GsonDateFormatTest.java

package com.leviwilson.test;

import static org.hamcrest.CoreMatchers.is;
import static org.hamcrest.CoreMatchers.not;
import static org.junit.Assert.assertThat;

import java.util.Date;

import org.junit.Test;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class GsonDateFormatTest {

    @Test
    public void itFailsToGoBackAndForthByDefault() {
        Date now = new Date();
        Gson defaultGson = new Gson();
        
        final String jsonDate = defaultGson.toJson(now, Date.class);
        
        // This is the problem that you were seeing before
        assertThat(defaultGson.fromJson(jsonDate, Date.class), is(not(now)));
    }
    
    @Test
    public void itSucceedsWhenIncludingMillisecondsByDefault() {
        Date now = new Date();
        
        // I think this would fix the issue
        Gson gsonWithDateFormat = new GsonBuilder()
            .setDateFormat("yyyy-MM-dd HH:mm:ss.SSS")
            .create();
        
        final String jsonDate = gsonWithDateFormat.toJson(now, Date.class);
        
        assertThat(gsonWithDateFormat.fromJson(jsonDate, Date.class), is(now));
    }

    @Test
    public void itUsesDefaultsForDateAndTime() {
        Date now = new Date();
        Gson defaultGson = new Gson();
        Gson builderGson = new GsonBuilder()
            .setDateFormat(DateFormat.DEFAULT, DateFormat.DEFAULT)
            .create();
        
        assertThat(builderGson.toJson(now, Date.class), is(defaultGson.toJson(now, Date.class)));
    }
}