java/util/Properties.java

/*
 * Copyright (c) 1995, 2015, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.util;

import java.io.IOException;
import java.io.PrintStream;
import java.io.PrintWriter;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.Reader;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.io.BufferedWriter;

import jdk.internal.util.xml.PropertiesDefaultHandler;

/**
 * The {@code Properties} class represents a persistent set of
 * properties. The {@code Properties} can be saved to a stream
 * or loaded from a stream. Each key and its corresponding value in
 * the property list is a string.
 * <p>
 * A property list can contain another property list as its
 * "defaults"; this second property list is searched if
 * the property key is not found in the original property list.
 * <p>
 * Because {@code Properties} inherits from {@code Hashtable}, the
 * {@code put} and {@code putAll} methods can be applied to a
 * {@code Properties} object.  Their use is strongly discouraged as they
 * allow the caller to insert entries whose keys or values are not
 * {@code Strings}.  The {@code setProperty} method should be used
 * instead.  If the {@code store} or {@code save} method is called
 * on a "compromised" {@code Properties} object that contains a
 * non-{@code String} key or value, the call will fail. Similarly,
 * the call to the {@code propertyNames} or {@code list} method
 * will fail if it is called on a "compromised" {@code Properties}
 * object that contains a non-{@code String} key.
 *
 * <p>
 * The {@link #load(java.io.Reader) load(Reader)} <tt>/</tt>
 * {@link #store(java.io.Writer, java.lang.String) store(Writer, String)}
 * methods load and store properties from and to a character based stream
 * in a simple line-oriented format specified below.
 *
 * The {@link #load(java.io.InputStream) load(InputStream)} <tt>/</tt>
 * {@link #store(java.io.OutputStream, java.lang.String) store(OutputStream, String)}
 * methods work the same way as the load(Reader)/store(Writer, String) pair, except
 * the input/output stream is encoded in ISO 8859-1 character encoding.
 * Characters that cannot be directly represented in this encoding can be written using
 * Unicode escapes as defined in section 3.3 of
 * <cite>The Java&trade; Language Specification</cite>;
 * only a single 'u' character is allowed in an escape
 * sequence.
 *
 * <p> The {@link #loadFromXML(InputStream)} and {@link
 * #storeToXML(OutputStream, String, String)} methods load and store properties
 * in a simple XML format.  By default the UTF-8 character encoding is used,
 * however a specific encoding may be specified if required. Implementations
 * are required to support UTF-8 and UTF-16 and may support other encodings.
 * An XML properties document has the following DOCTYPE declaration:
 *
 * <pre>
 * &lt;!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd"&gt;
 * </pre>
 * Note that the system URI (http://java.sun.com/dtd/properties.dtd) is
 * <i>not</i> accessed when exporting or importing properties; it merely
 * serves as a string to uniquely identify the DTD, which is:
 * <pre>
 *    &lt;?xml version="1.0" encoding="UTF-8"?&gt;
 *
 *    &lt;!-- DTD for properties --&gt;
 *
 *    &lt;!ELEMENT properties ( comment?, entry* ) &gt;
 *
 *    &lt;!ATTLIST properties version CDATA #FIXED "1.0"&gt;
 *
 *    &lt;!ELEMENT comment (#PCDATA) &gt;
 *
 *    &lt;!ELEMENT entry (#PCDATA) &gt;
 *
 *    &lt;!ATTLIST entry key CDATA #REQUIRED&gt;
 * </pre>
 *
 * <p>This class is thread-safe: multiple threads can share a single
 * <tt>Properties</tt> object without the need for external synchronization.
 *
 * @author  Arthur van Hoff
 * @author  Michael McCloskey
 * @author  Xueming Shen
 * @since   1.0
 */
public
class Properties extends Hashtable<Object,Object> {
    /**
     * use serialVersionUID from JDK 1.1.X for interoperability
     */
     private static final long serialVersionUID = 4112578634029874840L;

    /**
     * A property list that contains default values for any keys not
     * found in this property list.
     *
     * @serial
     */
    protected Properties defaults;

    /**
     * Creates an empty property list with no default values.
     */
    public Properties() {
        this(null);
    }

    /**
     * Creates an empty property list with the specified defaults.
     *
     * @param   defaults   the defaults.
     */
    public Properties(Properties defaults) {
        this.defaults = defaults;
    }

    /**
     * Calls the <tt>Hashtable</tt> method {@code put}. Provided for
     * parallelism with the <tt>getProperty</tt> method. Enforces use of
     * strings for property keys and values. The value returned is the
     * result of the <tt>Hashtable</tt> call to {@code put}.
     *
     * @param key the key to be placed into this property list.
     * @param value the value corresponding to <tt>key</tt>.
     * @return     the previous value of the specified key in this property
     *             list, or {@code null} if it did not have one.
     * @see #getProperty
     * @since    1.2
     */
    public synchronized Object setProperty(String key, String value) {
        return put(key, value);
    }


    /**
     * Reads a property list (key and element pairs) from the input
     * character stream in a simple line-oriented format.
     * <p>
     * Properties are processed in terms of lines. There are two
     * kinds of line, <i>natural lines</i> and <i>logical lines</i>.
     * A natural line is defined as a line of
     * characters that is terminated either by a set of line terminator
     * characters ({@code \n} or {@code \r} or {@code \r\n})
     * or by the end of the stream. A natural line may be either a blank line,
     * a comment line, or hold all or some of a key-element pair. A logical
     * line holds all the data of a key-element pair, which may be spread
     * out across several adjacent natural lines by escaping
     * the line terminator sequence with a backslash character
     * {@code \}.  Note that a comment line cannot be extended
     * in this manner; every natural line that is a comment must have
     * its own comment indicator, as described below. Lines are read from
     * input until the end of the stream is reached.
     *
     * <p>
     * A natural line that contains only white space characters is
     * considered blank and is ignored.  A comment line has an ASCII
     * {@code '#'} or {@code '!'} as its first non-white
     * space character; comment lines are also ignored and do not
     * encode key-element information.  In addition to line
     * terminators, this format considers the characters space
     * ({@code ' '}, {@code '\u005Cu0020'}), tab
     * ({@code '\t'}, {@code '\u005Cu0009'}), and form feed
     * ({@code '\f'}, {@code '\u005Cu000C'}) to be white
     * space.
     *
     * <p>
     * If a logical line is spread across several natural lines, the
     * backslash escaping the line terminator sequence, the line
     * terminator sequence, and any white space at the start of the
     * following line have no affect on the key or element values.
     * The remainder of the discussion of key and element parsing
     * (when loading) will assume all the characters constituting
     * the key and element appear on a single natural line after
     * line continuation characters have been removed.  Note that
     * it is <i>not</i> sufficient to only examine the character
     * preceding a line terminator sequence to decide if the line
     * terminator is escaped; there must be an odd number of
     * contiguous backslashes for the line terminator to be escaped.
     * Since the input is processed from left to right, a
     * non-zero even number of 2<i>n</i> contiguous backslashes
     * before a line terminator (or elsewhere) encodes <i>n</i>
     * backslashes after escape processing.
     *
     * <p>
     * The key contains all of the characters in the line starting
     * with the first non-white space character and up to, but not
     * including, the first unescaped {@code '='},
     * {@code ':'}, or white space character other than a line
     * terminator. All of these key termination characters may be
     * included in the key by escaping them with a preceding backslash
     * character; for example,<p>
     *
     * {@code \:\=}<p>
     *
     * would be the two-character key {@code ":="}.  Line
     * terminator characters can be included using {@code \r} and
     * {@code \n} escape sequences.  Any white space after the
     * key is skipped; if the first non-white space character after
     * the key is {@code '='} or {@code ':'}, then it is
     * ignored and any white space characters after it are also
     * skipped.  All remaining characters on the line become part of
     * the associated element string; if there are no remaining
     * characters, the element is the empty string
     * {@code ""}.  Once the raw character sequences
     * constituting the key and element are identified, escape
     * processing is performed as described above.
     *
     * <p>
     * As an example, each of the following three lines specifies the key
     * {@code "Truth"} and the associated element value
     * {@code "Beauty"}:
     * <pre>
     * Truth = Beauty
     *  Truth:Beauty
     * Truth                    :Beauty
     * </pre>
     * As another example, the following three lines specify a single
     * property:
     * <pre>
     * fruits                           apple, banana, pear, \
     *                                  cantaloupe, watermelon, \
     *                                  kiwi, mango
     * </pre>
     * The key is {@code "fruits"} and the associated element is:
     * <pre>"apple, banana, pear, cantaloupe, watermelon, kiwi, mango"</pre>
     * Note that a space appears before each {@code \} so that a space
     * will appear after each comma in the final result; the {@code \},
     * line terminator, and leading white space on the continuation line are
     * merely discarded and are <i>not</i> replaced by one or more other
     * characters.
     * <p>
     * As a third example, the line:
     * <pre>cheeses
     * </pre>
     * specifies that the key is {@code "cheeses"} and the associated
     * element is the empty string {@code ""}.
     * <p>
     * <a name="unicodeescapes"></a>
     * Characters in keys and elements can be represented in escape
     * sequences similar to those used for character and string literals
     * (see sections 3.3 and 3.10.6 of
     * <cite>The Java&trade; Language Specification</cite>).
     *
     * The differences from the character escape sequences and Unicode
     * escapes used for characters and strings are:
     *
     * <ul>
     * <li> Octal escapes are not recognized.
     *
     * <li> The character sequence {@code \b} does <i>not</i>
     * represent a backspace character.
     *
     * <li> The method does not treat a backslash character,
     * {@code \}, before a non-valid escape character as an
     * error; the backslash is silently dropped.  For example, in a
     * Java string the sequence {@code "\z"} would cause a
     * compile time error.  In contrast, this method silently drops
     * the backslash.  Therefore, this method treats the two character
     * sequence {@code "\b"} as equivalent to the single
     * character {@code 'b'}.
     *
     * <li> Escapes are not necessary for single and double quotes;
     * however, by the rule above, single and double quote characters
     * preceded by a backslash still yield single and double quote
     * characters, respectively.
     *
     * <li> Only a single 'u' character is allowed in a Unicode escape
     * sequence.
     *
     * </ul>
     * <p>
     * The specified stream remains open after this method returns.
     *
     * @param   reader   the input character stream.
     * @throws  IOException  if an error occurred when reading from the
     *          input stream.
     * @throws  IllegalArgumentException if a malformed Unicode escape
     *          appears in the input.
     * @throws  NullPointerException if {@code reader} is null.
     * @since   1.6
     */
    public synchronized void load(Reader reader) throws IOException {
        Objects.requireNonNull(reader, "reader parameter is null");
        load0(new LineReader(reader));
    }

    /**
     * Reads a property list (key and element pairs) from the input
     * byte stream. The input stream is in a simple line-oriented
     * format as specified in
     * {@link #load(java.io.Reader) load(Reader)} and is assumed to use
     * the ISO 8859-1 character encoding; that is each byte is one Latin1
     * character. Characters not in Latin1, and certain special characters,
     * are represented in keys and elements using Unicode escapes as defined in
     * section 3.3 of
     * <cite>The Java&trade; Language Specification</cite>.
     * <p>
     * The specified stream remains open after this method returns.
     *
     * @param      inStream   the input stream.
     * @exception  IOException  if an error occurred when reading from the
     *             input stream.
     * @throws     IllegalArgumentException if the input stream contains a
     *             malformed Unicode escape sequence.
     * @throws     NullPointerException if {@code inStream} is null.
     * @since 1.2
     */
    public synchronized void load(InputStream inStream) throws IOException {
        Objects.requireNonNull(inStream, "inStream parameter is null");
        load0(new LineReader(inStream));
    }

    private void load0 (LineReader lr) throws IOException {
        char[] convtBuf = new char[1024];
        int limit;
        int keyLen;
        int valueStart;
        char c;
        boolean hasSep;
        boolean precedingBackslash;

        while ((limit = lr.readLine()) >= 0) {
            c = 0;
            keyLen = 0;
            valueStart = limit;
            hasSep = false;

            //System.out.println("line=<" + new String(lineBuf, 0, limit) + ">");
            precedingBackslash = false;
            while (keyLen < limit) {
                c = lr.lineBuf[keyLen];
                //need check if escaped.
                if ((c == '=' ||  c == ':') && !precedingBackslash) {
                    valueStart = keyLen + 1;
                    hasSep = true;
                    break;
                } else if ((c == ' ' || c == '\t' ||  c == '\f') && !precedingBackslash) {
                    valueStart = keyLen + 1;
                    break;
                }
                if (c == '\\') {
                    precedingBackslash = !precedingBackslash;
                } else {
                    precedingBackslash = false;
                }
                keyLen++;
            }
            while (valueStart < limit) {
                c = lr.lineBuf[valueStart];
                if (c != ' ' && c != '\t' &&  c != '\f') {
                    if (!hasSep && (c == '=' ||  c == ':')) {
                        hasSep = true;
                    } else {
                        break;
                    }
                }
                valueStart++;
            }
            String key = loadConvert(lr.lineBuf, 0, keyLen, convtBuf);
            String value = loadConvert(lr.lineBuf, valueStart, limit - valueStart, convtBuf);
            put(key, value);
        }
    }

    /* Read in a "logical line" from an InputStream/Reader, skip all comment
     * and blank lines and filter out those leading whitespace characters
     * (\u0020, \u0009 and \u000c) from the beginning of a "natural line".
     * Method returns the char length of the "logical line" and stores
     * the line in "lineBuf".
     */
    class LineReader {
        public LineReader(InputStream inStream) {
            this.inStream = inStream;
            inByteBuf = new byte[8192];
        }

        public LineReader(Reader reader) {
            this.reader = reader;
            inCharBuf = new char[8192];
        }

        byte[] inByteBuf;
        char[] inCharBuf;
        char[] lineBuf = new char[1024];
        int inLimit = 0;
        int inOff = 0;
        InputStream inStream;
        Reader reader;

        int readLine() throws IOException {
            int len = 0;
            char c = 0;

            boolean skipWhiteSpace = true;
            boolean isCommentLine = false;
            boolean isNewLine = true;
            boolean appendedLineBegin = false;
            boolean precedingBackslash = false;
            boolean skipLF = false;

            while (true) {
                if (inOff >= inLimit) {
                    inLimit = (inStream==null)?reader.read(inCharBuf)
                                              :inStream.read(inByteBuf);
                    inOff = 0;
                    if (inLimit <= 0) {
                        if (len == 0 || isCommentLine) {
                            return -1;
                        }
                        if (precedingBackslash) {
                            len--;
                        }
                        return len;
                    }
                }
                if (inStream != null) {
                    //The line below is equivalent to calling a
                    //ISO8859-1 decoder.
                    c = (char) (0xff & inByteBuf[inOff++]);
                } else {
                    c = inCharBuf[inOff++];
                }
                if (skipLF) {
                    skipLF = false;
                    if (c == '\n') {
                        continue;
                    }
                }
                if (skipWhiteSpace) {
                    if (c == ' ' || c == '\t' || c == '\f') {
                        continue;
                    }
                    if (!appendedLineBegin && (c == '\r' || c == '\n')) {
                        continue;
                    }
                    skipWhiteSpace = false;
                    appendedLineBegin = false;
                }
                if (isNewLine) {
                    isNewLine = false;
                    if (c == '#' || c == '!') {
                        isCommentLine = true;
                        continue;
                    }
                }

                if (c != '\n' && c != '\r') {
                    lineBuf[len++] = c;
                    if (len == lineBuf.length) {
                        int newLength = lineBuf.length * 2;
                        if (newLength < 0) {
                            newLength = Integer.MAX_VALUE;
                        }
                        char[] buf = new char[newLength];
                        System.arraycopy(lineBuf, 0, buf, 0, lineBuf.length);
                        lineBuf = buf;
                    }
                    //flip the preceding backslash flag
                    if (c == '\\') {
                        precedingBackslash = !precedingBackslash;
                    } else {
                        precedingBackslash = false;
                    }
                }
                else {
                    // reached EOL
                    if (isCommentLine || len == 0) {
                        isCommentLine = false;
                        isNewLine = true;
                        skipWhiteSpace = true;
                        len = 0;
                        continue;
                    }
                    if (inOff >= inLimit) {
                        inLimit = (inStream==null)
                                  ?reader.read(inCharBuf)
                                  :inStream.read(inByteBuf);
                        inOff = 0;
                        if (inLimit <= 0) {
                            if (precedingBackslash) {
                                len--;
                            }
                            return len;
                        }
                    }
                    if (precedingBackslash) {
                        len -= 1;
                        //skip the leading whitespace characters in following line
                        skipWhiteSpace = true;
                        appendedLineBegin = true;
                        precedingBackslash = false;
                        if (c == '\r') {
                            skipLF = true;
                        }
                    } else {
                        return len;
                    }
                }
            }
        }
    }

    /*
     * Converts encoded &#92;uxxxx to unicode chars
     * and changes special saved chars to their original forms
     */
    private String loadConvert (char[] in, int off, int len, char[] convtBuf) {
        if (convtBuf.length < len) {
            int newLen = len * 2;
            if (newLen < 0) {
                newLen = Integer.MAX_VALUE;
            }
            convtBuf = new char[newLen];
        }
        char aChar;
        char[] out = convtBuf;
        int outLen = 0;
        int end = off + len;

        while (off < end) {
            aChar = in[off++];
            if (aChar == '\\') {
                aChar = in[off++];
                if(aChar == 'u') {
                    // Read the xxxx
                    int value=0;
                    for (int i=0; i<4; i++) {
                        aChar = in[off++];
                        switch (aChar) {
                          case '0': case '1': case '2': case '3': case '4':
                          case '5': case '6': case '7': case '8': case '9':
                             value = (value << 4) + aChar - '0';
                             break;
                          case 'a': case 'b': case 'c':
                          case 'd': case 'e': case 'f':
                             value = (value << 4) + 10 + aChar - 'a';
                             break;
                          case 'A': case 'B': case 'C':
                          case 'D': case 'E': case 'F':
                             value = (value << 4) + 10 + aChar - 'A';
                             break;
                          default:
                              throw new IllegalArgumentException(
                                           "Malformed \\uxxxx encoding.");
                        }
                     }
                    out[outLen++] = (char)value;
                } else {
                    if (aChar == 't') aChar = '\t';
                    else if (aChar == 'r') aChar = '\r';
                    else if (aChar == 'n') aChar = '\n';
                    else if (aChar == 'f') aChar = '\f';
                    out[outLen++] = aChar;
                }
            } else {
                out[outLen++] = aChar;
            }
        }
        return new String (out, 0, outLen);
    }

    /*
     * Converts unicodes to encoded &#92;uxxxx and escapes
     * special characters with a preceding slash
     */
    private String saveConvert(String theString,
                               boolean escapeSpace,
                               boolean escapeUnicode) {
        int len = theString.length();
        int bufLen = len * 2;
        if (bufLen < 0) {
            bufLen = Integer.MAX_VALUE;
        }
        StringBuilder outBuffer = new StringBuilder(bufLen);

        for(int x=0; x<len; x++) {
            char aChar = theString.charAt(x);
            // Handle common case first, selecting largest block that
            // avoids the specials below
            if ((aChar > 61) && (aChar < 127)) {
                if (aChar == '\\') {
                    outBuffer.append('\\'); outBuffer.append('\\');
                    continue;
                }
                outBuffer.append(aChar);
                continue;
            }
            switch(aChar) {
                case ' ':
                    if (x == 0 || escapeSpace)
                        outBuffer.append('\\');
                    outBuffer.append(' ');
                    break;
                case '\t':outBuffer.append('\\'); outBuffer.append('t');
                          break;
                case '\n':outBuffer.append('\\'); outBuffer.append('n');
                          break;
                case '\r':outBuffer.append('\\'); outBuffer.append('r');
                          break;
                case '\f':outBuffer.append('\\'); outBuffer.append('f');
                          break;
                case '=': // Fall through
                case ':': // Fall through
                case '#': // Fall through
                case '!':
                    outBuffer.append('\\'); outBuffer.append(aChar);
                    break;
                default:
                    if (((aChar < 0x0020) || (aChar > 0x007e)) & escapeUnicode ) {
                        outBuffer.append('\\');
                        outBuffer.append('u');
                        outBuffer.append(toHex((aChar >> 12) & 0xF));
                        outBuffer.append(toHex((aChar >>  8) & 0xF));
                        outBuffer.append(toHex((aChar >>  4) & 0xF));
                        outBuffer.append(toHex( aChar        & 0xF));
                    } else {
                        outBuffer.append(aChar);
                    }
            }
        }
        return outBuffer.toString();
    }

    private static void writeComments(BufferedWriter bw, String comments)
        throws IOException {
        bw.write("#");
        int len = comments.length();
        int current = 0;
        int last = 0;
        char[] uu = new char[6];
        uu[0] = '\\';
        uu[1] = 'u';
        while (current < len) {
            char c = comments.charAt(current);
            if (c > '\u00ff' || c == '\n' || c == '\r') {
                if (last != current)
                    bw.write(comments.substring(last, current));
                if (c > '\u00ff') {
                    uu[2] = toHex((c >> 12) & 0xf);
                    uu[3] = toHex((c >>  8) & 0xf);
                    uu[4] = toHex((c >>  4) & 0xf);
                    uu[5] = toHex( c        & 0xf);
                    bw.write(new String(uu));
                } else {
                    bw.newLine();
                    if (c == '\r' &&
                        current != len - 1 &&
                        comments.charAt(current + 1) == '\n') {
                        current++;
                    }
                    if (current == len - 1 ||
                        (comments.charAt(current + 1) != '#' &&
                        comments.charAt(current + 1) != '!'))
                        bw.write("#");
                }
                last = current + 1;
            }
            current++;
        }
        if (last != current)
            bw.write(comments.substring(last, current));
        bw.newLine();
    }

    /**
     * Calls the {@code store(OutputStream out, String comments)} method
     * and suppresses IOExceptions that were thrown.
     *
     * @deprecated This method does not throw an IOException if an I/O error
     * occurs while saving the property list.  The preferred way to save a
     * properties list is via the {@code store(OutputStream out,
     * String comments)} method or the
     * {@code storeToXML(OutputStream os, String comment)} method.
     *
     * @param   out      an output stream.
     * @param   comments   a description of the property list.
     * @exception  ClassCastException  if this {@code Properties} object
     *             contains any keys or values that are not
     *             {@code Strings}.
     */
    @Deprecated
    public void save(OutputStream out, String comments)  {
        try {
            store(out, comments);
        } catch (IOException e) {
        }
    }

    /**
     * Writes this property list (key and element pairs) in this
     * {@code Properties} table to the output character stream in a
     * format suitable for using the {@link #load(java.io.Reader) load(Reader)}
     * method.
     * <p>
     * Properties from the defaults table of this {@code Properties}
     * table (if any) are <i>not</i> written out by this method.
     * <p>
     * If the comments argument is not null, then an ASCII {@code #}
     * character, the comments string, and a line separator are first written
     * to the output stream. Thus, the {@code comments} can serve as an
     * identifying comment. Any one of a line feed ('\n'), a carriage
     * return ('\r'), or a carriage return followed immediately by a line feed
     * in comments is replaced by a line separator generated by the {@code Writer}
     * and if the next character in comments is not character {@code #} or
     * character {@code !} then an ASCII {@code #} is written out
     * after that line separator.
     * <p>
     * Next, a comment line is always written, consisting of an ASCII
     * {@code #} character, the current date and time (as if produced
     * by the {@code toString} method of {@code Date} for the
     * current time), and a line separator as generated by the {@code Writer}.
     * <p>
     * Then every entry in this {@code Properties} table is
     * written out, one per line. For each entry the key string is
     * written, then an ASCII {@code =}, then the associated
     * element string. For the key, all space characters are
     * written with a preceding {@code \} character.  For the
     * element, leading space characters, but not embedded or trailing
     * space characters, are written with a preceding {@code \}
     * character. The key and element characters {@code #},
     * {@code !}, {@code =}, and {@code :} are written
     * with a preceding backslash to ensure that they are properly loaded.
     * <p>
     * After the entries have been written, the output stream is flushed.
     * The output stream remains open after this method returns.
     *
     * @param   writer      an output character stream writer.
     * @param   comments   a description of the property list.
     * @exception  IOException if writing this property list to the specified
     *             output stream throws an <tt>IOException</tt>.
     * @exception  ClassCastException  if this {@code Properties} object
     *             contains any keys or values that are not {@code Strings}.
     * @exception  NullPointerException  if {@code writer} is null.
     * @since 1.6
     */
    public void store(Writer writer, String comments)
        throws IOException
    {
        store0((writer instanceof BufferedWriter)?(BufferedWriter)writer
                                                 : new BufferedWriter(writer),
               comments,
               false);
    }

    /**
     * Writes this property list (key and element pairs) in this
     * {@code Properties} table to the output stream in a format suitable
     * for loading into a {@code Properties} table using the
     * {@link #load(InputStream) load(InputStream)} method.
     * <p>
     * Properties from the defaults table of this {@code Properties}
     * table (if any) are <i>not</i> written out by this method.
     * <p>
     * This method outputs the comments, properties keys and values in
     * the same format as specified in
     * {@link #store(java.io.Writer, java.lang.String) store(Writer)},
     * with the following differences:
     * <ul>
     * <li>The stream is written using the ISO 8859-1 character encoding.
     *
     * <li>Characters not in Latin-1 in the comments are written as
     * {@code \u005Cu}<i>xxxx</i> for their appropriate unicode
     * hexadecimal value <i>xxxx</i>.
     *
     * <li>Characters less than {@code \u005Cu0020} and characters greater
     * than {@code \u005Cu007E} in property keys or values are written
     * as {@code \u005Cu}<i>xxxx</i> for the appropriate hexadecimal
     * value <i>xxxx</i>.
     * </ul>
     * <p>
     * After the entries have been written, the output stream is flushed.
     * The output stream remains open after this method returns.
     *
     * @param   out      an output stream.
     * @param   comments   a description of the property list.
     * @exception  IOException if writing this property list to the specified
     *             output stream throws an <tt>IOException</tt>.
     * @exception  ClassCastException  if this {@code Properties} object
     *             contains any keys or values that are not {@code Strings}.
     * @exception  NullPointerException  if {@code out} is null.
     * @since 1.2
     */
    public void store(OutputStream out, String comments)
        throws IOException
    {
        store0(new BufferedWriter(new OutputStreamWriter(out, "8859_1")),
               comments,
               true);
    }

    private void store0(BufferedWriter bw, String comments, boolean escUnicode)
        throws IOException
    {
        if (comments != null) {
            writeComments(bw, comments);
        }
        bw.write("#" + new Date().toString());
        bw.newLine();
        synchronized (this) {
            for (Enumeration<?> e = keys(); e.hasMoreElements();) {
                String key = (String)e.nextElement();
                String val = (String)get(key);
                key = saveConvert(key, true, escUnicode);
                /* No need to escape embedded and trailing spaces for value, hence
                 * pass false to flag.
                 */
                val = saveConvert(val, false, escUnicode);
                bw.write(key + "=" + val);
                bw.newLine();
            }
        }
        bw.flush();
    }

    /**
     * Loads all of the properties represented by the XML document on the
     * specified input stream into this properties table.
     *
     * <p>The XML document must have the following DOCTYPE declaration:
     * <pre>
     * &lt;!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd"&gt;
     * </pre>
     * Furthermore, the document must satisfy the properties DTD described
     * above.
     *
     * <p> An implementation is required to read XML documents that use the
     * "{@code UTF-8}" or "{@code UTF-16}" encoding. An implementation may
     * support additional encodings.
     *
     * <p>The specified stream is closed after this method returns.
     *
     * @param in the input stream from which to read the XML document.
     * @throws IOException if reading from the specified input stream
     *         results in an <tt>IOException</tt>.
     * @throws java.io.UnsupportedEncodingException if the document's encoding
     *         declaration can be read and it specifies an encoding that is not
     *         supported
     * @throws InvalidPropertiesFormatException Data on input stream does not
     *         constitute a valid XML document with the mandated document type.
     * @throws NullPointerException if {@code in} is null.
     * @see    #storeToXML(OutputStream, String, String)
     * @see    <a href="http://www.w3.org/TR/REC-xml/#charencoding">Character
     *         Encoding in Entities</a>
     * @since 1.5
     */
    public synchronized void loadFromXML(InputStream in)
        throws IOException, InvalidPropertiesFormatException
    {
        Objects.requireNonNull(in);
        PropertiesDefaultHandler handler = new PropertiesDefaultHandler();
        handler.load(this, in);
        in.close();
    }

    /**
     * Emits an XML document representing all of the properties contained
     * in this table.
     *
     * <p> An invocation of this method of the form <tt>props.storeToXML(os,
     * comment)</tt> behaves in exactly the same way as the invocation
     * <tt>props.storeToXML(os, comment, "UTF-8");</tt>.
     *
     * @param os the output stream on which to emit the XML document.
     * @param comment a description of the property list, or {@code null}
     *        if no comment is desired.
     * @throws IOException if writing to the specified output stream
     *         results in an <tt>IOException</tt>.
     * @throws NullPointerException if {@code os} is null.
     * @throws ClassCastException  if this {@code Properties} object
     *         contains any keys or values that are not
     *         {@code Strings}.
     * @see    #loadFromXML(InputStream)
     * @since 1.5
     */
    public void storeToXML(OutputStream os, String comment)
        throws IOException
    {
        storeToXML(os, comment, "UTF-8");
    }

    /**
     * Emits an XML document representing all of the properties contained
     * in this table, using the specified encoding.
     *
     * <p>The XML document will have the following DOCTYPE declaration:
     * <pre>
     * &lt;!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd"&gt;
     * </pre>
     *
     * <p>If the specified comment is {@code null} then no comment
     * will be stored in the document.
     *
     * <p> An implementation is required to support writing of XML documents
     * that use the "{@code UTF-8}" or "{@code UTF-16}" encoding. An
     * implementation may support additional encodings.
     *
     * <p>The specified stream remains open after this method returns.
     *
     * @param os        the output stream on which to emit the XML document.
     * @param comment   a description of the property list, or {@code null}
     *                  if no comment is desired.
     * @param  encoding the name of a supported
     *                  <a href="../lang/package-summary.html#charenc">
     *                  character encoding</a>
     *
     * @throws IOException if writing to the specified output stream
     *         results in an <tt>IOException</tt>.
     * @throws java.io.UnsupportedEncodingException if the encoding is not
     *         supported by the implementation.
     * @throws NullPointerException if {@code os} is {@code null},
     *         or if {@code encoding} is {@code null}.
     * @throws ClassCastException  if this {@code Properties} object
     *         contains any keys or values that are not
     *         {@code Strings}.
     * @see    #loadFromXML(InputStream)
     * @see    <a href="http://www.w3.org/TR/REC-xml/#charencoding">Character
     *         Encoding in Entities</a>
     * @since 1.5
     */
    public void storeToXML(OutputStream os, String comment, String encoding)
        throws IOException
    {
        Objects.requireNonNull(os);
        Objects.requireNonNull(encoding);
        PropertiesDefaultHandler handler = new PropertiesDefaultHandler();
        handler.store(this, os, comment, encoding);
    }

    /**
     * Searches for the property with the specified key in this property list.
     * If the key is not found in this property list, the default property list,
     * and its defaults, recursively, are then checked. The method returns
     * {@code null} if the property is not found.
     *
     * @param   key   the property key.
     * @return  the value in this property list with the specified key value.
     * @see     #setProperty
     * @see     #defaults
     */
    public String getProperty(String key) {
        Object oval = super.get(key);
        String sval = (oval instanceof String) ? (String)oval : null;
        return ((sval == null) && (defaults != null)) ? defaults.getProperty(key) : sval;
    }

    /**
     * Searches for the property with the specified key in this property list.
     * If the key is not found in this property list, the default property list,
     * and its defaults, recursively, are then checked. The method returns the
     * default value argument if the property is not found.
     *
     * @param   key            the hashtable key.
     * @param   defaultValue   a default value.
     *
     * @return  the value in this property list with the specified key value.
     * @see     #setProperty
     * @see     #defaults
     */
    public String getProperty(String key, String defaultValue) {
        String val = getProperty(key);
        return (val == null) ? defaultValue : val;
    }

    /**
     * Returns an enumeration of all the keys in this property list,
     * including distinct keys in the default property list if a key
     * of the same name has not already been found from the main
     * properties list.
     *
     * @return  an enumeration of all the keys in this property list, including
     *          the keys in the default property list.
     * @throws  ClassCastException if any key in this property list
     *          is not a string.
     * @see     java.util.Enumeration
     * @see     java.util.Properties#defaults
     * @see     #stringPropertyNames
     */
    public Enumeration<?> propertyNames() {
        Hashtable<String,Object> h = new Hashtable<>();
        enumerate(h);
        return h.keys();
    }

    /**
     * Returns a set of keys in this property list where
     * the key and its corresponding value are strings,
     * including distinct keys in the default property list if a key
     * of the same name has not already been found from the main
     * properties list.  Properties whose key or value is not
     * of type <tt>String</tt> are omitted.
     * <p>
     * The returned set is not backed by the <tt>Properties</tt> object.
     * Changes to this <tt>Properties</tt> are not reflected in the set,
     * or vice versa.
     *
     * @return  a set of keys in this property list where
     *          the key and its corresponding value are strings,
     *          including the keys in the default property list.
     * @see     java.util.Properties#defaults
     * @since   1.6
     */
    public Set<String> stringPropertyNames() {
        Hashtable<String, String> h = new Hashtable<>();
        enumerateStringProperties(h);
        return h.keySet();
    }

    /**
     * Prints this property list out to the specified output stream.
     * This method is useful for debugging.
     *
     * @param   out   an output stream.
     * @throws  ClassCastException if any key in this property list
     *          is not a string.
     */
    public void list(PrintStream out) {
        out.println("-- listing properties --");
        Hashtable<String,Object> h = new Hashtable<>();
        enumerate(h);
        for (Enumeration<String> e = h.keys() ; e.hasMoreElements() ;) {
            String key = e.nextElement();
            String val = (String)h.get(key);
            if (val.length() > 40) {
                val = val.substring(0, 37) + "...";
            }
            out.println(key + "=" + val);
        }
    }

    /**
     * Prints this property list out to the specified output stream.
     * This method is useful for debugging.
     *
     * @param   out   an output stream.
     * @throws  ClassCastException if any key in this property list
     *          is not a string.
     * @since   1.1
     */
    /*
     * Rather than use an anonymous inner class to share common code, this
     * method is duplicated in order to ensure that a non-1.1 compiler can
     * compile this file.
     */
    public void list(PrintWriter out) {
        out.println("-- listing properties --");
        Hashtable<String,Object> h = new Hashtable<>();
        enumerate(h);
        for (Enumeration<String> e = h.keys() ; e.hasMoreElements() ;) {
            String key = e.nextElement();
            String val = (String)h.get(key);
            if (val.length() > 40) {
                val = val.substring(0, 37) + "...";
            }
            out.println(key + "=" + val);
        }
    }

    /**
     * Enumerates all key/value pairs in the specified hashtable.
     * @param h the hashtable
     * @throws ClassCastException if any of the property keys
     *         is not of String type.
     */
    private synchronized void enumerate(Hashtable<String,Object> h) {
        if (defaults != null) {
            defaults.enumerate(h);
        }
        for (Enumeration<?> e = keys() ; e.hasMoreElements() ;) {
            String key = (String)e.nextElement();
            h.put(key, get(key));
        }
    }

    /**
     * Enumerates all key/value pairs in the specified hashtable
     * and omits the property if the key or value is not a string.
     * @param h the hashtable
     */
    private synchronized void enumerateStringProperties(Hashtable<String, String> h) {
        if (defaults != null) {
            defaults.enumerateStringProperties(h);
        }
        for (Enumeration<?> e = keys() ; e.hasMoreElements() ;) {
            Object k = e.nextElement();
            Object v = get(k);
            if (k instanceof String && v instanceof String) {
                h.put((String) k, (String) v);
            }
        }
    }

    /**
     * Convert a nibble to a hex character
     * @param   nibble  the nibble to convert.
     */
    private static char toHex(int nibble) {
        return hexDigit[(nibble & 0xF)];
    }

    /** A table of hex digits */
    private static final char[] hexDigit = {
        '0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F'
    };
}