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The coolest XML library for Java around. Define typesafe views (projections) to xml. Use XPath to read and write XML. Bind XML to Java collections. Requires at least Java6, supports Java8 features and has no further runtime dependencies.
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/**
 *  Copyright 2013 Sven Ewald
 *
 *  Licensed 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.xmlbeam.externalizer;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.lang.reflect.Method;
import java.util.Properties;

import org.xmlbeam.exceptions.XBException;
import org.xmlbeam.exceptions.XBIOException;

/**
 * A full working example for an Externalizer implementation. This Externalizer reads the XPaths
 * from a property file instead from the projection annotations.
 */
public class PropertyFileExternalizer implements Externalizer {

    /**
     *
     */
    private static final long serialVersionUID = -2752447643606854521L;
    private final File propertyFile;
    private final Properties props = new Properties();
    private final boolean useXmlFormat;
    private long lastReadTS = 0;
    private String encodingName = "ISO8859-1";

    /**
     * Constructor for a given property file.
     *
     * @param propertyFile
     */
    public PropertyFileExternalizer(final File propertyFile) {
        this.propertyFile = propertyFile;
        this.useXmlFormat = false;
    }

    /**
     * Constructor for a given property file with the option to choose XML format.
     *
     * @param propertyFile
     * @param useXmlFormat
     */
    public PropertyFileExternalizer(final File propertyFile, final boolean useXmlFormat) {
        this.propertyFile = propertyFile;
        this.useXmlFormat = useXmlFormat;
    }

    /**
     * Setter for file encoding.
     *
     * @param encodingName
     * @return this for convenience
     */
    public PropertyFileExternalizer setEncoding(final String encodingName) {
        this.encodingName = encodingName;
        return this;
    }

    private void updateProps() {
        if (!propertyFile.canRead()) {
            throw new XBException("Can not read file '" + propertyFile + "'");
        }
        long fileTS = propertyFile.lastModified();
        if (lastReadTS > fileTS) {
            return;
        }
        props.clear();
        FileInputStream inputStream = null;
        try {
            inputStream = new FileInputStream(propertyFile);
            if (useXmlFormat) {
                props.loadFromXML(inputStream);
                return;
            }
            @SuppressWarnings("resource")
            InputStreamReader reader = new InputStreamReader(inputStream, encodingName);
            props.load(reader);
        } catch (IOException e) {
            throw new XBIOException("Error while reading file '" + propertyFile + "'", e);
        } finally {
            lastReadTS = fileTS;
            if (inputStream != null) {
                try {
                    inputStream.close();
                } catch (IOException e) {
                    throw new XBIOException("Can not close file '" + propertyFile + "'", e);
                }
            }
        }
    }

    @Override
    public String resolveXPath(final String annotationValue, final Method method, final Object[] args) {
        updateProps();
        return findProperty(annotationValue, method, args);
    }

    @Override
    public String resolveURL(final String annotationValue, final Method method, final Object[] args) {
        updateProps();
        return findProperty(annotationValue, method, args);
    }

    /**
     * @param key
     * @param method
     * @param args
     * @return
     */
    protected String findProperty(final String key, final Method method, final Object[] args) {
        String[] propNameCandidates = new String[] {//
        method.getDeclaringClass().getName() + "." + method.getName(),//
                method.getDeclaringClass().getSimpleName() + "." + method.getName(),//
                method.getName(),//
                key //
        };
        for (String propName : propNameCandidates) {
            if (props.containsKey(propName)) {
                return props.getProperty(propName);
            }
        }
        throw new IllegalArgumentException("Expected to find property with key '" + key + "' in file " + propertyFile.getAbsolutePath() + " for method " + method + ". But it does not exist.");
    }

}