{"id":20961,"date":"2014-01-25T20:41:38","date_gmt":"2014-01-25T18:41:38","guid":{"rendered":"http:\/\/www.javacodegeeks.com\/?p=20961"},"modified":"2014-01-25T20:41:38","modified_gmt":"2014-01-25T18:41:38","slug":"understanding-spring-web-initialization","status":"publish","type":"post","link":"https:\/\/www.javacodegeeks.com\/2014\/01\/understanding-spring-web-initialization.html","title":{"rendered":"Understanding Spring Web Initialization"},"content":{"rendered":"

Few years ago majority of us were used to write XML config files everywhere, to setup even simple Java EE application. Today using Java or Groovy to configure projects is becoming preferred way – you just need to take a look at Gradle or functionalities introduced in further versions of the Spring Framework to gen up on this.<\/p>\n

Now I’ll deal with configuring Spring contexts for web application.<\/p>\n

Java EE provides ServletContainerInitializer<\/a> interface, which allows libraries to be notified of a web application startup. Since Spring 3.1<\/b> we have SpringServletContainerInitializer<\/a> class which handles WebApplicationInitializer<\/a> by instantiating all found classes implementing this interface, sorting them basing on @Order<\/a> annotation (non-annotated classes gets the highest possible order, so they are processed at the end) and invoking onStartup()<\/i> method.<\/p>\n

Spring since version 3.2<\/b> provides us a few classes implementing WebApplicationInitializer<\/a>\u00a0interface, from which first is AbstractContextLoaderInitializer<\/a>. This class included in spring-web module uses abstract createRootApplicationContext()<\/i> method to create application context, delegates it to ContextLoaderListener<\/a> which then is being registered in the ServletContext<\/a> instance. Creating application context using this class looks as follows:

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public class SpringAnnotationWebInitializer\r\n  extends AbstractContextLoaderInitializer {\r\n\r\n  @Override\r\n  protected WebApplicationContext createRootApplicationContext() {\r\n    AnnotationConfigWebApplicationContext applicationContext =\r\n      new AnnotationConfigWebApplicationContext();\r\n    applicationContext.register(SpringAnnotationConfig.class);\r\n    return applicationContext;\r\n  }\r\n\r\n}<\/pre>\n
That was the simplest way to start up Spring web context. But if we want to experience benefits provided by Spring MVC and don’t want to manually register DispatcherServlet<\/a> it’ll be better to use another class: AbstractDispatcherServletInitializer<\/a>. It extends previous class and adds two abstract methods: createServletApplicationContext()<\/i> and getServletMappings()<\/i>. \u00a0First method returns WebApplicationContext<\/a> that will be passed to DispatcherServlet<\/a>, which will be automatically added into\u00a0container ServletContext<\/a>. Please notice that this context will be established as a child of the context returned by createRootApplicationContext()<\/i> method. Second method – as you have probably already deduced – returns mappings that are used during servlet registration. You can also override getServletFilters()<\/i> method if you need any custom filters, because default implementation returns just empty array. Exemplary implementation using this class could be:<\/p>\n
public class SpringWebMvcInitializer\r\n  extends AbstractDispatcherServletInitializer {\r\n\r\n  @Override\r\n  protected WebApplicationContext createRootApplicationContext() {\r\n    AnnotationConfigWebApplicationContext applicationContext =\r\n      new AnnotationConfigWebApplicationContext();\r\n    applicationContext.register(SpringRootConfig.class);\r\n    return applicationContext;\r\n  }\r\n\r\n  @Override\r\n  protected WebApplicationContext createServletApplicationContext() {\r\n    AnnotationConfigWebApplicationContext applicationContext =\r\n      new AnnotationConfigWebApplicationContext();\r\n    applicationContext.register(SpringMvcConfig.class);\r\n    return applicationContext;\r\n  }\r\n\r\n  @Override\r\n  protected String[] getServletMappings() {\r\n    return new String[]{\"\/*\"};\r\n  }\r\n\r\n}<\/pre>\n
And now last but definitely not least class: AbstractAnnotationConfigDispatcherServletInitializer<\/a>. Here we can see further step in simplifying Spring initialization – we don’t need to manually create contexts but just set appropriate config classes in getRootConfigClasses()<\/i> and getServletConfigClasses()<\/i> methods. I hope you are already familiar with those names, because they works exactly like in the former case. Of course due to this class extends AbstractDispatcherServletInitializer<\/a>\u00a0we can still override getServletFilters()<\/i>. Finally we can implement our configuration in the\u00a0following way:<\/p>\n
public class SpringWebMvcSimpleInitializer\r\n  extends AbstractAnnotationConfigDispatcherServletInitializer {\r\n\r\n  @Override\r\n  protected Class<?>[] getRootConfigClasses() {\r\n    return new Class[] {SpringRootConfig.class};\r\n  }\r\n\r\n  @Override\r\n  protected Class<?>[] getServletConfigClasses() {\r\n    return new Class[] {SpringMvcConfig.class};\r\n  }\r\n\r\n  @Override\r\n  protected String[] getServletMappings() {\r\n    return new String[]{\"\/*\"};\r\n  }\r\n\r\n}<\/pre>\n