SpringMVC RESTful

1. RESTful CRUD实现流程

参数传递形式	RESTful风格
/getUser?id=1	GET /user/1
/updateUser?id=1	PUT /user/1
/deleteUser?id=1	DELETE /user/1
/saveUser	POST /user/1

1.1. 编写HTML

页面只能发送GET/POST请求。为了实现RESTful，SpringMVC提供了一种机制：页面可以发送POST请求，通过”_method”参数设置实际要发送的请求类型，SpringMVC会根据”_method”的值转变为对应的请求进行处理

<a href="/user/1">查询</a>
<form action="/user" method="post">
    <button type="submit">添加</button>
</form>
<form action="/user/1" method="post">
    <input type="hidden" name="_method" value="delete">
    <button type="submit">删除</button>
</form>
<form action="/user/1" method="post">
    <input type="hidden" name="_method" value="put">
    <button type="submit">更新</button>
</form>

1.2. 设置HiddenHttpMethodFilter（实现请求类型转变的原理）

浏览器 form 表单只支持 GET 与 POST 请求，而DELETE、PUT 等 method 并不支持，Spring3.0 添加了一个过滤器，可以将这些请求转换为标准的 http 方法，使得支持 GET、POST、PUT 与 DELETE 请求。

SpringMVC有一个内置的HiddenHttpMethodFilter，可以根据请求参数中的”_method”的值，转化为对应的请求方式，并进行处理

编辑web.xml，添加Filter

<filter>
    <filter-name>HiddenHttpMethodFilter</filter-name>
    <filter-class>org.springframework.web.filter.HiddenHttpMethodFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>HiddenHttpMethodFilter</filter-name>
    <!-- 拦截所有请求 -->
    <url-pattern>/*</url-pattern>
</filter-mapping>

1.3. 编写Controller

@Controller
public class UserController {
    @GetMapping("/user/{id}")
    public void getUser(@PathVariable Integer id) {
        System.out.println("查询id=" + id);
    }
    
    @PutMapping("/user/{id}")
    public void updateUser(@PathVariable Integer id) {
        System.out.println("更新id=" + id);
    }
    
    @DeleteMapping("/user/{id}")
    public void deleteUser(@PathVariable Integer id) {
        System.out.println("删除id=" + id);
    }
    
    @PostMapping("/user")
    public void saveUser() {
        System.out.println("添加");
    }
}

2. HiddenHttpMethodFilter源码分析（5.1.5.RELEASE）

public class HiddenHttpMethodFilter extends OncePerRequestFilter {
    /** 允许的请求方法：PUT/DELETE/PATCH */
    private static final List<String> ALLOWED_METHODS = Collections
            .unmodifiableList(Arrays.asList(HttpMethod.PUT.name(), HttpMethod.DELETE.name(), HttpMethod.PATCH.name()));

    /** 默认method参数的名称 Default method parameter: {@code _method}. */
    public static final String DEFAULT_METHOD_PARAM = "_method";

    private String methodParam = DEFAULT_METHOD_PARAM;

    /**
     * Set the parameter name to look for HTTP methods.
     * @see #DEFAULT_METHOD_PARAM
     */
    public void setMethodParam(String methodParam) {
        Assert.hasText(methodParam, "'methodParam' must not be empty");
        this.methodParam = methodParam;
    }

    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
            throws ServletException, IOException {

        HttpServletRequest requestToUse = request;

        // 只处理POST请求，即只允许POST请求转化为其它类型的请求
        if ("POST".equals(request.getMethod()) && request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) == null) {
            // 获取"_method"值
            String paramValue = request.getParameter(this.methodParam);
            // 判断值是否为空，为空就不处理
            if (StringUtils.hasLength(paramValue)) {
                // 统一将值转换为大写，如put==>PUT, delete==>DELETE 
                String method = paramValue.toUpperCase(Locale.ENGLISH);
                // 如果值是PUT/DELETE/PATCH，就将当前的HttpServletRequest转为HttpMethodRequestWrapper
                // HttpMethodRequestWrapper就是写在下面的静态内部类
                if (ALLOWED_METHODS.contains(method)) {
                    requestToUse = new HttpMethodRequestWrapper(request, method);
                }
            }
        }
        // 最后统一放行所有请求
        filterChain.doFilter(requestToUse, response);
    }

    /**
     * HttpMethodRequestWrapper 继承了HttpServletRequest
     *     1、传入一个method参数，该值就是之前处理过的PUT/DELETE/PATH
     *  2、重载了getMethod方法
     */
    /**
     * Simple {@link HttpServletRequest} wrapper that returns the supplied method for
     * {@link HttpServletRequest#getMethod()}.
     */
    private static class HttpMethodRequestWrapper extends HttpServletRequestWrapper {

        private final String method;

        public HttpMethodRequestWrapper(HttpServletRequest request, String method) {
            super(request);
            this.method = method;
        }

        @Override
        public String getMethod() {
            return this.method;
        }
    }

}