JsonEncoder.java

/*
 * Copyright 2013, 2014 Deutsche Nationalbibliothek
 *
 * 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.culturegraph.mf.json;

import java.io.IOException;
import java.io.StringWriter;

import org.culturegraph.mf.framework.FluxCommand;
import org.culturegraph.mf.framework.MetafactureException;
import org.culturegraph.mf.framework.ObjectReceiver;
import org.culturegraph.mf.framework.StreamReceiver;
import org.culturegraph.mf.framework.annotations.Description;
import org.culturegraph.mf.framework.annotations.In;
import org.culturegraph.mf.framework.annotations.Out;
import org.culturegraph.mf.framework.helpers.DefaultStreamPipe;

import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonStreamContext;
import com.fasterxml.jackson.core.SerializableString;
import com.fasterxml.jackson.core.io.CharacterEscapes;
import com.fasterxml.jackson.core.io.SerializedString;

/**
 * Serialises an object as JSON. Records and entities are represented
 * as objects unless their name ends with []. If the name ends with [],
 * an array is created.
 *
 * @author Christoph Böhme
 * @author Michael Büchner
 *
 */
@Description("Serialises an object as JSON")
@In(StreamReceiver.class)
@Out(String.class)
@FluxCommand("encode-json")
public final class JsonEncoder extends
		DefaultStreamPipe<ObjectReceiver<String>> {

	public static final String ARRAY_MARKER = "[]";

	private final JsonGenerator jsonGenerator;
	private final StringWriter writer = new StringWriter();

	public JsonEncoder() {
		try {
			jsonGenerator = new JsonFactory().createGenerator(writer);
			jsonGenerator.setRootValueSeparator(null);
		} catch (final IOException e) {
			throw new MetafactureException(e);
		}
	}

	public void setPrettyPrinting(final boolean prettyPrinting) {
		if (prettyPrinting) {
			jsonGenerator.useDefaultPrettyPrinter();
		} else {
			jsonGenerator.setPrettyPrinter(null);
		}
	}

	public boolean getPrettyPrinting() {
		return jsonGenerator.getPrettyPrinter() != null;
	}

	/**
	 * By default JSON output does only have escaping where it is strictly
	 * necessary. This is recommended in the most cases. Nevertheless it can
	 * be sometimes useful to have some more escaping.
	 *
	 * @param escapeCharacters an array which defines which characters should be
	 *                         escaped and how it will be done. See
	 *                         {@link CharacterEscapes}. In most cases this should
	 *                         be null. Use like this:
	 *                         <pre>{@code int[] esc = CharacterEscapes.standardAsciiEscapesForJSON();
	 * 	                       // and force escaping of a few others:
	 * 	                       esc['\''] = CharacterEscapes.ESCAPE_STANDARD;
	 *                         JsonEncoder.useEscapeJavaScript(esc);
	 *                         }</pre>
	 */
	public void setJavaScriptEscapeChars(final int[] escapeCharacters) {

		final CharacterEscapes ce = new CharacterEscapes() {

			private static final long serialVersionUID = 1L;

			@Override
			public int[] getEscapeCodesForAscii() {
				if (escapeCharacters == null) {
					return CharacterEscapes.standardAsciiEscapesForJSON();
				}
				return escapeCharacters;
			}

			@Override
			public SerializableString getEscapeSequence(final int ch) {
				final String jsEscaped = escapeChar((char) ch);
				return new SerializedString(jsEscaped);
			}

		};

		jsonGenerator.setCharacterEscapes(ce);
	}

	@Override
	public void startRecord(final String id) {
		final StringBuffer buffer = writer.getBuffer();
		buffer.delete(0, buffer.length());
		startGroup(id);
	}

	@Override
	public void endRecord() {
		endGroup();
		try {
			jsonGenerator.flush();
		} catch (final IOException e) {
			throw new MetafactureException(e);
		}
		getReceiver().process(writer.toString());
	}

	@Override
	public void startEntity(final String name) {
		startGroup(name);
	}

	@Override
	public void endEntity() {
		endGroup();
	}

	@Override
	public void literal(final String name, final String value) {
		try {
			final JsonStreamContext ctx = jsonGenerator.getOutputContext();
			if (ctx.inObject()) {
				jsonGenerator.writeFieldName(name);
			}
			if (value == null) {
				jsonGenerator.writeNull();
			} else {
				jsonGenerator.writeString(value);
			}
		} catch (final JsonGenerationException e) {
			throw new MetafactureException(e);
		}
		catch (final IOException e) {
			throw new MetafactureException(e);
		}
	}

	private void startGroup(final String name) {
		try {
			final JsonStreamContext ctx = jsonGenerator.getOutputContext();
			if (name.endsWith(ARRAY_MARKER)) {
				if (ctx.inObject()) {
					jsonGenerator.writeFieldName(name.substring(0, name.length() - ARRAY_MARKER.length()));
				}
				jsonGenerator.writeStartArray();
			} else {
				if (ctx.inObject()) {
					jsonGenerator.writeFieldName(name);
				}
				jsonGenerator.writeStartObject();
			}
		} catch (final JsonGenerationException e) {
			throw new MetafactureException(e);
		}
		catch (final IOException e) {
			throw new MetafactureException(e);
		}
	}

	private void endGroup() {
		try {
			final JsonStreamContext ctx = jsonGenerator.getOutputContext();
			if (ctx.inObject()) {
				jsonGenerator.writeEndObject();
			} else if (ctx.inArray()) {
				jsonGenerator.writeEndArray();
			}
		} catch (final JsonGenerationException e) {
			throw new MetafactureException(e);
		}
		catch (final IOException e) {
			throw new MetafactureException(e);
		}
	}

	private String escapeChar(char ch) {
		final String namedEscape = namedEscape(ch);
		if (namedEscape != null) {
			return namedEscape;
		}
		if (ch < 0x20 || 0x7f < ch ) {
			return unicodeEscape(ch);
		}
		return Character.toString(ch);
	}

	private String namedEscape(char ch) {
		switch(ch) {
			case '\b': return "\\b";
			case '\n': return "\\n";
			case '\t': return "\\t";
			case '\f': return "\\f";
			case '\r': return "\\r";
			case '\'': return "\\'";
			case '\\': return "\\\\";
			case '"': return "\\\"";
			case '/': return "\\/";
			default:
				return null;
		}
	}

	private String unicodeEscape(char ch) {
		return String.format("\\u%4H", ch).replace(' ', '0');
	}

}