Struts2 インタビューの質問と回答

Struts2は、Javaでウェブアプリケーションを開発するための有名なフレームワークの一つです。最近、私はたくさんのStruts2チュートリアルを書いてきましたが、この投稿では、面接で役立ついくつかの重要なStruts2のインタビュー問題と回答をリストアップしています。

Struts2のインタビューの質問

1. Struts2とは何ですか？

 

1. Struts1とStruts2の違いや、Struts2がStruts1より優れている点は何ですか？

 

1. Struts2のコアコンポーネントは何ですか？

 

1. Struts2のインターセプターとは何ですか？

 

1. Struts2インターセプターではどのデザインパターンが実装されていますか？

 

1. Struts2でActionクラスを作成する方法は何種類ありますか？

 

1. Struts2のアクションとインターセプターはスレッドセーフですか？

 

1. Struts2のフロントコントローラークラスは何ですか？

 

1. Struts2のインターセプターのメリットは何ですか？

 

1. ValueStackとOGNLとは何ですか？

 

1. Struts2で導入された便利なアノテーションをいくつか挙げてください。

 

1. 利用したことのある重要なStruts2の定数をいくつか教えてください。

 

1. Struts2のアクションマッピングにおける名前空間の利用目的は何ですか？

 

1. リクエストパラメータをアクションクラスのJava Beanのプロパティにマッピングするのに責任を持つインターセプターはどれですか？

 

1. i18nサポートを提供するのに責任を持つインターセプターはどれですか？

 

1. アクションクラスにおいてActionインターフェースとActionSupportクラスを使用することの違いは何ですか？どちらをお好みですか？

 

1. アクションクラスでServlet APIのRequest、Response、HttpSessionなどのオブジェクトを取得する方法は何ですか？

 

1. execAndWaitインターセプターの使用目的は何ですか？

 

1. tokenインターセプターの使用目的は何ですか？

 

1. Struts2アプリケーションにlog4jを統合する方法は何ですか？

 

1. Struts2の異なるタグは何ですか？どのように使用しますか？

 

1. Struts2でのカスタムタイプコンバーターとは何ですか？

 

1. 自分でインターセプターを作成し、アクションにマップする方法は何ですか？

 

1. インターセプターのライフサイクルは何ですか？

 

1. インターセプタースタックとは何ですか？

 

1. struts-defaultパッケージとは何で、その利点は何ですか？

 

1. Struts2アクションURIのデフォルトサフィックスは何ですか？それを変更する方法はありますか？

 

1. 結果ページのデフォルトの場所はどこですか？それを変更する方法はありますか？

 

1. Struts2アプリケーションでファイルをアップロードする方法は何ですか？

 

1. Struts2アプリケーションを開発する際のベストプラクティスは何ですか？

 

アプリケーションがスローした例外をStruts2でどのように処理しますか？

Struts2の面接質問と回答

What is Struts2?
Apache Struts2 is an open source framework to build web applications in Java. Struts2 is based on OpenSymphony WebWork framework. It’s highly improved from Struts1 and that makes it more flexible, easy to use and extend. The core components of Struts2 are Action, Interceptors and Result pages. Struts2 provides many ways to create Action classes and configure them via struts.xml or through annotations. We can create our own interceptors for common tasks. Struts2 comes with a lot of tags and uses OGNL expression language. We can create our own type converters to render result pages. Result pages can be JSPs and FreeMarker templates.

What are the differences between Struts1 and Struts2 or how Struts2 is better than Struts1?
Struts2 is designed to overcome the shortcomings of Struts1 and to make it more flexible, extendable. Some of the noticeable differences are:

Components
Struts1
Struts2

Action Classes
Struts1 action classes are forced to extend an Abstract Class that makes it not extendable.
Struts2 action classes flexible and we can create them by implementing Action interface, extending ActionSupport class or just by having execute() method.

Thread Safety
Struts1 Action Classes are Singleton and not thread safe, that makes extra care on developer side to avoid any side effects because of multithreading.
Struts2 action classes gets instantiated per request, so there is no multithreading and makes them thread safe.

Servlet API coupling
Struts1 APIs are tightly coupled with Servlet API and Request and Response objects are passed to action classes execute() method.
Struts2 API is loosely coupled with Servlet API and automatically maps the form bean data to action class java bean properties that we mostly use. If however we need reference to Servlet API classes, there are *Aware interfaces for that.

Testing
Struts1 action classes are hard to test because of Servlet API coupling.
Struts2 Action classes are like normal java classes and we can test them easily by instantiating them and setting their properties.

Request Parameters mapping
Struts1 requires us to create ActionForm classes to hold request params and we need to configure it in the struts configuration file.
Struts2 request params mapping is done on the fly and all we need is to have java bean properties in action classes or implement ModelDriven interface to provide the java bean class name to be used for mapping.

Tag Support
Struts1 uses JSTL Tags and hence are limited.
Struts2 uses OGNL and provide different kinds of UI, Control and Data Tags. It’s more versatile and easy to use.

Validation
Struts1 supports validation through manual validate() method
Struts2 support both manual validation as well as Validation framework integration.

Views Rendering
Struts1 uses standard JSP technology for providing bean values to JSP pages for views.
Struts2 uses ValueStack to store request params and attributes and we can use OGNL and Struts2 tags to access them.

Modules support
Struts1 modules are complex to design and looks like separate projects
Struts2 provides “namespace” configuration for packages to easily create modules.

What are Struts2 core components?
Struts2 core components are:

Action Classes
Interceptors
Result Pages, JSP of FreeMarker templates
ValueStack, OGNL and Tag Libraries

1. Struts2のインターセプタとは何ですか？

 

1. インターセプタはStruts2フレームワークの中核です。Struts2のインターセプタは、リクエストパラメータをアクションクラスに渡したり、Servlet APIのリクエスト、レスポンス、セッションをアクションクラスで利用可能にしたり、バリデーション、i18nサポートなど、フレームワークによるほとんどの処理を担当しています。ActionInvocationは、アクションクラスとインターセプタをカプセル化し、順番に実行する役割を担っています。ActionInvocationで最も重要なメソッドはinvoke()メソッドであり、インターセプタのチェーンを追跡し、次のインターセプタまたはアクションを呼び出します。これは、Java EEフレームワークのChain of Responsibilityパターンの最も良い例の一つです。

Struts2のインターセプタはどのデザインパターンを実装していますか？

Struts2 interceptors are based on intercepting filters design pattern. The invocation of interceptors in interceptor stack closely resembles Chain of Responsibility design pattern.

Struts2でアクションクラスを作成するための異なる方法は何ですか？

Struts2 provide different ways to create action classes.
1.  By implementing Action interface
2.  Using Struts2 @Action annotation
3.  By extending ActionSupport class
4.  Any normal java class with execute() method returning String can be configured as Action class.

Struts2 のアクションとインターセプターはスレッドセーフですか？

Struts2 Action classes are thread safe because an object is instantiated for every request to handle it. Struts2 interceptors are singleton classes and a new thread is created to handle the request, so it's not thread safe and we need to implement them carefully to avoid any issues with shared data.

ストラット2のフロントコントローラーのクラスはどれですか？

`org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter` is the Front Controller class in Struts2 and every request processing starts from this class. Earlier versions of Struts2 uses `org.apache.struts2.dispatcher.FilterDispatcher` as Front Controller class.

Struts2におけるインターセプターの利点は何ですか？

Some of the benefits of interceptors are:
-   Interceptor plays a crucial role in achieving high level of separation of concerns.
-   Struts2 interceptors are configurable, we can configure it for any action we want.
-   We can create our own custom interceptors to perform some common tasks such as request params logging, authentication etc. This helps us in taking care of common tasks at a single location, achieving low maintenance cost.
-   We can create interceptors stack to use with different actions.

ValueStackとは何ですか？ OGNLとは何ですか？

ValueStack is the storage area where the application data is stored by Struts2 for processing the client requests. The data is stored in `ActionContext` objects that use ThreadLocal to have values specific to the particular request thread. Object-Graph Navigation Language (OGNL) is a powerful Expression Language that is used to manipulate data stored on the ValueStack. As you can see in architecture diagram, both interceptors and result pages can access data stored on ValueStack using OGNL.

Struts2で導入された便利なアノテーションをいくつか挙げてください。

Some of the important annotations introduced in Struts2 are:
1.  @Action to create action class
2.  @Actions to configure single class for multiple actions
3.  @Namespace and @Namespaces for creating different modules
4.  @Result for result pages
5.  @ResultPath for configuring result pages location

使用した重要なStruts2の定数を教えてください。

Some of the Struts2 constants that I have used are:

1.  **struts.devMode** to run our application in development mode. This mode does reload properties files and provides extra logging and debugging feature. It's very useful while developing our application but we should turn it off while moving our code to production.
2.  **struts.convention.result.path** to configure the location of result pages. By default Struts2 look for result pages at {WEBAPP-ROOT}/{Namespace}/ and we can change the location with this constant.
3.  **struts.custom.i18n.resources** to define global resource bundle for i18n support.
4.  **struts.action.extension** to configure the URL suffix to for Struts2 application. Default suffix is .action but sometimes we might want to change it to .do or something else.

We can configure above constants in the struts.xml file like below.

```
<constant name="struts.devMode" value="true"></constant>
<constant name="struts.action.extension" value="action,do"></constant>
<constant name="struts.custom.i18n.resources" value="global"></constant>
<constant name="struts.convention.result.path" value="/"></constant>
```

Struts2におけるアクションマッピングでの名前空間の使い方は何ですか？

Struts2 namespace configuration allows us to create modules easily. We can use namespace to separate our action classes based on their functionality, for example admin, user, customer etc.

どのインターセプターが、リクエストパラメーターをアクションクラスのJava Beanプロパティにマッピングする責任を持っていますか？

`com.opensymphony.xwork2.interceptor.ParametersInterceptor` interceptor is responsible for mapping request parameters to the Action class java bean properties. This interceptor is configured in struts-default package with name "params". This interceptor is part of basicStack and defaultStack interceptors stack.

i18n サポートを担当しているインターセプターはどれですか？

`com.opensymphony.xwork2.interceptor.I18nInterceptor` interceptor is responsible for i18n support in Struts2 applications. This interceptor is configured in struts-default package with name "i18n" and it's part of i18nStack and defaultStack.

私たちのアクションクラスでActionインターフェースとActionSupportクラスを使う場合の違いは何ですか？どちらをお好みですか？

We can implement Action interface to create our action classes. This interface has a single method execute() that we need to implement. The only benefit of using this interface is that it contains some constants that we can use for result pages, these constants are SUCCESS, ERROR, NONE, INPUT and LOGIN. ActionSupport class is the default implementation of Action interface and it also implements interfaces related to Validation and i18n support. ActionSupport class implements Action, Validateable, ValidationAware, TextProvider and LocaleProvider interfaces. We can override the validate() method of ActionSupport class to include field level validation login in our action classes. Depending on the requirements, we can use any of the approaches to creating Struts 2 action classes, my favorite is ActionSupport class because it helps in writing validation and i18n logic easily in action classes.

アクションクラスでServlet APIのRequest、Response、HttpSessionなどのオブジェクトを取得する方法はありますか？

Struts2 action classes don't provide direct access to Servlet API components such as Request, Response, and Session. However, sometimes we need these access in action classes such as checking HTTP method or setting cookies in response. That's why Struts2 API provides a bunch of \*Aware interfaces that we can implement to access these objects. Struts2 API uses dependency injection to inject Servlet API components in action classes. Some of the important Aware interfaces are SessionAware, ApplicationAware, ServletRequestAware, and ServletResponseAware. You can read more about them in How to get [Servlet API Session in Struts2 Action Classes](/community/tutorials/get-servlet-session-request-response-context-attributes-struts-2-action) tutorial.

execAndWaitインターセプターの使用方法は何ですか？

Struts2 provides execAndWait interceptor for long running action classes. We can use this interceptor to return an intermediate response page to the client and once the processing is finished, final response is returned to the client. This interceptor is defined in the struts-default package and implementation is present in `ExecuteAndWaitInterceptor` class. Check out [Struts2 execAndWait interceptor example](/community/tutorials/struts2-execandwait-interceptor-example-for-long-running-actions) to learn more about this interceptor and how to use it.

Struts2におけるトークンインターセプタの目的は何ですか？

One of the major problems with web applications is the double form submission. If not taken care, double form submission could result in charging double amount to customer or updating database values twice. We can use a token interceptor to solve the double form submission problem. This interceptor is defined in the struts-default package but it's not part of any interceptor stack, so we need to include it manually in our action classes. Read more at [Struts2 token interceptor](/community/tutorials/struts2-token-interceptor-example) example.

ストラッツ2アプリケーションにおいて、どのようにlog4jを統合することができますか？

Struts2 provides easy integration of log4j API for logging purpose, all we need to have is log4j configuration file in the WEB-INF/classes directory. You can check out the sample project at [Struts2 Log4j integration](/community/tutorials/struts2-and-log4j-integration-example-project).

Struts2の異なるタグは何ですか？どのように使用することができますか？

Struts2 provides a lot of custom tags that we can use in result pages to create views for client request. These tags are broadly divided into three categories- Data tags, Control tags and UI tags. We can use these tags by adding these in JSP pages using taglib directive.

```
<%@ taglib uri="/struts-tags" prefix="s" %>
```

Some of the important Data tags are property, set, push, bean, action, include, i18n and text tag. Read more at [Struts2 Data Tags](/community/tutorials/struts-2-data-tags-example-tutorial). Control tags are used for manipulation and navigation of data from a collection. Some of the important Control tags are if-elseif-else, iterator, append, merge, sort, subset and generator tag. Read more at [Struts2 Control Tags](/community/tutorials/struts-2-control-tags-example-tutorial). Struts2 UI tags are used to generate HTML markup language, binding HTML form data to action classes properties, type conversion, validation, and i18n support. Some of the important UI tags are form, textfield, password, textarea, checkbox, select, radio and submit tag. Read more about them at [Struts2 UI Tags](/community/tutorials/struts-2-ui-tags-form-checkbox-radio-select-submit).

ストラット2では、カスタムタイプコンバータとは何ですか？

Struts2 support OGNL expression language and it performs two important tasks in Struts 2 – data transfer and type conversion. OGNL is flexible and we can easily extend it to create our own custom converter class. Creating and configuring custom type converter class is very easy. The first step is to fix the input format for the custom class. The second step is to implement the converter class. Type converter classes should implement `com.opensymphony.xwork2.conversion.TypeConverter` interface. Since in web application, we always get the request in form of String and send the response in the form of String, Struts 2 API provides a default implementation of TypeConverter interface, StrutsTypeConverter. StrutsTypeConverter contains two abstract methods – convertFromString to convert String to Object and convertToString to convert Object to String. For implementation details, read [Struts2 OGNL Example Tutorial](/community/tutorials/struts2-ognl).

私たちはどのように独自のインターセプターを作成し、アクションにマッピングすることができますか？

We can implement `com.opensymphony.xwork2.interceptor.Interceptor` interface to create our own interceptor. Once the interceptor class is ready, we need to define that in struts.xml package where we want to use it. We can also create interceptor stack with our custom interceptor and defaultStack interceptors. After that we can configure it for action classes where we want to use our interceptor. One of the best example of using custom interceptor is to validate session, read more about it at [Struts2 Interceptor Tutorial](/community/tutorials/struts-2-interceptor-example).

インターセプターのライフサイクルは何ですか？(Intāseputā no raifusakuru wa nan desu ka?)

Interceptor interface defines three methods - init(), destroy() and intercept(). init and destroy are the life cycle methods of an interceptor. Interceptors are Singleton classes and Struts2 initialize a new thread to handle each request. init() method is called when interceptor instance is created and we can initialize any resources in this method. destroy() method is called when application is shutting down and we can release any resources in this method. intercept() is the method called every time client request comes through the interceptor.

インターセプタースタックとは何ですか？(Intāseputā Sutakku to wa nan desu ka?)

An interceptor stack helps us to group together multiple interceptors in a package for further use. struts-default package creates some of the mostly used interceptor stack - basicStack and defaultStack. We can create our own interceptor stack at the start of the package and then configure our action classes to use it.

ストラットのデフォルトパッケージとは何であり、その利点は何ですか？

struts-default is an abstract package that defines all the Struts2 interceptors and commonly used interceptor stack. It is advisable to extend this package while configuring our application package to avoid configuring interceptors again. This is provided to help developers by eliminating the trivial task of configuring interceptor and result pages in our application.

ストラット2のアクションURIのデフォルトの接尾辞は何ですか？それを変更する方法はありますか？

The default URI suffix for Struts2 action is .action, in Struts1 default suffix was .do. We can change this suffix by defining struts.action.extension constant value in our Struts2 configuration file as:

```
<constant name="struts.action.extension" value="action,do"></constant>
```

結果ページのデフォルトの保存場所はどこであり、それを変更する方法はありますか？

By default Struts2 looks for result pages in {WEBAPP-ROOT}/{Namespace}/ directory but sometimes we want to keep result pages in another location, we can provide struts.convention.result.path constant value in Struts2 configuration file to change the result pages location. Another way is to use @ResultPath annotation in action classes to provide the result pages location.

Struts2アプリケーションでファイルをアップロードする方法は何ですか？ (Sutorutsu2アプリケーションでどのようにファイルをアップロードできますか？)

File Upload is one of the common tasks in a web application. That's why Struts2 provides built-in support for file upload through FileUploadInterceptor. This interceptor is configured in the struts-default package and provide options to set the maximum size of a file and file types that can be uploaded to the server. Read more about FileUpload interceptor at [Struts2 File Upload Example](/community/tutorials/struts-2-file-upload-example).

Struts2アプリケーションを開発する際には、どのようなベストプラクティスを守るべきですか？

Some of the best practices while developing Struts2 application are:
1.  Always try to extend struts-default package while creating your package to avoid code redundancy in configuring interceptors.
2.  For common tasks across the application, such as logging request params, try to use interceptors.
3.  Always keep action classes java bean properties in a separate bean for code reuse and implement ModelDriven interface.
4.  If you have custom interceptor that you will use in multiple actions, create interceptor stack for that and then use it.
5.  Try to divide your application in different modules with namespace configuration based on functional areas.
6.  Try to use Struts2 tags in result pages for code clarify, if needed create your own type converters.
7.  Use development mode for faster development, however make sure production code doesn't run in dev mode.
8.  Use Struts2 i18n support for resource bundles and to support localization.
9.  Struts2 provides a lot of places where you can have resource bundles but try to keep one global resource bundle and one for action class to avoid confusion.
10.  struts-default package configures all the interceptors and creates different interceptor stacks. Try to use only what is needed, for example if you don't have localization requirement, you can avoid i18n interceptor.

ストラット2でアプリケーションによって投げられた例外をどのように処理すればよいですか？ (Sutoratto ni de apurikēshon ni yotte nagerareta reigai o donoyō ni shori sureba yoidesu ka?)

Struts2 provides a very robust framework for exception handling. We can specify global results in packages and then map specific exceptions to these result pages. The exception mapping can be done at the global package level as well as the action level. It's a good idea to have exception result pages to provide some information to the user when some unexpected exception occurs that is not processed by the application. The sample configuration in the struts.xml file looks like below.

```
<package name="user" namespace="/" extends="struts-default">
 
<global-results>
    <result name="exception">/exception.jsp</result>
    <result name="runtime_exception">/runtime_exception.jsp</result>
    <result name="error">/error.jsp</result>
</global-results>
 
<global-exception-mappings>
    <exception-mapping exception="java.lang.Exception" result="exception"></exception-mapping>
    <exception-mapping exception="java.lang.Error" result="error"></exception-mapping>
    <exception-mapping exception="java.lang.RuntimeException" result="runtime_exception"></exception-mapping>
</global-exception-mappings>
 
    <action name="myaction" class="com.scdev.struts2.exception.MyAction">
    </action>
    <action name="myspecialaction" class="com.scdev.struts2.exception.MySpecialAction">
    <exception-mapping exception="java.io.IOException" result="login"></exception-mapping>
    <result name="login">/error.jsp</result>
    </action>
</package>
```

Read more at [Struts2 Exception Handling Example](/community/tutorials/struts2-exception-handling-example-tutorial).

これがStruts2のインタビューの質問と回答のすべてです。もし私が見落とした重要な質問があれば、コメントで教えてください。