ((bytesRead = in.read(buf)) != -1) {
				out.write(buf, 0, bytesRead);
			}
			out.close();

			return out.toByteArray();
		} catch (IOException ignored) {
			return null;
		}
	}

	public static void main(String[] args) throws Exception {
		if (args.length != 1) {
			System.err.println("Usage: java Base64Decoder fileToDecode");
			return;
		}

		Base64Decoder decoder = null;
		try {
			decoder = new Base64Decoder(new BufferedInputStream(new FileInputStream(args[0])));
			byte[] buf = new byte[4 * 1024]; // 4K buffer
			int bytesRead;
			while ((bytesRead = decoder.read(buf)) != -1) {
				System.out.write(buf, 0, bytesRead);
			}
		} finally {
			if (decoder != null)
				decoder.close();
		}
	}
	
}