src/java.logging/share/classes/java/util/logging/Handler.java

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package java.util.logging;

import java.util.Objects;
import java.io.UnsupportedEncodingException;

/**
 * A {@code Handler} object takes log messages from a {@code Logger} and
 * exports them.  It might for example, write them to a console
 * or write them to a file, or send them to a network logging service,
 * or forward them to an OS log, or whatever.
 * <p>
 * A {@code Handler} can be disabled by doing a {@code setLevel(Level.OFF)}
 * and can  be re-enabled by doing a {@code setLevel} with an appropriate level.
 * <p>
 * {@code Handler} classes typically use {@code LogManager} properties to set
 * default values for the {@code Handler}'s {@code Filter}, {@code Formatter},
 * and {@code Level}.  See the specific documentation for each concrete
 * {@code Handler} class.
 *
 * <h2><a id=threadSafety>Thread Safety and Deadlock Risk in Handlers</a></h2>
 *
 * Implementations of {@code Handler} should be thread-safe. Handlers are
 * expected to be invoked concurrently from arbitrary threads. However,
 * over-use of synchronization may result in unwanted thread contention,
 * performance issues or even deadlocking.
 * <p>
 * In particular, subclasses should avoid acquiring locks around code which
 * calls back to arbitrary user-supplied objects, especially during log record
 * formatting. Holding a lock around any such callbacks creates a deadlock risk
 * between logging code and user code.
 * <p>
 * As such, general purpose {@code Handler} subclasses should not synchronize
 * their {@link #publish(LogRecord)} methods, or call {@code super.publish()}
 * while holding locks, since these are typically expected to need to process
 * and format user-supplied arguments.
 *
 * @since 1.4
 */
public abstract class Handler {
    private static final int offValue = Level.OFF.intValue();

    // ensure log manager is initialized
    private final LogManager manager = LogManager.getLogManager();

    // We're using volatile here to avoid synchronizing getters, which
    // would prevent other threads from calling isLoggable()
    // while publish() is executing.
    // On the other hand, setters will be synchronized to exclude concurrent
    // execution with more complex methods, such as StreamHandler.publish().
    // We wouldn't want 'level' to be changed by another thread in the middle
    // of the execution of a 'publish' call.
    private volatile Filter filter;
    private volatile Formatter formatter;
    private volatile Level logLevel = Level.ALL;
    private volatile ErrorManager errorManager = new ErrorManager();
    private volatile String encoding;

    /**
     * Default constructor.  The resulting {@code Handler} has a log
     * level of {@code Level.ALL}, no {@code Formatter}, and no
     * {@code Filter}.  A default {@code ErrorManager} instance is installed
     * as the {@code ErrorManager}.
     */
    protected Handler() { }

    /**
     * Package-private constructor for chaining from subclass constructors
     * that wish to configure the handler with specific default and/or
     * specified values.
     *
     * @param defaultLevel       a default {@link Level} to configure if one is
     *                           not found in LogManager configuration properties
     * @param defaultFormatter   a default {@link Formatter} to configure if one is
     *                           not specified by {@code specifiedFormatter} parameter
     *                           nor found in LogManager configuration properties
     * @param specifiedFormatter if not null, this is the formatter to configure
     */
    Handler(Level defaultLevel, Formatter defaultFormatter,
            Formatter specifiedFormatter) {
        this();

        LogManager manager = LogManager.getLogManager();
        String cname = getClass().getName();

        final Level level = manager.getLevelProperty(cname + ".level", defaultLevel);
        final Filter filter = manager.getFilterProperty(cname + ".filter", null);
        final Formatter formatter = specifiedFormatter == null
                ? manager.getFormatterProperty(cname + ".formatter", defaultFormatter)
                : specifiedFormatter;
        final String encoding = manager.getStringProperty(cname + ".encoding", null);

        setLevel(level);
        setFilter(filter);
        setFormatter(formatter);
        try {
            setEncoding(encoding);
        } catch (Exception ex) {
            try {
                setEncoding(null);
            } catch (Exception ex2) {
                // doing a setEncoding with null should always work.
                // assert false;
            }
        }
    }

    /**
     * Publish a {@code LogRecord}.
     * <p>
     * The logging request was made initially to a {@code Logger} object,
     * which initialized the {@code LogRecord} and forwarded it here.
     * <p>
     * The {@code Handler}  is responsible for formatting the message, when and
     * if necessary.  The formatting should include localization.
     *
     * @apiNote To avoid the risk of deadlock, implementations of this method
     * should avoid holding any locks while calling out to application code,
     * such as the formatting of {@code LogRecord}.
     *
     * @param  record  description of the log event. A null record is
     *                 silently ignored and is not published
     */
    public abstract void publish(LogRecord record);

    /**
     * Flush any buffered output.
     */
    public abstract void flush();

    /**
     * Close the {@code Handler} and free all associated resources.
     * <p>
     * The close method will perform a {@code flush} and then close the
     * {@code Handler}.   After close has been called this {@code Handler}
     * should no longer be used.  Method calls may either be silently
     * ignored or may throw runtime exceptions.
     */
    public abstract void close();

    /**
     * Set a {@code Formatter}.  This {@code Formatter} will be used
     * to format {@code LogRecords} for this {@code Handler}.
     * <p>
     * Some {@code Handlers} may not use {@code Formatters}, in
     * which case the {@code Formatter} will be remembered, but not used.
     *
     * @param newFormatter the {@code Formatter} to use (may not be null)
     */
    public synchronized void setFormatter(Formatter newFormatter) {
        formatter = Objects.requireNonNull(newFormatter);
    }

    /**
     * Return the {@code Formatter} for this {@code Handler}.
     * @return the {@code Formatter} (may be null).
     */
    public Formatter getFormatter() {
        return formatter;
    }

    /**
     * Set the character encoding used by this {@code Handler}.
     * <p>
     * The encoding should be set before any {@code LogRecords} are written
     * to the {@code Handler}.
     *
     * @param encoding  The name of a supported character encoding.
     *        May be null, to indicate the default platform encoding.
     * @throws  UnsupportedEncodingException if the named encoding is
     *          not supported.
     */
    public synchronized void setEncoding(String encoding)
            throws java.io.UnsupportedEncodingException {
        if (encoding != null) {
            try {
                if(!java.nio.charset.Charset.isSupported(encoding)) {
                    throw new UnsupportedEncodingException(encoding);
                }
            } catch (java.nio.charset.IllegalCharsetNameException e) {
                throw new UnsupportedEncodingException(encoding);
            }
        }
        this.encoding = encoding;
    }

    /**
     * Return the character encoding for this {@code Handler}.
     *
     * @return  The encoding name.  May be null, which indicates the
     *          default encoding should be used.
     */
    public String getEncoding() {
        return encoding;
    }

    /**
     * Set a {@code Filter} to control output on this {@code Handler}.
     * <P>
     * For each call of {@code publish} the {@code Handler} will call
     * this {@code Filter} (if it is non-null) to check if the
     * {@code LogRecord} should be published or discarded.
     *
     * @param   newFilter  a {@code Filter} object (may be null)
     */
    public synchronized void setFilter(Filter newFilter) {
        filter = newFilter;
    }

    /**
     * Get the current {@code Filter} for this {@code Handler}.
     *
     * @return  a {@code Filter} object (may be null)
     */
    public Filter getFilter() {
        return filter;
    }

    /**
     * Define an ErrorManager for this Handler.
     * <p>
     * The ErrorManager's "error" method will be invoked if any
     * errors occur while using this Handler.
     *
     * @param em  the new ErrorManager
     */
    public synchronized void setErrorManager(ErrorManager em) {
        if (em == null) {
           throw new NullPointerException();
        }
        errorManager = em;
    }

    /**
     * Retrieves the ErrorManager for this Handler.
     *
     * @return the ErrorManager for this Handler
     */
    public ErrorManager getErrorManager() {
        return errorManager;
    }

   /**
     * Protected convenience method to report an error to this Handler's
     * ErrorManager.
     *
     * @param msg    a descriptive string (may be null)
     * @param ex     an exception (may be null)
     * @param code   an error code defined in ErrorManager
     */
    protected void reportError(String msg, Exception ex, int code) {
        try {
            errorManager.error(msg, ex, code);
        } catch (Exception ex2) {
            System.err.println("Handler.reportError caught:");
            ex2.printStackTrace();
        }
    }

    /**
     * Set the log level specifying which message levels will be
     * logged by this {@code Handler}.  Message levels lower than this
     * value will be discarded.
     * <p>
     * The intention is to allow developers to turn on voluminous
     * logging, but to limit the messages that are sent to certain
     * {@code Handlers}.
     *
     * @param newLevel   the new value for the log level
     */
    public synchronized void setLevel(Level newLevel) {
        if (newLevel == null) {
            throw new NullPointerException();
        }
        logLevel = newLevel;
    }



    /**
     * Get the log level specifying which messages will be
     * logged by this {@code Handler}.  Message levels lower
     * than this level will be discarded.
     * @return  the level of messages being logged.
     */
    public Level getLevel() {
        return logLevel;
    }

    /**
     * Check if this {@code Handler} would actually log a given {@code LogRecord}.
     * <p>
     * This method checks if the {@code LogRecord} has an appropriate
     * {@code Level} and  whether it satisfies any {@code Filter}.  It also
     * may make other {@code Handler} specific checks that might prevent a
     * handler from logging the {@code LogRecord}. It will return false if
     * the {@code LogRecord} is null.
     *
     * @param record  a {@code LogRecord} (may be null).
     * @return true if the {@code LogRecord} would be logged.
     *
     */
    public boolean isLoggable(LogRecord record) {
        final int levelValue = getLevel().intValue();
        if (record == null) return false;
        if (record.getLevel().intValue() < levelValue || levelValue == offValue) {
            return false;
        }
        final Filter filter = getFilter();
        if (filter == null) {
            return true;
        }
        return filter.isLoggable(record);
    }

}