GsonHttpMessageConverter.java

/*
* Copyright 2002-2014 the original author or authors.
*
* 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  info.xudshen.basic.util.gson;

import com.google.gson.Gson;
import com.google.gson.JsonIOException;
import com.google.gson.JsonParseException;
import com.google.gson.JsonSyntaxException;
import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpInputMessage;
import org.springframework.http.HttpOutputMessage;
import org.springframework.http.MediaType;
import org.springframework.http.converter.AbstractHttpMessageConverter;
import org.springframework.http.converter.HttpMessageNotReadableException;
import org.springframework.http.converter.HttpMessageNotWritableException;
import org.springframework.util.Assert;

import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.Reader;
import java.lang.reflect.Type;
import java.nio.charset.Charset;


/**
 * Created by xudshen on 14/8/1.
 */
public class GsonHttpMessageConverter extends AbstractHttpMessageConverter<Object> {
    public static final Charset DEFAULT_CHARSET = Charset.forName("UTF-8");
    private Gson gson;
    private Type type = null;
    private boolean prefixJson = false;
    private Class[] excludeClazz = {};

    /**
     * Construct a new {@code GsonHttpMessageConverter} with a default {@link Gson#Gson() Gson}.
     */
    public GsonHttpMessageConverter() {
        this(GsonSingleton.getInstance().getGson());
    }

    public GsonHttpMessageConverter(Class[] excludeClazz) {
        this(GsonSingleton.getInstance().getGson());
        this.excludeClazz = excludeClazz;
    }

    /**
     * Construct a new {@code GsonHttpMessageConverter}.
     *
     * @param gson a customized {@link Gson#Gson() Gson}
     */
    public GsonHttpMessageConverter(Gson gson) {
        super(new MediaType("application", "json", DEFAULT_CHARSET));
        setGson(gson);
    }


    /**
     * Sets the {@code Gson} for this view. If not set, a default
     * {@link Gson#Gson() Gson} is used.
     * <p>Setting a custom-configured {@code Gson} is one way to take further control of the JSON serialization
     * process.
     *
     * @throws IllegalArgumentException if gson is null
     */
    public void setGson(Gson gson) {
        Assert.notNull(gson, "'gson' must not be null");
        this.gson = gson;
    }

    public Type getType() {
        return type;
    }

    public void setType(Type type) {
        this.type = type;
    }

    /**
     * Indicates whether the JSON output by this view should be prefixed with "{} &&". Default is false.
     * <p> Prefixing the JSON string in this manner is used to help prevent JSON Hijacking. The prefix renders the string
     * syntactically invalid as a script so that it cannot be hijacked. This prefix does not affect the evaluation of JSON,
     * but if JSON validation is performed on the string, the prefix would need to be ignored.
     */
    public void setPrefixJson(boolean prefixJson) {
        this.prefixJson = prefixJson;
    }

    @Override
    public boolean canRead(Class<?> clazz, MediaType mediaType) {
        return canRead(mediaType);
    }

    @Override
    public boolean canWrite(Class<?> clazz, MediaType mediaType) {
        for (Class exclude : excludeClazz) {
            if (exclude.equals(clazz))
                return false;
        }
        return canWrite(mediaType);
    }

    @Override
    protected boolean supports(Class<?> clazz) {
// should not be called, since we override canRead/Write instead
        throw new UnsupportedOperationException();
    }

    @Override
    protected Object readInternal(Class<?> clazz, HttpInputMessage inputMessage)
            throws IOException, HttpMessageNotReadableException {

        Reader json = new InputStreamReader(inputMessage.getBody(), getCharset(inputMessage.getHeaders()));
        Object ret = null;

        try {
            Type typeOfT = getType();
            if (typeOfT != null) {
                ret = this.gson.fromJson(json, typeOfT);
            } else {
                ret = this.gson.fromJson(json, clazz);
            }

            if (ret == null) {
                throw new HttpMessageNotReadableException("Could not read JSON: null json");
            } else {
                return ret;
            }
        } catch (JsonSyntaxException ex) {
            throw new HttpMessageNotReadableException("Could not read JSON: " + ex.getMessage(), ex);
        } catch (JsonIOException ex) {
            throw new HttpMessageNotReadableException("Could not read JSON: " + ex.getMessage(), ex);
        } catch (JsonParseException ex) {
            throw new HttpMessageNotReadableException("Could not read JSON: " + ex.getMessage(), ex);
        }
    }

    @Override
    protected void writeInternal(Object o, HttpOutputMessage outputMessage)
            throws IOException, HttpMessageNotWritableException {

        OutputStreamWriter writer = new OutputStreamWriter(outputMessage.getBody(), getCharset(outputMessage.getHeaders()));

        try {
            if (this.prefixJson) {
                writer.append("{} && ");
            }
            Type typeOfSrc = getType();
            if (typeOfSrc != null) {
                this.gson.toJson(o, typeOfSrc, writer);
            } else {
                this.gson.toJson(o, writer);
            }
            writer.close();
        } catch (JsonIOException ex) {
            throw new HttpMessageNotWritableException("Could not write JSON: " + ex.getMessage(), ex);
        }
    }

    // helpers
    private Charset getCharset(HttpHeaders headers) {
        if (headers != null && headers.getContentType() != null
                && headers.getContentType().getCharSet() != null) {
            return headers.getContentType().getCharSet();
        }
        return DEFAULT_CHARSET;
    }

}

Why we have this writer.close() here and not having it on a finally block?