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Apache Commons Exec is a library to reliably execute external processes from within the JVM.
There is a newer version: 1.4.0
/* 
 * 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.exec;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;

/**
 * Base class to connect a logging system to the output and/or
 * error stream of then external process. The implementation
 * parses the incoming data to construct a line and passes
 * the complete line to an user-defined implementation.
 *
 * @version $Id: LogOutputStream.java 1636056 2014-11-01 21:12:52Z ggregory $
 */
public abstract class LogOutputStream
  extends OutputStream {

    /** Initial buffer size. */
    private static final int INTIAL_SIZE = 132;

    /** Carriage return */
    private static final int CR = 0x0d;

    /** Linefeed */
    private static final int LF = 0x0a;

    /** the internal buffer */
    private final ByteArrayOutputStream buffer = new ByteArrayOutputStream(
      INTIAL_SIZE);

    private boolean skip = false;

    private final int level;

    /**
     * Creates a new instance of this class.
     * Uses the default level of 999.
     */
    public LogOutputStream() {
        this(999);
    }

    /**
     * Creates a new instance of this class.
     *
     * @param level loglevel used to log data written to this stream.
     */
    public LogOutputStream(final int level) {
        this.level = level;
    }

    /**
     * Write the data to the buffer and flush the buffer, if a line separator is
     * detected.
     *
     * @param cc data to log (byte).
     * @see java.io.OutputStream#write(int)
     */
    @Override
    public void write(final int cc) throws IOException {
        final byte c = (byte) cc;
        if (c == '\n' || c == '\r') {
            if (!skip) {
                processBuffer();
            }
        } else {
            buffer.write(cc);
        }
        skip = c == '\r';
    }

    /**
     * Flush this log stream.
     *
     * @see java.io.OutputStream#flush()
     */
    @Override
    public void flush() {
        if (buffer.size() > 0) {
            processBuffer();
        }
    }

    /**
     * Writes all remaining data from the buffer.
     *
     * @see java.io.OutputStream#close()
     */
    @Override
    public void close() throws IOException {
        if (buffer.size() > 0) {
            processBuffer();
        }
        super.close();
    }

    /**
     * @return the trace level of the log system
     */
    public int getMessageLevel() {
        return level;
    }

    /**
     * Write a block of characters to the output stream
     *
     * @param b the array containing the data
     * @param off the offset into the array where data starts
     * @param len the length of block
     * @throws java.io.IOException if the data cannot be written into the stream.
     * @see java.io.OutputStream#write(byte[], int, int)
     */
    @Override
    public void write(final byte[] b, final int off, final int len)
            throws IOException {
        // find the line breaks and pass other chars through in blocks
        int offset = off;
        int blockStartOffset = offset;
        int remaining = len;
        while (remaining > 0) {
            while (remaining > 0 && b[offset] != LF && b[offset] != CR) {
                offset++;
                remaining--;
            }
            // either end of buffer or a line separator char
            final int blockLength = offset - blockStartOffset;
            if (blockLength > 0) {
                buffer.write(b, blockStartOffset, blockLength);
            }
            while (remaining > 0 && (b[offset] == LF || b[offset] == CR)) {
                write(b[offset]);
                offset++;
                remaining--;
            }
            blockStartOffset = offset;
        }
    }

    /**
     * Converts the buffer to a string and sends it to {@code processLine}.
     */
    protected void processBuffer() {
        processLine(buffer.toString());
        buffer.reset();
    }

    /**
     * Logs a line to the log system of the user.
     *
     * @param line
     *            the line to log.
     */
    protected void processLine(final String line) {
        processLine(line, level);
    }

    /**
     * Logs a line to the log system of the user.
     *
     * @param line the line to log.
     * @param logLevel the log level to use
     */
    protected abstract void processLine(final String line, final int logLevel);
}