jackson使用方法总结

 一、转换java对象为json

第一步：创建Jackson的核心对象 ObjectMapper

第二步：使用转换方法

常用的有：

1、writeValue(参数一，参数二)；

参数一：

　　(1) File：将obj对象转换为JSON字符串，并保存到指定的文件中

　　(2) Writer：将obj对象转换为JSON字符串，并将json数据填充到字符输出流中(通常可以使用respon的getWrite()方法，获取该类)

　　(3) OutputStream：将obj对象转换为JSON字符串，并将json数据填充到字节输出流中

参数二：

　　要转换为JSON的对象　

2、writeValueAsString(obj)：将对象转为json字符串

---

示例代码：

@WebServlet("/testJsonServlet")
public class TestJsonServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        Map<String, String[]> map = request.getParameterMap();
        //使用BeanUtils封装TestUser
        TestUser testUser = new TestUser();
        try {
            BeanUtils.populate(testUser, map);
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
        System.out.println(testUser);

        ObjectMapper mapper = new ObjectMapper();
        response.setContentType("application/json;charset=utf-8");
        mapper.writeValue(response.getWriter(),testUser);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }
}

访问 /testJsonServlet?name=tom&phone=123

页面返回的内容为

说明服务器成功将参数以json的格式发送到客户端。

二、转换list集合为json

与将java对象转换为json格式大同小异，这里是将obj替换为list集合对象

看一下代码：

@WebServlet("/testJsonServlet")
public class TestJsonServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

/*
        Map<String, String[]> map = request.getParameterMap();
        //使用BeanUtils封装TestUser
        TestUser testUser = new TestUser();
        try {
            BeanUtils.populate(testUser, map);
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
        System.out.println(testUser);

        ObjectMapper mapper = new ObjectMapper();
        response.setContentType("application/json;charset=utf-8");
        mapper.writeValue(response.getWriter(),testUser);
*/

        TestUser testUser1 = new TestUser();
        testUser1.setName("tom");
        testUser1.setPhone(123);

        TestUser testUser2 = new TestUser();
        testUser2.setName("tom");
        testUser2.setPhone(123);

        TestUser testUser3 = new TestUser();
        testUser3.setName("tom");
        testUser3.setPhone(123);

        List<TestUser> list = new ArrayList<TestUser>();
        list.add(testUser1);
        list.add(testUser2);
        list.add(testUser3);

        ObjectMapper mapper = new ObjectMapper();
        response.setContentType("application/json;charset=utf-8");
        String json = mapper.writeValueAsString(list);
        response.getWriter().write(json);

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }
}

访问 /testJsonServlet

此时返回的是一个数组，数组中为三个json对象 

三、转换Map集合为json

同样是将对象换做map集合，

代码如下：

@WebServlet("/testJsonServlet")
public class TestJsonServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        TestUser testUser1 = new TestUser();
        testUser1.setName("tom");
        testUser1.setPhone(123);

        TestUser testUser2 = new TestUser();
        testUser2.setName("tom");
        testUser2.setPhone(123);

        TestUser testUser3 = new TestUser();
        testUser3.setName("tom");
        testUser3.setPhone(123);

        List<TestUser> list = new ArrayList<TestUser>();
        list.add(testUser1);
        list.add(testUser2);
        list.add(testUser3);

        Map<Integer, TestUser> map = new HashMap<Integer, TestUser>();
        map.put(1,testUser1);
        map.put(2,testUser2);
        map.put(3,testUser3);

        ObjectMapper mapper = new ObjectMapper();
        response.setContentType("application/json;charset=utf-8");
        String json = mapper.writeValueAsString(map);
        response.getWriter().write(json);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }
}

访问 /testJsonServlet

结果如下图

返回的内容为，json对象，其中每个键值对应的value值也是一个json对象

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PS

response.setContentType(MIME)的作用是使客户端浏览器，区分不同种类的数据，并根据不同的MIME调用浏览器内不同的程序嵌入模块来处理相应的数据。
例如web浏览器就是通过MIME类型来判断文件是GIF图片。通过MIME类型来处理json字符串。