Pengenalan ke Spring DispatcherServlet

1. Perkenalan

Sederhananya, dalam pola desain Pengontrol Depan , satu pengontrol bertanggung jawab untuk mengarahkan HttpRequests yang masuk ke semua pengontrol dan penangan aplikasi lainnya .

DispatcherServlet Spring mengimplementasikan pola ini dan, oleh karena itu, bertanggung jawab untuk mengoordinasikan HttpRequests dengan benar ke handler kanan mereka.

Pada artikel ini, kita akan memeriksa alur kerja pemrosesan permintaan Spring DispatcherServlet dan cara menerapkan beberapa antarmuka yang berpartisipasi dalam alur kerja ini.

2. Pemrosesan Permintaan DispatcherServlet

Pada dasarnya, DispatcherServlet menangani HttpRequest yang masuk , mendelegasikan permintaan, dan memproses permintaan tersebut sesuai dengan antarmuka HandlerAdapter yang dikonfigurasi yang telah diterapkan dalam aplikasi Spring bersama dengan anotasi yang menyertai yang menentukan penangan, titik akhir pengontrol, dan objek respons.

Mari kita pelajari lebih dalam tentang bagaimana DispatcherServlet memproses sebuah komponen:

  • yang WebApplicationContext terkait dengan DispatcherServlet bawah kunci DispatcherServlet.WEB_APPLICATION_CONTEXT_ATTRIBUTE adalah mencari dan tersedia untuk semua elemen dari proses
  • The DispatcherServlet menemukan semua implementasi dari HandlerAdapter antarmuka dikonfigurasi untuk petugas operator menggunakan getHandler () - masing-masing menemukan dan dikonfigurasi menangani pelaksanaan permintaan melalui handle () melalui sisa proses
  • yang LocaleResolver adalah opsional terikat dengan permintaan untuk mengaktifkan unsur-unsur dalam proses untuk menyelesaikan lokal yang
  • yang ThemeResolver adalah opsional terikat permintaan untuk membiarkan unsur-unsur, seperti pandangan, menentukan tema untuk digunakan
  • jika MultipartResolver ditentukan, permintaan diperiksa untuk MultipartFile s - setiap yang ditemukan dibungkus dalam MultipartHttpServletRequest untuk diproses lebih lanjut
  • Implementasi HandlerExceptionResolver yang dideklarasikan di WebApplicationContext mengambil pengecualian yang dilempar selama pemrosesan permintaan

Anda dapat mempelajari lebih lanjut tentang semua cara untuk mendaftar dan menyiapkan DispatcherServlet di sini.

3. Antarmuka HandlerAdapter

The HandlerAdapter antarmuka memfasilitasi penggunaan pengendali, servlets, HttpRequests , dan HTTP jalur melalui beberapa antarmuka khusus. The HandlerAdapter antarmuka dengan demikian memainkan peran penting melalui banyak tahapan DispatcherServlet alur kerja memproses permintaan .

Pertama, setiap implementasi HandlerAdapter ditempatkan ke dalam HandlerExecutionChain dari metode getHandler () petugas operator Anda . Kemudian, masing-masing implementasi menangani () yang HttpServletRequest objek sebagai hasil rantai eksekusi.

Pada bagian berikut, kita akan menjelajahi beberapa HandlerAdapters yang paling penting dan umum digunakan secara lebih rinci.

3.1. Pemetaan

Untuk memahami pemetaan, pertama-tama kita perlu melihat bagaimana memberi anotasi pada pengontrol karena pengontrol sangat penting untuk antarmuka HandlerMapping .

The SimpleControllerHandlerAdapter memungkinkan untuk pelaksanaan controller secara eksplisit tanpa @Controller penjelasan.

The RequestMappingHandlerAdapter mendukung metode dijelaskan dengan @RequestMapping penjelasan .

Kita akan fokus pada anotasi @Controller di sini, tetapi sumber daya yang berguna dengan beberapa contoh menggunakan SimpleControllerHandlerAdapter juga tersedia.

The @RequestMapping penjelasan menetapkan titik akhir tertentu di mana penangan akan tersedia dalam WebApplicationContext yang terkait dengan itu.

Mari kita lihat contoh Pengontrol yang mengekspos dan menangani titik akhir '/ pengguna / contoh' :

@Controller @RequestMapping("/user") @ResponseBody public class UserController { @GetMapping("/example") public User fetchUserExample() { // ... } }

Jalur yang ditentukan oleh anotasi @RequestMapping dikelola secara internal melalui antarmuka HandlerMapping .

Struktur URL secara alami relatif terhadap DispatcherServlet itu sendiri - dan ditentukan oleh pemetaan servlet.

Jadi, jika DispatcherServlet dipetakan ke '/', semua pemetaan akan tercakup dalam pemetaan itu.

Namun, jika pemetaan servlet adalah ' / dispatcher ', maka setiap anotasi @ RequestMapping akan menjadi relatif terhadap URL root tersebut.

Ingatlah bahwa '/' tidak sama dengan '/ *' untuk pemetaan servlet! '/' adalah pemetaan default dan menampilkan semua URL ke area tanggung jawab operator.

‘/*' is confusing to a lot of newer Spring developers. It does not specify that all paths with the same URL context are under the dispatcher's area of responsibility. Instead, it overrides and ignores the other dispatcher mappings. So, ‘/example' will come up as a 404!

For that reason, ‘/*' shouldn't be used except in very limited circumstances (like configuring a filter).

3.2. HTTP Request Handling

The core responsibility of a DispatcherServlet is to dispatch incoming HttpRequests to the correct handlers specified with the @Controller or @RestController annotations.

As a side note, the main difference between @Controller and @RestController is how the response is generated – the @RestController also defines @ResponseBody by default.

A writeup where we go into much greater depth regarding Spring's controllers can be found here.

3.3. The ViewResolver Interface

A ViewResolver is attached to a DispatcherServlet as a configuration setting on an ApplicationContext object.

A ViewResolver determines both what kind of views are served by the dispatcher and from where they are served.

Here's an example configuration which we'll place into our AppConfig for rendering JSP pages:

@Configuration @EnableWebMvc @ComponentScan("com.baeldung.springdispatcherservlet") public class AppConfig implements WebMvcConfigurer { @Bean public UrlBasedViewResolver viewResolver() { UrlBasedViewResolver resolver = new UrlBasedViewResolver(); resolver.setPrefix("/WEB-INF/view/"); resolver.setSuffix(".jsp"); resolver.setViewClass(JstlView.class); return resolver; } }

Very straight-forward! There are three main parts to this:

  1. setting the prefix, which sets the default URL path to find the set views within
  2. the default view type which is set via the suffix
  3. setting a view class on the resolver which allows technologies like JSTL or Tiles to be associated with the rendered views

One common question involves how precisely a dispatcher's ViewResolverand the overall project directory structure are related. Let's take a look at the basics.

Here's an example path configuration for an InternalViewResolver using Spring's XML configuration:

For the sake of our example, we'll assume that our application is being hosted on:

//localhost:8080/

This is the default address and port for a locally hosted Apache Tomcat server.

Assuming that our application is called dispatcherexample-1.0.0, our JSP views will be accessible from:

//localhost:8080/dispatcherexample-1.0.0/jsp/

The path for these views within an ordinary Spring project with Maven is this:

src -| main -| java resources webapp -| jsp WEB-INF

The default location for views is within WEB-INF. The path specified for our InternalViewResolver in the snippet above determines the subdirectory of ‘src/main/webapp' in which your views will be available.

3.4. The LocaleResolver Interface

The primary way to customize session, request, or cookie information for our dispatcher is through the LocaleResolver interface.

CookieLocaleResolver is an implementation allowing the configuration of stateless application properties using cookies. Let's add it to AppConfig.

@Bean public CookieLocaleResolver cookieLocaleResolverExample() { CookieLocaleResolver localeResolver = new CookieLocaleResolver(); localeResolver.setDefaultLocale(Locale.ENGLISH); localeResolver.setCookieName("locale-cookie-resolver-example"); localeResolver.setCookieMaxAge(3600); return localeResolver; } @Bean public LocaleResolver sessionLocaleResolver() { SessionLocaleResolver localeResolver = new SessionLocaleResolver(); localeResolver.setDefaultLocale(Locale.US); localResolver.setDefaultTimeZone(TimeZone.getTimeZone("UTC")); return localeResolver; } 

SessionLocaleResolver allows for session-specific configuration in a stateful application.

The setDefaultLocale() method represents a geographical, political, or cultural region, whereas setDefaultTimeZone( ) determines the relevant TimeZone object for the application Bean in question.

Both methods are available on each of the above implementations of LocaleResolver.

3.5. The ThemeResolver Interface

Spring provides stylistic theming for our views.

Let's take a look at how to configure our dispatcher to handle themes.

First, let's set up all the configuration necessary to find and use our static theme files. We need to set a static resource location for our ThemeSource to configure the actual Themes themselves (Theme objects contain all of the configuration information stipulated in those files). Add this to AppConfig:

@Override public void addResourceHandlers(ResourceHandlerRegistry registry) { registry.addResourceHandler("/resources/**") .addResourceLocations("/", "/resources/") .setCachePeriod(3600) .resourceChain(true) .addResolver(new PathResourceResolver()); } @Bean public ResourceBundleThemeSource themeSource() { ResourceBundleThemeSource themeSource = new ResourceBundleThemeSource(); themeSource.setDefaultEncoding("UTF-8"); themeSource.setBasenamePrefix("themes."); return themeSource; } 

Requests managed by the DispatcherServlet can modify the theme through a specified parameter passed into setParamName() available on the ThemeChangeInterceptor object. Add to AppConfig:

@Bean public CookieThemeResolver themeResolver() { CookieThemeResolver resolver = new CookieThemeResolver(); resolver.setDefaultThemeName("example"); resolver.setCookieName("example-theme-cookie"); return resolver; } @Bean public ThemeChangeInterceptor themeChangeInterceptor() { ThemeChangeInterceptor interceptor = new ThemeChangeInterceptor(); interceptor.setParamName("theme"); return interceptor; } @Override public void addInterceptors(InterceptorRegistry registry) { registry.addInterceptor(themeChangeInterceptor()); } 

The following JSP tag is added to our view to make the correct styling appear:


     

The following URL request renders the example theme using the ‘theme' parameter passed into our configured ThemeChangeIntercepter:

//localhost:8080/dispatcherexample-1.0.0/?theme=example

3.6. The MultipartResolver Interface

A MultipartResolver implementation inspects a request for multiparts and wraps them in a MultipartHttpServletRequest for further processing by other elements in the process if at least one multipart is found. Add to AppConfig:

@Bean public CommonsMultipartResolver multipartResolver() throws IOException { CommonsMultipartResolver resolver = new CommonsMultipartResolver(); resolver.setMaxUploadSize(10000000); return resolver; } 

Now that we've configured our MultipartResolver bean, let's set up a controller to process MultipartFile requests:

@Controller public class MultipartController { @Autowired ServletContext context; @PostMapping("/upload") public ModelAndView FileuploadController( @RequestParam("file") MultipartFile file) throws IOException { ModelAndView modelAndView = new ModelAndView("index"); InputStream in = file.getInputStream(); String path = new File(".").getAbsolutePath(); FileOutputStream f = new FileOutputStream( path.substring(0, path.length()-1) + "/uploads/" + file.getOriginalFilename()); int ch; while ((ch = in.read()) != -1) { f.write(ch); } f.flush(); f.close(); in.close(); modelAndView.getModel() .put("message", "File uploaded successfully!"); return modelAndView; } }

We can use a normal form to submit a file to the specified endpoint. Uploaded files will be available in ‘CATALINA_HOME/bin/uploads'.

3.7. The HandlerExceptionResolver Interface

Spring's HandlerExceptionResolver provides uniform error handling for an entire web application, a single controller, or a set of controllers.

To provide application-wide custom exception handling, create a class annotated with @ControllerAdvice:

@ControllerAdvice public class ExampleGlobalExceptionHandler { @ExceptionHandler @ResponseBody public String handleExampleException(Exception e) { // ... } }

Any methods within that class annotated with @ExceptionHandler will be available on every controller within dispatcher's area of responsibility.

Implementations of the HandlerExceptionResolver interface in the DispatcherServlet's ApplicationContext are available to intercept a specific controller under that dispatcher's area of responsibility whenever @ExceptionHandler is used as an annotation, and the correct class is passed in as a parameter:

@Controller public class FooController{ @ExceptionHandler({ CustomException1.class, CustomException2.class }) public void handleException() { // ... } // ... }

The handleException() method will now serve as an exception handler for FooController in our example above if either exception CustomException1 or CustomException2 occurs.

Here's an article that goes more in-depth about exception handling in a Spring web application.

4. Conclusion

In this tutorial, we've reviewed Spring's DispatcherServlet and several ways to configure it.

As always, the source code used in this tutorial is available over on Github.