https://ant.apache.org/: master POM (The Apache Software Foundation) 
/*
 *  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
 *
 *      https://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.tools.ant.filters;

import java.io.IOException;
import java.io.Reader;
import java.util.LinkedList;

import org.apache.tools.ant.types.Parameter;
import org.apache.tools.ant.util.LineTokenizer;


Reads the last n lines of a stream. (Default is last10 lines.)
Example:

<tailfilter lines="3"/>

Or:

<filterreader classname="org.apache.tools.ant.filters.TailFilter">
  <param name="lines" value="3"/>
</filterreader>


/**
 * Reads the last <code>n</code> lines of a stream. (Default is last10 lines.)
 *
 * Example:
 *
 * <pre>&lt;tailfilter lines=&quot;3&quot;/&gt;</pre>
 *
 * Or:
 *
 * <pre>&lt;filterreader classname=&quot;org.apache.tools.ant.filters.TailFilter&quot;&gt;
 *   &lt;param name=&quot;lines&quot; value=&quot;3&quot;/&gt;
 * &lt;/filterreader&gt;</pre>
 *
 */
public final class TailFilter extends BaseParamFilterReader
    implements ChainableReader {
    
Parameter name for the number of lines to be returned. 
/** Parameter name for the number of lines to be returned. */
    private static final String LINES_KEY = "lines";

    
Parameter name for the number of lines to be skipped. 
/** Parameter name for the number of lines to be skipped. */
    private static final String SKIP_KEY = "skip";

    
Default number of lines to show 
/** Default number of lines to show */
    private static final int DEFAULT_NUM_LINES = 10;

    
Number of lines to be returned in the filtered stream. 
/** Number of lines to be returned in the filtered stream. */
    private long lines = DEFAULT_NUM_LINES;

    
Number of lines to be skipped. 
/** Number of lines to be skipped. */
    private long skip = 0;

    
Whether or not read-ahead been completed. 
/** Whether or not read-ahead been completed. */
    private boolean completedReadAhead = false;

    
A line tokenizer 
/** A line tokenizer */
    private LineTokenizer lineTokenizer = null;

    
the current line from the input stream 
/** the current line from the input stream */
    private String    line      = null;
    
the position in the current line 
/** the position in the current line */
    private int       linePos   = 0;

    private LinkedList<String> lineList = new LinkedList<>();

    
Constructor for "dummy" instances.

See Also:
  • BaseFilterReader.BaseFilterReader()
/**
     * Constructor for "dummy" instances.
     *
     * @see BaseFilterReader#BaseFilterReader()
     */
    public TailFilter() {
        super();
    }

    
Creates a new filtered reader.

Params:
  • in – A Reader object providing the underlying stream.
          Must not be null.
/**
     * Creates a new filtered reader.
     *
     * @param in A Reader object providing the underlying stream.
     *           Must not be <code>null</code>.
     */
    public TailFilter(final Reader in) {
        super(in);
        lineTokenizer = new LineTokenizer();
        lineTokenizer.setIncludeDelims(true);
    }

    
Returns the next character in the filtered stream. If the read-ahead
has been completed, the next character in the buffer is returned.
Otherwise, the stream is read to the end and buffered (with the buffer
growing as necessary), then the appropriate position in the buffer is
set to read from.

Throws:
  • IOException – if the underlying stream throws an IOException
during reading
Returns:the next character in the resulting stream, or -1
if the end of the resulting stream has been reached
/**
     * Returns the next character in the filtered stream. If the read-ahead
     * has been completed, the next character in the buffer is returned.
     * Otherwise, the stream is read to the end and buffered (with the buffer
     * growing as necessary), then the appropriate position in the buffer is
     * set to read from.
     *
     * @return the next character in the resulting stream, or -1
     * if the end of the resulting stream has been reached
     *
     * @exception IOException if the underlying stream throws an IOException
     * during reading
     */
    public int read() throws IOException {
        if (!getInitialized()) {
            initialize();
            setInitialized(true);
        }

        while (line == null || line.isEmpty()) {
            line = lineTokenizer.getToken(in);
            line = tailFilter(line);
            if (line == null) {
                return -1;
            }
            linePos = 0;
        }

        int ch = line.charAt(linePos);
        linePos++;
        if (linePos == line.length()) {
            line = null;
        }
        return ch;
    }

    
Sets the number of lines to be returned in the filtered stream.

Params:
  • lines – the number of lines to be returned in the filtered stream
/**
     * Sets the number of lines to be returned in the filtered stream.
     *
     * @param lines the number of lines to be returned in the filtered stream
     */
    public void setLines(final long lines) {
        this.lines = lines;
    }

    
Returns the number of lines to be returned in the filtered stream.

Returns:the number of lines to be returned in the filtered stream
/**
     * Returns the number of lines to be returned in the filtered stream.
     *
     * @return the number of lines to be returned in the filtered stream
     */
    private long getLines() {
        return lines;
    }

    
Sets the number of lines to be skipped in the filtered stream.

Params:
  • skip – the number of lines to be skipped in the filtered stream
/**
     * Sets the number of lines to be skipped in the filtered stream.
     *
     * @param skip the number of lines to be skipped in the filtered stream
     */
    public void setSkip(final long skip) {
        this.skip = skip;
    }

    
Returns the number of lines to be skipped in the filtered stream.

Returns:the number of lines to be skipped in the filtered stream
/**
     * Returns the number of lines to be skipped in the filtered stream.
     *
     * @return the number of lines to be skipped in the filtered stream
     */
    private long getSkip() {
        return skip;
    }

    
Creates a new TailFilter using the passed in
Reader for instantiation.

Params:
  • rdr – A Reader object providing the underlying stream.
           Must not be null.
Returns:a new filter based on this configuration, but filtering
        the specified reader
/**
     * Creates a new TailFilter using the passed in
     * Reader for instantiation.
     *
     * @param rdr A Reader object providing the underlying stream.
     *            Must not be <code>null</code>.
     *
     * @return a new filter based on this configuration, but filtering
     *         the specified reader
     */
    public Reader chain(final Reader rdr) {
        TailFilter newFilter = new TailFilter(rdr);
        newFilter.setLines(getLines());
        newFilter.setSkip(getSkip());
        newFilter.setInitialized(true);
        return newFilter;
    }

    
Scans the parameters list for the "lines" parameter and uses
it to set the number of lines to be returned in the filtered stream.
also scan for "skip" parameter.

/**
     * Scans the parameters list for the "lines" parameter and uses
     * it to set the number of lines to be returned in the filtered stream.
     * also scan for "skip" parameter.
     */
    private void initialize() {
        Parameter[] params = getParameters();
        if (params != null) {
            for (Parameter param : params) {
                final String paramName = param.getName();
                if (LINES_KEY.equals(paramName)) {
                    setLines(Long.parseLong(param.getValue()));
                } else if (SKIP_KEY.equals(paramName)) {
                    skip = Long.parseLong(param.getValue());
                }
            }
        }
    }

    
implement a tail filter on a stream of lines.
line = null is the end of the stream.

Returns:"" while reading in the lines,
        line while outputting the lines
        null at the end of outputting the lines
/**
     * implement a tail filter on a stream of lines.
     * line = null is the end of the stream.
     * @return "" while reading in the lines,
     *         line while outputting the lines
     *         null at the end of outputting the lines
     */
    private String tailFilter(String line) {
        if (!completedReadAhead) {
            if (line != null) {
                lineList.add(line);
                if (lines == -1) {
                    if (lineList.size() > skip) {
                        return lineList.removeFirst();
                    }
                } else {
                    long linesToKeep = lines + (skip > 0 ? skip : 0);
                    if (linesToKeep < lineList.size()) {
                        lineList.removeFirst();
                    }
                }
                return "";
            }
            completedReadAhead = true;
            if (skip > 0) {
                for (int i = 0; i < skip; ++i) {
                    lineList.removeLast();
                }
            }
            if (lines > -1) {
                while (lineList.size() > lines) {
                    lineList.removeFirst();
                }
            }
        }
        if (lineList.size() > 0) {
            return lineList.removeFirst();
        }
        return null;
    }
}