利用Jackson对Object,Map,List,数组,枚举,日期类等转化为json

利用Jackson中的ObjectMapper对象，转为对象为json字符串，springside对Jackson的简单封装 JsonBinder ：

public class JsonBinder {

	private static Logger logger = LoggerFactory.getLogger(JsonBinder.class);

	private ObjectMapper mapper;

	public JsonBinder(Inclusion inclusion) {
		mapper = new ObjectMapper();
		//设置输出包含的属性
		mapper.getSerializationConfig().setSerializationInclusion(inclusion);
		//设置输入时忽略JSON字符串中存在而Java对象实际没有的属性
		mapper.getDeserializationConfig().set(
				org.codehaus.jackson.map.DeserializationConfig.Feature.FAIL_ON_UNKNOWN_PROPERTIES, false);
	}

	/**
	 * 创建输出全部属性到Json字符串的Binder.
	 */
	public static JsonBinder buildNormalBinder() {
		return new JsonBinder(Inclusion.ALWAYS);
	}

	/**
	 * 创建只输出非空属性到Json字符串的Binder.
	 */
	public static JsonBinder buildNonNullBinder() {
		return new JsonBinder(Inclusion.NON_NULL);
	}

	/**
	 * 创建只输出初始值被改变的属性到Json字符串的Binder.
	 */
	public static JsonBinder buildNonDefaultBinder() {
		return new JsonBinder(Inclusion.NON_DEFAULT);
	}

	/**
	 * 如果JSON字符串为Null或"null"字符串,返回Null.
	 * 如果JSON字符串为"[]",返回空集合.
	 * 
	 * 如需读取集合如List/Map,且不是List这种简单类型时使用如下语句:
	 * List beanList = binder.getMapper().readValue(listString, new TypeReference>() {});
	 */
	public  T fromJson(String jsonString, Class clazz) {
		if (StringUtils.isEmpty(jsonString)) {
			return null;
		}

		try {
			return mapper.readValue(jsonString, clazz);
		} catch (IOException e) {
			logger.warn("parse json string error:" + jsonString, e);
			return null;
		}
	}

	/**
	 * 如果对象为Null,返回"null".
	 * 如果集合为空集合,返回"[]".
	 */
	public String toJson(Object object) {

		try {
			return mapper.writeValueAsString(object);
		} catch (IOException e) {
			logger.warn("write to json string error:" + object, e);
			return null;
		}
	}

	/**
	 * 设置转换日期类型的format pattern,如果不设置默认打印Timestamp毫秒数.
	 */
	public void setDateFormat(String pattern) {
		if (StringUtils.isNotBlank(pattern)) {
			DateFormat df = new SimpleDateFormat(pattern);
			mapper.getSerializationConfig().setDateFormat(df);
			mapper.getDeserializationConfig().setDateFormat(df);
		}
	}

	/**
	 * 取出Mapper做进一步的设置或使用其他序列化API.
	 */
	public ObjectMapper getMapper() {
		return mapper;
	}
}

在其他类中定义jsonBinder对象：

private static JsonBinder binder = JsonBinder.buildNonDefaultBinder();

以下是对象到json字符串以及json字符串到对象的转化：

/**
	 * 序列化对象/集合到Json字符串.
	 */
	@Test
	public void toJson() throws Exception {
		//Bean
		TestBean bean = new TestBean("A");
		String beanString = binder.toJson(bean);
		System.out.println("Bean:" + beanString);
		assertEquals("{\"name\":\"A\"}", beanString);

		//Map
		Map map = Maps.newLinkedHashMap();
		map.put("name", "A");
		map.put("age", 2);
		String mapString = binder.toJson(map);
		System.out.println("Map:" + mapString);
		assertEquals("{\"name\":\"A\",\"age\":2}", mapString);

		//List
		List stringList = Lists.newArrayList("A", "B", "C");
		String listString = binder.toJson(stringList);
		System.out.println("String List:" + listString);
		assertEquals("[\"A\",\"B\",\"C\"]", listString);

		//List
		List beanList = Lists.newArrayList(new TestBean("A"), new TestBean("B"));
		String beanListString = binder.toJson(beanList);
		System.out.println("Bean List:" + beanListString);
		assertEquals("[{\"name\":\"A\"},{\"name\":\"B\"}]", beanListString);

		//Bean[]
		TestBean[] beanArray = new TestBean[] { new TestBean("A"), new TestBean("B") };
		String beanArrayString = binder.toJson(beanArray);
		System.out.println("Array List:" + beanArrayString);
		assertEquals("[{\"name\":\"A\"},{\"name\":\"B\"}]", beanArrayString);
	}

	/**
	 * 从Json字符串反序列化对象/集合.
	 */
	@Test
	@SuppressWarnings("unchecked")
	public void fromJson() throws Exception {
		//Bean
		String beanString = "{\"name\":\"A\"}";
		TestBean bean = binder.fromJson(beanString, TestBean.class);
		System.out.println("Bean:" + bean);

		//Map
		String mapString = "{\"name\":\"A\",\"age\":2}";
		Map map = binder.fromJson(mapString, HashMap.class);
		System.out.println("Map:");
		for (Entry entry : map.entrySet()) {
			System.out.println(entry.getKey() + " " + entry.getValue());
		}

		//List
		String listString = "[\"A\",\"B\",\"C\"]";
		List stringList = binder.getMapper().readValue(listString, List.class);
		System.out.println("String List:");
		for (String element : stringList) {
			System.out.println(element);
		}

		//List
		String beanListString = "[{\"name\":\"A\"},{\"name\":\"B\"}]";
		List beanList = binder.getMapper().readValue(beanListString, new TypeReference>() {
		});
		System.out.println("Bean List:");
		for (TestBean element : beanList) {
			System.out.println(element);
		}
	}

在我的实际项目中，遇到一个有意思的问题，我把一个对象转为为json字符串时，调用binder.toJson(bean) ，这个方法，方法总是返回null，后面发现该bean中没有默认的构造函数，加上某人构造函数就ok了。