一、方法一：

使用jackson-databind直接返回对象。

二、方法二

使用Gson将对象转换成String再返回，要设置contentType为application/json。

三、方法三

将对象转换成json，然后直接写入到HttpServletResponse中。

例子如下：Controller

@Controller
public class GreetController {
    private static final String template ="Hello, %s" ;
    private final AtomicLong counter = new AtomicLong();
 
    @RequestMapping("/greeting")
    @ResponseBody
    public Greeting greeting(@RequestParam(value = "name", defaultValue = "World")String name ) {
        return new Greeting(counter.incrementAndGet(), String.format(template, name));
    }
 
    @RequestMapping("/greeting1")
    public void greeting1(@RequestParam(value = "name",defaultValue = "world")String name, HttpServletResponse resp){
        Greeting greeting = new Greeting(counter.incrementAndGet(),String.format(template,name ));
        resp.setContentType("application/json");
        resp.setCharacterEncoding("UTF-8");
        try {
            PrintWriter writer = resp.getWriter() ;
            writer.print(JsonUtil.toJson(greeting));
        }catch (IOException e){
            e.printStackTrace();
        }
    }
 
    @RequestMapping(value = "/greeting2",method = RequestMethod.GET, produces = "application/json")
    @ResponseBody
    public String greeting2(@RequestParam(value = "name",defaultValue = "lisj")String name){
        Greeting greeting = new Greeting(counter.incrementAndGet(),String.format(template,name));
        return JsonUtil.toJson(greeting);
    }
 
}

　　JsonUtil如下：

public class JsonUtil {
    private static Gson gson = new Gson();
 
    public static String toJson(Object src){
        if (src== null){
            return gson.toJson(JsonNull.INSTANCE);
        }
        return gson.toJson(src);
    }
}