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The Pax Logging API Library is to allow for the Pax Logging Service to be reloaded without stopping the many dependent bundles. It also contains the OSGi Log Service API and the Knopflerfish Log API.
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/*
 * 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.logging.log4j.message;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.text.Format;
import java.text.MessageFormat;
import java.util.Arrays;
import java.util.regex.Pattern;

/**
 * Handles messages that contain a format String. Dynamically determines if the format conforms to
 * MessageFormat or String.format and if not then uses ParameterizedMessage to format.
 */
public class FormattedMessage implements Message {

    private static final long serialVersionUID = -665975803997290697L;
    private static final int HASHVAL = 31;
    private static final String FORMAT_SPECIFIER = "%(\\d+\\$)?([-#+ 0,(\\<]*)?(\\d+)?(\\.\\d+)?([tT])?([a-zA-Z%])";
    private static final Pattern MSG_PATTERN = Pattern.compile(FORMAT_SPECIFIER);

    private String messagePattern;
    private transient Object[] argArray;
    private String[] stringArgs;
    private transient String formattedMessage;
    private final Throwable throwable;
    private Message message;

    public FormattedMessage(final String messagePattern, final Object[] arguments, final Throwable throwable) {
        this.messagePattern = messagePattern;
        this.argArray = arguments;
        this.throwable = throwable;
    }

    public FormattedMessage(final String messagePattern, final Object[] arguments) {
        this(messagePattern, arguments, null);
    }

    /**
     * Constructor with a pattern and a single parameter.
     * @param messagePattern The message pattern.
     * @param arg The parameter.
     */
    public FormattedMessage(final String messagePattern, final Object arg) {
        this(messagePattern, new Object[] {arg}, null);
    }

    /**
     * Constructor with a pattern and two parameters.
     * @param messagePattern The message pattern.
     * @param arg1 The first parameter.
     * @param arg2 The second parameter.
     */
    public FormattedMessage(final String messagePattern, final Object arg1, final Object arg2) {
        this(messagePattern, new Object[]{arg1, arg2});
    }


    /**
     * Returns the formatted message.
     * @return the formatted message.
     */
    @Override
    public String getFormattedMessage() {
        if (formattedMessage == null) {
            if (message == null) {
                message = getMessage(messagePattern, argArray, throwable);
            }
            formattedMessage = message.getFormattedMessage();
        }
        return formattedMessage;
    }

    /**
     * Returns the message pattern.
     * @return the message pattern.
     */
    @Override
    public String getFormat() {
        return messagePattern;
    }

    /**
     * Returns the message parameters.
     * @return the message parameters.
     */
    @Override
    public Object[] getParameters() {
        if (argArray != null) {
            return argArray;
        }
        return stringArgs;
    }

    protected Message getMessage(final String msgPattern, final Object[] args, final Throwable aThrowable) {
        try {
            final MessageFormat format = new MessageFormat(msgPattern);
            final Format[] formats = format.getFormats();
            if (formats != null && formats.length > 0) {
                return new MessageFormatMessage(msgPattern, args);
            }
        } catch (final Exception ignored) {
            // Obviously, the message is not a proper pattern for MessageFormat.
        }
        try {
            if (MSG_PATTERN.matcher(msgPattern).find()) {
                return new StringFormattedMessage(msgPattern, args);
            }
        } catch (final Exception ignored) {
            // Also not properly formatted.
        }
        return new ParameterizedMessage(msgPattern, args, aThrowable);
    }

    @Override
    public boolean equals(final Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }

        final FormattedMessage that = (FormattedMessage) o;

        if (messagePattern != null ? !messagePattern.equals(that.messagePattern) : that.messagePattern != null) {
            return false;
        }
        if (!Arrays.equals(stringArgs, that.stringArgs)) {
            return false;
        }

        return true;
    }

    @Override
    public int hashCode() {
        int result = messagePattern != null ? messagePattern.hashCode() : 0;
        result = HASHVAL * result + (stringArgs != null ? Arrays.hashCode(stringArgs) : 0);
        return result;
    }


    @Override
    public String toString() {
        return "FormattedMessage[messagePattern=" + messagePattern + ", args=" +
            Arrays.toString(argArray) + ']';
    }

    private void writeObject(final ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
        getFormattedMessage();
        out.writeUTF(formattedMessage);
        out.writeUTF(messagePattern);
        out.writeInt(argArray.length);
        stringArgs = new String[argArray.length];
        int i = 0;
        for (final Object obj : argArray) {
            stringArgs[i] = obj.toString();
            ++i;
        }
    }

    private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        formattedMessage = in.readUTF();
        messagePattern = in.readUTF();
        final int length = in.readInt();
        stringArgs = new String[length];
        for (int i = 0; i < length; ++i) {
            stringArgs[i] = in.readUTF();
        }
    }

    /**
     * Always returns null.
     *
     * @return null
     */
    @Override
    public Throwable getThrowable() {
        if (throwable != null) {
            return throwable;
        }
        if (message == null) {
            message = getMessage(messagePattern, argArray, null);
        }
        return message.getThrowable();
    }
}