Java Web应用技术

Jakarta EE Servlet API

Servlet+Gradle实例：

  Java Servlet 技术入门

                    plugins {
                        id "java"
                        id "org.gretty" version "4.1.1"
                    }
                    repositories { mavenCentral() }
                    dependencies {
                        compileOnly 'jakarta.servlet:jakarta.servlet-api:6.0.0'
                    }
                
                    //  首页位置 - src/main/webapp/index.html
                    //  http://localhost:8080/app/xx/any
                    //@MultipartConfig(maxFileSize = 1024 * 1024 * 10) // form上传模式enctype="multipart/form-data"; 注意 - GAE getParts()时报"Error: bad multipart"可用commons-fileupload2-jakarta-servlet6库
                    @WebServlet(urlPatterns = {"/x", "/xx/*"}) // urlPatterns等同value = {"/y", "/yy/*"}，但两者互斥。
                    public class NewClass extends HttpServlet {
                      @Override
                      protected void doGet(HttpServletRequest request, HttpServletResponse response)
                            throws ServletException, IOException {
                        try ( var out = response.getWriter()) {
                            out.println(request.getServletPath());
                        }
                      }
                    }

Servlet的异步和非阻塞:

                    //  http://localhost:8080/gradle-servlet-template/aa/any
                    //  若配置了 loadOnStartup = 1 则也必须配置 value = "/must-be-specified"，web.xml则为可选。
                    @WebServlet(asyncSupported = true, urlPatterns = {"/a", "/a/*"})
                    public class NewServlet extends HttpServlet {
                      private static final ExecutorService ES = Executors.newFixedThreadPool(9);
                      @Override
                      protected void doGet(HttpServletRequest request, HttpServletResponse response)
                            throws ServletException, IOException {
                        // 或指定非当前Servlet的请求响应入参；注意 - GAE不支持无入参的startAsync()。
                        var ac = request.startAsync(); // request.startAsync(request, response);
                        ac.setTimeout(10 * 1000);

                        // ac.start(() -> {...}); // 或 new Thread(() -> {...}).start(); // 线程利用率无提升，应换用线程池：
                        ES.execute(() -> {
                            try {
                                TimeUnit.SECONDS.sleep(2); // 模拟业务耗时
                                ac.getResponse().getWriter().write("ok");
                                ac.complete(); // 通知异步上下文请求处理完成
                            } catch (IOException | InterruptedException e) {
                                System.err.println(e);
                            }
                        });
                
                      }
                    }

                    //  http://localhost:8080/gradle-servlet-template/aa/any
                    @WebServlet(asyncSupported = true, urlPatterns = {"/a", "/a/*"})
                    public class NewServlet extends HttpServlet {
                      private static final ExecutorService ES = Executors.newFixedThreadPool(9);
                      @Override
                      protected void doGet(HttpServletRequest request, HttpServletResponse response)
                            throws ServletException, IOException {

                       if (request.getDispatcherType() == DispatcherType.ASYNC) { response.getWriter().write("from ac.dispatch()."); } else {
                        var ac = request.isAsyncStarted() ? request.getAsyncContext() : request.startAsync(request, response);
                        ac.setTimeout(10 * 1000);

                        //ac.addListener(new AsyncListener() {});
                        // 异步非阻塞方式读取或响应HTTP Body大尺寸内容
                        //var is = request.getInputStream();
                        //is.setReadListener(new MyRL(is, asyncContext));
                        //var os = response.getOutputStream();
                        //os.setWriteListener(new MyWL(os, asyncContext));
                        // 若业务线程需要Body参数可在MyRL的onAllDataRead()回调中拿取

                        // new Thread(() -> {...}).start(); // 线程利用率无提升，应换用线程池：
                        ES.execute(() -> { // 或 ac.start(() -> {...});
                            try {
                                TimeUnit.SECONDS.sleep(2); // 模拟业务耗时
                                //if (complete.compareAndSet(false, true)){ ac.complete(); } // 避免已完成。
                                ac.getResponse().getWriter().write("ok");
                                ac.complete(); // 通知异步上下文请求处理完成
                                //ac.dispatch(); // 或转发至当前Servlet
                            } catch (IOException | InterruptedException e) {
                                System.err.println(e);
                            }
                        });
                       }
                      }
                    }

    启动 - gradle appRun
注意 - 执行前确保 src\main\webapp\ 网站根目录存在。

  extends HttpServlet的doGet(...)方法并删掉IDE生成的响应了SC_METHOD_NOT_ALLOWED的super.doGet(req, resp);
  输出文本 - response.getWriter().append("hi！");
  响应状态- resp.setStatus(HttpServletResponse.SC_MOVED_TEMPORARILY); resp.addHeader("Location", "/"); // 永久转向用SC_MOVED_PERMANENTLY
  转向语句r.sendRedirect(path)会附加全URL至302临时转向的Location头

注入Servlet对象：@Context private HttpServletRequest request;

响应返回值：
  说明 - 
    return Response.ok().build() 比 return "非泛型对象" 更细节可控。
    默认响应HTML，其他MIME则必须写注解 @Produces(MediaType.APPLICATION_JSON) 或 Response.ok().type(MediaType.TEXT_PLAIN).build()
    抛异常 - throw new WebApplicationException(Response.Status.NOT_FOUND); 或 @Provider ... implements ExceptionMapper


    @GET
    @Path("http-204-no-content")
    public void void204() { // HTTP/1.1 204 No Content
        System.out.println("等同 return null; 或 return Response.noContent().build();");
    }

    @GET
    @Path("http-200")
    @Produces(MediaType.APPLICATION_JSON)
    public Response http200() {
        return Response.ok().build() // 返回http 200但无body; 或 return Response.ok("body对象").build()
        //return Response.ok(new GenericEntity<List<String>>( List.of("解决泛型擦除问题") ){}).build();
    }
    public record ResultRecord(String text) { } // 支持响应record对象


    @PUT
    @Path("upload")
    @Produces(MediaType.APPLICATION_OCTET_STREAM)
    @Consumes(MediaType.MULTIPART_FORM_DATA)
    public Response upload(@FormParam("uploadedFile") EntityPart uf) {
        System.out.println(uf.getFileName().orElse(null));

        // 或 fis.transferTo(os); 底层基于缓冲型的Transfer-Encoding: chunked
        StreamingOutput so = (os) -> { try (os) { os.write("xyz".getBytes()); } };
        return Response.ok(so).build();

        // Quarkus则必须用不注解的(MultipartFormDataInput uf)入参：
        //System.out.println(uf.getFormDataMap().get("uploadedFile").getFirst());

        // 让浏览器下载另存为(若设定了mimeType则无需@Produces注解)：
        //return Response.ok((Object) new File("x.txt"))
        //  .header("Content-Disposition", "attachment; filename=\"download.txt\"").type(MediaType.TEXT_PLAIN).build();
    }

JSF大部分情况下，只需对小于号转义： 将 < 写为 &lt;
整块儿转义：
    <script>//<![CDATA[
        document.getElementById("x").innerHTML = "<a target='_blank' href='https://example.com/?" + 1 + "'>" + document.title + "</a><br />";
    //]]> console.log("CDATA之外脚本");</script>

转义终极解决办法 - 将脚本独立为x.js文件！
或 x.innerHTML = `<a target='_blank' href='https://example.com/path/` + x.id + `'>` + x.name + `</a>`;
转义用 HTML编辑器

Expression Language (EL) 前缀 & 的实体引用必须以分号(;)结尾，否则报“实体名称必须紧跟在 '&' 后面”；符号&自身的实体引用写为“&amp;”。
  若用到 && 符号，则可换为单词 and 替代：
    <ui:fragment rendered="#{!request.getServerName().equals('localhost') and request.getHeader('X-Real-IP') != null}">x</ui:fragment>

其他：
  解决中文乱码(Java18后则不需要了/默认已是UTF-8) - response.setContentType("text/plain; charset=utf-8");