JAVA RESTful WebService实战笔记(三)

前言

AOP(Aspect oriented Programming,面向切面编程),其实现原理就是代理被调用的方法,在其被执行的方法前后,增加额外的业务功能,AOP的实现机制就是通过注解或者XML配置,就这些配置,动态的生成字节码(bytecode).使被调用代码对应的字节码被环绕注入新的功能;或者使用Java的动态代理机制,完成对被调用方法的增强.

REST请求流程

在以下的图中,请求流程中存在3种角色,分别是用户、REST客户端和REST服务器。请求始于请求的发送,止于调用Response类的readeEntity()方法,获取响应实体。

• 1、用户提交请求数据,客户端接收请求,进入第一个扩展点:”客户端请求过滤器ClientRequestFilter实现类“的filter()方法。
• 2、请求过滤器处理完毕之后,流程进入第二个扩展点:”客户端写拦截器WriterInterceptor实现类”的aroundWriteTo()方法,实现对客户端序列化操作的拦截
• 3、”客户端消息体写处理器MessageBodyWriter“执行序列化,流程从客户端过渡到服务器端
• 4、服务器接收请求,流程进入第三个扩展点:”服务器前置请求过滤器ContainerRequestFilter实现类“的filter()方法。
• 5、过滤处理完毕后,服务器根据请求匹配资源方法,如果匹配到相应的资源方法,流程进入第四个扩展点:”服务器后置请求过滤器ContainerRequestFilter实现类“的filter()方法
• 6、后置请求过滤器处理完毕之后,流程进入第五个扩展点:”服务器读拦截器ReaderInterceptor实现类的”aroundReadFrom()方法,拦截服务器端反序列化操作
• 7、”服务器消息体读处理器MessageBodyReader”完成对客户端数据流的反序列化。服务器执行匹配的资源方法
• 8、REST请求资源处理完毕之后,流程进入第六个扩展点:”服务器相应过滤器ContainerResponseFilter实现类“的filter()方法
• 9、过滤器处理完毕之后,流程进入第七个扩展点:”服务器写拦截器WriterInterceptor实现类“的aroundWriteTo()方法,对服务器端序列化到客户端这个操作的拦截
• 10、”服务器消息体写处理器MessageBodyWriter“执行序列化,流程返回到客户端一侧。
• 11、客户端接收响应,流程进入第八个扩展点:”客户端响应过滤器ClientResponseFilter实现类”的filter()方法
• 12、过滤器处理完毕后,客户端响应实例response返回到用户一侧,用户执行response.readEntity()流程进入第九个扩展点:”客户端读取拦截器ReaderInterceptor实现类“的aroundReadFrom()方法,对客户端反序列化进行拦截
• 13、”客户端消息体赌徒处理器MessageBodyReader”执行反序列化,将Java类型的对象最终作为readEntity()方法的返回值。到此,一次REST请求处理器的完整流程完毕
  

  这期间如果出现异常或者资源不匹配情况,会从出错点开始结束流程

REST过滤器

1、ClientRequestFilter

• 客户端请求过滤器(ClientRequestFilter)定义的过滤方法filter()包含一个输入参数,是客户端请求的上下文类ClientRequestFilter。从该上下文中可以获取请求信息,典型的示例包括获取请求context.getMethod(),获取请求资源地址context.getUri()和获取请求头信息context.getHeaders()等。过滤器的实现类中可以利用这些信息,覆写改方法以实现特有的过滤功能。ClientRequestFilter接口的实现类如下:

2、ContainerRequestFilter

• 针对过滤切面,服务器请求过滤接口ContainerRequestFilter的实现类可以定义为预处理和后处理,默认情况下,采用后处理方式。及先执行容器接收请求操作.当服务器接收并处理请求后.流程才进入过滤器实现类的filter()方法。而预处理是在服务器处理接收到的请求之前就执行过滤。如果希望实现一个预处理的过滤器实现类,需要在类名上定义注解@PreMatching

• 服务器请求过滤器定义的过滤方法filter()包含一个输入参数,即容器请求上下文类ContainerRequestContext。ContainerRequestFilter接口的实现类如下:
  

• 如下代码展示了ContainerRequestFilter接口的实现类,我们以CsrfProtectionFilter为例来说明,示例代码如下所示:

public class CsrfProtectionFilter implements ClientRequestFilter {    /**     * Name of the header this filter will attach to the request.     */    public static final String HEADER_NAME = "X-Requested-By";    private static final Set<String> METHODS_TO_IGNORE;    static {        HashSet<String> mti = new HashSet<String>();        mti.add("GET");        mti.add("OPTIONS");        mti.add("HEAD");        METHODS_TO_IGNORE = Collections.unmodifiableSet(mti);    }    private final String requestedBy;    /**     * Creates a new instance of the filter with X-Requested-By header value set to empty string.     */    public CsrfProtectionFilter() {        this("");    }    /**     * Initialized the filter with a desired value of the X-Requested-By header.     *     * @param requestedBy Desired value of X-Requested-By header the filter     *                    will be adding for all potentially state changing requests.     */    public CsrfProtectionFilter(final String requestedBy) {        this.requestedBy = requestedBy;    }    @Override    public void filter(ClientRequestContext rc) throws IOException {        if (!METHODS_TO_IGNORE.contains(rc.getMethod()) && !rc.getHeaders().containsKey(HEADER_NAME)) {            rc.getHeaders().add(HEADER_NAME, requestedBy);        }    }}

上述代码中,CsrfProtectionFilter定义了一个特殊的头信息”X-Requested-By”和CSRF忽略监控的方法集合。在过滤器的filter()方法中,首先从上下文中获取头信息rc.getHeaders()和请求方法信息rc.getMethod(),然后判断头信息是否包含“X-Requested-By”,方法信息是否是安全的请求方法,即”GET”、”OPTIONS”或”HEAD”,如果两个条件不成立,过滤器会抛出一个运行时异常BadRequestException

3、ContainerResponseFilter

• 服务器响应过滤器接口ContainerResponseFilter定义的过滤方法filter()包含两个输入参数,一个是容器请求上下文类ContainerRequestContext,另一个是容器响应上下文类ContainerResponseContext。ContainerResponseFilter接口的实现类如下图所示:

我们通过EncodingFilter为例来说明。

 @Override    public void filter(ContainerRequestContext request, ContainerResponseContext response) throws IOException {        if (!response.hasEntity()) {            return;        }        // add Accept-Encoding to Vary header        List<String> varyHeader = response.getStringHeaders().get(HttpHeaders.VARY);        if (varyHeader == null || !varyHeader.contains(HttpHeaders.ACCEPT_ENCODING)) {            response.getHeaders().add(HttpHeaders.VARY, HttpHeaders.ACCEPT_ENCODING);        }        // if Content-Encoding is already set, don't do anything        if (response.getHeaders().getFirst(HttpHeaders.CONTENT_ENCODING) != null) {            return;        }        // retrieve the list of accepted encodings        List<String> acceptEncoding = request.getHeaders().get(HttpHeaders.ACCEPT_ENCODING);        // if empty, don't do anything        if (acceptEncoding == null || acceptEncoding.isEmpty()) {            return;        }        // convert encodings from String to Encoding objects        List<ContentEncoding> encodings = Lists.newArrayList();        for (String input : acceptEncoding) {            String[] tokens = input.split(",");            for (String token : tokens) {                try {                    ContentEncoding encoding = ContentEncoding.fromString(token);                    encodings.add(encoding);                } catch (ParseException e) {                    // ignore the encoding that could not parse                    // but log the exception                    Logger.getLogger(EncodingFilter.class.getName()).log(Level.WARNING, e.getLocalizedMessage(), e);                }            }        }        // sort based on quality parameter        Collections.sort(encodings);        // make sure IDENTITY_ENCODING is at the end (since it accepted if not explicitly forbidden        // in the Accept-Content header by assigning q=0        encodings.add(new ContentEncoding(IDENTITY_ENCODING, -1));        // get a copy of supported encoding (we'll be modifying this set, hence the copy)        SortedSet<String> acceptedEncodings = Sets.newTreeSet(getSupportedEncodings());        // indicates that we can pick any of the encodings that remained in the acceptedEncodings set        boolean anyRemaining = false;        // final resulting value of the Content-Encoding header to be set        String contentEncoding = null;        // iterate through the accepted encodings, starting with the highest quality one        for (ContentEncoding encoding : encodings) {            if (encoding.q == 0) {                // ok, we are down at 0 quality                if ("*".equals(encoding.name)) {                    // no other encoding is acceptable                    break;                }                // all the 0 quality encodings need to be removed from the accepted ones (these are explicitly                // forbidden by the client)                acceptedEncodings.remove(encoding.name);            } else {                if ("*".equals(encoding.name)) {                    // any remaining encoding (after filtering out q=0) will be acceptable                    anyRemaining = true;                } else {                    if (acceptedEncodings.contains(encoding.name)) {                        // found an acceptable one -> we are done                        contentEncoding = encoding.name;                        break;                    }                }            }        }        if (contentEncoding == null) {            // haven't found any explicit acceptable encoding, let's see if we can just pick any of the remaining ones            // (if there are any left)            if (anyRemaining && !acceptedEncodings.isEmpty()) {                contentEncoding = acceptedEncodings.first();            } else {                // no acceptable encoding can be sent -> return NOT ACCEPTABLE status code back to the client                throw new NotAcceptableException();            }        }        // finally set the header - but no need to set for identity encoding        if (!IDENTITY_ENCODING.equals(contentEncoding)) {            response.getHeaders().putSingle(HttpHeaders.CONTENT_ENCODING, contentEncoding);        }    }

EncodingFilter过滤器的filter()方法通过对请求头信息”Accept-Encoding”的分析,先后为响应头信息”Vary”和”Content-Encoding”赋值,以实现编码部分的内容协商。

4、ClientResponseFilter

客户端响应过滤器(ClientResponseFilter)定义的过滤方法filter()包含两个参数,一个是客户端请求上下文类ClientRequestContext,另一个是客户端响应的上下文类ClientResponseContext。ClientResponseFilter接口的实现类如下图所示:

我们以HTTP摘要认证过滤器类HttpAuthenticationFilter为例。

 @Override    public void filter(ClientRequestContext request) throws IOException {        if ("true".equals(request.getProperty(REQUEST_PROPERTY_FILTER_REUSED))) {            return;        }        if (request.getHeaders().containsKey(HttpHeaders.AUTHORIZATION)) {            return;        }        Type operation = null;        if (mode == HttpAuthenticationFeature.Mode.BASIC_PREEMPTIVE) {            basicAuth.filterRequest(request);            operation = Type.BASIC;        } else if (mode == HttpAuthenticationFeature.Mode.BASIC_NON_PREEMPTIVE) {            // do nothing        } else if (mode == HttpAuthenticationFeature.Mode.DIGEST) {            if (digestAuth.filterRequest(request)) {                operation = Type.DIGEST;            }        } else if (mode == HttpAuthenticationFeature.Mode.UNIVERSAL) {            Type lastSuccessfulMethod = uriCache.get(getCacheKey(request));            if (lastSuccessfulMethod != null) {                request.setProperty(REQUEST_PROPERTY_OPERATION, lastSuccessfulMethod);                if (lastSuccessfulMethod == Type.BASIC) {                    basicAuth.filterRequest(request);                    operation = Type.BASIC;                } else if (lastSuccessfulMethod == Type.DIGEST) {                    if (digestAuth.filterRequest(request)) {                        operation = Type.DIGEST;                    }                }            }        }        if (operation != null) {            request.setProperty(REQUEST_PROPERTY_OPERATION, operation);        }    }