Commons IO example source code file (ReaderInputStream.java)

This example Commons IO source code file (ReaderInputStream.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Commons IO tags/keywords

bytebuffer, charset, coderresult, default_buffer_size, default_buffer_size, inputstream, io, ioexception, ioexception, nio, override, override, reader, readerinputstream, readerinputstream, string

The Commons IO ReaderInputStream.java source code

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.io.input;

import java.io.IOException;
import java.io.InputStream;
import java.io.Reader;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.charset.Charset;
import java.nio.charset.CharsetEncoder;
import java.nio.charset.CoderResult;

/**
 * {@link InputStream} implementation that reads a character stream from a {@link Reader}
 * and transforms it to a byte stream using a specified charset encoding. The stream
 * is transformed using a {@link CharsetEncoder} object, guaranteeing that all charset
 * encodings supported by the JRE are handled correctly. In particular for charsets such as
 * UTF-16, the implementation ensures that one and only one byte order marker
 * is produced.
 * <p>
 * Since in general it is not possible to predict the number of characters to be read from the
 * {@link Reader} to satisfy a read request on the {@link ReaderInputStream}, all reads from
 * the {@link Reader} are buffered. There is therefore no well defined correlation
 * between the current position of the {@link Reader} and that of the {@link ReaderInputStream}.
 * This also implies that in general there is no need to wrap the underlying {@link Reader}
 * in a {@link java.io.BufferedReader}.
 * <p>
 * {@link ReaderInputStream} implements the inverse transformation of {@link java.io.InputStreamReader};
 * in the following example, reading from <tt>in2 would return the same byte
 * sequence as reading from <tt>in (provided that the initial byte sequence is legal
 * with respect to the charset encoding):
 * <pre>
 * InputStream in = ...
 * Charset cs = ...
 * InputStreamReader reader = new InputStreamReader(in, cs);
 * ReaderInputStream in2 = new ReaderInputStream(reader, cs);</pre>
 * {@link ReaderInputStream} implements the same transformation as {@link java.io.OutputStreamWriter},
 * except that the control flow is reversed: both classes transform a character stream
 * into a byte stream, but {@link java.io.OutputStreamWriter} pushes data to the underlying stream,
 * while {@link ReaderInputStream} pulls it from the underlying stream.
 * <p>
 * Note that while there are use cases where there is no alternative to using
 * this class, very often the need to use this class is an indication of a flaw
 * in the design of the code. This class is typically used in situations where an existing
 * API only accepts an {@link InputStream}, but where the most natural way to produce the data
 * is as a character stream, i.e. by providing a {@link Reader} instance. An example of a situation
 * where this problem may appear is when implementing the {@link javax.activation.DataSource}
 * interface from the Java Activation Framework.
 * <p>
 * Given the fact that the {@link Reader} class doesn't provide any way to predict whether the next
 * read operation will block or not, it is not possible to provide a meaningful
 * implementation of the {@link InputStream#available()} method. A call to this method
 * will always return 0. Also, this class doesn't support {@link InputStream#mark(int)}.
 * <p>
 * Instances of {@link ReaderInputStream} are not thread safe.
 * 
 * @see org.apache.commons.io.output.WriterOutputStream
 * 
 * @author <a href="mailto:veithen@apache.org">Andreas Veithen
 * @since Commons IO 2.0
 */
public class ReaderInputStream extends InputStream {
    private static final int DEFAULT_BUFFER_SIZE = 1024;

    private final Reader reader;
    private final CharsetEncoder encoder;

    /**
     * CharBuffer used as input for the decoder. It should be reasonably
     * large as we read data from the underlying Reader into this buffer.
     */
    private final CharBuffer encoderIn;

    /**
     * ByteBuffer used as output for the decoder. This buffer can be small
     * as it is only used to transfer data from the decoder to the
     * buffer provided by the caller.
     */
    private final ByteBuffer encoderOut = ByteBuffer.allocate(128);

    private CoderResult lastCoderResult;
    private boolean endOfInput;

    /**
     * Construct a new {@link ReaderInputStream}.
     * 
     * @param reader the target {@link Reader}
     * @param charset the charset encoding
     * @param bufferSize the size of the input buffer in number of characters
     */
    public ReaderInputStream(Reader reader, Charset charset, int bufferSize) {
        this.reader = reader;
        encoder = charset.newEncoder();
        encoderIn = CharBuffer.allocate(bufferSize);
        encoderIn.flip();
    }

    /**
     * Construct a new {@link ReaderInputStream} with a default input buffer size of
     * 1024 characters.
     * 
     * @param reader the target {@link Reader}
     * @param charset the charset encoding
     */
    public ReaderInputStream(Reader reader, Charset charset) {
        this(reader, charset, DEFAULT_BUFFER_SIZE);
    }

    /**
     * Construct a new {@link ReaderInputStream}.
     * 
     * @param reader the target {@link Reader}
     * @param charsetName the name of the charset encoding
     * @param bufferSize the size of the input buffer in number of characters
     */
    public ReaderInputStream(Reader reader, String charsetName, int bufferSize) {
        this(reader, Charset.forName(charsetName), bufferSize);
    }

    /**
     * Construct a new {@link ReaderInputStream} with a default input buffer size of
     * 1024 characters.
     * 
     * @param reader the target {@link Reader}
     * @param charsetName the name of the charset encoding
     */
    public ReaderInputStream(Reader reader, String charsetName) {
        this(reader, charsetName, DEFAULT_BUFFER_SIZE);
    }

    /**
     * Construct a new {@link ReaderInputStream} that uses the default character encoding
     * with a default input buffer size of 1024 characters.
     * 
     * @param reader the target {@link Reader}
     */
    public ReaderInputStream(Reader reader) {
        this(reader, Charset.defaultCharset());
    }

    /**
     * Read the specified number of bytes into an array.
     * 
     * @param b the byte array to read into
     * @param off the offset to start reading bytes into
     * @param len the number of bytes to read
     * @return the number of bytes read or <code>-1
     *         if the end of the stream has been reached
     * @throws IOException if an I/O error occurs
     */
    @Override
    public int read(byte[] b, int off, int len) throws IOException {
        int read = 0;
        while (len > 0) {
            if (encoderOut.position() > 0) {
                encoderOut.flip();
                int c = Math.min(encoderOut.remaining(), len);
                encoderOut.get(b, off, c);
                off += c;
                len -= c;
                read += c;
                encoderOut.compact();
            } else {
                if (!endOfInput && (lastCoderResult == null || lastCoderResult.isUnderflow())) {
                    encoderIn.compact();
                    int position = encoderIn.position();
                    // We don't use Reader#read(CharBuffer) here because it is more efficient
                    // to write directly to the underlying char array (the default implementation
                    // copies data to a temporary char array).
                    int c = reader.read(encoderIn.array(), position, encoderIn.remaining());
                    if (c == -1) {
                        endOfInput = true;
                    } else {
                        encoderIn.position(position+c);
                    }
                    encoderIn.flip();
                }
                lastCoderResult = encoder.encode(encoderIn, encoderOut, endOfInput);
                if (endOfInput && encoderOut.position() == 0) {
                    break;
                }
            }
        }
        return read == 0 && endOfInput ? -1 : read;
    }

    /**
     * Read the specified number of bytes into an array.
     * 
     * @param b the byte array to read into
     * @return the number of bytes read or <code>-1
     *         if the end of the stream has been reached
     * @throws IOException if an I/O error occurs
     */
    @Override
    public int read(byte[] b) throws IOException {
        return read(b, 0, b.length);
    }

    /**
     * Read a single byte.
     *
     * @return either the byte read or <code>-1 if the end of the stream
     *         has been reached
     * @throws IOException if an I/O error occurs
     */
    @Override
    public int read() throws IOException {
        byte[] b = new byte[1];
        return read(b) == -1 ? -1 : b[0] & 0xFF;
    }

    /**
     * Close the stream. This method will cause the underlying {@link Reader}
     * to be closed.
     * @throws IOException if an I/O error occurs
     */
    @Override
    public void close() throws IOException {
        reader.close();
    }
}